Heat Recovery Systems Vol. 4, No. 5, pp. 317-322, 1984 0198-7593,'84 $3.00 + .00 Printed in Great Britain. All rights reserved Pergamon Press Ltd

MUNICIPAL WASTE DERIVED FUELS. PRODUCTION COMBUSTION AND ENVIRONMENTAL ASPECTS

A. PORTEOUS Reader in Engineering Mechanics, The Open University, Walton Hall, Milton Keynes, U.K.

Abstract--This paper reviews the production of Waste Derived Fuel (WDF) from the waste paper content of municipal solid waste in the U.K. The state of the art of the technology, it's advantages over recuperative incineration, plant flow-sheets, outline economics, and the environmental and corrosion aspects of the fuels are considered. It is suggested that WDF production is an alternative to dumping municipal solid waste, provided there is sufficient waste paper to make fuel production viable.

I N T R O D U C T I O N

In the U.K., waste paper reclamation for repulping has shown a marked down-turn in recent years. The extent of this market decline is readily appreciated when the 1980 figures are broken down. Davis (1981) reported that the 1980 total British waste paper consumption would be down to 2 million tonnes, a drop of 99/o on the 1979 figures.

Of these 2 million tonnes of waste paper, around 100,000 tonnes per year are collected by local authorities from solid waste. The great majority is reclaimed by specialist waste paper merchants. Hence, in the U.K., of the 19 million tonnes of municipal solid waste (also called refuse) which contains over 5 million tonnes of paper, only 2 ~ is actually collected for repulping due to market conditions. It is little wonder that other means of utilizing this vast quantity of paper are being sought. Waste derived fuel production is one of these means.

The U.K. Waste Management Advisory Council [1] states that the calorific value of typical, as received, municipal waste is in the range of 9000-11000 kJkg -t . The calorific value of U.K. industrial coal approximates to 24,500-28,000kJkg -1 . Hence, the 19 million plus tonnes of municipal solid wastes can be deemed to be equivalent to 7-8 million tonnes of industrial coal worth up to £350 million per year, in gross terms, at 1983 prices.

The economic potential of the solid waste, when used as a fuel, has attracted two separate routes of exploitation, namely recuperative incineration which is described briefly below, and waste derived fuel or WDF, which comprises the bulk of this contribution.

R E C U P E R A T I V E I N C I N E R A T I O N

Municipal solid waste can be burnt in the "as received" state in specially constructed steam raising incinerators and the steam raised used for district heat and/or electric power generation.

The U.K. has, at Edmonton, London, one of the world's largest power generating incinerators which consumes around l l00 tonnes per day of municipal waste (based on 80~o plant availability) generating 16 MW for sale or 387 MWh day. -1. This equates to 2.84 tonnes of solid waste consumed per MWh produced at an estimated overall thermal efficiency of 12.3~o based on the calorific value of London's waste [2]. This figure should be compared with the U.K.'s more efficient thermal power stations whose efficiencies are in the region of 36~. Thus, if i tonne of waste derived fuel is burnt as a solid fuel supplement in an efficient power station boiler, as opposed to an all solid waste fired incinerator, it can be utilized three times more effectively. This is one argument for WDF production, namely much more efficient use of the fuel content. The second major argument is that of cost. As WDF production has a lower capital cost cf. that of building and running recuperative incineration.

LOOSE WDF P R O D U C T I O N

The use of shredded (sometimes referred to as pulverised) solid waste, with the ferrous metals content extracted, is the basis of two very successful industrial applications in Britain, namely the

317

318 A. PORTEOUS

Scteev

UnOersTze

C~ered ~,toroqe-

Ferrous rnetol seoorotors

v

Pe~rou5 rnetOl% ( ~ PtOqltqnf tO kdn

Pneurnot~c l,eeeler

Fig. 1. Loose WDF production system (courtesy Blue Circle Oroup).

Blue Circle Project [3] and the Imperial Metal Industries-Witton Project [4]. Both processes are similar in that the solid waste is shredded to 75 mm particle size, screened, ferrous metals are then extracted and, in the case of Blue Circle method, further fine shredding to less than 50 mm takes place. Figure 1 shows the Blue Circle flowsheet. The IMI one is similar, but a primary crusher only is used.

The end uses of the WDF in these flowsheets are entirely different. The Blue Circle uses the WDF as a fuel supplement in cement kilns (at least 10~o fuel replacement), whereas the IMI method makes use of chain grate boilers with a 50~o shredded solid waste-50~o coal mix, on a heat input basis.

The economics of both projects are closely guarded secrets. However, Porteous [5] has estimated that, in the case of IMI process, the 1980 costs of a one stage shredder and solid waste handling facility processing 150,000 tonnes year. are of order £500,000. As 2,5 tonnes of unprocessed municipal waste may replace 1 tonne of coal, the annual fuel cost savings can be as much as £300,000. There will also be the running and labour costs of the processing plant, plus the costs of tipping any residue.

The area of minimal preparation of the solid waste for immediate burning as a fuel supplement in existing chain grate/vibratory hearth boiler plants or cement kilns appears to have a promising future as fuel costs escalate. Indeed, a 40 tonne- ~ plant, embodying the processing features already described, has recently been installed at the Grimsby works of Courtaulds Ltd. The roam contractors for the plant are ToUemache Ltd. [6]. However. not all districts have large boilers or cement kilns near at hand, and therefore a more amenable upgraded fuel is required which can be burnt in a wide variety of installations. This is the province of pelletised WDF.

PELLETISED WDF PRODUCTION

The pelletising of the finely shredded combustible portion ofsohd waste means that a fuel is made which can be stored, transported and used at will in someone else's boiler plant. This removes the major objection to loose W D F which requires immediate combustion. The pellets can also be burned in a wide range of solid fuel appliances, ranging from small district heating boilers to massive industrial installations, with minor performance down-rating. Indeed, it is claimed that there may be no performance loss on some installations (7).

Basically, pelletising plants have the following components in their flowsheets: solid waste reception, coarse shredding (pulverising), screening to remove fine ash and glass particles, magnetic separation of ferrous metals, air classification to remove the light combustibles (paper, plastics), further fine shredding of the air classified 'lights' followed by pelletising in a variety of pdletising processes. A drying c~rcuit is often incorporated as well. The result is a pellet which is composed

Municipal waste der ived fuels 319

Reception h a t L ~ St•rage area Process ore•

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~ - Rotary -12 rnrn + 150 mm ! o c r e e n ~ l r e l m l ~ * J ~ ' ~ • TO air CLassifier

• • .................. , . . ' T ' ~ i ~ J . . ,

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± ± Fines rejec'(s "¢o Oversize rejects Land recLomo'~ion 4:o reprocessing or LOndf'iLL

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• . . . . . . .~, • • o o o e e m c e e s • e a s e r~arv ~ . " | ~ V U U ' ~ ~ * , , " ~r I ~ • steel_ inclu~ry " • ~ • ~ 0000000 screen ~ ] " - " - - - ~ : ~ ' • * . ._ ee ~ Ab. a CLassifier

, / * "~** * ~'.OSpreaderl I ~ - I - b StO~O~ ~ L j eeeoo • e'~o • ~ |HeaW rejects'co

• **~eee* . . . . . . . . . eee4t '~ ~ l t a n c l reel.am•tie, c Process • s.;J Air classifier e~ F

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c .,_...°*°tae ~ to district hoa*ing

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• ~ * ~ - J " ~ - J • IEWasto derivod~ /~ ' t . .~ lL ~ pass : *" S.con~or~ _ " = = = = " I I / ~ ; ~ ' ; t ~ r ~ , ' - ~ J ~ J ~ I n t L._X • • e • • o ~ . . . . . . . . , . , , . r i~ . . . . , . . . . ! . . . * JI~/;Y, TY2,/A ~Fq~x~

Fig. 2. Byker pelletised W D F flowsheet (Cour tesy Tyne & W e a r C o u n t y Council) .

mainly of paper and plastic with an approximate 14~o ash content• Pellet properties are described later.

Space does not permit a full description of all plants, and a "typical" flowsheet is shown in Fig. 2. for the Byker Plant (Hewitt, 1981) owned and operated by the Tyne and Wear County Council• This plant is designed to handle 1,000 tonnes of solid waste per week. It was built at a total cost of £4.5 million and can produce up to 200 tonnes of WDF per week, some of which is burned in the Tyne & Wear County Council's own district heating boilers• For this reason, the Tyne & Wear County Council can pay itself the full energy value of its WDF of around £30•00 per tonne for the WDF it consumes• But when put on sale on the open market, it can only fetch £15.00 per tonne due to the need to offer a substantial discount to encourage it's use. So, if existing solid waste disposal costs are high, as they are in Tyne & Wear, pelletised WDF production can indeed have much to commend it, particularly where the pellets can replace bought in fuel in municipally owned or operated boiler installations. This effects a net reduction in solid waste disposal costs•

Table 1 gives a cost breakdown on the basis of a generalised pelletised WDF plant handling 75,000 tonnes solid waste per year, costed on an annuity basis (12½~ o interest) with an assumed life of 15 yr for mechanical plant, 30 yr for buildings and 5 yr for vehicles. The WDF is assumed to have a ready market of 85~o of the coal equivalent price•

This compares favourably with the costs of solid waste disposal via long haul transfer stations provided the WDF selling price of £20/tonne can be realised. The current Byker disposal costs

Table 1. Estimated (1983) costs associated with a 75,000 tonne yr -I pelleted WDF plant

Total capital cost £5,000,000

Total annual costs, including loan charges, residue disposal etc. £300,000

Total annual revenue from the sale of: Pelleted waste-derived fuel (22,000 tonne yr -~ at £20 tonne -L) £440,000

Net operating costs per year at £10.13 tonne -j of solid waste disposed. £760,000

320 A. PORTEOUS

Fuel

Table 2, Comparison of the properties of WDF and coal [10]

CV MJkg - ~ BTu Volatile "'as received" I b ~ Moisture °o Ash matter

15.7 6740 16,8 15.6 64 mean mean mean mean

1,2 4800 26 27* mean mean mean

WSL pellets ex Doncaster City solid waste

Shredded WDF produced in U S A ,

Typical British industrial coal 30 12,800 6 6 34

*High ash attributed to hammer milling of raw solid waste at start of sorting process,

(allowing for drying of the pellets) and pellet price of £15.00 per tonne, are estimated as £14.00 per tonne of MSW [9]. Given the experimental nature of this plant, the economics of Table I may be realisable once the market is fully established.

WDF PELLET CHARACTERISTICS

The characteristics of most interest in pelletized W D F are: pellet stability, calorific value and combustion properties.

(a) Pellet stability is influenced by the moisture content and, provided that it does not exceed 20~ (preferably 15~), on a wet basis, a stable pellet can be made which can be stored; under cover for months. However, U.K. solid waste can have up to 35~ moisture content if the weather has been very wet, and thus provision has to be made in the pelletising circuit for drying the pellets if long term storage is envisaged.

(b) A comparison of the calorific values of WDF. and coal is given in Table 2 [10]. It Should be stated that other processes claim to make a higher calorific value pellet with less moisture This shows the importance of fuel analysis.

Combustion properties are such that slightly lowered boiler etticiencies result when, say, 50,/50 coal/WDF pellets are burned, but this is a function of the mix used and the pellet anaiysis.

ENVIRONMENTAL EMISSIONS

Environmental emissions may be conveniently summarised by reporting the results of Olexsey et al. [1 l] test firing of WDF and U.S. coal, on boilers at Ames, IA. The boilers were set at ~0" load and Table 3 gives the results for boiler number 5, which was used for the WDF tests,

Table 3. Ames. IA WDF Emissions Tests Bo!!e,r Number 5 [tl] .

. . . . . . . . . . . . . . . . . . . . . . Percent WDF b y H e a t Input Parameter (units) 0 20 50

Uncontrolled particulate (gMJ - ~ ) 3.6 4.0 3.4 NO I (stack) (mgMJ -~) 80 72 68 Sulphur (gMJ-~ ) 2.3 t.7 1.6 Formaldehyde (mgMJ - ~ ) 0:2 0.6 3,0 Cyanide (mgMJ-~) 0,22 0.17 0.19 Chloride (mgMJ- J ) 12 78 93 ............. _ . . . . . . . . . . . . . . ,, ,, ...... . . . . . . . . . . . . . . .

The interesting feature is that particulate emissions are virtually independent of WDF variation, Sulphur emissions were reduced. Chlorides increased (due to the high proportion of plastics and chlorine compounds in the US solid waste), but a t m ~ h e r i c concentrations were claimed to be within air quality standards. U.K. solid waste has a much lower chlorine Content of. U.S. sotid waste, and no U.K. Chloride emission pr0blcms have been found at any installations buining WD F as a fuel supplement. Indeed; U.K. WDF pellets have very 10w levels Of bothehtor i~ and sulphur which renders these emissions entirely satisfactory.

CORROSION

One of the major disadvantages of all solid waste firing, as practised in municipal incinerators, is that if energy recovery is attempted, there is the possibility of substantial boiler eroSionicorrosion

020

'~: O J5 --

"~ O, IG 2

-6 k~ c 0.0~

Municipal waste derived fuels

o

Metal temperoture - I 4

• 500 F (260 C) ~ o 9 0 0 F ( 4 8 2 C)

3

1 ° I 2

I I

/ I

0 / / / / / I 0

- " | 0

25 50 ?5 I00

Weight percent refuse with 3 % S cool

Fig. 3. Initial corrosion rates (8 h) for A106. Carbon steel as a function of percentage of refuse by weight in fuel [13] (Battelle Memorial Institute and US Environmental Protection Agency) (copyright ASME).

321

from the corrosive gases produced. Krause et al. [12] have effectively demonstrated that the gases SO2, HC1 and Cl2 are responsible for the metal wastage and, in order to minimise these effects, incinerators should be operated at the relatively low metal temperature of 260°C and high excess air which, of course, restricts thermal efficiency.

Where WDF is used as a fuel supplement, Krause et al. [13] found that corrosion rates were only one tenth to one thirtieth as great as those in previous all solid waste fired tests. This is summarized in Fig. 3 which shows no significant increase in corrosion rate until a 75% solid waste, 25% coal (by weight) mix is exceeded. It was also found that a high sulphur coal could be beneficial in reducing corrosion as, in the presence of sufficient SO2, the metal chlorides in the combustion products are converted to sulphates in the flue gas stream. A full review of WDF corrosion and combustion properties is given by Porteous [5].

D I S C U S S I O N

This paper has been confined to WDF that can be burnt as a coal (or oil) supplement in conventional installations--space has not permitted a look at the other, not yet commercial developments of pyrolysis for gas production, or cellulose hydrolysis for ethanol production. That said, both loose and pelletized WDF production look very promising as viable means of solid waste disposal/recycling in areas of high solid waste disposal costs. Already in the U.K., solid waste disposal costs of up to £15.00 per tonne are encountered where incineration or long haul transfer stations are in use, and WDF production can, it is projected, do better than this in cost terms.

Investment in WDF plant can provide a constructive alternative disposal method which offers a product whose selling price should match inflation, thereby containing solid waste disposal costs.

R E F E R E N C E S

1. Waste Management Advisory Council, (1980) Waste Statistics. HMSO, London (1979). 2. A. Porteous, Recycling, Resources, Refuse. Longman, London (1977). 3. D. Watson, Combustible gases and low grade fuels--their use in cement manufacture, Proceedings of Rock Products

14th International Cement Seminar, Chicago 0978). 4. J. E. Marshall and K. Harvey, The use of refuse as a supplementary fuel, Paper 24, Proceedings Institute of Municipal

Engineers, Public Works Congress, Birmingham (1976). 5. A. Porteous, Refuse Derived Fuels. Applied Science Publishers, London (1981). 6. Tollemache Ltd., Industrial energy from domestic waste. The Gilbey Road WDF Plant, Grimsby (Technical brochure)

(1983). 7. R. Taylor, The preparation of solid waste for fuel usage. Proceedings of the Symposium on Energy from Waste Burning,

Portsmouth (1979). 8. J. M. Hewitt, The Byker Reclamation Plant. Proc. Symposium: The Practical Implications of the Reuse of Solid Waste.

I.C.E., London (1981). 9. J. M. Hewitt, Personal Communication (1983).

10. Warren Spring Laboratory., Production and firing of refuse derived fuel (Technical brochure) (1980). 11. R. A. Olexsey, C. Wiles, C. L Kulwicki, A. W. Joensen and J. L. Hall, Operation of the stoker boiler firing coal and

refuse at Ames, Iowa. Proceedings of the 5th National Conference on Energy and the Environment. Cincinatti, OH (1977).

322 A. POR'rEOUS

12. H. H. Krause, D. A. Vaughan and P. D. Miller, Corrosion and deposits from combustion of solid waste, J. Engng Pr. 95, 45-52 (1973).

13. H. H. Krause, D. A. Vaughan, P. W. Cover and W. K. Boyd, Corrosion and deposits from combustion of solid waste. part 6---processed refuse as a supplementary fuel in a stoker fired boiler, A S M E J. Engng Pr~ t01, 592-5 (t979).