Heating value of municipal solid waste

Heating value of municipal solid waste

Waste Management & Research (1987) 5, 141-145 HEATING VALUE OF MUNICIPAL SOLID WASTE C . Finet (Received 10 March 1985) This paper describes the p...

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Waste Management & Research (1987) 5, 141-145

HEATING VALUE OF MUNICIPAL SOLID WASTE C . Finet (Received 10 March 1985) This paper describes the processes that are used to calculate the heating value of municipal solid waste in France . The calculation can be done either by using the thermal-balance method of a furnace or a furnace-boiler unit, or by sorting the refuse and calculating the heating value of the homogeneous components . Both methods are described in this paper . The first method measures the heating value of the refuse that is injected into the furnace ; the furnace becomes a calorimeter in which thermal balance is achieved on measuring the input and output heats . The second method consists of sorting a 100 kg refuse sample into piles that are as homogeneous as possible so that it can be considered as unchanging . The heating value is determined for each component and the lower heating value of the whole sample is calculated . The advantages and drawbacks of both methods are discussed . Key Words-Municipal solid waste, incineration, lower heating value, higher heating value, thermal balance, sorting, recoverable heat .

1 . Introduction When municipal solid waste (MSW) is to be burnt, it is necessary to know the thermal energy or heating value contained in it . Both the engineering design of the incineration plant and the energy that this plant may generate depend on the estimate of the heating value of the refuse . The heating value must be continuously calculated when the plant is operating, in order to control the operation of the incinerator and energy-generation equipment with respect to the changes which occur in the MSW . This paper describes some of the terms and the processes that are used to calculate the heating value .

1 .1 . Lower and higher heating values The higher heating value, HHV M , is the quantity of heat, (in joules, J) emitted during the complete combustion of 1 kg of MSW (called HHV-1 in ASTM standards) . During incineration, water contained in the refuse and water generated from hydrogen in the combined state remains in the form of steam . Therefore, when calculating the lower heating value it is assumed that water stays in the form of steam (this assumption is used especially in Europe) . Conversely, when calculating the higher heating value, the water is assumed to be condensed . The higher heating value is measured in the laboratory when the products of combustion are cooled to standard conditions . The lower heating value, which assumes that steam is not condensed, is the part of the heat that can really be utilized . For American practice see Hasselriis (1985) .

1 .2 . Reference conditions The heating value obtained depends on the conditions under which it was measured, i .e . pressure and temperature. Therefore, these conditions must be specified when

* T .I .R .U .S .A . (Traitement Industriel des Residus Urbains), 134 Bld Haussmann, 75008 Paris, France . 0734-242X/87/020141 + 05 $0 .300/0

© 1987 ISWA



C . Finet

142

quoting a heating value . Generally, the standard atmospheric pressure 101 325 N m -2 , approximately 10 5 N M-2 and the ISO standard temperature (25°C) are used . In a furnace, combustion is achieved under conditions that are close to atmospheric pressure . The ambient temperature near the storage bunker represents the reference temperature, i .e . it represents the temperature of the refuse when it is injected into the furnace . It also represents the combustion air temperature which is usually taken from inside the bunker . The difference between the ambient temperature and the standard temperature (25°C) does not usually exceed a few °C : this difference only involves a very small change in the heating value figure .

2 . Measurements The processes that are used to calculate the MSW heating value aim at reducing the variability that is due to the heterogeneous composition of the fuel, either by achieving thermal balance of the incineration plant, or by sorting the refuse into homogeneous piles in order to measure each component .

2 .1 . Thermal balance process of a furnace or a furnace boiler unit 2 .1 .1 . Principle This process is used to calculate the heating value of the refuse that is injected into the furnace or the furnace-boiler unit, which becomes a calorimeter when thermal balance is achieved, by measuring the input and output heat (see Fig . 1) . These measurments last between 6 and 10 h and determine the thermal energy that is contained in a few tons or several hundreds of tons of waste-according to the size of the furnace .

Radiation and convection

i MSW

-

-3o Reheated fluid exit of the recovery exchanger

Combustion air

Supplemental fuel

0

Cold-fluid recovery exchanger entrance

3

Furnace boiler unit

Imbalance Fig . 1 . Diagrammatic representation of an incineration-plant thermal balance .



143

Heating value of municipal solid waste

2 .1 .2 . Description The formula used for calculating heat production in an incineration furnace with heat recovery is illustrated in Fig . 2 . If no heat is recovered, this formula becomes simplified . As shown in section 1 .2, if the ambient temperature in the bunker is chosen as a

M LHV M Q QM CA LHV C Q QE

QF Qv Qs Qs F QRC QB

MSW MSW lower heating value . Quantity of heat that is available in refuse . Quantity of heat that is available in combustion air . Supplemental fuel input . Lower heating value of the supplement fuel . Quantity of heat that is available in the supplement fuel . Quantity of heat conveyed by the cold fluid towards the recovery exchanger entrance . Quantity of heat brought away by the hot fluid at the recovery exchanger exit . Quantity of heat contained in the clinker . Quantity of heat contained in the fly ash . Quantity of heat contained in the flue gases . Quantity of heat conveyed by the furnace walls (radiation-convection) . Quantity of heat that shows the need of balance between the final and initial states .

At thermal balance, the input and the output heats are equal . MLHVM+QM+QA+CLHVC+Qc+QE=Qv+Q

QK

Qs + QF + Q RC + QB

Fig. 2 . Quantities used to determine the heat production of an incinerator

reference, then QM = Qq = 0 when the operation of the furnace-boiler units is stabilized and Q B = 0 . The formula of the lower heating value LHV M becomes : LHVM = (QV - Q E - Q C - C LHVC + Q K + Qs + Q F + Q RC )/M The quantities of heat that figure at the numerator are determined by throughput measures (volume or mass), temperature measures and gas analyses . The quantity M of MSW that is burnt is measured by a balance that is placed in the refuse conveyor . 2 .1 .3 . Advantages and drawbacks of the process This process reduces the variations due to the heterogeneous materials contained in the refuse because it uses important quantities of MSW. However, it requires a lot of complicated equipment in order to measure very accurately the different components of the thermal balance, and, therefore, a qualified staff is necessary and the process is very expensive . Furthermore, whilst this process continuously controls the operation of an existing plant, it can only rarely be used to determine the MSW lower heating value of a collection area prior to building an incineration plant in order to find the main features of the future plant . This can only be done when an existing plant is close to the collection area being studied . 2 .2 . Refuse sorting process and homogeneous-components heating value calculation 2 .2 .1 . Principle The refuse-sorting process uses a 100 kg refuse sample which is sorted into piles that must be as homogeneous as possible so that the heating values can be considered



1 44

C. Finer

constant ; the lower heating value of the whole sample probe is then calculated . A laboratory determination of the heating value of unsorted refuse would require very careful grinding and mixing to obtain a representative sample of only I g . 2 .2 .2 . Description The refuse sample (100 kg) was taken from the collection vehicle as follows: (1) the collection vehicle was emptied and its contents mixed by a dozer ; (2) 1-1 .5 tonne was sampled and piled up by the dozer in a long row ; (3) the pile was twice vertically cut and the middle part taken as the sample probe . The sample was then sorted by two or three persons, which took about 4 h . The following components were piled separately : fine particles, less than 20 mm diameter ; paper and cardboard ; fermentable substances (food wastes, etc .) ; fabrics ; bones ; plastics; miscellaneous combustible components (wood, leather, etc .) ; miscellaneous non-combustible materials (metal, glass, etc .) . The various combustible components of the first sample, the proportion of which is expressed by the Xi value, were then sampled and dried in order to determine their moisture content, H 2 01 . The dry samples were then shredded to 200 µm and were analysed in the laboratory for hydrogen content, H i , and higher heating value, HHV .. The lower heating value of each component, LHV i was calculated from the HHV 1 and the hydrogen content, H i, using the equation : LHVi = HHV i (I - H 2 O i) - 2440 (H 2 0 1 + 9H i) . and the lower heating value, LHV, of the total sample is given by : PCI = Exi[PCI]

LHV = Ei(LHV i)

where : the heat of vaporization of the water at 25°C is 2440 kJ kg' ; 9 H i means that for each hydrogen mass unit burnt, nine water mass units are formed ; H i is the hydrogen content in the different raw components (i .e . before they are dried) and -H 2 Oi is the corresponding face water content . Table 1 shows this calculation for the MSW collected in Paris (June 1984) .

TABLE I Calculation of the lower heating value (LHV) by sorting of the components of the MSW that was collected in Paris (June 1984) X~ of

X . of Component Fine particles Paper and cardboard Fermentable components Fabrics Bones Plastics Miscellaneous burning components Total * LHV = r i ( LHV i) =

8300

kJ kg - ' .

H (%)

LHV (kJ kg - '

LHH i

(%)

H ZO i (%)

9 .6 45 16 3 .6 0 .7 7 .4 3 .2

40 28 70 27 .5 42 11 .8 11 .5

2.2 4.6 2 .1 5 .4 3 .1 9.4 6 .1

3900 10,000 3300 12,500 7200 28,000 14,300

370 4300 520 450 50 2100 460

HHV '-

HHV1

(kJ kg- ') 8900 16,500 18,000 19,500 15,500 34,500 19,500

(kJ)

8300



Heating value of municipal solid waste

145

2 .2 .3 . Comments The accuracy of the process improves on increasing the number of 100 kg sorted samples . In one urban district, pattern areas are chosen (city centre, scattered residential area, dense residential area, industrial and commercial areas) and a route is mapped out within each pattern area in which 100 kg of refuse is sampled . Each sample is sorted according to the process that has already been described . The total lower heating value of the urban district is calculated by weighting each 100-kg sample lower heating value by the quantity of refuse generated within each pattern area . A calorimetric bomb is used to measure the higher heating value of the components . While the measurement is being done, the volume of the bomb is assumed to remain unchanged, the reference temperature is the laboratory one . Measurement is done under an oxygen pressure of 30 bar (30 MN M -2 ) . These conditions are very different from the ones in which the heating value is used, i .e . when combustion really takes place in a furnace at atmospheric pressure . In fact, the change in the reference conditions does not affect the heating value very much and the differences are nearly as important as the errors made while the heating value is calculated . They may, then, be disregarded . 2 .2 .4 . Advantages and drawbacks of the process The sorting does not require a lot of equipment (a weighing machine, a 60-1 bin, a manual 20-mm screen) and does not require qualified staff . The processing of the components that have been sorted (e .g . shredding) requires a more specific equipment . A laboratory, equipped for the analyses of fuels, required for the measurement of the higher and lower heating values . Compared to the means required for the thermalbalance process described previously, the requirements of this method are small and, therefore, its cost is moderate . The accuracy of the data obtained depends on the choice of the samples . For this choice to be good, knowledge of the urban district features is required : i .e . the type of housing, the socio-cultural behaviour, the quantities of refuse that are generated, etc .

3 . Conclusion The processes used to calculate the MSW heating value have briefly been described . Of the two methods, that of sorting and determining moisture and heat contents of the different categories (paper, plasters, food, etc .) is found to be more useful than attempting to operate an entire incinerator as a calorimeter .

References Hasselriis, F . (1985), How to compute the energy content of solid waste, Waste Age, 16, 213, 216, 218, 220 .