Fuel 89 (2010) 3579–3589

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How much of the target for biofuels can be met by biodiesel generatedfrom residues in Ireland?

T. Thamsiriroj, J.D. Murphy *

Department of Civil and Environmental Engineering, University College Cork, Cork, Ireland

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 February 2009Received in revised form 11 January 2010Accepted 3 June 2010Available online 17 June 2010

Keywords:BiodieselTallowUsed cooking oil

0016-2361/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.fuel.2010.06.009

* Corresponding author. Tel.: +353 21 490 2286; faE-mail address: jerry.murphy@ucc.ie (J.D. Murphy

This study focuses on biodiesel production from residues in particular tallow and used cooking oil (UCO).Ireland has 8% of the EU cattle herd with less than 1% of the EU population. Thus a significant quantity ofslaughter residues is available for energy production. The total energy potential associated with slaughterwastes and UCO is estimated to be 7.07 PJ/a; 61% of which is suitable for biodiesel production. The poten-tial quantity of biodiesel is equivalent to 2.1% of predicted transport fuel use in 2010; three quarters ofthe 2010 biofuels target. Biodiesel production from these two residue streams may be expressed asequivalent to 22% of arable land in Ireland under oilseed rape.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Ireland has set national targets to substitute conventional fuelswith biofuels in the transport sector. These targets included 5.75%substitution by 2010 and 10% by 2020, to comply with the gov-ernment’s White Paper ‘Delivering a Sustainable Energy Futurefor Ireland’, published in March 2007 [1]. An obligation of 5% bio-fuels in retailed petrol and diesel was scheduled to be introducedto the transportation market by 2009. However, in September2008 the Irish government revised the 5.75% target, and loweredit to 3% by 2010. The target of 10% by 2020 still remains as acommitment to achieving the EU targets [2]. The main reasonsfor this revision are the increase in global food prices, which islinked to the transfer of land from food to energy crops and alsodoubts as to whether biofuels are as environmentally friendly asoriginally thought [2]. The Republic’s biofuels sector producesonly 0.6% of all fuel, with a greater quantity being imported tomeet the biofuels target of 2.2% by 2008 [3,4]. A paper writtenby the authors [5] shows that 1% of total agricultural land is re-quired to satisfy 1% substitution of transport fuels with rapeseedbiodiesel. To achieve 5.75% substitution according to the previoustarget would thus require 5.75% of all agricultural land under oil-seed rape, which is equivalent to 63% of arable land in Ireland.Food crops are grown on arable land and hence this raises theconflict of land for food versus land for fuel.

The EU Renewable Energy Directive [6] states that biofuels to beused in 2020 will only be classified as biofuels if they contribute

ll rights reserved.

x: +353 21 427 6648.).

greenhouse-gas (GHG) emissions savings of at least 60% comparedto conventional fuels. Using a life cycle analysis approach withoutallocation to co-products or by-products, Thamsiriroj and Murphy[5] calculated a GHG emissions reduction of 28.8% for biodieselproduced from Irish-grown rapeseed. The energy used in agricul-ture (e.g. ploughing, fertilizing) is the major energy-consumingprocess in rapeseed biodiesel production, releasing a significantamount of GHG emissions in the biodiesel life cycle [5]. Recently,interest has been focused on transport fuels produced from resi-dues, wastes, and by-products. These feedstocks do not have theassociated GHG emissions from crop production; for example agri-culture is responsible for 14% of the parasitic energy demand in ra-peseed biodiesel and 79% of the GHG emissions in the life cycleanalysis. Biofuels produced from wastes, residues, and by-productshave a significantly better energy balance with less GHG emissionsthan biofuels produced from energy crops.

Tallow and used cooking oil (UCO) are considered to be renew-able and sustainable feedstocks for biodiesel production. Their rel-atively low costs contribute to a low biodiesel price and allowbiodiesel to compete economically with diesel. In the USA, one ofthe largest fats and oils trading countries, inedible tallow and recy-cled greases have remained the lowest priced among the varioustypes of oil and fat, since records began in 1990 [7,8]. The producedquantity of tallow and greases is predicted to increase every year[9] as a result of increasing population, rising food consumptionand an increased demand for industrial products. It is estimatedthat the potential of UCO collectable in the EU is between700,000 and 1,000,000 t/a [10], which could account for up to20% of biodiesel produced in the EU (based on 4,890,000 t/a biodie-sel production in 2006 [11]).

Table 1SFA and FFA contents in some oils and fats.

Oils and fats % Saturated fatty acids % Free fattyacids

Soybean 14.3 [22], 15.5 [23] 0.1% [22]Palm 43.4 [23], 47.0 [24] 0.4% [29]Rapeseed 4.1 [24], 5.1 [25] 0.02% [30]Jatropha 21.1 [23], 22.9 [26] 0.1% [26]Animal fats Lard 39.3 [19], chicken fat 32.0 [16], beef

tallow 45.6 [19], mutton tallow 61.1 [16]Inedibletallow: 11.2–20% [31]

Used cooking 11.8 [27], 13.6 [22], 90.9 [28] 1.8 [28], 5.6%

3580 T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589

The 1st generation feedstocks for biofuels produced from en-ergy crops have resulted in concerns over change in land use andfood price increase. The next generation feedstocks need to be trulybeneficial in a more sustainable manner; food supply should not beadversely affected and net environmental benefits should result.Biodiesel production from tallow and used cooking oil has beendeveloped to a commercial scale in Ireland, and a facility with aproduction capacity of 30,000 t/a has been operational since mid-2008 [12]. Biofuels produced from residues (such as tallow andused cooking oil) are allowed a double count towards the biofuelstargets according to the Renewable Energy Directive [6].

oil [32]

O O

CH2 – O – C – R1 CH3 – O – C – R1 CH2 – OH

O O

CH – O – C – R2 + 3 CH3OH CH3 – O – C – R2 + CH – OH

O O

CH2 – O – C – R3 CH3 – O – C – R3 CH2 – OH

Triglyceride Methanol Methyl ester Glycerol

(Catalyst)

Fig. 1. Transesterification reaction.

2. Methodology and objectives

Initially this paper reviews the available processes for generat-ing biodiesel in the form of fatty acid methyl ester (FAME) fromresidues such as used cooking oil (UCO) and tallow, which are highin either free fatty acids (FFA) or saturated fatty acids (SFA) as com-pared to pure plant oil (PPO). An analysis is undertaken to estimatethe quantity of residues, which are readily available in Ireland andto estimate the potential production of biodiesel from suchsources. Of particular interest is the quantity of land that wouldneed to be under rapeseed to produce a similar quantity of biodie-sel; previous work by the authors [5] showed that 1355 L of biodie-sel is the average production of rapeseed biodiesel per hectare perannum in Ireland. This is especially important as only 9% of land inIreland is arable and thus there is extreme competition betweenfood and fuel. A sensitivity analysis is carried out by examiningthe readily available feedstock. Thus the paper has a number ofobjectives:

� quantify animal by-products and used cooking oil inIreland;

� estimate the proportion of the national biofuels target thatcan be met by biodiesel from residues;

� calculate the land under oilseed rape required to producethe same quantity of biodiesel.

3. Biodiesel produced from animal fats and used cooking oil

3.1. Terminology

The term ‘fats’ may be briefly defined either as ‘animal tissuedistended with greasy or oily matter’, or as ‘glyceride compoundsof fatty acids’ [13]. The former definition, which is mostly used inthis paper, specifies that fats are animal products. In the latter def-inition, the entire range of fats and oils from vegetables and ani-mals is included. Greases generally refer to solid or thick oil.Greases can be classified as yellow and brown greases. Yellowgrease is UCO collected from commercial or industrial cookingbusinesses, and brown grease is the grease recovered from greasetraps and interceptors [14]. Finally, tallow is the term used for ani-mal fats obtained by either the melting process or rendering pro-cess [15]. Inedible tallow, generally produced from the renderingprocess, is a competitive feedstock in biodiesel production due toits low cost [14,16–18].

3.2. Saturated fatty acid (SFA) content

Fats and oils are made up of 1 mol of glycerol and 3 mol of fattyacids, and are commonly referred to as triglycerides. There are alsominor quantities of monoglycerides and diglycerides in fats andoils [19–21]. Primarily, there are two types of fatty acids, saturatedand unsaturated fatty acids. SFA content in animal fats is typicallyhigher than in most type of vegetable oils. However, in UCO it can

vary by a wide range, depending on the usage and properties of theoriginal oil. Table 1 shows the SFA content of some fats and oils. Alarge variation can be seen in UCO (11–90% SFA). High SFA contentcauses the oil to solidify at room temperature.

3.3. Free fatty acid (FFA) content

The FFA content of oil feedstocks is a key parameter in deter-mining the viability of the transesterification process. For fatsand oils with an FFA content of less than 2–3%, feedstock pretreat-ment is not required and alkaline-catalyzed transesterification canbe simply performed. The higher the acidity of the oil, which isassociated with higher FFA content, the lower the conversion effi-ciency of biodiesel through alkaline catalysis [14,33]. Rendered tal-low and UCO normally contain a significant quantity of FFA (seeTable 1). The upper value is between 15% and 20% FFA for renderedtallow [15,34]. For UCO, the FFA content is typically less than 15%[14]. With these high FFA content feedstocks, alternative transeste-rification methods to the simple one-step, alkali-catalyzed processshould be considered in order to achieve a high yield of biodiesel.

3.4. Transesterification methods

Transesterification is a chemical reaction which involves themixing of a fat or oil with an alcohol in the presence of a catalystto produce fatty acid esters (biodiesel) and glycerol. It transformsthe large, branched molecular structure of the bio-oils into smaller,straight-chain molecules, with fuel characteristics similar to thoseof regular diesel. Methanol is the most commonly used alcohol be-cause of its low price compared to other alcohols [14,17,19,35–37].The transesterification reaction of a triglyceride and methanol isoutlined by the chemical reaction in Fig. 1.

3.5. Reduced yield from alkaline-catalyzed transesterification of highFFA feedstock

Alkalines, e.g. KOH and NaOH, are the most common catalystsused in the one-step process. The reaction is faster foralkaline- than for acid-catalyzed processes (only 30 min comparedto 1–8 h for the acid catalysis) [21,25]. However, the yield ofmethyl ester will substantially decrease if high FFA content feed-

Fig. 2. Yield of methyl esters related to %FFA [14].

(a) Dehydrolysis reaction

O O

HO – C – R + KOH K+ -O – C – R + H2O

Free fatty acid Potassium Hydroxide Potassium soap Water

(b) Hydrolysis reaction

O O

CH3 – O – C – R + H2O HO – C – R + CH3OH

Methyl ester Water Free fatty acid Methanol

(c) Saponification reaction

O O

CH2 – O – C – R1 K+ -O – C – R CH2 – OH

O O

CH – O – C – R2 + 3 KOH K+ -O – C – R + CH – OH

O O

CH2 – O – C – R3 K+ -O – C – R CH2 – OH

Triglyceride Potassium Hydroxide Potassium soap Glycerol

Fig. 3. (a) Dehydrolysis reaction, (b) hydrolysis reaction and (c) saponificationreaction.

T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589 3581

stocks are used. An example of the reduced yield in the one-step,alkaline-catalyzed transesterification is outlined in Fig. 2. Whenadding an alkaline catalyst, FFA reacts with the catalyst to formsoap and water in a dehydrolysis reaction (Fig. 3a) [38]. Watercan further react with methyl esters to produce more FFA

(a) Esterification reaction

O

HO – C – R + CH3OH CH

Free fatty acid Methanol M

(b) Combined esterification and transes

O

CH2–O–C–R1

O O

n CH–O–C–R2 + m HO–C–R4 + (3n+m) CH3OH

O

CH2–O–C–R3

Triglyceride Free fatty acid Methanol

(H2SO4)

(Lip

Fig. 4. (a) Esterification reaction and (b) combined

(Fig. 3b), thus resulting in additional soap products. The presenceof water causes further saponification (Fig. 3c), which also pro-duces more soap [21]. Hence, one-step, alkaline-catalyzed transe-sterification is not an appropriate process for high FFA contentfeedstocks.

3.6. Acid-catalysed processes for feedstock with high FFA

A one-step transesterification process may be applied to highFFA content feedstocks using an acid as a catalyst. Sulfuric acid(H2SO4) is a commonly chosen catalyst. The reaction of the one-step, acid-catalyzed transesterification process is also based onFig. 1. Although the reaction can produce a high yield of biodiesel,the reaction is relatively slow and a reactor is required to with-stand an acidic environment [39]. Alkaline-catalyzed transesterifi-cation is much more effective in terms of reaction rate. Thisadvantage of alkaline catalysts can be applied to high FFA contentfeedstocks through a two-step transesterification process. In thefirst step, pretreatment known as ‘esterification’ (Fig. 4a) uses anacid catalyst to convert FFA into biodiesel; the remainder of the tri-glycerides are subsequently transesterified to biodiesel in the sec-ond step using an alkaline catalyst [21,38].

3.7. Alternative transesterification methods for feedstock with high FFA

The supercritical methanol method is an alternative to thetransesterification of oil feedstocks with high FFA content. A cata-lyst is not required in the process; as a result, the wastewater con-taining acid or alkaline can be avoided in the washing process. Thereaction rate is relatively high; a residence time of between 12.5and 50 min is required. However, the reaction has a high energydemand as high temperatures and pressures are required (at least250 �C and 10 MPa) [35,40].

Enzymatic catalysts like lipases are also used in transesterifica-tion. Triglycerides and FFA can be enzymatically transesterified tobiodiesel in a one-step process because lipases catalyze bothtransesterification and esterification reactions (Fig. 4b) [41]. Hence,FFA contained in oil feedstocks can be completely converted to bio-diesel, and the difficulty of recovering glycerol is also overcome asno soap is produced in the process. On the other hand, the produc-tion cost of lipase catalysts is higher than that for alkaline catalysts[33,42]. Renewable diesel may also be generated using hydrogena-tion; the end product is not FAME but synthetic diesel. It is veryattractive to petroleum refining companies but is not discussedfurther in this paper.

Table 2 outlines some studies dealing with high FFA content tal-low and UCO. Most of the processes are viable for these oil feed-

O

3 – O – C – R + H2O

ethyl ester Water

terification reactions

CH2–OH

O

(3n+m) CH3–O–C–Rn + n CH–OH + m H2O

CH2–OH

Methyl ester Glycerol Water

ase)

esterification and transesterification reactions.

Table 2Processing conditions and biodiesel yield.

Alkaline catalysis (one-step) [31]

Acid catalysis (one-step) [43]

Acid, alkaline catalysis (two-step) [44]

Supercriticalmethanol [43]

Lipase catalysis[45]

Feedstocks Tallow UCO Tallow UCO UCO%FFA in feedstock 20 5.6 9 5.6 8.5Process

temperature(�C)

55 65 60 350 50

Process pressure(MPa)

0.1 0.1 0.1 43 0.1

Catalyst used KOH H2SO4 H2SO4, NaOCH3 No Immobilized lipasePS-30

Residence time 1 h 48 h 1 h, 1 h, 8 ha 4 min 18 hBiodiesel yield (%) 34.5 97.8 90.2 96.9 94

a Two stages of pretreatment by acid-catalyzed esterification with residence time of 1 h each, and 8 h for alkaline-catalyzed transesterification.

Table 3Fuel properties of biodiesel.

Properties Tallow UCO Rape seed oil EU biodiesel standard

Density (kg/L) 0.877 [47] 0.882 [32] 0.882 [50] 0.86–0.90Viscosity (mm2/s) 5.0 [48] 4.68 [32] 4.58 [50] 3.5–5.0Cetane number 58.0 [47] 54.5 [32] 52.9 [50] >51.0Cold filter plugging point (�C) 8 [48] 1 [49] �10 [51] –Pour point (�C) 9 [48] �3 [32] �15 [52] –Cloud point (�C) 11 [48] 1 [32] 0 [52] –Acid value (mg KOH/g) 0.44 [48] 0.5 [32] 0.16 [51] <0.5Iodine value 53.6 [48] 85.83 [32] 97.4 [50] <120

3582 T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589

stocks, except the one-step, alkaline-catalyzed transesterificationprocess. The yield of biodiesel is unacceptably low and this processis not therefore feasible for commercial production.

3.8. Fuel quality

Biodiesel is required to comply with the EN 14214 standard[46]. The fuel properties of biodiesel produced from tallow andUCO can be different from those of biodiesel from virgin oils be-cause of the higher FFA and SFA contents of recycled oils. FFAand SFA affect many important fuel parameters including cetanenumber (CN), cold flow properties, acid value, and iodine value.Some fuel properties are presented in Table 3 and discussed below:

Cetane number indicates the ignition properties of the fuel. It isgenerally dependent on the composition of the fuel and can impactthe engine’s startibility, noise level, and exhaust emissions. Thehigher the cetane number, the more efficient the ignition [53,54].Biodiesel from high SFA content feedstocks has a higher cetanenumber, which has a positive impact on the diesel engine [55].

Iodine value is a measure of total unsaturation within a mixtureof fatty acids. It is expressed in grams of iodine which reacts withdouble bonds in a 100 g oil sample. The limitation of unsaturatedfatty acid is necessary due to the fact that heating the unsaturatedfatty acids results in polymerization of glycerides. This can lead tothe formation of deposits or deterioration of the lubrication. Theiodine value is also important in measuring the oxidation stabilityof fuels. The oxidation stability decreases with the increasing con-tent of polyunsaturated methyl esters [32,51]. Biodiesel from highSFA content feedstocks has a lower iodine value, which also has apositive impact on the diesel engine.

Acid value is a measure of the number of acidic functionalgroups in a sample, and is measured in terms of the quantity ofpotassium hydroxide required to neutralize the sample [14]. Thepresence of acid affects fuel aging. High acid value in fuel may becaused by either high FFA content in oil feedstocks or by theamount of acid added in the transesterification process [21,33]. Acase of high acid value caused by the amount of added acid wasstudied by Rice and Frohlich [31]; a two-stage transesterification

process was selected in the study, in reverse order to the typicalprocess, i.e. alkaline-catalyzed transesterification in the first step,followed by acid-catalyzed esterification in the second step. Thebiodiesel product had an acid value as high as 1.5 mg KOH/g (lim-ited to less than 0.5 mg KOH/g in the EN 14214 standard). Biodieselfrom high FFA content feedstocks has a higher acid value, whichhas a negative impact on the diesel engine.

Cold flow properties are characterized by three temperaturemeasures including cloud point (CP), pour point (PP), and cold filterplugging point (CFPP). CP is the temperature at which the fuelshows a haze from the formation of crystals; PP is the lowest tem-perature at which the liquid will flow; and finally, CFPP is the tem-perature at which the crystals formed will cause the plugging ofthe filters [56].

Among these fuel properties, cold flow properties and acid va-lue are the most critical and require control measures (Table 3).Additives may be used to reduce cold-starting problems [57].Blending biodiesel with conventional diesel will also improve thecold flow properties [56]. For biodiesel produced from tallow, PPis reduced to �11.1 �C and CP to �9.9 �C when neat biodiesel is dis-placed by B20 biodiesel [48]. The high acid value can be reducedwith a two-step process, consisting of acid-catalyzed esterificationfollowed by alkaline-catalyzed transesterification. Most of the acidresidues are neutralized in the second step. However, the use of anadditional alkaline catalyst for neutralization increases the cost ofbiodiesel [58].

3.9. Emissions from biodiesel fuel

Biodiesel produced from high SFA content feedstocks such astallow results in a higher combustion efficiency due to its in-creased cetane number, and thus in a reduction in the emissionsof CO, HC, and white smoke [33]. Studies of engine tests[48,59,60] show that a higher degree of unsaturation in biodieselresults in an increase in NOx emission levels. NOx emissions fromtallow B20 were found to be as low as emissions from diesel fuel,while from soybean B20, NOx emissions were 6.2% above dieselfuel [48]. Methyl esters high in saturated fat produce a high flame

Raw materials

Sizing Heat processing

(Time x Temperature)

Press

Fat clean-up

Protein

Grinding

T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589 3583

temperature. This could lead to an increase in combustion temper-ature and NOx emission levels [61]. However, a model simulationby Yuan et al. [59] shows that the increased NOx emissions are alsoinfluenced by earlier start of injection and shorter ignition delay.These factors result in higher NOx emission levels for methyl estershigh in unsaturated fat than for methyl esters high in saturated fat[59]. Biodiesel from high SFA content feedstocks therefore has apositive impact on NOx emission levels.

Particulate matter (PM) emissions relate to the oxygen contentof fuels. There is no significant difference in PM emissions betweenhighly saturated and unsaturated methyl esters; blending dieselwith biodiesel helps to reduce PM emissions because of the in-creased oxygen content [60].

Storage/Load out

Fig. 5. Rendering process [67].

3.10. Technology at commercial scale

It is found that most of the commercial biodiesel processingplants are currently based on the homogenous, alkaline-catalyzedprocess. Sodium and potassium hydroxides are traditional cata-lysts, but potassium hydroxide is preferable as potassium soap issofter than sodium soap and, unlike sodium soap, does not blockthe bottom of the separation funnel. Another advantage of potas-sium hydroxide is the potential to recover potash fertilizer, whichcreates an added value. The use of sodium or potassium methox-ides is also increasing. They have an advantage over the sodiumand potassium hydroxides in that they reduce side reactions, suchas saponification, resulting in an improved biodiesel yield [14,62].

The acid-catalyzed transesterification process has been provento be technically feasible for high FFA content oils. It is less com-plex than the alkaline-catalyzed transesterification process and istherefore a competitive alternative to commercial biodiesel pro-duction using the alkaline-catalyzed process [63]. However, be-cause of the slow reaction rate, acid catalysts are only used inthe esterification reaction in the pretreatment stage of the two-step process. In commercial biodiesel production, the two-stepprocess commonly uses sulfuric acid to esterify oil feedstocks priorto the alkaline-catalyzed transesterification. This process offers alarge potential to process high FFA content oils. Some examplesof industrial technology using the two-step process include BDI,CMB, DSB, and Energea [62].

Heterogeneous catalyzed transesterification through solid alka-line or solid acid catalysts has also been developed to the produc-tion stage at commercial scale. The technology known as Esterfip-H process was developed by Institut Français du Pétrole (IFP) andcommercialized by Axens. The main advantages of the processare the production of high quality glycerol with over 98% purityand the fact that the disposal of salts resulting from catalyst usageis not required. The biodiesel yield is close to 100%. However, it isonly economically feasible at a scale above 100,000 t/a, and onlyvirgin vegetable oils with a maximum FFA content of 0.25% aresuitable for the process [62,64].

4. Tallow, and used cooking oil (UCO)

4.1. The rendering process

Rendering is the recycling of raw material tissue from animalsused for food and waste cooking fats and oils from all types of eat-ing establishments into a variety of value-added products. Duringthe rendering process, heat, separation technology, and filteringare applied to the material using various processes. Heat is usedin the cooking process. The temperature and duration of the cook-ing process are critical and so are the primary determinants of thequality of the finished products [65,66]. The diagram in Fig. 5 illus-trates the rendering process. The principle rendering process used

in the EU is pressure-cooking. According to the EU Regulation EC1774/2002 [68], animal by-products that are not intended for hu-man consumption must be cooked using one of five different op-tions. The most applicable option is cooking at 133 �C and 3 barfor 20 min, with the particle size of starting material 650 mm [68].

4.2. Tallow, and meat and bone meal (MBM)

All rendering system technologies include the collection andsanitary transport of raw material to a facility, where it is pro-cessed into a consistent particle size and conveyed to a cookingvessel, which can be either continuous-flow or of batch configura-tion. Fat is separated from the cooked material via a screw presswithin a closed vessel. The fat known as tallow is stored and trans-ported in tanks. After the cooking and fat separation stages, thecrackling (which includes protein, minerals, and some residualfat) is further processed by moisture removal and grinding to pro-duce meat and bone meal (MBM) [65,69]. MBM is currently used asa substitute for fossil fuel. It is often co-incinerated in power gen-erating plants or used in cement manufacturing [34]. The ban onthe feeding of MBM to farmed animals in the EU has been in placesince its introduction in 2000 [70].

4.3. Animal by-products

Three categories of animal by-products are defined in the EURegulation EC 1774/2002 [68].

Category 1 materials are animal by-products that are regarded asa high-risk of TSE (Transmissible spongiform encephalopathies).All category 1 materials must be either incinerated; rendered andsubsequently incinerated; co-incinerated; or buried under strictconditions.

Category 2 materials are animal by-products that still possess arisk (but not a TSE risk). This category covers materials not in-cluded in category 1 or 3. There are more options for the disposalof category 2 materials than for category 1.

Category 3 materials are animal by-products that possess a lowrisk. These by-products are derived from animals considered fitbut not chosen for human consumption. This category includesby-products from the slaughter process: degreased bones; formerfoodstuffs of animal origin devoid of an infectious risk to humansand animals; fresh fish by-products; and catering waste other thanthat of international transport origin. Materials in this third cate-gory can be used for pet-food production, animal feed for non-food-producing animals, or technical products after appropriateprocessing in an approved category 3 processing plant [69].

Table 4Carcass weight of animals slaughtered in Ireland [78].

(’000 t) 2000 2001 2002 2003 2004 2005 2006 Average

Cattle 577 579 540 568 564 546 572 563.7Pig 230 240 230 219 204 205 209 219.6Sheep 83 78 67 63 72 73 70 72.3Poultry 123 123 122 123 128 129 123 124.4Total 1013 1020 959 973 968 953 974 980

3584 T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589

4.4. Use of tallow from animal by-products as a source of biodiesel

Tallow produced from all three categories is permitted to be usedfor biodiesel production according to the EU Regulation EC 92/2005[71]. Scientific studies in recent years have confirmed that crude,unfiltered tallow produced by a rendering procedure is safe frominfectivity, even if the starting materials contain a high concentra-tion of pathogenic prions and the 133 �C/3 bar/20 min condition isnot achieved. The downstream biodiesel is considered safe [72,73].

4.5. Cleaning process for used cooking oil (UCO)

UCO can be used as a raw material for many applications includ-ing biodiesel and a variety of oleochemical products such as surfac-tants, plasticizers, cosmetics and lubricants [74]. It can be useddirectly as fuel in a modified diesel engine. In animal feed produc-tion, the use of UCO as a raw material has been banned in the EUsince 2004 [31]. Most of the UCO collected from restaurants andfried food processing lines is currently delivered to biofuel andoleochemical industries. The quality of this UCO may be expectedto vary depending on the original oil types, cooking practices, wasteoil storage methods, and collection systems [75]. After collection,the oil is transferred to large holding tanks where it is blended withother waste oils; however, the quality is very inconsistent. Theimpurities present in UCO consist primarily of FFA, polymers, chlo-rides, and phospholipids [76]. A cleaning process separates thewater and solid portion from the oil, but the impurities in the oil re-quire further processing (e.g. transesterification and separationsteps) to make it suitable for industrial applications. The cleaningprocess includes steam injection (heating the oil), coarse screening,and centrifugation to eliminate the fine portion and water [31]. Thewater-soluble impurities which dissolve in water can be removedby washing the oil with water or water vapour. The oil may alsobe neutralized by adding alkali but this must be in a small quantityin order to avoid the production of a lot of soap. More detail on oilcleaning techniques can be found in Ref. [77].

5. Potential energy associated with selected residues in Ireland

5.1. Tallow and MBM

The total number of livestock (i.e. cattle, pigs, and sheep)slaughtered in Ireland is estimated to be about 9 million annually(Fig. 6). This is twice the population of the country (4.42 million

0

2000

4000

6000

8000

10000

12000

1990 1991 1992 1993 1994 1995 1996 1997 199Y

No.

of a

nim

als

slau

ghte

red

(thou

sand

hea

ds)

Fig. 6. Number of livestock sla

people [78]). About 90% of beef produced in Ireland is exported,mainly to EU countries. More than 85% of Irish cattle outputs areslaughtered in Ireland; the remainder are exported live [79]. Morethan 50% of the total carcass weight of livestock and poultry ani-mals slaughtered in Ireland is from the slaughtering of cattle (seeTable 4).

There are currently nine licensed rendering plants in Ireland.These plants are classified as follows: five category 1 and four cat-egory 3. There are no category 2 plants currently licensed in thecountry [80]. In 2006, 558,000 t/a of animal by-products were usedto produce 151,000 t/a of MBM and 88,000 t/a of tallow in Ireland[81]. Based on these figures, approximately 16% of the by-productquantity can be converted to tallow and 27% to MBM, while 57% islost in the process [34]. The percentage of losses in the renderingprocess is high, because the major component of animal by-prod-ucts is water. Animal carcasses can contain up to 68% water [82].Rendering plants accept animal carcasses, such as heads, feet, offal,excess fat, excess meat, hides, skin, feathers, and bones. In Ger-many, on average 35% of the live weight of all animal species istransported to inedible rendering plants [82]. If this figure is trea-ted as the quantity of animal by-products available to the render-ing process, the tallow and MBM potentially available in Irelandare calculated as 92,600 and 156,300 t/a respectively (Table 5).This compares favourably with the 2006 production figures(88,000 and 151,000 t/a).

5.2. Biogas from paunch content

If we consider slaughter waste, it is important to also considerthe element of slaughter waste which is suitable for digestionand conversion to biogas/biomethane. Belly grass, which is diges-tive tract content (stomach content) obtained from cattle, is nor-mally removed in slaughterhouses in the form of dischargedslurry. Belly grass is classified as a category 2 material under the

8 1999 2000 2001 2002 2003 2004 2005 2006 2007ear

CattlePigSheepTotal

ughtered in Ireland [78].

Table 5Potential of animal by-products in Ireland.

(Data 2000–2006) Cattle Pig Sheep Poultry Total

Average carcass wt (’000 t/a)a 563.7 219.6 72.3 124.4 980Carcass wt as % of live wt (%)b 54 77 49 70Average live wt (’000 t/a) 1043.9 285.2 147.6 177.7 1654.435% live wt as by-products (’000 t/a) 57916% by-products as tallow (’000 t/a) 92.627% by-products as MBM (’000 t/a) 156.3

a From Table 4.b Average values in Europe modified from [82].

T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589 3585

EU Regulation EC 1774/2002. Land-spreading of this material ispermissible in Ireland under the same regulations as manure[83]. It can be used in pet-food production [82] and also pos-sesses the potential to produce biogas through anaerobic diges-tion. About 92 kg of stomach content may be expected perbovine, 7 kg per pig and 5 kg per sheep [84,85]. The dry solids(DS) content of the material from the various animals is verysimilar, i.e. 11% DS for cattle and sheep, and 10% DS for pig’sstomach content. For all animals, the figures of 80% and 85%are used for volatile dry solids (VS) content and volatile dry sol-ids destroyed (VSdest) respectively [84]. The biogas potential fromthe stomach content of these farmed animals in Ireland is esti-mated to be 15.21 million mn

3/a of biogas @55.5% CH4 (8.45 mil-lion mn

3 CH4/a) (Box 1). This is equivalent to 0.32 PJ/a.

Box 1. Biogas potential from animals’ stomach content.CattleAverage No. of cattle slaughtered (2000–2006): 1,814,000

cattle/a

Stomach content @92 kg/cattle @11%DS @80%VS

@85%VSdest: 6.88 kg VSdest/cattle

Destruction of 1 kg VS = 1 mn3 biogas @55.5% CH4

Biogas produced from stomach content: 12.48 mil-

lion mn3/a (6.93 million mn

3 CH4/a)

SheepAverage No. of sheep slaughtered (2000–2006): 3,593,000

sheep/a

Stomach content @5 kg/sheep @11%DS @80%VS

@85%VSdest: 0.37 kg VSdest/sheep

Biogas produced from stomach content: 1.34 million mn3/

a (0.75 million mn3 CH4/a)

PigAverage No. of pigs slaughtered (2000–2006): 2,918,000

pig/a

Stomach content @7 kg/pig @10%DS @80%VS

@85%VSdest: 0.48 kg VSdest/pig

Biogas produced from stomach content: 1.39 million mn3/

a (0.77 million mn3 CH4/a)

Total biogas potential:15.21 million mn

3/a (8.45 million mn3 CH4/a)

5.3. Used cooking oil (UCO)

A study done by the Irish Universities Nutrition Alliance [86]investigating the habitual food consumption of Irish people from1997 to 1999 showed that on average Irish people had a daily fatintake of 87.1 g/day (31.79 kg/a). About 82% of this fat intake(26.07 kg/a) is considered to be from vegetable oils and animal fats,which are used in a variety of food categories including meats,spreads, cake and biscuits, confectionery, breads, potato products,and others. The remaining 18% of fat intake is fats from milk and

yogurt, cheeses, and vegetables. Records from the FAO (Fig. 7) indi-cate that the average quantity of oils and fats used in Ireland forfood consumption activities (both direct and indirect), betweenthe same period (1997–1999) was 32.93 kg/capita/a. Of this quan-tity, 16.16 kg/capita/a was from vegetable oils and 16.77 kg/capita/a was from animal fats [87]. The estimation of recoverable cookingoil is outlined in Table 6 to be 6.86 kg/capita/a, accounting for20.8% of the total quantity for food consumption activities. The cal-culated figure (20.8%) will be further used to estimate a more up-dated quantity of recoverable cooking oil in the followingparagraph.

Fig. 7, which outlines the quantity of vegetable oils and animalfats consumed per person from 1990 to 2003, indicates a signifi-cant drop in animal fat consumption after 1998. This is due to achange in people’s habitual consumption. The decrease in animalfat consumption has been associated with an increase in vegetableoil consumption. The historical data show that the total oil and fatconsumption has never dropped below 25 kg/capita/a. Hence, thisfigure may be used as the bottom line to calculate the potentialrecoverable quantity. Most oils and fats associated with food resi-dues collected from households are disposed to landfill sites, com-posting plants, and wastewater treatment systems. As a result, notall of the total quantity produced is collectable. Only around two-thirds are believed to be realistically collectable in Ireland [88]. Thepotential collectable UCO in Ireland is thus estimated to be15,300 t/a (see Box 2), compared to the reported collected quantityof 9381 t in 2006 [89]. Hence, a further 5919 t/a could be collectedwith a better collection network and a more economically viableusage.

Box 2. Potential of UCO in Ireland.

Oils/fats used for food activities: 25 kg/capita/a

Recoverable quantity @20.8%: 5.2 kg/capita/a

Collectable quantity @66.67%: 3.46 kg/capita/a

Population of Ireland 2008: 4,422,100 people

Potential collectable UCO in Ireland: 15.3 thousand t/a

5.4. Total energy produced from animal by-products and UCO

Three different biofuels can be produced from animal by-prod-ucts including inedible tallow, MBM, and biogas from animals’stomach content. Tallow can be used as a direct fuel in boilers (ex-cept tallow produced from category 1 material under the EU regu-lation EC 1774/2002 which strictly requires incineration or co-incineration) and for biodiesel production, while MBM is a goodbiomass energy source [82]. Biogas can be upgraded to biomethaneand consequently to bio-CNG which is used as a transport fuel. It isalso a source of energy for heat production and combined heat andpower (CHP) production systems [90–94]. The lower heating value(LHV) of tallow is reported to be 39.8 GJ/t; the figure may also be

0

5

10

15

20

25

30

35

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003Year

Oils

an

d f

ats

qu

anti

ties

/cap

ita/

a (k

g)

Total oils and fatsVegetable oilsAnimal fats

Fig. 7. Quantity of oils and fats used for food in Ireland [87].

Table 6Recoverable quantity of cooking oil.

Period 1997–1999 (kg/capita/a) Total Vegetableoils

Animal fats

Fat eaten 31.79Fat eaten sourced from oils/fats

@82%26.07

Quantity used for food activities 32.93 16.16 (49.1%) 16.77 (50.9%)Recoverable quantity 6.86 3.37a 3.49a

% average recoverable oils/fats = 20.8% (i.e. 6.86/32.93)

a Assuming a direct relationship between the quantity used for food activitiesand the recoverable quantity.

3586 T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589

assumed for UCO. For MBM, the LHV is about 15.7 GJ/t [82], whilethe energy content of biogas (@55.5% CH4) is approximately 21 MJ/mn

3 [84]. The total energy potential is estimated to be 6.46 PJ/afrom animal by-products and 7.07 PJ/a if UCO is included (Box 3).

Box 3. Total energy potential.

Tallow @92,600 t/a @39.8 GJ/t: 3.69 PJ/a

MBM @156,300 t/a @15.7 GJ/t: 2.45 PJ/a

Biogas @15.21 million mn3/a @21 MJ/ mn

3: 0.32 PJ/a

Total energy from animal by-products: 6.46 PJ/a

UCO @15,300 t/a @39.8 GJ/t: 0.61 PJ/a

Total energy from animal by-products and UCO: 7.07 PJ/a

Table 7Feedstock potential for biodiesel production.

(’000 t/a) Tallow UCO Total

Potential quantity 92.6 15.3 107.9Existing productiona 88 9.4 97.4

a Based on the figures in 2006.

Of this energy potential, 60.8% (4.3 PJ/a) is associated with tal-low and UCO. Therefore, tallow and UCO are the most significantproducts among the residues to produce biofuels in Ireland.Paunch content is considered to be an excellent and plentiful feed-stock for biomethane production through anaerobic digestion[84,90–94]. However, it may produce up to only 4.5% (0.32 PJ/a)of the total energy potential explored here. MBM accounts for34.7% (2.45 PJ/a) of the total energy potential, which is significant.However it has found favour as a displacement for conventionalfuels for thermal energy production in industries such as cementmanufacturing.

5.5. Substitution of biodiesel based on potential production

Final energy consumption in the road transport sector in Irelandis predicted to be 188.15 PJ/a in 2010 [95]. With the new national

target of 3% biofuel substitution by 2010, this equates to 5.64 PJ/ain Ireland. If the target was to be met solely by biodiesel, 164.5 mil-lion L/a would be required (based on the biodiesel energy contentof 34.32 MJ/L). The existing production of tallow and UCO is about88,000 and 9400 t/a, respectively (Table 7). About 95% of the po-tential quantity of tallow is met by the existing production of tal-low, while only 61% is met by the existing collection of UCO. Basedon the conversion efficiency of 95% from oils/fats to biodiesel,102,505 t/a of biodiesel could be potentially produced from thefeedstocks (scenario 1, Table 8). Such biodiesel quantity could sub-stitute 2.1% of transport fuels in 2010, which would account forabout three quarters of the target (3% substitution). This is alsoequivalent to 85,970 ha of arable land (21.5% of the country’s ara-ble land) under oilseed rape if used for biodiesel production inIreland.

5.6. Sensitivity analysis

Three scenarios are examined in relation to the quantities of tal-low and UCO used for biodiesel production.

Existing collection of oils and fats. The most up to date data arethe collected tallow and UCO (Table 7). This is available fromtwo national reports; the 2007 EPA national waste report [89]and the 2003 SEI report on UCO and animal fats [88]. These figuresare considered the mean case for production (not all produced fatis collected) and are included as scenario 2 in the middle column ofTable 8.

Potential production of oils and fats. The collected quantity ofUCO according to the EPA report [89] is 9381 t/a. The SEI report[88] states that only two-thirds of the produced UCO is col-lected; this would yield a potential production figure of about14,000 t/a. Our analysis generates (Section 5.3) a theoretical fig-ure for recoverable UCO of 15,300 t/a, which indicates that ouranalysis is close to the expected value. The collection networkmay improve if a viable UCO biodiesel market exists. Our analy-sis on tallow indicates that production exceeds collection by 5%

Table 8Potential contribution of tallow and UCO to biodiesel based on three scenarios.

Scenario 1: potential production of oilsand fats

Scenario 2: existing collection of oilsand fats

Scenario 3: oils and fats presentlyexported

Total oils/fats quantity (t/a) 107,900 97,400 49,486Total biodiesel produced @95% (t/a) 102,505 92,530 47,010Biodiesel energy value (GJ/t) 39 39 39Biodiesel gross energy (PJ/a) 4.0 3.61 1.83Transport fuel required in 2010 (PJ/a) 188.15 188.15 188.15Biodiesel substitution in 2010 (%)a 2.1 1.9 1.0Equivalent arable land required under oilseed

rape (kha/a)b85.97 77.61 39.43

Equivalent percent arable land (%)c 21.5 19.4 9.9Equivalent percent agricultural land (%)d 1.9 1.7 0.9

a Based on 188.15 PJ of the transport energy (petrol and diesel) required in 2010 [95].b Based on gross energy of biodiesel produced from rape seed at 46.5 GJ/ha/a [5].c Based on the total 400 kha of arable land in Ireland [90].d Based on the total 4.45 million ha of agricultural land in Ireland [90].

T. Thamsiriroj, J.D. Murphy / Fuel 89 (2010) 3579–3589 3587

(Table 7); this is not unlikely. These values are included as sce-nario 1 in Table 8.

Oils and fats presently exported. Tallow may be used in animalfeed, soap manufacture, biodiesel production and oleochemicalindustries. In Ireland in 2003, 54% of collected tallow was usedas a substitute for mineral oil in the rendering industry while theremainder was exported [96]. Thus, conservatively 44,800 t/a oftallow should be readily available for biodiesel production. In2006, 50% of UCO (4686 t) was exported. This should also be read-ily available for biodiesel production.

Range of values. With respect to Table 8 it may be noted that thebiodiesel may substitute between 1% and 2.1% of transport fuelwith scenarios ranging from present export levels to collection oftotal produced oils.

6. Conclusions

This paper focuses on the use of residues to produce biodiesel.Tallow and used cooking oil (UCO) are examined as sources of bio-diesel which may substitute for oilseed rape. Rapeseed biodieselrequires extensive quantities of land to meet biofuel targets. Ire-land has a significant quantity of livestock in comparison to itspopulation. The slaughter industry is a very important industry;for example beef exports account for 90% of beef production. ThusIreland has a rich source of biodiesel in the form of slaughterwastes. Tallow and meat and bone meal (MBM) can be producedfrom animal by-products through the rendering process. Biogascan also be produced from the paunch content removed in theslaughterhouse as a raw material. It is estimated in this study that92,600 t/a of tallow and 156,300 t/a of MBM are potentially avail-able in Ireland. There is also potential for biogas from 205,000 t/aof paunch content. Of the energy potential in these residues,60.8% is associated with tallow and UCO and is suitable for biodie-sel production. Biomethane from the paunch content is limited to4.5% of the total energy potential explored. MBM accounts for34.7% of the total energy potential, which is significant. HoweverMBM has found favour as a displacement for conventional fuelsfor thermal energy production in industries such as cementmanufacturing.

The potential quantity of tallow and UCO produced in Irelandcould be used to produce 116.5 million L/a of biodiesel and effecta substitution of 2.1% of transport fuels (based on the predicted2010 energy demands). This is equivalent to 85,970 ha of arableland under oilseed rape, if it was grown to produce biodiesel in Ire-land. Thus energy content in tallow and UCO may be expressed asequivalent to 21.5% of all arable land in Ireland.

Acknowledgement

The research is funded by the Higher Education Authority (HEA)under the HEA PRTLI Cycle 4 programme.

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