and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 11 (2009), Issue 1, p-21-30
Odgazowanie mieszanin węgla z osadem ściekowym
Nadziakiewicz J.
Katedra Technologii i Urządzeń Zagospodarowania Odpadów Politechnika Śląska w Gliwicach
ul. Konarskiego 18
tel. +48 32 237 12 14, fax +48 32 237 11 67, e-mail: jan.nadziakiewicz@polsl.pl
Streszczenie
Piroliza odgrywa istotną rolę w procesie spalania odpadów. Produkty pirolizy i odgazowania są palne i biorą udział w zapłonie i spalaniu paliwa (odpadu). Z tego powodu znajomość parametrów odgazowania i właściwości produktów pirolizy jest istotna dla technologii spalania odpadów. W artykule przedstawiono badania stosunku odgazowania, wartości opałowej karbonizatu i części lotnych wyznaczonych dla węgla, osadu ściekowego oraz dla mieszanek tych dwóch substancji.
Abstract
Devolatilization of coal – sewage sludge mixtures
Pyrolysis is one of the thermal processes and knowing its parameters is important for technology of waste utilization. Pyrolysis and simultaneous process of degassing (devolatilization) play important role in incineration process of waste since the products are combustible and take part in ignition and oxidation of fuel. In the paper important thermal parameters are determined for coal, sewage sludge and their mixture of various proportions. Devolatization ratio, heating value of char and heating values of volatile matter were determined by measurements or by subsequent calculations.
1. Introduction
One of possibilities of waste utilization is co-combustion of waste with fuel of good thermal parameters like coal. Process of combustion of solid waste is complicated because of non-homogeneous waste. The understanding of such mixture combustion process is even more complicated because both components undergo similar processes in combustion chamber at the same time.
Process of combustion of fuels consists of a series of consecutive processes: drying, devolatilization, ignition and final burning of char. The sequence of processes is determined mostly by the temperature of the fuel and kinetics of processes. These processes are even
more complicated in the case of combustion of mixture of fuel (like coal) and waste (co-combustion). The sequence of the processes is disturbed by various temperatures defining the processes limits for all components of the mixture. From the other side for the analysis of combustion process of such fuels the knowledge of summarized processes is essential, especially for the energy and mass balances of the process. The possible question is if the analyzed processes undergoing in the components of the fuel mixture influence each other. Pyrolysis is the process of thermal decomposition of material (mainly hydrocarbons and organic matter) that takes place during heating of the fuel material at non oxidative conditions. If no air is present the final products of this process are gaseous products and solid substance (char), in other case both gaseous products and char will be oxydized to the combustible gas or flue gases. The process of release of gaseous parts from the fuel is called devolatilization. The gaseous parts consists of carbon dioxide, carbon monoxide, hydrogen, methane, nitrogen, water and some heavy hydrocarbons (tar) and the solid products (the char) consists mainly of carbon and mineral matter. The process of devolatilization plays important role in the process of ignition of fuel – ignition of gaseous parts and final ignition of the char itself.
In this paper the consequent devolatilization processes for fuel-waste mixture was investigated and results shown in the form of diagrams.
Since no general theory of devolatilization exists the investigation of processes undergoing in the fuel is possible only by experiment. The investigated parameters are: devolatilization ratio, temperature of devolatilization and thermal parameters of products.
2. Devolatilization characteristcs of fuel
The parameter that characterizes devolatilization process is devolatilization ratio, defined as a ratio of gaseous substances released in given temperature to the initial mass of substance [1]: ZT= 0 1
m
m
* 100 [%] (1)Where: m1-mass of volatile matter, m0-initial mass of the dry, ash free (daf) substance. Devolatilization ratio depends on the temperature, on the rate of temperature increase of the substance and on the time of residence of substance in the final temperature. The full release of gaseous substances reached for given temperature after some minimal time of residence is called full devolatilization. The maximum release of gaseous parts takes place when all organic substances have been decomposed in maximum temperature (usually 600 – 9000C), and this process is called complete devolatilization.
The devolatilization ratio is the qualitative measure of the intensity of process. Knowing its value for various temperatures makes possible to find total mass of gaseous products released from the fuel in pyrolysis process during heating. In practice it is important parameter in mass and energy balance of combustion process of fuel particles on the grate [4].
Archives of Waste Management and Environmental Protection, vol. 11 issue 1 (2009) 23 3. Method of investigation
The subject of investigation was the dependence of ZT on temperature for given fuel and fuel mixtures. For the investigation two kinds of fuel material was analyzed: coal – as fossil fuel, and sewage sludge (dry) as waste. During investigation the standard test methods were applied (Polish Standard PN-G-04516:1998). The crucibles with lids containing tested materials were placed in the oven of stabilized temperature for a certain period of time and mass of char was measured. The residence time was chosen according to previous tests to ensure the total devolatilization of tested material (usually it was 9 – 11 mins).
The other parameter that has been tested was the heat of combustion Qs [kJ/kg] of the residue char – this measurement was the basis for estimating the heat of combustion of gaseous parts of pyrolysis.
The tests were performed for following fuel mixtures: • coal,
• sewage sludge from the municipal wastewater treatment plant (dry), • 75% coal + 25% sewage sludge (sample A),
• 50% coal + 50% sewage sludge (sample B), • 25% coal + 75% sewage sludge (sample C). 3.1. Results of measurements
At first all samples were homogenized and their moisture and mass of combustible matter were measured. The results were following:
Moisture of coal: wc = 2,145%, combustible matter of coal: 92,8%,
Moisture of sewage sludge: ws = 6,115%, combustible matter of sludge: 56,3%.
The mixtures of these components were prepared and measurements of devolatilization process were performed in temperatures 350, 550, 750 and 8500C. Three samples were measured each time and average value was taken as result.
As first the water content, combustible matter and heating value of fuel components were measured by standard procedures. The results are given in Table 1.
Table 1. Parameters of the components of fuel: coal and sewage sludge [1].
Parameter Unit Coal Sludge
Water content % 2,145 6,115
Combustible matter % 92,8 56,3
The next step was determination of full devolatilization ratio of above mentioned mixtures of fuel components in various temperatures. The residence time of test samples in the oven was chosen to ensure full devolatilization. The results for coal, fuel mixtures A, B and C and for sewage sludge are presented in Table 2.
Table 2. Results of investigations of devolatilization ratio [1]. Devolatilization ratio ZT. [%] Temperature [C°] Coal 25% Sewage sludge 50% Sewage sludge 75% Sewage sludge Sewage sludge 0 0 0 0 0 0 350 0,021696 0,071255 0,134226 0,198054 0,264412 550 0,249475 0,280867 0,302146 0,347432 0,413553 750 0,315544 0,336933 0,357207 0,408018 0,459608 850 0,319443 0,346878 0,380959 0,421614 0,482399
The results are also presented in Fig.1.
0 0,1 0,2 0,3 0,4 0,5 0,6 0 200 400 600 800 1000 Temperature, C ZT Coal 25% 50% 75% Sludge
Fig.1. Full devolatilization ratio as function of temperature and composition of fuel samples.
As can be seen from these drawing the general shape of the curves for coal, sewage sludge and their mixtures are similar. The differences are in the temperatures of beginning of
Archives of Waste Management and Environmental Protection, vol. 11 issue 1 (2009) 25 devolatilization process and the maximal value of devolatilization ratio (complete devolatization ratio).
To analyze the influence of the mixture composition on the devolatilization process one can show the above mentioned results in the different coordinate system ZT = f(sludge mass fraction, temperature) – Fig.2.
0 0,1 0,2 0,3 0,4 0,5 0,6 0 25 50 75 100
Sludge mass fraction, % ZT
350 550 750 850
Fig.2. Devolatilization ratio as function of sewage sludge fraction and temperature.
As can be seen from this drawing the devolatilization ratio of the fuel composed of coal and sludge is almostlinearly proportional to the mass fractions of both components of the mixture. This result means that both coal and sludge do not interact chemically in this process and they behave as separate components not influencing each other.
4. Thermal parameters of devolatilization products
Beside the devolatilization ratio the knowledge of other parameters of products of pyrolysis are also important especially when energy balance of the process is formulated. The most important parameters are physical and chemical enthalpy of the products.
To determine these parameters at the same time, when the devolatilization ratio was determined other investigations were performed dealing with products of the process. The aim of this investigations was determining the thermal parameters of the products of devolatilization process realized in various temperatures.
To assess the energy balance of the pyrolysis process one can analyze the fuel as substrate and products (volatile and tar) with the temperature of all substances constant. For this analysis the pyrolysis of fuel can be described by following scheme:
Organic material →→→ char +volatile matter →
In this scheme the volatile matter means both gas products and tar in the gas phase (raw gas) (this is usually so, because of high temperatures of process). If the temperatures of organic material (fuel) and products before and after pyrolysis process are the same that means that only enthalpy of substrates and products change in expense of heat introduced to the system.
4.1. Heating value of char(s)
To determine the enthalpy of volatile matter the heating value of char(s) obtained in various temperatures after complete devolatilization was measured. Values of these parameters are given in Table 3.
Table 3. Measured high heating values of char for various fuel mixtures. High heating value of char(s) Wg,s. [MJ/kg]
Temperature [C°] Coal 25% Sewage sludge 50% Sewage sludge 75% Sewage sludge Sewage sludge 25 34,951 28,29 24,417 18,782 12,925 350 33,88 26,98 22,186 16,136 9,148 550 32,787 26,293 21,369 16,215 6,658 750 31,866 24,907 20,348 13,042 4,974 850 31,619 24,669 19,276 12,845 5,129
The drawing of these values is given in Fig. 3 and Fig.4.
0 5 10 15 20 25 30 35 40 0 25 50 75 100
Sludge mass fraction %
W g ,s M J /k g 20C 350C 850C
Archives of Waste Management and Environmental Protection, vol. 11 issue 1 (2009) 27 0 10 20 30 40 0 200 400 600 800 1000 Temperature, C W g ,s M J /k g Coal 25% 50% 75% Sludge
Fig.4. Measured high heating value of char(s) as a function of temperature.
As can be seen from these drawings the calorific value of the char(s), originated from the mixture of coal and sewage sludge, depend linearly on the sludge mass ratio in the fuel mixture.
4.2. Heating value of volatile matter
Determination of heating value of volatile matter is very complicated because of the difficulty in measurement the composition of the gas outgoing the heated fuel and because of small amounts of the matter to be analyzed. The other point is the temperature of the gas phase – in high temperature all components are in gas phase while in ambient temperature (temperature of analysis) some of them form liquid phase (oils) and solid phase (tars). The approximate value of enthalpy of volatile matter can be determined from the energy balance of the pyrolysis process. The energy balance of the process, in which fuel is decomposed into char and volatile matter in determined temperature can be formulated as follows:
)
(
)
(
)
(
p g,p g g g,g s s g,s pi
W
Q
m
i
W
m
i
W
m
+
+
=
+
+
+
(2)
Where: mp, mg, ms [kg] – amounts of fuel, gas products and solid products (char) of process,
ip, ig, is, Wg,p, Wg,g, Wg,s [kJ/kg] – physical enthalpies and high heating values of fuel, gas
and solid products, Q [kJ] – amount of heat added to the process. As physical enthalpies the temperature function was taken:
i
=
c
p⋅
t
, cp is specific heat capacity [kJ/kg].As chemical enthalpies the high heating values Wg are taken because these values, unlike the low heating values, are possible to measure in the calorimeter and it is not necessary to know the ultimate analysis of the material, which is often difficult to make for non-homogeneous waste material. Another issue is that low heating value is less dependent on the water content in the analyzed material.
In the equation of energy balance (2) one can determine by measurement of temperature values of physical enthalpies and one can measure values of high heating values by straight calorimetry. The only two parameters in this equation that can not be directly measured are heat of reaction Q and heating value of gas fraction. Determination of heat of reaction is possible only by thermogravimetric methods, but this can be done only for very small, uniform samples, which is impossible for waste material. The heating value of gas phase would be possible to determine by analyze of its composition in the time of experiment, which is very difficult because of many compounds, often hydrocarbons, present in this phase. Nevertheless it is possible to determine with satisfactory accuracy the heating value of gas products by using of other values from the energy balance equation.
Having this in mind one can estimate the high heating value of gas phase from relation:
[
]
{
gp T gk T g T k p p}
T g gq
W
Z
W
Z
c
Z
c
c
t
Z
W
,=
1
⋅
+
,−
(
1
−
)
⋅
,−
⋅
+
(
1
−
)
⋅
−
⋅
(3)In this relation ZTwas introduced as in eq.1.and the value of
p
m
Q
q =
. Data given in literature show, that the value of heat of pyrolysis q is not big in comparison with heating values of the components (it makes 5 to 10% of the heating value of fuel [2]). Beside this in this relation the heat consumption for heating the products from ambient temperature to the temperature of process is taken into account which make the contribution of the heat q in the total heat balance even lower.Table. 4. Calculated high heating value of volatile matter Wg,l [MJ/kg] High heating value of volatile matter Wg,l [MJ/kg] Temperature [C°] Coal 25% Sewage sludge 50% Sewage sludge 75% Sewage sludge Sewage sludge 20 34,95 28,29 24,42 18,78 12,93 350 58,09 40,63 36,37 27,90 22,28 550 39,53 31,72 29,91 22,30 20,76 750 39,64 33,10 30,01 25,65 21,02 850 39,81 33,08 30,96 25,34 19,95
Archives of Waste Management and Environmental Protection, vol. 11 issue 1 (2009) 29 If one assume that
q
=
0
than from this relation, chemical enthalpy Wg,g of gas products ofpyrolysis process can be calculated. The value of this enthalpy bears the uncertainity of all components, nevertheless it is the value of some use, especially when no accurate data is known (as in the case of waste material).
The calculated values are presented in Fig. 4.
0 10 20 30 40 50 60 70 300 400 500 600 700 800 900 Temperature, C W g ,l M J /k g Coal 25% 50% 50% Sludge
Fig.4. Calculated high heating values of volatile matter Wg,l [MJ/kg].
The calculated high heating values of volatile matter are in the range 20 – 40 MJ/kg with the except for coal at 3500C. These values are of the range given in the literature for these types of fuel [2,3]. One should mention that values of Wg,l for volatile matter calculated from eq. (3) bears high level of uncertainity caused by small values of devolatilization ratio. The higher ZT the more trustworthy values of Wg,l are obtained.
5. Conclusions
Several conclusions can be drawn from these measurements:
• The devolatilization ratio of the fuel composed of coal and sludge is linearly proportional to the mass fractions of both components of the mixture. This means that both coal and sludge do not interact chemically in this process and they behave as separate components not influencing each other.
• The heating value of the char from the mixture of coal and sludge depend linearly on the sludge mass ratio in the fuel mixture.
• The calculated values of heating value of volatile matter are in the range given in the literature for these types of fuel which means, that omission the heat of pyrolysis is not the source of great uncertainity.
• The best method of determining the heating value of volatile matter is by measurement of its composition and calculating according thermal parameters of the components. This method is unfortunately difficult to apply when small amounts of fuel are investigated and char particles are present in the gas phase. Literature
[1]. Nadziakiewicz J., Wacławiak K., Stelmach S.: Procesy termiczne utylizacji odpadów. Wyd. Pol. Śl. Gliwice 2007.
[2]. Termochemiczne przetwórstwo węgla i biomasy. Red. Ściążko M., Zieliński H. Wyd. IChPW. Zabrze-Kraków 2003.
[3]. Szargut J.: Termodynamika techniczna. Wyd. Pol.Śl. Gliwice 1997.
[4]. Falińska A.: Badanie procesu odgazowania osadów i ich mieszanek z biomasą. MSc thesis. Pol.Śl. Gliwice. 2007.
