and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 11 (2009), Issue 2 p. 09-18
Investigation of calorific value of volatile matter from the pyrolysis process of waste
Nadziakiewicz J.
Katedra Technologii i Urządzeń Zagospodarowania Odpadów, Politechnika Śląska, ul. Konarskiego 18, 44-100 Gliwice, tel. (+48 32 237 21 14), fax (+48 32) 237 12 13, e-mail jan.nadziakiewicz@polsl.pl
Streszczenie
Badania ciepła spalania lotnych produktów pirolizy odpadów
Piroliza jest jednym z ważnych procesów zachodzących podczas termicznej utylizacji odpadów. Sporządzenie bilansu energii takich procesów jest możliwe gdy znane są wartości entalpii wszystkich substancji biorących udział lub powstających w procesie. Poniżej zdefiniowano entalpie złożonych substancji oraz metody ich wyznaczania. Przedstawiono wyniki pomiarów ciepła spalania osadów ściekowych oraz karbonizatu otrzymanego w różnych temperaturach i na tej podstawie obliczono wartości ciepła spalania części lotnych. Otrzymane wartości porównano z wartościami ciepła spalania części lotnych otrzymanych z węgla oraz z odpadów komunalnych. Porównanie to wykazało, że zastosowana metoda określania wartości ciepła spalania części lotnych jest wystarczająco dokładna dla obliczeń bilansu energii.
Abstract
Pyrolysis is one of important processes undergoing during incineration of waste. To make energy balance for these processes one must know enthalpies of all substances taking part or emerging in processes. The enthalpies of all substances are defined in the paper and method of their investigation is presented. Measurements of higher calorific values HCV of sewage sludge and char were made and respective values for volatile matter were calculated. Values obtained by measurement and calculations are compared with the values of other substances like coal and municipal waste. The comparison prove that the method of determining HCV from the parameters of other products of pyrolysis process is fairly accurate.
1. Introduction
Pyrolysis is one of methods of thermal waste utilization applied mainly for special types of waste like rubber tyres, sewage sludge etc. Pyrolysis is also one of important processes undergoing during incineration of waste. To make the energy balances for these processes one must know enthalpies of all substances taking part or emerging in analyzed processes. The incineration process starts from the drying of the fuel (waste) and the consequent process is pyrolysis resulted by devolatilization of the fuel. These processes are of special interest because of their heat consumption which influence the energy balance of incineration and decides of self-sufficiency of incineration process. This is the reason for putting special interest in investigation of enthalpies of all substances of the processes. Pyrolysis is the process of thermal destruction of organic matter with no oxygenation (no presence of oxygen). The waste heated up in the process chamber undergoes decomposition to volatile matter and solid residue called char. As a result three substances are present during pyrolysis process: initial fuel (waste), volatile matter and char. To make energy balance for this process total enthalpies of these substances should be defined [1].
The enthalpy of each substance in chemical process is understood as composed of two parts: physical if and chemical ich enthalpies:
ch
f
i
i
i
=
+
(1)The physical enthalpy depends mainly on specific heat cp and temperature t of the
substance:
t
c
i
f=
p⋅
(2) The chemical enthalpy depends on the composition of the substance and is determined in the normal temperature. The measure of the chemical enthalpy often used is highercalorific value (HCV) designed as Qs.
s
ch
Q
i
=
(3)This value for solid substances can be easily measured in calorimetric bomb, but for gaseous products of pyrolysis it is difficult to make straight measurement because of the small amount of the gas released during pyrolysis process.
2. The aim of investigation
The aim of presented investigation was comparison of the values of chemical enthalpies of volatile matters obtained from three different substances during pyrolysis undergoing in various temperatures. Such comparison can show the changes of the analyzed values with temperature and, which is even more important, to authenticate the approximate values for sewage sludge obtained by experimental method.
Three substances were taken for comparison. For one of them: municipal waste, the values of higher calorific values were determined basing on the literature values, while for sewage sludge and for coal the values were determined experimentally.
Archives of Waste Management and Environmental Protection , vol.11 issue 2 (2009) 11
3. Method of investigation
The aim of the investigation was determination of chemical enthalpies of volatile products of pyrolysis of waste material in various temperatures. The investigated sample of material was put in the crucible and placed into the oven with controlled temperature. The first measurement was the mass of the char that was collected after devolatilization. The devolatilization ratio was then determined.
0
)
(
m
m
T
Z
=
l (4)Devolatilization ratio of substance depends mainly on the temperature. The full release of gaseous substances reached for given temperature after the time necessary to release all volatile matter is called full devolatilization Zf. The maximum release of gaseous parts takes
place when all organic substances have been decomposed in maximum temperature and this process is called complete devolatilization Zc. The presented investigations were performed
for full devolatilization in the range of temperatures from the beginning of devolatilization up to the temperature of complete devolatilization. The method of investigation was described in [2]and [3].
The next step of investigation was the measurement of higher calorific value of the char residue after devolatilization process – this was made by in the calorimetric bomb. The measured value was used for approximate calculation of the higher calorific value of volatile matter.
3.1. Energy balance of pyrolysis process
The energy balance of pyrolysis process can be described as function of enthalpies of all substances that take part in the process, ie. initial substance to be pyrolysed (fuel, waste), solid products of pyrolysis (char) and volatile products of pyrolysis. The energy balance of the process, in which fuel is decomposed into char and volatile matter in given temperature can be formulated as follows:
)
(
)
(
)
(
p s,p g g s,g s s s,s pi
Q
Q
m
i
Q
m
i
Q
m
+
+
=
+
+
+
(5)where mp – initial mass of the fuel, mg – mass of the gaseous products of pyrolysis, ms –
mass of the solid products (char), indexes p, s and g stand for fuel, char and volatile matter respectively.
The unknown element of the balance equation is heat of reaction Q needed to brake up the organic matter to lighter volatile substances. If all parameters in the equation (5) were known, the heat of reaction can be easily determined. Unfortunately determination of chemical enthalpy is fairly easy only for solid substances like substance pyrolyzed (Qs,p)
and for solid products of pyrolysis (Qs,s) (the char). Unlike for solid substance
determination of calorific value of volatile matter is very complicated because of the difficulty in measurement the composition of the gas released from the fuel and because of small amounts of the gas to be analyzed in laboratory conditions.
The other point is the temperature of the gas phase – in high temperatures all components are in the gas phase while in ambient temperature (temperature of analysis) some of them form tar or solid phase.
The approximate value of enthalpy of volatile matter can be determined from the energy balance of the pyrolysis process (5).
)
(
)
(
, , , s ss p s g s g T p s pi
Q
m
m
Q
i
Z
q
Q
i
+
+
=
+
+
+
(6)The meanings of symbols in these equations are: mp, mg, ms [kg] – amounts of input
material, gas products and solid products (char) of process, ip, ig, is, Qs,p, Qs,g, Qs,s [kJ/kg] –
physical enthalpies and higher calorific values of material, gas and solid products, q [kJ/kg] – amount of heat added to the process per unit mass of fuel.
As physical enthalpies the temperature function was taken:
i
=
c
p⋅
t
, where cp [kJ/kg] isspecific heat capacity of the substance.
The value of heat q is low in comparison with the calorific values of substrates Qs,p [eg. 6]
and for the present investigation it was neglected:
[
]
{
sp T ss T g T s p p}
T g sQ
Z
Q
Z
c
Z
c
c
t
Z
Q
,≈
1
⋅
,−
(
1
−
)
⋅
,−
⋅
+
(
1
−
)
⋅
−
⋅
(7) In the calculations also the temperatures of all products was assumed as ambient so the physical enthalpies can be neglected.4. Results of measurements 4.1. Sewage sludge
The experiments for dry sewage sludge were performed in the laboratory of the Chair of Technologies and Systems for Waste Utilization in Silesian Technical University [2]. The results of measurements of devolatilization ratio and values of higher calorific values of char obtained in this process are given in Table 1. The values of calculated according eq. 7. calorific values of volatile matter are also given in this table. The calorific value of raw sewage sludge was measured as Qs,p = 14,95 MJ/kg.
As can be seen from the drawing the calorific values of volatile matter Qs,g does not change
Archives of Waste Management and Environmental Protection , vol.11 issue 2 (2009) 13
Table 1. Measured values of devolatilization ratio ZT , HCV of char and calculated values
of HCV for volatile matter for sewage sludge [2].
T [0C] ZT Qs,s [MJ/kg] Qs,g [MJ/kg] 0 0,01 14,95 14,85 200 0,1335 14,41 18,45 300 0,279 14,5 16,11 400 0,4214 11,56 19,60 500 0,4852 9,7 20,52 600 0,5269 9,6 19,75 700 0,5381 8 20,91 800 0,5544 7,98 20,55 900 0,5711 6,83 21,05
These values are also presented in Fig.1.
0 5 10 15 20 25 30 0 200 400 600 800 1000 Temperature, 0C Qs , M J /k g 0 0,1 0,2 0,3 0,4 0,5 0,6 ZT Qs,s Qs,g ZT (T)
Fig.1. Values of devolatilization ratio and calorific values of products of pyrolysis of sewage sludge.
4.2. Coal
Similarly to the measurements made for sewage sludge experiments for coal were carried out. Devolatilization ratio was measured for temperatures up to 9000C. Because the values of devolatilization ratio for temperatures lower than 4000C are low and bearing high inaccuracy the results for these temperatures were not taken into further calculations. Calorific values of char obtained from coal in these temperatures were as well measured in
a calorimeter. The values of measured parameters for coal are given in Table 2. and in Fig.2. The measured higher calorific value of coal was: Q s,p = 25,99 MJ/kg.
Table 2. Measured values of devolatilization ratio, HCV of char and calculated values of HCV for volatile matter for coal [2].
T [0C] ZT Q s,s [MJ/kg] Qs,g [MJ/kg] 30 0,0001 25,99 25,99 300 0,0019 24,27 930,59 400 0,0329 22,12 139,81 500 0,1583 22,06 46,90 600 0,2298 21,36 41,52 700 0,2514 21,35 39,81 800 0,2614 19,92 43,15 900 0,2824 19,9 41,47
These values have been presented in Fig.2 as functions of pyrolysis temperature.
0 10 20 30 40 50 60 400 500 600 700 800 900 1000 Temperature 0C Qs , M J /k g 0 0,05 0,1 0,15 0,2 0,25 0,3 ZT Qs,s Qs,g ZT
Fig.2. Values of devolatilization ratio and calorific values of products of pyrolysis of coal. In this case the calorific value of char does not change in the investigated range of temperatures. This can be explained by the almost constant composition of the char after most of the volatile matter being released. One must stress, that the temperature range of these investigations start from 5000C. This is because for lower temperatures devolatilization ratio is very low and obtained values bear high level of inaccuracy.
Archives of Waste Management and Environmental Protection , vol.11 issue 2 (2009) 15 4.3. Municipal waste
In the presented research pyrolysis of sewage sludge, coal and their mixtures were investigated. To have some more data for comparison the values for dry municipal waste was taken from literature [4]. These values were obtained in a special experimental device with controlled temperature and gas sampling. The obtained values are: devolatilization ratio and volume composition of volatile matter measured by gas chromatograph method. From the gas composition the higher calorific values of volatile matter for particular temperatures were calculated. The values of gas components concentrations and devolatilization ratio are given in Table 3 and in Fig.3.
Table 3. Composition of volatile matter from pyrolysis of dry municipal waste as functions of temperature [4]. Temp. [0C] ZT H2 CH4 CO CO2 C2H4 C2H6 482 0,123 5,56 12,43 33,5 44,77 0,45 3,03 648 0,186 16,58 15,91 30,49 31,78 2,18 3,06 815 0,237 28,55 13,73 34,12 20,59 2,24 0,77 926 0,244 32,48 10,45 35,25 18,31 2,43 1,07 0 10 20 30 40 50 400 500 600 700 800 900 1000 Temperature 0C C o n c e n tr a ti o n % H2 CH4 CO CO2 C2H4 C2H6
Fig. 3. Composition of gas products of pyrolysis of municipal waste.
The calculated values of higher calorific values of gas substance are presented in Table 4. and in Fig.4. Two values are presented in this table: values related to the MJ/m3n –
calculated straight from the gas volumetric composition and other values dimensioned in MJ/kg that were calculated taking into account the gas density in normal conditions. Table 4. Calculated values of HCV of volatile matter from the dry municipal waste.
Temp. [ 0C] ZT Qs,g [MJ/m3] Qs,g [MJ/kg] 482 0,123 21,50 3918,7 648 0,186 28,90 3483,9 815 0,237 36,35 3438,8 926 0,244 41,30 3795,1 0 4 8 12 16 20 400 500 600 700 800 900 1000 Temperature 0C Qs ,g 0,00 0,08 0,16 0,24 0,32 0,40 ZT Qs,g MJ/m3 Qs,g MJ/kg ZT
Fig.4. Values of devolatilization ratio and calorific values of products of pyrolysis of municipal waste.
5. Comparison and results of investigations
The values of higher calorific value determined experimentally for sewage sludge and coal were compared with the values determined for municipal waste by the composition of volatile matter in various temperatures.
The values of higher calorific values for above mentioned substances and for coal for range of temperatures are presented in Fig.5. As a control point the calorific value of coke gas was shown, its value was calculated from the composition of that gas [7].
Archives of Waste Management and Environmental Protection , vol.11 issue 2 (2009) 17 0 10 20 30 40 50 0 200 400 600 800 1000 Temperature 0C Qs ,g M J /k g Qs,g-sludge Qs,g-waste Qs,g-coal Qs,g-coke
Fig.5. Comparison of measured and calculated values of HCV for analysed substances. As can be seen from the drawing the higher calorific values of volatile matters are different, but this can be explained by the nature of the analyzed substances. Coal have many hydrocarbons and volatile matter released from coal have more higher hydrocarbons which results in high calorific values. The municipal waste has rather lighter organic substances and consequently the calorific values are lowest of the analyzed substances. The values obtained for sewage sludge are in between of these of the coal and the waste. The value for coke gas lies close to the values for coal, but slightly lower, which can be explained by removing tar substances in the gas cleaning process.
The interesting point of these investigations was comparison of the calorific values of volatile matter from various pyrolysed substances. The omission of the heat of pyrolysis does not influence seriously the calculated values for volatile matter because as can be seen from the drawing, the calculated values are not far from real values. The analysis of equation (3) and consequent equation (7) show, that taking into calculations heat of reaction of the order 5% of Qs,p changes the result of calculations of HCV of volatile matter about
the same order.
References
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[2] Chludzińska H.: Badania stosunku odgazowania osadów i ich mieszanek z węglem. MSc Thesis. Gliwice 2005
[3] Nadziakiewicz J.: Devolatilization of coal – sludge mixture. AGO. No I.2009.
[4] Tome-Kozmiensky K.J.: Thermische Abfallbehandlung. EF-Verlag Umwelttechnik 1994.
[5] Termochemiczne przetwórstwo węgla i biomasy. Ściążko M., Zieliński H. red.:Wyd. IChPW. Zabrze 2003.
[6] Rath J., Wolfinger M.G., Steiner G., Krammer G., Bartonini F., Cozzani V.: Heat of wood pyrolysis. Fuel. 82 (2003) 81-91.
