Peat Briquetting by J. MARTIN

T H E decision by Bord na Mona to proceed with the erection of two new peat briquette factories will result in the appearance of this attractive form of

fuel in many districts not hitherto capable of being supplied from the production of the existing briquette factory at Lullymore, Co. Kildare.

Established by a private Company, the Peat Fuel Co. Ltd., in 1935, the Lullymore factory was designed to produce 50,000 tons of briquettes per year, using milled peat from a 500 acre bog as raw material. Following a period of financial difficulty in 1938/39, the Company went into liquidation and Lully-more came into the possession of the Turf Development Board Ltd. Production recommenced and was continued during the war years, when the briquettes proved of considerable use in many urgent cases of national need. Considerable experience was gained in operating the plant and various modifications were made. After the war the bog production area was trebled so as to produce about 150,000 tons of milled peat per annum, and in 1956/57 the factory output had been pushed up to just short of the planned 50,000 tons of briquettes per annum.

Development of the demand for briquettes during recent years, coupled with a recession in the demand for milled peat by the Electricity Supply Board in early 1956 resulted in the decision in July, 1957, to proceed with the immediate ordering and construction of the first of two briquette factories to produce 100,000 tons per annum each. These projects were assisted by a financial loan made available by Messrs. Arthur Guinness Son and Co. Ltd. The second factory was put in hands in July, 1958.

The first factory, known as " Derrinlough ", lies between Birr and Cloghan, Co. Offaly, and will be supplied with milled peat from the Boora group of bogs which now supply the Electricity Supply Board power station at Ferbane with about 500,000 tons of milled peat per annum. The briquette factory requirements of milled peat will be approximately 250,000 tons per annum.

The site for the second factory, called " Croghan", lies between Eden-derry and Daingean, Co. Offaly. It will be supplied with the necessary 250,000 tons of milled peat from the Mount Lucas and other adjacent sections of the group of bogs controlled by Derrygreenagh Works. Other sections of Derry-greenagh bog will supply about 700,000 tons of milled peat per annum to the Electricity Supply Board power station now under construction at Rhode.

BRIQUETTE TYPES AND SPECIFICATION Experience gained at Lullymore in meeting sales demands suggests that a

very considerable market exists for briquettes sold in strapped bales of approxi-mately 28 lbs. per bale. Some 75% of the Lullymore production is in the form of bales and so it is arranged that at both of the new factories there will be

Milled Peat Harvesting Machine.

Milled Peat Loading Machine

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facilities for baling any required proportion of the output. Only actual experience of a market to which full supplies of briquettes are available will determine the extent to which the planned baling facilities will be required. Provision has also been made to supply loose briquettes as requiredall the storing and out-loading from store being carried out by mechanical means.

Arrangements have been made to produce briquettes of the same general appearance as those from Lullymore, except that portion of the production will be of briquettes with two grooves pressed in the top and two more on the bottom of each briquette, so that these briquettes can be broken into three approximately equal pieces, each measuring about 2" x 2' x 1". Such breaking will be done at the factory should the customer wish for a supply of smaller sized briquettes for specially constructed combustion devices designed to suit smaller fuel sizes. Otherwise, the briquette will be supplied as heretofore, either loose "Saloon" size, or in bales.

Briquettes have a calorific value of about 8,500 BTUs at 10% moisture content. When well made their cross breaking strength is 425/500 lbs./sq. in. and crushing strength is 2,000/2,600 lbs./sq. in. according to the test procedure used in testing brown coal briquettes. Density is usually between 1.18 to 1.22 and no fissures or malformation should be present. This freedom from splitting or Assuring is particularly important in the case of loose briquettes as it enables them to withstand the several handlings that may occur between the factory and the consumer.

In general it can be said that the briquette represents the best effort to date at converting peat into a fuel of standard appearance, combustion characteristics, moisture content, ash value and calorific value. The appearance of the finished product is pleasingly polished, regular in shape and of satisfactory bulk density. Briquettes in bales can be stacked to give about 35-40 cubic feet per ton and when briquettes are piled in loose form, will bulk at 55-60 cubic feet per ton.

MILLED PEAT SUPPLY The necessary quantity of 500,000 tons of milled peat for the two new

factories requires a nett production area of some 5,000 acres, together with a further 800 acres for bog railways and stacking grounds (railfields) for the peat. This area of bog is already well advanced in development to meet the start-of-production date in early 1960 for the first factory at Derrinlough and for late 1960 for the second factory at Croghan.

Each factory will require about 1,000 tons of milled peat a day transported from the bog. Loading of the peat will take place over a period of 8-10 hours per day. Delivery will be taken at the factory during a similar period.

STORAGE AND BLENDING On arrival at the factory the peat will be emptied from the wagons by a

rotary tippler and delivered to the storage-cum-blending bunker where the peat of various qualities and moisture contents arriving from different parts of the bog will be laid down in alternate horizontal layers.

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This storage and blending bunker, is 492 feet long, 33 feet wide and 36 feet high, and is so designed that one half of it can hold sufficient milled peat for one day's (24 hours') briquetting at a rate of 18 tons of briquettes an hour, requiring 45 tons of milled peat at what is known as an " input-to-output ratio " of 2.5 tons milled peat to 1 ton briquettes. This ratio holds for peat of an incoming moisture content of 55%, being lower for drier peat and higher for wetter peat.

As the peat in any given half of the bunker is being removed for use in the factory, the other half of the bunker is refilled for use the next day.

The peat is removed from the bunker by means of a self-propelled rail-supported carriage on which a series of revolving spirals cut into the pile of peat, and having loosened it, pull it down to a belt conveyor running under the floor of the bunker. The forward speed of the carriageand hence the rate of delivery of peat to the factoryis controlled by high/low level indicators fitted on a control bunker installed between the blending bunker and the factory proper.

The method of alternate horizontal layering of different peat qualities and moistures in the bunker, and the removal of this peat vertically by the special carriage gives a good blending of the various peats. Blending is very necessary when preparing such a variable raw material as milled peat for a drying process in which constant conditions are vital in order to obtain optimum production. The same holds true to a great extent when preparing milled peat for firing to boilers specially designed to burn milled peat.

Milled peat varies almost from wagon to wagon and varies considerably in bulk density and moisture content, depending upon the section or sub-section of bog on which loading is taking place. It is desirable in the central midland bogs to use up to three mechanical loaders located on different areas so as to obtain three different peat qualities for blending. The desire to use more loaders than three has to be balanced against the higher capital and operating costs involved. The moisture content of milled peat varies not alone with peat quality, but also with the season of the year. Blacker, denser peat tends to have a lower moisture content than the lighter types, although this is not invariably the case. Milled peat tends to increase in moisture in the storage piles during autumn, winter and spring.

It will thus be seen that the successful regulation of the final moisture and bulk density of milled peat from the blending bunker will involve careful selection of the quantity of peat taken from each of the various loading points on the bog. Depending upon the known general condition of the peat piles on the bog a long term target of blended peat moisture and bulk density is set by the management for the guidance of the transport and bunker staff, and control of the day-to-day figures is kept by the factory laboratory staff. Any tendency for the daily blend figure to drift away from the target has to be corrected during the next filling of the bunker.

Generally speaking, a variation in the incoming milled peat of from 40% MC to 60% MC and of 12 lbs./cub. ft. or less to 25 lbs./cub. ft. or more in bulk density can be blended to produce an average peat whose tested

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moisture or bulk density does not vary more than about 1% from the mathe-matical average of the various peat moistures and bulk densities constituting the blend for any period of 24 hours, i.e., for a half-fill of the blending bunker.

It should be mentioned here that moisture and bulk density are not the only variables in the incoming peatit can also vary in regard to its fibre content, particle size distribution, flow characteristics, timber debris content, etc. The blending bunker also helps to level out the incidence of these factors.

PREPARATION, SCREENING AND DRYING The preparation of the peat by disintegration, the screening process, the

drying by use of the Peco process, and briquetting using brown-coal type string presses is essentially the same for Derrinlough and Croghan as at the older plant at Lullymore. A number of constructional variations have been introduced at the new factories, mainly with the object of giving economy in building, or of making control of the process automatic to the greatest possible degree.

Whilst the screens have been accommodated within the factory building, it has been found possible to arrange the dryers in the open and also most of the larger cyclones, fans and associated motors. The boiler has been arranged with its controls and fuel preparation portions in the factory building, but its furnace and superheated portions, air and water preheaters and induced draught fan will be installed in the open air. In all cases of open air construction, suitable precautions have to be taken to insulate hot surfaces more fully than if the equipment had been erected within a building. The outdoor hot surfaces at Derrinlough and Croghan will be insulated against a temperature of 5C below freezing point.

The placing out-of-doors of the equipment described above is not novel, but it remains to be seen whether in our climate maintenance or operation of the equipment will necessitate some extra weather protection over that provided for, but only experience will decide this point.

The main disintegrator has been located at ground level and after passing through this, the peat is screened on six resonance type screens, each measuring 16 ft. long by 4 ft. wide, and the fines from these screens are then conveyed pneumatically through the dryers of the Peco drying circuit. Having been dried to 10% MC the peat is then sent to the briquette presses. The number of dryers fiveis the same as at Lullymore, but the through-put rate is double that at Lullymore: the extra drying area has been obtained by increasing the cross sectional area and lengthening each dryer. The dryers are 60 ft. high, as com-pared with 35 ft. at Lullymore and as a result a considerably longer drying path is available. This is a particularly important point when dealing with the less decomposed elements in Irish peat, as these tend to be slow in parting with their moisture.

The problem of chokage of the dryers' tubes at Lullymore has, it is hoped, been solved for the new factories by making the internal diameter of the new tubes 2(3/4)" as compared with the old internal diameter of 1-9/16" at Lully-

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more. The spiralled or rifled design of the tube bore has been retained so as to hold the heavier, moister particles of peat against the hot inside wall of the tube, whilst facilitating the more rapid passage of the finerand usually drier particles through the dryers.

In order to reduce the ejection of peat dust from the dryer's cyclone to atmo-sphere, and to reduce peat dust losses in waste liquors from the factory, the fitting of secondary cyclones has been found necessary. The effectiveness of such cyclones is high and apart from the prevention of pollution mentioned above, a useful quantity of peat dust can be recovered. The recovered dust has hitherto been mixed with the dry peat passing to the presses, but this practice is under investigation at present. The available evidence tends to suggest that when the percentage of very fine dust rises above a certain amount, difficulties arise in the production of good quality briquettes. An alternative use for the dust is to pass it to the boiler for combustion purposes, having previously mixed it with the reject peat from the screens.

The reject peat from the screens along with sufficient extra peat from the blending bunker and together totalling about 20-30% of the total input of peat to the factory, is fired in a PF type boiler equipped with mill and produces sufficient steam for all factory requirements. This steam at 500 lbs./sq. inch and 780F is passed to the 2,400 kW turbo-alternator to generate power and also supply steam heat to the dryers. The steam heat is obtained by operating the turbine at a back pressure of 40 lbs./sq. inch and 400F temperature desuper-heated to 280F. At the inlet and outlet pressures and temperatures quoted above, the turbine provides the 2,050 kW required for all factory processes and also sufficient steam heat per hour to evaporate the moisture from 36 tons of 55% moisture content peat to yield 18 tons of peat at 10% moisture content for briquettingan estimated evaporation of 18 tons of water per hour for a boiler output of 20 tons of steam/hour. This high rate of water evaporation per ton of steam is due to the double effect Peco drying system.

WATER SUPPLIES The question of steam supply raises the matter of water supply for boiler,

condenser and for other Derrinlough Factory services. This has provided a problem and in order to obtain the 10,000 gals per hour required, a micro-filter plant has been installed to filter the water from a local stream with an excessive solids content. This filtered water will provide most of the requirements, but a deep bore well will provide drinking, cooking and toilet, as well as boiler water needs. At Croghan the matter of water supplies is somewhat simpler as the Philipstown River runs close to the site and provides an adequate supply. Drinking water will be obtained from a deep bore well here also.

BRIQUETTING The dry peat at 10% MC and 200F temperature is removed from the

Peco drying circuit by a cyclone placed above the five presses. A quantity of this dry peat variable at will can be conveyed back to the screens house for pre-mixing with the incoming milled peat should this be above 50%MC.

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By this means the peat sent for screening can be preconditioned to give best screening results.

Peat is dropped from the cyclone into a horizontal Redler type conveyor, which takes the peat round and round above the presses until it eventually flows down for briquetting. This conveyor not alone conveys, but acts also as a storage reservoir, holding about twenty minutes' supply of peat ready for pressing. By this means minor fluctuations in supply are evened out and a steady feed of peat obtained for the presses.

The five presses are each of the double ram simple crank string type, as used in brown coal briquetting. Thus there are ten separate strings of briquettes being produced. Owing to the relatively low bulk density of Irish peat, it has proved necessary to arrange that a special form of pre-compression device be fitted to the presses so as to obtain a briquette of a minimum thickness of 1.5 inches from dried peat bulking at 17 lbs./cub. ft. The other dimensions are 7.2 inches approx and 2.8 inches between the polished faces.

The briquettes are formed in each press by simple ram pressure and depend for their adhesion on moisture content. After issuing from the press mouth, the briquettes are pushed by the power of the ram along steel slideways or runners as far as either the position where bales are assembled and strapped, or further along into a loose storage bunker designed to hold about 25,000 tons against fluctuations in demand between summer and winter.

In the case of briquettes destined for loose storage it is highly desirable that the core temperature of the briquette be reduced from the 200F or so at time of pressing to about 110F before dumping the briquettes on to the pile, other-wise spalling or splitting will tend to occur. As the loose briquettes are mechanically handled after that for loading, the German brown coal regulation in this regard is of importance" adequate briquette cooling is a pre-requisite for mechanical handling."

LOOSE BRIQUETTE BUNKER The runners guide the briquette strings a short distance into the bunker

and about 30 feet above ground level. At this point the briquettes roll off the runners on to one or other of three slow moving rubber belt conveyors. By this means a simple form of pressure-face cooling is arranged. Experience will dictate whether the amount of cooling given in this way will be sufficient to consolidate the briquettes against self-splitting and the rigours of the subsequent dumping and mechanical rehandling operations. On the briquettes reaching the point of dumping, they are scooped off the belt and fall on to the advancing face of the loose briquette pile.

The bunker dimensions are 492 feet long by 66 feet wide and 36 feet high to the eaves. The design of the very strong side walls has had to take into account casual loading. Great side pressure is capable of being exerted by loose briquettes, especially in such high piles, as when loose briquettes are first laid down their angle of repose is very small prior to consolidation. As the peat

F

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blending bunker and the 25,000 tons briquette bunker had the same lengths it was decided to combine both into a single building with a dividing wall. Some economy in construction was obtained thereby.

OUT-LOADING OF LOOSE BRIQUETTES The recovery of the loose briquettes from the pile is effected by means of

a special device which is probably novel in this country. The loose briquettes fall through a 3 ft. wide by 490 ft. long longitudinal slot in the floor of the loose briquette bunker and then lie on a 20-degree sloped support about four feet above the basement floor. A heavy pusher carriage runs on rails on this under-ground basement floor, and beside the sloped briquette support. A special cir-cular pushing device on the carriage pushes the briquettes gently off the sloped support and onto a belt conveyor running parallel to the support and on the opposite side to the carriage. The conveyor takes the loose briquettes away to the lorry loading point. The special carriage and the conveyor are remote-controlled by the loading attendant. By means of this device the problem of out-loading briquettes from even the remotest parts of the briquette bunker will be rendered relatively simple and safe.

On arrival by belt conveyor from the briquette bunker, the loose briquettes slide down across a rough screen and so all fines and smalls are separated and taken away by special rail wagon for re-circulation through the factory.

The screened briquettes are then shuttle-belt loaded on to the waiting lorry. This shuttle-belt can be dropped down to the bottom of the lorry and as the briquette pile builds up, the belt is raised and slowly moved backwards on rails whilst loading continues. By this means the briquettes are deposited as gently as possible in the lorry so as to reduce impact damage and production of smalls to the minimum.

There is a weighbridge of 35 tons capacity located under the stationary lorry being loaded, and by this means it will be possible to give any pre-determined quantity of briquettes to a customer. This is a necessity, as many customers arrive with prefilled cheques, or with instructions to take loads of a given tonnage.

BALED BRIQUETTE MANUFACTURE The production of baled briquettes was carried out by the Irish Peat

Fuel Company at Lullymore in 1936-1938, prior to the Turf Development Board taking over. These bales were wire bound. When baling was restarted by the Board after the War, use was made, however, of steel strapping of 5/16" width x 0.012" thick tied by hand-operated machines. Automatic strapping machines are being tested at present for this work.

At Derrinlough and Croghan factories a special floor has been provided where the briquette runners enter the main bunker building and here are placed facilities for baling either all or any proportion of the outputs from the ten separate briquette runners.

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When the bales have been tied they are deposited on a belt conveyor and sent to a special bale storage shed for loading on to lorry as required.

If baling is not taking place for any particular reason, then the briquettes fall from the runners on to belt conveyors which take them to the loose storage previously described.

The making of a bale is a simple operation. The bale assembler removes a predetermined length of briquettes from one advancing briquette string and quickly puts this length on top of a similar length of briquettes on the front of the advancing string in an adjacent runner. The untied bale of two rows of briquettes is then pushed to the bale strapper, who wraps the steel strapping end-to-end about the bale and clips the joint after first putting a safe amount of tension on the strap. He then pushes the bale off the strapping jig on to the conveyor to stores.

The consumption of steel strapping reaches high figuresan annual pro-duction of 50,000 tons of bales, as anticipated at Derrinlough, will require about 3,200 miles of strapping per year, or 170 tons.

PRODUCTION COSTS The economics of briquetting are tied up in great degree with that of its

raw material, milled peat, as will be seen from the following table: ESTIMATED PERCENTAGE PRODUCTION COSTS OF BRIQUETTES

Percentage of total cost Milled peat delivered to Factory ... ... ... 69 % Factory Production cost ... ... ... ... 11.7% Factory Capital charge and H.O. Charge ... ... 19.3%

100% FUTURE TRENDS

Some consideration is being given to the development of several of our smaller bogs for briquette production. Smaller briquette factoriesprobably of the order of 15,000 tons per yearare envisaged, and as this size of factory is rather small for the incorporation of the Peco double effect steam dryer system, investigations are being made into systems such as the disintegrator mill with circulated hot gas, and others.

In the case of the disintegrator mill type briquette factory, the capital cost is much lower than a Peco plant, but high running charges just out-balance the saving in capital, mainly caused by the need to buy power, as electricity is not generated as a by-product as in the case of the Peco drying system. The disintegrator mill plant, however, is attractive because of its simpler mechanical equipment. Plans are being made for a test of a prototype plant of this type.

J. MARTIN.