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THE USE OF BIOMASS WASTES TO FABRICATE

CHARCOAL SUBSTITUTES IN KENYA

Feasibility Study

forming part of the Shell Foundation-supported project on charcoal briquetting in Kenya

March 2004

Chardust Ltd. and Spectrum Technical Services P.O. Box 24371 P.O. Box 69993 Nairobi Nairobi

Executive Summary This study was designed by Chardust Ltd. and implemented jointly by Chardust and Spectrum Technical Services, a Nairobi-consulting firm. The aim was to determine the viability of producing briquetted charcoal fuel from biomass wastes in Kenya. The study was funded by the Shell Foundation and conducted over a two month period in late 2003. The viability of producing fuel commercially from biomass wastes was assessed from four perspectives: (a) Availability: Existence and accessibility of biomass in bulk, preferably with

no competing uses. The study began with a list of 28 potential wastes, which was narrowed down to 20 and then to just ten, according to a ranking system based upon basic availability and accessibility.

(b) Conversion Potential: Physical suitability for drying, carbonisation and

briquetting. Samples of the ten short-listed wastes were sourced and delivered to Chardust in Nairobi, where production trials were carried out. Based on these trials, a ranking system was devised for comparing the wastes in terms of their suitability for fuel production.

(c) Fuel Quality: Energy value and general performance of fabricated fuel. The

third part of the study comprised fuel quality testing, during which the strength, heat output, ash content and other performance characteristics of the various fuels were determined. A further ranking system was drawn up based on the results of these tests.

(d) Enterprise Potential: Willingness and capacity of the waste owner or

producer to collaborate in a briquetting venture. This final component looked at the capacity and interest of the various waste ‘owners’ in collaborating in commercial briquetting enterprise.

From these combined perspectives, it was concluded that there are five biomass wastes available in Kenya with real potential for commercial fuel production. These are macadamia nut shell, sawdust, wattle bark, coffee husk and bagasse. Research is continuing on other materials.

Table of Contents

1. Study Overview................................................................................................ 1 1.1 Introduction ................................................................................................. 1 1.2 Objectives..................................................................................................... 1

2. Biomass Waste Assessment............................................................................ 2 2.1 Introduction ................................................................................................. 2 2.2 Initial Short-listing ..................................................................................... 2 2.3 Further Short-Listing for Production Trials .............................................. 3 2.4 Discussion of Short-listed Wastes............................................................... 6

2.4.1 Sawdust ....................................................................................................... 6 2.4.2 Bagasse........................................................................................................ 8 2.4.3 Wattle Bark................................................................................................. 9 2.4.4 Coffee Husk ............................................................................................... 10 2.4.5 Coconut Husk............................................................................................ 11 2.4.6 Macadamia Nut Shell ............................................................................... 12 2.4.7 Pineapple Pulp .......................................................................................... 13 2.4.8 Sisal Fibre ................................................................................................. 14 2.4.9 Rice Husk .................................................................................................. 15 2.4.10 Maize Stover.............................................................................................. 16

2.5 Conclusions................................................................................................ 17

3. Fuel Production Trials ................................................................................. 18 3.1 Introduction ............................................................................................... 18 3.2 Fuel Production Methodology ................................................................... 18

3.2.1 Drying and Milling.................................................................................... 18 3.2.2 Carbonisation ............................................................................................ 19 3.2.3 Quenching and Mixing with Binders ....................................................... 21 3.2.4 Binding and Briquetting........................................................................... 24 3.2.5 Drying........................................................................................................ 25

3.3 Fuel Production Findings.......................................................................... 26 3.3.1 Sawdust ..................................................................................................... 26 3.3.2 Bagasse...................................................................................................... 27 3.3.3 Wattle Bark............................................................................................... 29 3.3.4 Coffee Husk ............................................................................................... 29 3.3.5 Coconut Husk............................................................................................ 30 3.3.6 Macadamia Nut Shell ............................................................................... 32 3.3.7 Pineapple Pulp .......................................................................................... 32 3.3.8 Sisal Fibre ................................................................................................. 33 3.3.9 Rice Husk .................................................................................................. 34 3.3.10 Maize Stover.............................................................................................. 35

3.4 Fuel Production Rankings ........................................................................ 35

4. Fuel Performance Tests ............................................................................... 37 4.1 Introduction ............................................................................................... 37 4.2 Findings of Fuel Performance Tests......................................................... 38

4.2.1 Friability Tests.......................................................................................... 38 4.2.2 Water Boiling Tests .................................................................................. 39 4.2.3 Overall Performance Ranking.................................................................. 40

5. Business Potential ......................................................................................... 41

5.1.1 Introduction............................................................................................... 41 5.1.2 Ranking Process........................................................................................ 41

6. Conclusions ..................................................................................................... 43 6.1 Overall Viability Rankings ....................................................................... 43 6.2 Summary ................................................................................................... 44

List of Tables Table 1 : Annual Availability and Ash Content of 20 Selected Biomass Wastes............... 5 Table 2 : Preliminary Feasibility Ranking of 20 Biomass Wastes...................................... 6 Table 3 : Ranking of Biomass Wastes by Suitability for Fuel Production ....................... 36 Table 4 : Results of Fuel Friability Tests ........................................................................... 38 Table 5 : Results of Water Boiling Tests ............................................................................ 39 Table 6 : Combined Performance Rankings: Friability and Water Boiling...................... 40 Table 7 : Ranking of Business Potential ............................................................................ 41 Table 8 : Overall Feasibility Ranking for Biomass Wastes............................................... 43 Annexes

Annex A : Biomass Waste Availability ............................................................... i

Annex B : Carbonisation Trial Raw Data ........................................................ii

Annex C : Friability Test Raw Data ................................................................ iii

Annex D : Combustion Test Raw Data.............................................................iv

Feasibility Study: Production of Charcoal Briquettes from Biomass Wastes in Kenya Page 1

1. Study Overview 1.1 Introduction Chardust Ltd. produces charcoal briquettes commercially in Nairobi and Chemelil (Nyanza Province) from two types of waste. The Nairobi operation is based on lumpwood charcoal waste salvaged from charcoal traders in low-income areas of the city. The resulting fuel, known as the Vendors Waste Briquette (VWB), costs less than regular charcoal by weight and burns for longer with no smoke, sparks or smell. The Chemelil operation is based on bagasse, a by-product of sugar processing. The resulting fuel is known as CaneCoal, and offers a direct substitute for lumpwood charcoal in western Kenya markets. Chardust is committed to investigating the viability of additional raw materials for briquette production. This study was commissioned and funded by the Shell Foundation to assess which raw materials might be most viable in the Kenyan context, with the eventual hope of establishing new briquetting enterprises based upon those with the best prospects. 1.2 Objectives The specific objectives of the study were as follows: (a) Biomass Assessment: Determine the types and amounts of convertible

waste biomass available in Kenya. (b) Fuel Production Trials: Carry out trials to produce carbonised, briquetted

fuels using the most promising of these waste materials. (c) Performance Survey: Assess the comparative quality and acceptance of the

fuels produced. (d) Enterprise Assessment: Determine the most viable materials from a

business development point of view. This report summarises the findings of the study, which took place over a two month period in late 2003. The fieldwork was carried out by Richard Kibulo of Spectrum Technical Services and Matthew Owen of Chardust Ltd. Elsen Karstad and John Njuguna of Chardust supervised the production trials and fuel combustion tests.


2. Biomass Waste Assessment 2.1 Introduction For the purposes of this study, biomass wastes were defined as organic residues, surpluses and by-products of agro-industrial processing, available in bulk. A preliminary assessment of biomass waste in Kenya was conducted by means of internet research and a multi-industry desk study to identify potential sources, followed by field visits to selected producers. A total of 28 potential wastes were initially identified. 2.2 Initial Short-listing It was necessary to narrow the list down to a more manageable set of materials with real potential. A desk study was therefore conducted involving literature review at the Ministry of Agriculture headquarters and the Kenya Agricultural Research Institute (KARI) libraries in Kangemi and Muguga. It became apparent that a number of the potential wastes were available in such small quantities that commercial briquetting would definitely not be viable. Other wastes were scattered over such large areas that there would be no means of centralising them for processing, particularly in the extensive cropping systems of the Kenyan drylands. Therefore a total of eight wastes were discounted at this preliminary stage. These were: • barley straw • cashewnut shells • millet stalks • pyrethrum stalks • rapeseed stalks • sesame stalks • soya bean stalks • tobacco waste The 20 wastes remaining for further consideration were: • bagasse • cassava stem • coconut husk • coconut shell • coffee husk • cotton stalk • groundnut shell • macadamia nut shell • maize cob • maize stalk • pigeon pea stalk


• pineapple pulp • rice husk • rice straw • sawdust • sisal fibre • soybean stalk • sunflower straw • wattle bark • wheat straw 2.3 Further Short-Listing for Production Trials In order to permit a direct comparison of viability across the full range of these 20 wastes and residues, a scoring system was devised. This entailed ranking each of the wastes according to a set of six parameters. The sum of the rankings was used to generate an overall ranking of preliminary suitability. The six parameters chosen were: (a) total quantity available; (b) degree of centralisation; (c) clarity of ownership; (d) absence of annual or seasonal variations in supply; (e) absence of pre-existing or competing uses; (f) low ash content. (a) Total quantity available: The most important parameter to consider for

fuel production is the total amount of biomass potentially available across the country. This will define the potential upper and lower limits of production. At least 5,000 t. per annum of a given waste would need to be available to run a modest fuel production facility at commercial levels, meaning that total national production would need to be several times greater than this if multiple production units were to be supported. The 20 biomass wastes were ranked according to total annual availability, with the most abundant (maize stover) assigned a value of 1 and the least abundant (sisal fibre) a value of 201. This was done quantitatively using crop production statistics and known residue percentages derived from the available literature and consultation with experts within the respective sectors.

(b) Degree of centralisation: Certain types of biomass are centrally

processed and hence their residues and wastes are readily accessible at industrial-scale plants. This applies, for example, to bagasse, coffee husk and sisal waste, each centralised at fewer than 10 locations within Kenya. It also applied to sawdust and rice husk under the regulated processing

1 Therefore a lower score denotes greater potential. For any parameter where a tied ranking occurred, the average of the rankings was applied.


arrangements that existed until recently, but the availability situation has changed with economic liberalisation. Other wastes (such as maize stover and rice straw) are generated at field level and remain dispersed, whatever the processing system employed. Such wastes available only at small-scale processing centres or in fields post-harvest were assigned a higher score than those more readily accessible at a limited number of sites.

(c) Clarity of ownership: The ownership of a particular biomass waste must

be clear and a supply arrangement agreed upon if a fuel production facility is to maintain control over its raw material. Certain biomass wastes have no defined owners and a user could be exposed to opportunistic pricing. This could apply to sawdust, often dumped in ‘public’ heaps but probably susceptible to price imposition if a user were to materialise. In other cases a waste might have multiple owners (such as post-harvest residues produced by smallholder farmers) and this would make procurement both complex and potentially costly. Meanwhile biomass that is produced by large industry is available from assured sources on a relatively reliable basis and a supply can be guaranteed through written contract. Such biomass was hence assigned a lower score in the ranking exercise.

(d) Absence of annual or seasonal variations in supply: Where the

ultimate intention is to establish a full-time fuel production facility based on a particular type of waste, it is important that the waste should be available in consistent quantities on a year-round basis. Significant seasonal variations would either necessitate cessation of production for extended periods or would demand an enormous fuel storage capacity to enable year-round demand to be satisfied. Either eventuality would need to be avoided if possible. Hence lower scores were assigned to materials available throughout the year.

(e) Absence of pre-existing or competing uses: A given biomass waste

might be produced in large quantities but could have a pre-existing use. Maize stover, for example, is an abundant resource in Kenya but is used as dry season cattle fodder and is sold for up to Sh 1,750 (USD 232)/t. in dairying regions. Maize cobs are widely used as a firewood substitute, as are pigeon pea stalks. Many crop residues are left in the field after harvest to enhance moisture conservation and recycle nutrients. True wastes are those with no value whatsoever. Some may actually have an associated disposal cost and therefore represent a negative asset for their producers (e.g. bagasse). These were given the lowest rankings.

(f) Low ash content: Low ash content is more conducive to good quality fuel

production than high ash content. Whilst up to 10% ash in a raw material is acceptable, any more than this will result in an end product with as much as 50% ash after the concentration of these non-combustible elements during the carbonisation process and the subsequent addition of binding

2 KSH 75 per USD (Jan 2004).


agents. Such concentrations would inhibit lighting of the fuel and demand significantly more ventilation than is found in standard stoves and appliances. Low ash is therefore generally desirable and wastes with less ash were ranked accordingly.

Parameters (a) and (f) were quantified and the findings are summarised in Table 1.

Table 1: Annual Availability and Ash Content of 20 Selected Biomass Wastes

Biomass Waste Quantity

Available (t/yr dry weight)

Ash Content

Bagasse 109,400 7.0%Cassava stem 89,880 6.0%Coconut husk 31,806 6.0%Coconut shell 13,965 0.8%Coffee husk 215,517 6.0%Cotton stalk 46,998 17.2%Groundnut shell 5,580 4.4%Macadamia nut shell 4,800 4.4%Maize cob 393,071 1.7%Maize stover 1,758,771 4.9%Pigeon pea stalk 1,320,900 2.0%Pineapple waste 46,200 7.0%Rice husk 12,000 16.5%Rice straw 103,250 19.2%Sawdust 226,800 2.0%Sisal fibre 135 6.0%Soybean stalk 1,554 5.0%Sunflower straw 13,720 4.0%Wattle bark 1,890 4.0%Wheat straw 279,116 8.5%

Note: Quantities are quoted on air dry basis (13% moisture content).

Full details and sources at Annex A. For each of the six criteria, the biomass wastes were ranked from 1 (most promising) down to 20 (least promising). The sum of the rankings was used to generate an overall ‘score’ denoting preliminary feasibility. Table 2 summarises this overall ranking of the 20 materials.


Table 2: Preliminary Feasibility Ranking of 20 Biomass Wastes

Biomass Waste Total

Quantity Available

Degree of Centralisation

Clarity of Ownership

Lack of Seasonal

Variability

Absence of Existing Uses or Values

Ash Content

TOTAL RANKING

Sawdust* 5 7 8 2 2 3 27

Bagasse* 7 3 2.5 2 1 16 31.5

Wattle bark* 18 5 2.5 2 8 5 40.5

Coffee husk* 6 2 2.5 14.5 5 12 42

Macadamia nut shell* 17 4 2.5 14.5 9 7 54

Coconut shell 12 9 6 4.5 10 13 54.5

Pineapple pulp* 11 1 2.5 7 19 15 55.5

Sisal fibre* 20 8 7 6 4 11 56

Rice husk* 15 6 9 8 3 18 59

Wheat straw 4 11 10 14.5 6 17 62.5

Coconut husk* 13 16 13 4.5 18 1 65.5

Maize cob 3 20 14.5 14.5 15 2 69

Groundnut shell 16 10 12 14.5 11 8 71.5

Pigeon pea stalk 2 19 17 14.5 15 4 71.5

Rice straw 8 12 11 14.5 7 20 72.5

Maize stover* 1 17 14.5 14.5 20 9 76

Sunflower straw 14 15 16 14.5 12 6 77.5

Cassava stem 9 18 20 14.5 15 14 90.5

Cotton stalk 10 14 18 14.5 15 19 90.5

Soybean stalk 19 13 19 14.5 15 10 90.5

* - selected for subsequent fuel production trials From this initial ranking exercise, ten biomass wastes were selected (as indicated) for fuel production trials. Maize stover, though not one of the top-ranked materials, was included at the request of a farmer who had generously donated a large quantity for conversion tests. 2.4 Discussion of Short-listed Wastes This section gives additional information on the ten biomass wastes that were short-listed for the fuel production trials, in order of apparent suitability as per the initial ranking. 2.4.1 Sawdust There are approximately 350 sawmills in Kenya, the majority of them in Central, Eastern, Western and Rift Valley Provinces. They process approximately one million cu.m. of pine and cypress clearfell per annum, the majority of it from government-owned plantations, in addition to an unknown quantity of thinnings and clearfell of other species and farm-grown trees.


The industry is characterised by a large number of small sawmills, 90% of which process less than 5,000 m3 roundwood over-bark per annum. They generally use out-dated and inefficient equipment based on circular sawing technology. The result is an average conversion efficiency of just 32%, meaning that 68% of the roundwood timber processed ends up as off-cuts, wood chips, shavings or sawdust.

Circular saw at Kibirichia, Meru: Affordable but wasteful technology

It is estimated that Kenyan sawmills generate up to 230,000 t. of sawdust each year (air dry basis), adding to an unknown quantity in existing sawdust mountains across the country. Some mills are able to sell small quantities as domestic fuel, mulch, animal bedding or as floor covering for bars and restaurants. There is also a localised demand in Nyeri for mosquito coil production. But in most cases the material has no economic value and is burned in situ, used to repair access roads or simply taken away and dumped. This often has a cost implication for sawmills. In wet environments there is also the potential for leaching of tannic acid from sawdust heaps into groundwater.

Waste sawdust heap in Elburgon


The primary drawback to the use of sawdust for fuel production is lack of clarity of ownership, with sawmillers tending to dump the material adjacent to their premises or on nearby roadsides. In the absence of a buyer it has no value and no clear owner. But with the establishment of a sawdust-based briquetting operation it could begin to attract a charge. The largest sawmills with the highest volumes of available waste under assured ownership would therefore represent the most viable partners for a fuel production operation, as binding supply agreements could be entered into for assured provision of free or fixed-priced raw material. The timber industry is currently in a state of turmoil with heavy restrictions imposed on access to government forests. The amount of sawdust currently produced is therefore abnormally low. But with an anticipated shift toward more controlled, sustainable logging practices the industry is expected to consolidate progressively with fewer, larger timber processors and more centralised (if somewhat diminished) availability of sawdust. This will ultimately improve the accessibility of the material and bring more of it under clearly defined ownership, which is a desirable trend in terms of bulk fuel production. In summary, sawdust is a highly attractive raw material from the point of view of commercial briquette production. It is concentrated in large volumes at point sources and is relatively easy to process, as section 3.3.1 below outlines. Current uncertainty in the forestry sector is affecting overall availability, however, and issues of ownership need to be clarified. But it is hoped that a consolidation in the sawmilling sector will eventually lead to more assured availability at fewer sites. 2.4.2 Bagasse Bagasse is the fibrous residue of the sugarcane stalk after the extraction of sugar. At the time of the study there were four operational sugar factories in Kenya, all of them in the west of the country3. The largest of these, Mumias Sugar Company, employs a modern steam diffusion process that takes in finely chopped cane and produces a relatively powdery bagasse. The other factories use traditional crushing techniques that process more roughly chopped cane and consequently generate a more fibrous form of bagasse. This is somewhat easier to handle than the Mumias bagasse and was the variety used in the fuel production trials (see 3.3.2). 40% of the sugar cane processed in Kenya ends up as bagasse. About 85% of this bagasse is burned on site at the factories to generate steam for the evaporative extraction of sugar. Some of the steam is also used to produce electricity for factory operations and (in the case of Mumias) for sale to the local grid. Nevertheless, 15% of the bagasse is still typically surplus to requirements - a

3 Mumias, Chemelil, Nzoia and South Nyanza (SONY).


total of some 110,000 t./yr. (air dry weight). This bagasse accumulates in huge stacks close to the factories and is periodically hauled away by tractors and burned in fields. Sugar factories spend huge sums of money on this disposal process, which costs them at least Sh 150/t. The surplus bagasse not only has no use for them, it is actually a negative asset.

Heaps of surplus bagasse at Chemelil sugar factory, Nyanza Bagasse is a massive potential source of waste concentrated at a small number of locations. Given that sugar cane is largely grown as a commercial monoculture, there also tends to be a relative shortage of lumpwood charcoal in sugar growing areas. With high-priced charcoal markets close to such an abundant biomass resource, the potential for bagasse-based briquette production is further enhanced. In mid-2002, Chardust entered into a collaborative partnership with the Chemelil Sugar Company to establish a plant to convert bagasse into a charcoal briquettes. With the initial support of the UK Department for International Development (DFID), the ultimate aim of the venture is to sell 5 t. of briquettes per day, which will usefully dispose of 25 t. of bagasse. Similar potential may exist at other sugar factories in the region. 2.4.3 Wattle Bark The wattle tree (Acacia mearnsii) is an Australian species that was introduced to Kenya by the colonial government for commercial tannin production. Tannin, which is used in the treatment of leather, is extracted from the wattle bark through a high pressure steam process and the bark is then discarded. Up to the late 1990s there had been two significant tannin producers in Kenya: the East African Tannin Extract Company (EATEC) in Eldoret and the Kenya Tanning Extract Company in Thika. However EATEC went out of business and most of its wattle plantations were sold or settled. The Thika plant continues to operate, but productivity is low and the equipment is outdated and inefficient.


Raw material supply means that the plant is moribund and looks likely to soon close down. There is, however, a new tannin extract facility in the Athi River Export Processing Zone (EPZ) about 30 km east of Nairobi called Kenya Vegext (EPZ) Ltd. Waste wattle bark, from which the tannin has been extracted, is accumulating at this plant at rate of about 5 t. per day (dry weight). It seems to have no existing uses and brings a significant disposal problem for Kenya Vegext.

Wattle bark awaiting processing at Kenya Vegext Waste wattle bark after milling and steaming Wattle bark is therefore available from a reliable commercial outfit relatively close to Nairobi. Although it has quite high moisture content, as section 3.3.3 below outlines, it may have commercial conversion potential due to the consistency and volume of supply, though probably in combination with other ingredients. 2.4.4 Coffee Husk Coffee is a staple cash crop throughout the Kenyan highlands above 1,800 m. The processing of coffee generates two types of waste. The first is a pulp produced during the separation of the cherry from the bean. As this pulp is available only at dispersed coffee factories located close to the areas of cultivation and already has value as fertilizer and animal feed, it was not considered within this study. The second waste is the coffee husk that is separated during the milling process. Of the 50,000 t. of coffee that Kenya produces annually, some 25% is discarded as husk (also known as parchment). Coffee husk is fibrous, low in moisture, uniform in size and low in ash. This makes it well suited to carbonised fuel production. It is also readily available at relatively few locations - 80% of the husk is concentrated at just six sites.


The biggest milling organisation, processing 70% of Kenya’s coffee at five sites in Dandora, Sagana, Meru, Tala and Bungoma, is the Kenya Planters Cooperative Union (KPCU). Total husk availability from KPCU is about 9,000 t/yr, with the largest volumes available at the Dandora plant. The second biggest miller is Thika Coffee Mills, a subsidiary of the Kenya Nut Company, which processes a further 20% of the national output and generates some 2,500 t. of husk per annum. Among the smaller producers are SOCFINAF and the Komothai Coffee Growers Cooperative Union (KCGCU), both in Ruiru. Coffee husk is not regarded as waste. Some is sold to farmers for use as mulch, poultry bedding or cooking fuel. It is also bought by clay works for firing bricks and tiles. However the price is generally only Sh 300-500/t., and most of the mills consider its disposal a community relations exercise rather than a commercial venture. It could be available for briquetting if a mutually acceptable price and supply arrangement could be agreed upon with the mills. The husk is normally available throughout the year in reasonable quantity, with peak supply from January to May. It is worth mentioning a second type of coffee husk derived from ‘buni’ coffee. This is low grade coffee harvested at the wrong time and processed in a single stage that separates the pulp and the husk together. It is from buni husk that KPCU has been making a type of charcoal briquette known as ‘kahawa coal’ since 1988. This product has failed to penetrate the market in a significant way, partly due to poor marketing and packaging, and partly because it emits acrid fumes - probably due to incomplete carbonisation and the use of unbaked rice flour as a binder. Unsold stocks of Kahawa Coal held by KPCU are believed to exceed 600 t. The demand for any buni husk left over from the kahawa coal operation is rather high as it makes a good fertilizer. There appears to be good potential for profitable fuel production based on coffee husk, provided that the operation was more competitive and quality-oriented than that being run by KPCU. 2.4.5 Coconut Husk Coconut production in Kenya is concentrated in Coast Province up to 50 km inland. The outer husk of the coconut is a light-weight fibrous capsule that is removed from the nut after harvesting. In countries where the fibre (coir) is processed and sold this removal entails a lengthy process of pre-soaking to soften the fibres. In Kenya, where damage to the coir is not considered detrimental as it has no commercial value, the de-husking is carried out immediately after picking with a sharp stick or blade. The husk usually comes away in two or three sections approximately 20 cm long, 10 cm wide and 5 cm thick.


Fresh coconut husks after removal from the nut For further processing the husks must be broken up and milled. The resulting coir fibre was originally used for ropes and ship rigging but is now also used to produce mats, rugs, fishing nets, sacks, upholstery padding, insulation and potting compost, primarily in Asia. In Kenya the husks are generally left to rot in large heaps in the coastal plantations. They are therefore readily available for purposes such as fuel production.

Dry coconut husk and the coir fibre produced after milling The total volume of husk available is in excess of 30,000 t./yr. and the material is relatively low in moisture and ash, making it quite suitable for fuel production trials. 2.4.6 Macadamia Nut Shell Macadamia nut shell is available from the Kenya Nut Co. in Thika town, 45 km north-east of Nairobi. This material is high in energy, needs no pre-drying, is not hygroscopic and is of a good particle size for carbonisation using Chardust's open-bed system.


The Kenya Nut Company Ltd. produces various macadamia products under the ‘Nutfields’ and ‘Out of Africa’ brands, including roasted nuts, honey-coated nuts and chocolate-coated nuts. It is also involved, on a much smaller scale, in the processing of cashew nuts. The company cracks 5,000 t. of macadamia nuts per annum at 12% moisture content, of which 80-82% is discarded as waste. This amounts to 4,000 t./yr or 11 t./day. Output is expected to rise by 20% in 2004. At present, Thika Clothing Mills purchase some of the shells at Sh 1,000/t., but large volumes still have no productive use. The company even uses the shells to surface roads and car parks.

The car park at the Kenya Nut Company, made of macadamia nut shells The R&D Manager is keen to make more productive use of these shells and has various ideas for collaborative charcoal briquetting ventures. Cashew nut shells are also available, as is coffee husk from the adjacent Thika Coffee Mill (a subsidiary company). 2.4.7 Pineapple Pulp The processing of pineapples to produce juice and segmented fruit for canning results in the production of large quantities of waste pulp and fruit fibre. Pineapple processing is relatively inefficient and up to half of the fruit is actually discarded, resulting in annual pulp availability in Kenya of around 46,000 t. dry weight (140,000 t. fresh). This pulp is available exclusively from Del Monte Ltd. in Thika, the country’s only pineapple processing operation. As a bulk raw material readily available from a single source, pineapple pulp initially showed promising signs as a potential raw material for fuel production. However it was determined as the research progressed that pineapple pulp has a competitive alternative use as a fertilizer and soil improver on the Del Monte estates. Small quantities are also given away to dairy farmers, though its high acidity and milk-tainting effect make it suitable only as a minor feed supplement. Unsuccessful trials on pulp-based alcohol production were attempted some years ago and Del Monte is conducting research into vinegar production.


Pineapple pulp could represent a viable source of raw material for a briquetting plant if the economics of commercial production compared favourably with the cost of importing fertilizers to substitute for the soil maintenance functions that it currently performs. This question of competitive value notwithstanding, fuel production trials were still conducted as part of the technical research process (see 3.3.7 below). 2.4.8 Sisal Fibre Sisal is used mainly to produce bags, ropes and floor coverings. In Kenya the plant is grown by seven companies on ten different estates. These are located in Coast Province (Vipingo, Kilifi, Mwatate and Voi), Eastern Province (Kibwezi) and Rift Valley Province (just north of Nakuru). A number of wastes and low grade biomass materials are produced during the processing of sisal from the green leaf to the manufactured item: • Decorticator waste: Moist fibrous material produced at the first stage of

processing from the green leaf. It is used locally around sisal estates as cattle feed or left to dry and burned in the fields. It did not prove possible to determine the amount of decorticator waste potentially available in Kenya due to its dispersed nature.

• Brush Room waste: Low grade sisal fibre produced at the level of the

estate. There are 10 recognised grades of sisal fibre, the lowest of which are known as Tow I, Tow II and Flume Tow (in decreasing order of quality). Tow I and II represent about 6% of Kenya’s sisal production and Flume Tow just 62 t./yr. (less than 0.3% of total output). Tow I and II have a commercial value of around US$300/t. and cannot therefore be considered true ‘waste’. Meanwhile Flume Tow, while legitimately classifiable as waste, is available in such small quantities that it could not form the basis for any commercial fuel production.

• Processing waste: Most of Kenya’s 22,000 t. of sisal fibre is exported in a

semi-processed state. But there are three spinning factories in the country, which process the fibres further to manufacture mats, bags and ropes for the local market4. These factories all produce fibrous waste as a by-product of the combing, cleaning and spinning processes. The waste comes in the form of both a light fluffy lint and a coarse fibre. The latter is the most suitable for fuel production and is also available in larger quantities, though still only 2-3 t./day across the three sites.

Sisal processing waste is low in moisture, easily carbonised and low in ash (see section 3.3.8). It also available all year round at no cost. The only competing use seems to be local furniture makers who salvage a small percentage for cushion

4 Premier Bag and Cordage Ltd. (Juja), Teita Estate Ltd. (Nairobi) and Majani Mingi Ltd. (Nakuru).


stuffing. But its limited availability is a serious constraint. It would need to be combined with other raw materials available in the same locality (e.g. pineapple pulp in Juja or sawdust in Nakuru) if it were to be used as an input for commercial fuel production. 2.4.9 Rice Husk There are two by-products of rice processing that were initially considered in this study. The first was rice straw, which is left in the fields after harvesting. Collection and centralisation is impractical and the potential use of this residue was not pursued further. The second waste considered was rice husk, which has traditionally been available in large quantities in Kenya from two rice mills situated in each of the country’s main irrigation schemes, one at Ahero (Kisumu) and one at Mwea (Kirinyaga). 22% of the rice paddy which is processed in modern rice mills ends up as waste husks. The husks are used locally as a dry season feed supplement, horticultural mulch and to strengthen burned bricks, but large surpluses have always remained and been made available at a nominal price of Sh 1,000 per lorry load.

Waste rice husk at Ngurubani, Mwea Up until 1997, the Ahero and Mwea mills processed 80% of all Kenya’s rice. This was a total of some 30,000 t. of paddy per annum which generated 6,600 t. of husk. The husks were available in bulk right at the factory sites. A further 1,650 t. were generated by dispersed rice growers in rainfed cropping areas, mostly in Coast Province. Though rather high in silica (which represents ash during combustion), rice husks are already dried to just 13% moisture content during the milling process and hence represent a potentially promising raw material for fuel production. The husk availability situation changed drastically after 1997 following a dispute between rice farmers and the National Irrigation Board (NIB), the majority shareholder in both the Mwea and Ahero mills. Farmers have since virtually stopped processing their rice at these mills and their combined annual paddy


throughput is now well below 900 t. A large number of private and cooperative-run mills have instead sprung up in both localities. There are thought to be over 200 such mills in the Mwea area alone, many of them in Ngurubani town. The largest process no more than 4 t. of paddy per day. This has had major implications for rice husk availability. Not only is the material now dispersed over multiple sites and hence much harder to consolidate, but it is also of poorer quality as far as fuel production is concerned. This is because most small-scale processors carry out milling and polishing of the rice as a single step, whereas the large mills separate the two stages and produce rice husks and polishings as distinct by-products. The combined process results in a powdery bran rather than true rice husks. This material is much harder to handle than fibrous rice husks and would carbonise poorly. Other than the Mwea and Ahero mills, now operating at barely 3% of capacity, fewer than 20 mills in the country are believed to still separate the milling and polishing processes. Therefore a negligible quantity of true husk is currently available. The situation may change if the dispute between the farmers and the NIB can be resolved. Centralised milling could be resumed. Chardust therefore looked upon rice husk as a potentially viable fuel production material which may once more become available in large quantities at centralised locations - but only if a milling and purchasing arrangement acceptable to the farmers can be negotiated. 2.4.10 Maize Stover5 Maize stover comprises the stems and leaves of the maize plant, but excludes the cob and the maize grain itself. In most maize-growing areas of Kenya it is available twice annually just after the maize harvest, and may be cut standing or left to dry and harvested some weeks later. Maize stover was not in fact one of the highest-ranked wastes in the preliminary part of the Chardust study, but an offer of free material was made by an interested farmer in Naivasha so it was decided to carry out full production tests all the same. Aside from any technical aspect of handling and processing, the principal drawbacks with maize stover as far as fuel production is concerned are its wide dispersal (2-4 t. available per hectare), its seasonal variability in supply and, above all, its existing value as cattle fodder. Maize stover is in fact the principal dry season feed for dairy cattle in Kenya when napier grass is in short supply. It is traded at around Sh 1,750/t. This essentially precludes any alternative use for fuel production, but trials were nevertheless carried out for research purposes and to learn lessons for the processing of other, similar materials.

5 ‘Stover’ is a term used to describe the residues of thick or coarse-stemmed cereals such as maize, sorghum or millet, while the residues of slender-stemmed cereals such as rice, wheat, barley and oats are generally termed ‘straw’.


2.5 Conclusions Following a process of ranking and short-listing, a total of ten biomass wastes were considered worthy of inclusion in the fuel production trials and quality tests, as discussed in the preceding sections. Of these ten, bagasse, sawdust, coconut husk, coffee husk, wattle bark and macadamia nut shell already showed promising signs of commercial viability based on their availability within Kenya and general physical suitability. Meanwhile pineapple pulp, rice husk, sisal fibre and maize stover were considered less likely raw materials for commercial fuel production, based primarily on their more limited availability, but were nevertheless included for comparative purposes and to build experience and knowledge.


3. Fuel Production Trials 3.1 Introduction Fuel production trials were carried out on the ten most promising biomass wastes, as derived from the short-listing process described in section 2 above: • Sawdust • Bagasse • Wattle Bark • Coffee Husk • Coconut Husk • Macadamia Nut Shell • Pineapple Pulp • Sisal Fibre • Rice Husk • Maize Stover 3.2 Fuel Production Methodology 3.2.1 Drying and Milling The first processing stage was the drying of the raw biomass from its fresh state to around 13% moisture content. With some materials, such as coffee husk and coconut husk, drying was in fact unnecessary as they were already very low in moisture. With others, such as sawdust and bagasse, it was an important and sometimes lengthy process. The materials to be dried were laid out on the ground or, in the case of rain, on black plastic sheeting, to a depth of about 25 cm. With good weather and regular manual turning they mostly dried in 2-4 days. One of the materials tested in the trials (coconut husk) also required milling before the next stage of carbonisation. This was carried out using a standard hammer mill.


Sharpening the blades of the hammer mill for processing coconut husk 3.2.2 Carbonisation Carbonisation was carried out using an open pit downdraught system devised by Chardust.

Open pit downdraught carboniser for particulate biomass


Numerous modifications were made to the original carboniser, mainly to optimise the flaring of volatiles

Raw biomass was lit in the carboniser pits using a propane torch For all of the tests conducted, a wood fire was established in the combustion chamber two hours prior to carbonisation, which pre-heated the chamber and provided the flame for initial ignition.


Adding feedstock during the carbonisation process 3.2.3 Quenching and Mixing with Binders When the depth of carbonised material in the kilns had built up sufficiently to occlude airflow (between 10 and 25 cm. depth depending on particle size and density), the charcoal powder was removed and extinguished.

Carbonised material was removed from the pits and quenched Two methods of quenching were used. One was to seal the hot charcoal powder in airtight metal drums with a pin-prick hole in the lid to prevent internal vacuum and collapse of the drum upon cooling. This proved satisfactory for relatively dense materials such as coffee and rice husk. For bulky material such as sisal, coconut and bagasse, direct quenching with water from a sprinkler proved most efficient. The latter method could result in unacceptably high moisture levels for subsequent briquetting if excess water was applied. The following photographs depict each of the raw materials and the carbonised samples that were produced:


Sawdust

Bagasse

Wattle bark (no carbonised picture available)

Coffee husk


Coconut husk

Macadamia nut shell

Pineapple pulp

Sisal fibre


Rice husk

Maize stover

3.2.4 Binding and Briquetting Prior to the next stage of briquetting, the carbonised materials were mixed with wet binders. The selected binders were clay, molasses and gum arabic, in various combinations according to the material being processed.

The carbonised material was mixed with suitable binders prior to briquetting The mixture of wet carbonised material and binders was fed into Chardust’s standard roller briquetting machines, to produce pillow-shaped 35 gm. briquettes.


Briquetting was carried out using a roller briquettor Chardust’s briquetting machines are driven by 3-phase electric motors and produce up to 600 kg. of fuel per hour. 3.2.5 Drying The briquettes were sun-dried for 1½ to 3 days (depending on weather conditions) on open trestles at a density of 15 kg./sq. m.

The briquettes were dried on open trestles, with occasional turning


The final evaluation stage after drying was to carry out a number of performance tests on the fuel briquettes. These are described in section 4 below. 3.3 Fuel Production Findings This section describes the processing of each of the ten biomass wastes and any particular characteristics that emerged that were either conducive or detrimental to carbonised fuel production. 3.3.1 Sawdust Preparation Sawdust was collected from sawmills in Nanyuki and Limuru. Both samples were fresh and contained 42 and 46% moisture respectively. This amount of moisture made the material costly to transport, especially as sawdust has an inherently low bulk density. It also made the drying process difficult. The material was spread out in a layer approximately 15 cm thick on dry ground or plastic sheets (depending on the weather) and manually turned two or three times daily for up to a week before an acceptably low moisture level of less than 15% was achieved. Nairobi has relatively low humidity at most times which facilitates air/sun drying in such a fashion, but the labour required was still considerable. Sawdust was one of the hardest materials to dry. A forced-air dryer, preferably heated, would be required for a commercial-scale operation. Carbonisation The open pit downdraught method of carbonisation had initially been developed specifically for sawdust. Providing the material was below 15% moisture content (preferably even drier) and average particle size not below 2 mm, carbonisation was swift and conversion rates very good, averaging 34%. The widespread use of circular saws in Kenyan sawmills results in relatively large particle sizes of 2-4 mm. compared with sawdust produced by more efficient band saws. The downdraught system of carbonisation is better suited to these larger particle sizes. Wood type and species is not important. Lighting the first batch of sawdust proved tedious at times, but with the use of kindling material such as dry grass the pits could be fired up more rapidly and evenly. Sawdust particles reduced appreciably in size when carbonised, restricting airflow down through the kiln beds. The kilns therefore had to be emptied once the charred sawdust reached a depth of about 20 cm. Quantities of acrid, irritating smoke were produced if the kilns were agitated prematurely prior to being emptied. This problem was eventually eliminated with experience. Quenching was best achieved by transferring freshly carbonised material from the pits to 200 litre drums and capping them to exclude air. After a minimum of 20 minutes in the drums, the carbonised sawdust could be poured


out, though a careful watch had to be maintained to control re-ignition. Water was sprinkled on the occasional ‘hot spot’. Full day (8-hour) sawdust carbonisation sessions consistently produced in the region of 700 kg. of charcoal from 2 t. of raw sawdust - an input rate of 250 kg./hr., representing nearly double this amount of fresh wet material. Including the 15% clay binder, around 800 kg. of sawdust charcoal briquettes could be produced in one day at this rate using a single carboniser. Briquetting Freshly carbonised sawdust proved difficult to briquette, even with the normal addition of water and binding clay. This problem was overcome by allowing the mixture to sit for at least 8 hours prior to briquetting. This permitted the water to be properly absorbed into the charcoal and act as a lubricant during briquetting. The recipe for a 50 kg. batch of briquettes was standard: 20 kg. water + 7.5 kg. clay + 42.5 kg. carbonised sawdust. The carbonised sawdust briquettes were comparatively light and dried rapidly on the open trestles. Moisture content could be brought from 40% to less than 10% in dry sunny conditions within two days. Briquette strength and density was on the low side when compared with other briquette types produced, but remained acceptable. 3.3.2 Bagasse Preparation Two 5 t. loads of bagasse were collected from the Chemelil Sugar Company near Kisumu. The first was taken straight from the plant and was absolutely fresh, at 47% moisture when received in Nairobi. The second was collected from the large heap of dumped bagasse located adjacent to the factory and was significantly drier, at 24% moisture. Both samples were quite pithy and dusty, representing bagasse from immature cane due to a prevailing shortage of mature raw material. The bagasse particle size ranged from fine powder to fibrous chunks up to 8 cm long and 2 cm wide, with an average size of approximately 2.5 x 1 cm. The first bagasse consignment was received during a period of wet weather and took almost a week to dry. Tarpaulins were needed to cover it during rain. The material was spread out 25 cm deep and turned frequently for proper drying. Dry material was periodically collected and bagged from the top layer. The handling and drying process was therefore moderately difficult, and a mechanical drying system would be recommended for any commercial fuel production based on bagasse.


Sample of bagasse before drying Carbonisation Bagasse carbonised swiftly using the open-kiln downdraught method. It was easy to light and a flare was quickly established in the carboniser’s combustion chamber, which led to minimal smoke emissions. The work proceeded quickly and a good rhythm was established as the carbonised beds were alternately loaded, topped up and emptied. The smoke was found to be mild and non-irritant, with little escaping from the pits during the final stirring prior to emptying. An 8 hour shift, starting cold, produced 400 kg. of charcoal from 2 t. of dry bagasse - a fairly low conversion rate of 20%. Care had to be taken with the raw material to ensure that its original moisture level was below 15%, otherwise conversion rates could fall as low as 17% and ash levels in the briquetted fuel became proportionately higher. Quenching was best achieved using water administered by a garden-type sprinkler. Carbonised bagasse was found to be light, bulky and powdery, so not much could be fitted into the 200 litre steel drums used for storage. Care had to be taken not to over-quench as the charcoal powder soaked up as much as 60% water by weight - far more than would be desirable for briquetting. A further lesson learned was that freshly carbonised bagasse had a tendency to re-ignite and therefore had to be left in a pile for at least 8 hours if it was to be bagged, during which time it had to be turned once or twice to ensure that there were no hot spots. It proved preferable to mix in the binders and carry out briquetting immediately to avoid the need for this observation period and the risk of re-ignition. Overall, however, bagasse was the easiest of the eight materials to carbonise - though at just 20%, also the least efficient.


Briquetting ‘CaneCoal’ is the name given by Chardust to carbonised bagasse briquettes. Briquetting was easy and a smooth homogenous product was produced of intermediate strength (see section 4.2 for data on comparative friability). 15% clay was used for binding and open air drying was found to be rapid. 3.3.3 Wattle Bark Preparation A lorry load of wattle bark was provided by Kenya Vegext in the Athi River EPZ. Moisture content upon arrival was approximately 40% and open-air drying was possible, though was rather slow and obviously weather-dependant. Surplus heat seems to be available at the Athi River plant and may be a longer-term option for pre-drying at source. Carbonisation The wattle bark carbonised easily and quickly at a good 33% conversion rate - similar to bagasse. Briquetting Briquetting posed no problems and the briquettes were of medium-high density. 5% clay was used as a binder & release agent in addition to 2% gum arabic. 3.3.4 Coffee Husk Preparation A 9 t. load of coffee husk was collected from the Nairobi milling plant of the Kenya Planters Co-operative Union (KPCU). Coffee husk is a light coloured, flaky and dusty material similar in texture to fish scales with an average particle size of 3-4 mm. The use of dust masks during loading and off-loading was required. It is surprisingly dense, 50% more so than sawdust.

Unloading coffee husk at the Chardust plant


Carbonisation Some difficulty in initial lighting was experienced during carbonisation, resolved by the addition of a thin layer of bagasse on top of the coffee husk to act as kindling. Dried grass, straw, or even sawdust could also facilitate start-up. An even ignition over the entire surface of the pit proved important, as coffee husk tended to burn vertically (up and down) as opposed to horizontally. The material carbonised quickly once properly lit, and the flared volatiles produced tremendous heat in the combustion chamber. Due to the slow lateral spread of pyrolysis in the pits, there was a fair amount of uncarbonised material remaining when the pits were stirred prior to emptying. This resulted in a brief period of dense smoke output. Skilled operators would be able to reduce smoke at this time with practice, and indeed efficiencies increased over the four carbonisation sessions. Yield of carbonised material from the 9,100 kg. averaged an excellent 33%. Quenching was extremely easy, with the material auto-extinguishing upon removal from the influence of heat and airflow in the kiln beds. Very few hot spots were detected in the mounds of carbonised coffee husk removed, and these were quickly extinguished when sprinkled with water. Briquetting The carbonised coffee husk crumbled to dust and was easily shaped, allowing the clay to bind firmly. Briquetting was easily accomplished, with a dense hard briquette produced. Drying took a noticeably longer period of time, commensurate with the briquette’s density. The briquette was surprisingly homogenous, unlike the rough-grained sawdust briquette. 3.3.5 Coconut Husk Preparation Approximately 7 t. of coconut husk were collected at Kikambala on the Kenyan coast and delivered to Chardust. As de-husking of coconuts takes place at distinct locations, the material accumulates in piles and this facilitates collection and loading. As coconut husks are large and fibrous, it was necessary to mill them in a 15 kW fixed-beater hammer mill prior to carbonisation. In order to handle this extremely fibrous material, the hammer mill beater tips were replaced with specially sharpened steel. Even so, the milling rate was slow, with a maximum of 200 kg. of husk processed per hour. The operation was also extremely dusty and a special collection container had to be constructed to vent the material outside the milling building.


An attempt was made to contain the dust from coconut husk milling The sample of coconut husks was relatively dry upon delivery to Chardust. This obviated the need for the prolonged sun drying that was required for sawdust, bagasse and other materials. But the milling process still made the pre-preparation of the husks a fairly lengthy and difficult operation. Carbonisation The matted and bulky coir fibres proved problematic to carbonise. Lighting was easy and the rate of carbonisation acceptable, but the milled coconut husk was very low in density and carbonised in a rather patchy manner. Bags of coconut fibre weighed approximately half as much as same-size bags of sawdust and a third of coffee husk. Airflow was unimpeded through the material and this resulted in occasional flare-ups and much smoke. Quenching proved difficult and the carbonised husk was too bulky for sealing into drums. Much water was needed to extinguish the glowing charcoal powder and pre-drying was required prior to briquetting. Charcoal yield was on the low side at 22%.


Coconut husk carbonised unevenly and with a lot of smoke Briquetting No problems were encountered with briquetting carbonised coconut fibre, and the resulting briquettes were acceptably hard, homogenous and dense. 3.3.6 Macadamia Nut Shell Preparation A load of macadamia nut shell was provided by the Kenya Nut Company in Thika. It arrived in dry, clean and homogenous condition, and no pre-processing was required. Carbonisation Conversion to charcoal was excellent at between 37 and 41%, and carbonisation rate was high. The carbonised shell was brittle and glassy. It was milled to granular size before mixing with binders and briquetting. Briquetting A combination of 7% clay and 2% gum arabic was used for binding. In order to obtain proper adhesion during briquetting, the material had to be mixed with water and binders 12 hours prior to briquetting. Sun drying took slightly longer than normal due to the high density of the resulting briquette. 3.3.7 Pineapple Pulp Preparation A load of 3 t. of pineapple pulp was sourced from Del Monte in Thika. The pulp contained up to 80% moisture when fresh and was light yellow in colour and sweet-smelling. Its high water content demanded a prolonged period of drying prior to carbonisation. Fortunately the material proved surprisingly resistant to decay during this period, perhaps due to high acidity. The dried material was brown, flaky and had a fairly uniform particle size of around 1 cm.


Pineapple pulp after drying Carbonisation The dried pineapple pulp was one of the easiest materials to carbonise and flared cleanly in the carboniser chimney. The material was easy to light with a propane torch and equally easy to quench at the end of the process. The conversion efficiency was very high at 47%, reflecting a high ash content in the finished product - in fact the third highest of the materials tested, at over 50% (see section 4). High carbonisation efficiency is not a reliable indicator of suitability in itself, but needs to be considered in conjunction with the proportion of residual ash in the carbonised material. Briquetting Pineapple charcoal was one of the easier materials to briquette due its homogeneity. 3.3.8 Sisal Fibre Preparation 3 t. of rain-wetted sisal fibre were collected form the Premier Bag and Cordage Company near Thika. This factory produces sisal mats, carpets, ropes and bags from sisal fibre purchased directly from farm-based decortication facilities. Sisal fibre waste is dumped in a field adjacent to the plant. The light coloured fibre was about as thick as horse-hair and individual strands measured 5-15 cm in length. The matted rain-wetted material dried quickly in the sun with a minimum of turning. Preparation was therefore relatively simple. Carbonisation Of the ten materials investigated, sisal fibre was the most difficult to carbonise. In the pits the material had a tendency to either smoke excessively or burst into flames. The springy, bulky fibres did not carbonise homogenously at all. Top-dressing to damp out flames and reduce the acrid white smoke often just added to the problem. Conversion was nevertheless very good at 32%. But, as with


other materials carbonising with similarly impressive efficiency (such as rice husk and pineapple pulp), over 40% of the resulting charcoal was actually ash, increasing to over 50% with the addition of binders during briquetting. Quenching of the sisal fibre proved extremely difficult, with some hot spots still requiring drenching up to 12 hours later. It was difficult to stop the pyrolysis process at the charcoal stage and the fibre had a tendency to burn rapidly and completely.

Sisal fibre was by far the smokiest material during carbonisation Briquetting Briquetting sisal charcoal was the easiest of all the materials tested. A smooth, homogenous charcoal paste was produced with the addition of water and 15% clay binder. The carbonised sisal briquettes were lightweight but reasonably hard. 3.3.9 Rice Husk Preparation A load of 7 t. of rice husk was collected from Mwea Rice Mills in Kirinyaga District. The material was one of the easiest biomass wastes to handle, being of uniform particle size, low in dust and low in moisture. In fact the moisture content averaged only 13% and the husks therefore required no drying prior to carbonisation. This was a significant advantage over several of the other wastes that were tested. Carbonisation Rice husk needed to be lit on top with a propane torch to initiate carbonisation, but once started it combusted evenly and rapidly with good release of heat. It was also easy to quench, but the finished product was very high in ash. The impressive conversion efficiency of 60% must therefore be balanced with the fact that the ash levels were unacceptably high for fuel fabrication.


Briquetting The rice husk charcoal produced extremely hard and dense briquettes, but the ash was clearly visible as white flecks inside the fuel. The briquettes also took a long time to dry because of their density. 3.3.10 Maize Stover Preparation A 4 t. load of maize stover was obtained from Delamere Estates in Naivasha. The material was chopped, immature green maize and may not have truly represented maize stalk and leaf as found in the field post-harvest, but would have been very similar. The material was moist upon receipt and was laid out to dry. In mild weather this took five days with the material turned twice a day. A slightly alcoholic fermentation odour was emitted for the first three days. The final result was a very acceptable lightweight material similar to bagasse in all respects. Carbonisation Carbonisation was straightforward, again being very similar to bagasse, though with a better conversion rate of 31%. Briquetting Carbonised stover was similar in its physical characteristics to carbonised bagasse. The briquettes extruded smoothly and were homogenous, of intermediate hardness and dried quickly. 3.4 Fuel Production Rankings The comparative results of the fuel production trials are summarised in Table 3. A qualitative ranking system was used for three of the selected parameters and a quantitative ranking for efficiency and rate of carbonisation (given that these two parameters could be measured definitively).


Table 3: Ranking of Biomass Wastes by Suitability for Fuel Production

Biomass Waste

Ease of Preparation

Ease of Carbonisation

Rate of Carbonisation

Efficiency of Carbonisation

Ease of Briquetting

TOTAL RANKING

Whether drying or milling required, not too dusty, not too damp, dries fast, minimal labour needed

Lights easily, smoke flares, extinguishes

easily

kg. of raw material

carbonisedper hr

Ranking%

conversion Ranking

Minimal binder

required, cohesive,

homogenous, dries fast

Macadamia nut shell 2 3 239 2 40% 3 7 17.0

Rice husk 2 6 168 5 60% 1 5 19.0

Coffee husk 2 8 228 3 33% 5 2 20.0

Wattle bark 8.5 4 290 1 23% 8 4 25.5

Pineapple pulp 8.5 5 36 10 47% 2 3 28.5

Sawdust 8.5 2 178 4 34% 4 10 28.5

Sisal fibre 4.5 10 127 8 32% 6 1 29.5

Bagasse 8.5 1 129 7 20% 10 6 32.5

Maize stover 6 7 161 6 31% 7 9 35.0

Coconut husk 4.5 9 117 9 22% 9 8 39.5

Note: See Annex B for full carbonisation results. As the table shows, the wastes considered most easily processed were macadamia nut shells, rice husk and coffee husk, largely because of their dry nature, even particle size and high rate of carbonisation with good conversion efficiencies. Wet materials such as bagasse, pineapple pulp and maize stover were harder to process because of the need for pre-drying, and were thus ranked less favourably. Pineapple pulp also carbonised at an extremely slow rate of just 36 kg./hr.


4. Fuel Performance Tests 4.1 Introduction The preceding sections reviewed the availability of the various biomass wastes available in Kenya (section 2), followed by suitability for fuel production (section 3). This section focuses on the performance and quality of the ten fuel samples that were produced. These quality tests also included samples of lumpwood charcoal and Chardust’s Vendors’ Waste Briquette (VWB) for comparative purposes. Two types of tests were conducted. The first was a basic friability test to determine each fuel’s strength and durability. The second was a water boiling test to ascertain ease of lighting, length of burn, amount of water boiled and ash content.

Fuel testing underway

Comparative water boiling test in progress


4.2 Findings of Fuel Performance Tests 4.2.1 Friability Tests The friability tests were conducted by placing 250 gm. samples of each type of briquette in a plastic bag and dropping them five times onto a concrete floor from a height of 2 m. The remains were hand separated by size into three categories: • unbroken/halves • chips • dust Table 4 summarises the results of these tests.

Table 4: Results of Fuel Friability Tests

Type of Briquette

% Remaining Unbroken or as

Halves after Drop Tests

Macadamia nut shell (binder 7% clay; 2% gum arabic) 76% Wattle bark (binder 5% clay; 2% gum arabic) 76% Maize stover 68% Sisal fibre 60% Bagasse 52% Sawdust 52% Rice husk 48% Coffee husk 44% Coconut fibre 36% Pineapple pulp 28% Additional tests: Lumpwood charcoal 76% Coffee husk (binder 10% molasses) 60% VWB (1½” diameter, no binder) 40% VWB (1” diameter, no binder) 36%

Note: 15% clay used as binder, except where stated. Full details in Annex C. The results show that lumpwood charcoal was the strongest type of fuel when subjected to sudden impact, with only 24% breakage upon repeated dropping onto concrete. Of the fabricated briquettes, the ones produced from macadamia nut shell and wattle bark were particularly durable, with only 24% breakage. This could be attributable to the gum arabic binder as much as the material itself. Pineapple pulp briquettes and Chardust’s VWB were the weakest and 60-72% ended up as chippings or dust. It is worth noting that these were impact tests based on repeated dropping onto a hard surface. In the event of commercial production and marketing, fuel briquettes would also be subjected to abrasive damage during transportation and


handling. In terms of resistance to abrasion, the more brittle fuels such as lumpwood charcoal would show a less marked advantage under such circumstances. Indeed the Chardust VWB operation is founded upon the fact that up to 15% of all charcoal ends up as waste dust and fines, which is clear evidence of its susceptibility to abrasive forces - however well it may hold up under the force of sudden impact. 4.2.2 Water Boiling Tests Each water boiling test involved the combustion of a 1 kg. fuel sample in a standard ‘Kenya Ceramic Jiko’ (KCJ). 2 litres of water were brought to the boil in an aluminium pot without a lid. The water was kept at a simmer and more was added until the fuel could boil off no more. This test enabled Chardust to record a number of useful performance indicators for each fuel, including the time taken to achieve the first boil, the total length of burn, the total amount of water boiled off, and the proportion of ash remaining. A secondary performance indicator was derived by calculating the amount of water boiled off per minute of burn. This revealed the true heat output of the fuel in the kind of way that a cook would consider it. Table 5 summarises the results of the water boiling tests.

Table 5: Results of Water Boiling Tests

Type of Briquette

Time Taken to Boil (mins)

Performance (gm. of water

boiled off/ min.)

% Ash Remaining

after Combustion

Sawdust 25 17.5 27% Macadamia nut shell (binder 7% clay; 2% gum arabic) 18 15.1 9% Coffee husk 23 14.0 20% Bagasse 29 13.6 33% Wattle bark (binder 5% clay; 2% gum arabic) 22 13.4 19% Pineapple pulp 31 13.0 54% Sisal fibre 31 12.2 56% Coconut fibre 26 10.3 33% Maize stover 40 8.3 34% Rice husk 26 n/a 68% Additional tests: VWB (1” diam.) 29 23.1 34% Lumpwood charcoal 29 16.6 8% VWB (1½” diam.) 28 14.5 33% Bagasse (binder 5% clay, 10% molasses) 21 13.7 33% Coffee husk (binder 10% clay, 5% molasses) 29 13.6 17% Sawdust (binder 20% clay) 21 11.7 35%

Note: 15% clay used as binder except where stated.

Refer to Annex D for full results.


The results show that lumpwood charcoal had the lowest ash content at only 8%, which is not surprising as it contains no binder. More surprising, perhaps, was that lumpwood charcoal was not the quickest fuel in bringing water to the boil, neither was it the best performer in terms of water boiled per minute of burn. In both cases it was beaten by sawdust briquettes. Chardust’s small diameter VWB showed the most impressive heat output per minute of burn time, boiling off 23.1 gm. of water per minute. The poorest performers overall were the rice husk briquettes, which actually went out after some time due to their high ash content, and the briquettes made from maize stover, which took 40 mins. to bring the water to the boil and managed to boil off only 8.3 gm./min. of burn time. 4.2.3 Overall Performance Ranking Table 6 shows the combined rankings from the friability tests and the water boiling tests.

Table 6: Combined Performance Rankings: Friability and Water Boiling

Type of Briquette Friability Time

Taken to Boil Water

Performance (gm. of water boiled/min of

burn)

Ash Content

COMBINEDRANKING

Macadamia nut shell 2 1 3 2 8

Wattle bark 1 2 7 3 13

Lumpwood charcoal 3 9 2 1 15

Sawdust 7 4 1 5 17

Coffee husk 9 3 5 4 21

Bagasse 6 8 6 6 26

VWB (large diameter) 10 7 4 8 29

Coconut husk 11 5 10 7 33

Maize stover 4 12 11 9 36

Sisal fibre 5 11 9 11 36

Rice husk 8 6 12 12 38

Pineapple pulp 12 10 8 10 40

The table shows that the best quality fuel overall, by a significant margin, was the macadamia nut shell briquette. Briquettes made from wattle bark and sawdust were also good performers, along with standard lumpwood charcoal. Briquettes made from sisal fibre, rice husk and pineapple pulp were simply too high in ash to deliver any real heat output, and thus achieved the poorest ranking overall.


5. Business Potential 5.1.1 Introduction Practical suitability and technical feasibility are vital elements in any fuel production enterprise, but must obviously be considered in tandem with commercial viability. Before a private sector briquetting operation can be established using any of the short-listed wastes as a raw material, it is important to consider the most suitable locations and the likelihood of commercial partners being available. 5.1.2 Ranking Process A commercially viable charcoal briquetting facility probably needs to produce at least 5 t. of finished product per day. The establishment of a production facility at this scale in Kenya requires a capital investment of KSh 8-10 million (USD 100,000-130,000) with an expected payback of not less than four years. It also requires a committed sales and marketing effort, and a willingness and ability on the part of all partners to take risk. Furthermore, the owner of the material should be clearly known and acknowledged, and be cooperative in the allocation and conversion of the waste to fuel. Table 7 is an effort to rank each raw material in terms of business potential, based on a qualitative assessment of these factors; primarily, a stated willingness on the part of a waste producer to establish a commercially viable briquetting venture, combined with an apparent capacity to do so.

Table 7: Ranking of Business Potential

Biomass Waste Business Potential

Macadamia nut shell 1 Coffee husk 2 Wattle bark 3 Coconut husk 4 Bagasse 5 Sisal fibre 6 Rice husk 7 Pineapple pulp 8 Sawdust 9 Maize stover 10

As shown in the table, the highest-ranked materials that fall under the direct control of a reputable, private sector concern with a pro-active approach to waste conversion are macadamia nut shell, coffee husk and wattle bark. They are produced and owned by the Kenya Nut Company, Thika Coffee Mills and Kenya Vegext respectively. Meanwhile the lowest-ranked materials are those that are


widely dispersed, with no clear ownership or with high value for alternative applications, such as pineapple pulp. sawdust and maize stover. Given that the Kenya Nut Company owns Thika Coffee Mills, there is clearly some potential for a collaborative effort between the two to make productive use of both macadamia nut shells and coffee husk in a joint operation.


6. Conclusions 6.1 Overall Viability Rankings This study began with a list of 28 biomass wastes that were thought to have potential for conversion to charcoal fuel in Kenya. The list was narrowed down to 20 on the basis of straightforward bulk availability. This list was further reduced to ten for fuel production trials based on a scoring system that covered six physical parameters (section 2). Further parameters were drawn up for ranking the wastes based on production trials (section 3), performance tests (section 4) and commercial conversion potential (section 5). Combining the four scores that were assigned, an overall assessment may be derived that gives the final comparative rankings in terms of commercial viability of conversion to charcoal briquettes. This is summarised in Table 8.

Table 8: Overall Feasibility Ranking for Biomass Wastes

Type of Briquette

Availability & Suitability

Ranking (section 2; Table 2)

Fuel Production


Fuel Performance


Business Potential Ranking

(section 5; Table 7)

COMBINEDTOTAL

Macadamia nut shell 54.0 17.0 8.0 1.0 80.0

Sawdust 27.0 28.5 17.0 9.0 81.5

Wattle bark 40.5 25.5 13.0 3.0 82.0

Coffee husk 42.0 20.0 21.0 2.0 85.0

Bagasse 31.5 32.5 26.0 5.0 95.0

Rice husk 59.0 19.0 38.0 7.0 123.0

Sisal fibre 56.0 29.5 36.0 6.0 127.5

Coconut husk 54.5 39.5 33.0 4.0 131.0

Pineapple pulp 55.5 28.5 40.0 8.0 132.0

Maize stover 76.0 35.0 36.0 10.0 157.0

As the table shows, of the ten materials that were short-listed, there are five that have significantly higher overall potential than all of the rest. These are macadamia nut shell, sawdust, wattle bark, coffee husk and bagasse. The other raw materials lag behind significantly in the rankings. The result of this research is therefore a narrowed-down list of five biomass wastes that appear to be available cheaply in Kenya in large quantities, are technically suitable for conversion to charcoal fuel, can produce fuel of sufficiently high quality to compete with lumpwood charcoal and have the potential to form the basis of a viable briquetting business.


6.2 Summary The commercial production of charcoal briquettes from biomass waste is not a high profit enterprise and it will not yield rapid returns. But it does represent a significant long-term opportunity that can only grow as lumpwood charcoal in Kenya becomes of ever poorer quality and higher in price. Within the existing market environment there is a need for financial assistance in the establishment phase and initial stages of commercial production. Thereafter a production plant can be sustained from product sales. Considering the vast regional market for charcoal, there is a virtually inexhaustible demand for a good quality substitutes for unsustainably produced lump-wood charcoal. Chardust already makes briquettes from charcoal traders’ waste and sugar cane bagasse. This research has shown that commercial potential may also exist for the conversion of macadamia nut shell, sawdust, wattle bark and coffee husk to briquettes.

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Annex A: Biomass Waste Availability

Biomass Waste Output of Harvested Crop (t/yr)

Volume of Standing

Plant

% Residue

Production of Residue (t/yr,

wet basis)

Estimated Moisture Content

Production of Residue (t/yr, air

dry basis; 13% MC) Source

Maize stover 1,395,850 2,791,700 50% 1,758,771 Min. of Agric. Stats; Delamere Estates; KARI; internet research

Pigeon pea stalk 629,000 2,096,667 50% 1,320,900 Min. of Agric. Stats; internet research

Maize cob 1,395,850 446,672 25% 393,071 Min. of Agric. Stats; internet research

Wheat straw 162,750 284,813 15% 279,116 Min. of Agric. Stats; internet research

Sawdust 288,000 900,000 40% 360,000 50% 226,800 Forest Department; sawmiller interviews

Coffee husk 50,000 862,069 25% 215,517 13% 215,517 Coffee Board of Kenya; Thika Coffee Mills.; Min. of Agric. Stats

Bagasse 382,000 3,473,000 5% 173,650 50% 109,400 Chemelil Sugar Company; Mumias Sugar Company

Rice straw 59,000 103,250 13% 103,250 National Irrigation Board; Mwea Rice Mills

Cassava stem 428,000 142,667 50% 89,880 Min. of Agric. Stats; internet research

Cotton stalk 26,000 247,619 26% 64,381 40% 46,998 Min. of Agric. Stats; internet research

Pineapple waste 280,000 50% 140,000 80% 46,200 Del Monte Ltd.

Coconut husk 57,000 60% 34,200 20% 31,806 Min. of Agric. Stats; internet research

Coconut shell 57,000 25% 14,250 15% 13,965 Min. of Agric. Stats; internet research

Sunflower straw 8,000 14,000 15% 13,720 Min. of Agric. Stats; internet research

Rice husk ? 37,500 22% 12,000 13% 12,000 National Irrigation Board; Mwea Rice Mills

Groundnut shell 12,000 6,000 20% 5,580 Min. of Agric. Stats; internet research

Macadamia nut shell 6,000 80% 4,800 13% 4,800 Min. of Agric.; Kenya Nut Co.; internet research

Wattle bark 3,000 50% 1,890 Kenya Tanning Extract Co.; Kenya Vegext (EPZ) Ltd.

Soybean stalk 740 2,467 50% 1,554 Min. of Agric. Stats; internet research

Sisal fibre 21,672 135 13% 135 Kenya Sisal Board Note: Crop production figures are all 5 year averages (1993-97), except for macadamia (2004), coffee husk (2003), wattle bark (2003), pineapple (2000) and

sisal (averages for 1999 & 2000). Refer to Table 1 in text for abbreviated summary (page 5).

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Annex B: Carbonisation Trial Raw Data

Biomass Waste Input of Raw Material (kg.

air dry)

Output of Carbonised Material, incl. quenching

water (kg)

Percentage Water Added to

Quench

Output of Carbonised Material (kg. dry

weight)

Conversion Efficiency

Time Taken (hrs)

Amount of Raw Material Carbonised

per hr (kg)

Bagasse 420 126 34% 83 20% 3.25 129

Coconut fibre 820 296 38% 184 22% 7 117

Coffee husk 9,100 3,267 9% 2,973 33% 40 228 Macadamia nut shell 1,350 558 4% 536 40% 5.66 239

Maize stover 1,365 663 36% 424 31% 8.5 161

Pineapple pulp 280 154 15% 130 47% 7.75 36

Rice husk 2,680 1,801 11% 1,603 60% 16 168

Sawdust 1,425 690 30% 483 34% 8 178

Sisal fibre 1,015 364 10% 327 32% 8 127 Wattle bark 2,027 743 36% 476 23% 7 290 Note: Refer to Table 3 in text for abbreviated summary (page 36).

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Annex C: Friability Test Raw Data

Type of Briquette Binder(s) No. of

Briquettes in Sample

Amount Remaining Unbroken/Halves

(gm.)

Amount Broken into Chips (gm.)

Amount Broken into Dust (gm.)

Unbroken/Halves(%)

Chips (%)

Dust (%)

Bagasse Clay (15%) 12 130 70 50 52% 28% 20% Coconut fibre Clay (15%) 6 90 20 140 36% 8% 56% Coffee husk Molasses (10%) 13 150 60 40 60% 24% 16% Coffee husk Clay (15%) 9.5 110 85 55 44% 34% 22% Lumpwood charcoal none 9 190 60 0 76% 24% 0% Macadamia nut shell Clay (7%); Gum Arabic (2%) 8 190 30 30 76% 12% 12% Maize stover Clay (15%) 15 170 30 50 68% 12% 20% Pineapple pulp Clay (10%) 8 90 80 80 36% 32% 32% Pineapple pulp Clay (15%) 9 70 80 100 28% 32% 40% Rice husk Clay (15%) 5 120 90 40 48% 36% 16% Sawdust Clay (20%) 8 170 20 60 68% 8% 24% Sawdust Clay (15%) 11 130 60 60 52% 24% 24% Sisal fibre Clay (10%) 7 150 50 50 60% 20% 20% Sisal fibre Clay (15%) 6 150 80 20 60% 32% 8% VWB (1.5" diam.) None 3 100 70 80 40% 28% 32% VWB (1" diam.) None 6 90 90 70 36% 36% 28% Wattle bark Clay (5%); Gum Arabic (2%) 7 190 20 40 76% 8% 16%

Note: Test involved dropping 250 gm. fuel samples in a plastic bag onto concrete from a 2 m. height. Refer to Table 4 in text for abbreviated summary (page 38).

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Annex D: Combustion Test Raw Data

Type of Briquette Binder(s) No. of

Samples Tested

Time Taken to Boil (mins)

Water Loss (gm.)

Time Taken (mins)

Overall Performance (gm. of water lost/time

taken)

Ash Produced

(gm.)

Ash Produced

(%)

Bagasse Clay (10%); Molasses (5%) 2 21 1,420 104 13.7 330 33% Bagasse Clay (15%) 3 29 1,820 134 13.6 330 33% Coconut fibre Clay (15%) 2 26 1,480 143 10.3 330 33% Coffee husk Clay (10%); Molasses (5%) 2 29 2,900 214 13.6 170 17% Coffee husk Clay (15%) 3 23 2,775 198 14.0 200 20% Lumpwood charcoal n/a 2 29 2,740 165 16.6 80 8% Maize stover Clay (15%) 2 40 1,500 180 8.3 340 34% Pineapple pulp Clay (15%) 2 31 1,300 100 13.0 540 54% Rice husk Clay (15%) 2 26 685 n/a n/a 675 68% Sawdust Clay (15%) 2 25 2,340 134 17.5 270 27% Sawdust Clay (20%) 2 21 1,800 154 11.7 350 35% Sisal fibre Clay (15%) 2 31 1,130 93 12.2 560 56% VWB (1.5” diam.) n/a 2 28 2,000 138 14.5 330 33% VWB (1” diam.) n/a 4 29 2,730 118 23.1 340 34% Wattle bark Clay (5%); Gum Arabic (2%) 1 22 2,410 180 13.4 190 19% Macadamia nut shell Clay (7%); Gum Arabic (2%) 2 18 3,120 207 15.1 90 9%

Note: Each test involved the combustion of 1 kg. fuel samples in a standard Kenya Ceramic Jiko. 2 litres of water was brought to the boil in an aluminium pot without a lid. The water was kept at a simmer and more was added until the fuel could boil off no more. Refer to Table 5 in text for abbreviated summary (page 39).

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