and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 2 (2005), p-37-48
Equilibrium modeling of the gasification of solid wastes
Łuckoś A.
Mintek, Private Bag X3015, Randburg, South Africa
Abstract
Gasification is an attractive alternative for the thermal treatment of solid wastes. The number of possible applications of product gas allows gasification to be integrated with several industrial processes (production of transportation fuels, chemicals and hydrogen), as well as power generation systems. In this paper important physical and chemical characteristics of solid wastes as fuel are summarized and compared with conventional fossil fuels, primarily coal. Main conversion processes for municipal solid wastes (MSW) are briefly discussed. A non-stoichiometric equilibrium model is developed to predict the performance of an oxygen-blown MSW gasifier. The equilibrium composition and calorific value of product gas as a function of temperature, pressure and oxygen ratio are calculated. The predictions of the model show that oxygen-blown gasification of MSW can produce a medium heating-value gas suitable for both power generation and chemical synthesis.
Modelowanie równowagowe procesów gazyfikacji odpadów stałych
Streszczenie
Gazyfikacja jest alternatywą w stosunku do termicznej utylizacji odpadów. Duża liczba możliwych zastosowań gazowych produktów pozwala na jej zastosowanie w różnych procesach przemysłowych - takich jak np. produkcji paliw komunikacyjnych i innych związków chemicznych - oraz w systemach energetycznych. W artykule przedstawiono podsumowanie własności odpadów stałych oraz dokonano porównania z paliwami konwencjonalnymi – głównie węglem. Zostały również omówione ważniejsze procesy przekształcania odpadów stałych zostały. Nie-stechiometryeczny model równowagowy został stworzony aby określić sprawność gazyfikatorów odpadów stałych. Obliczono równowagowy skład i wartość opałowa produktów gazowych w funkcji temperatury, ciśnienia i współczynnika nadmiaru powietrza. Zaprezentowany model pokazuje, że możliwa jest gazyfikacja odpadów stałych w celu produkcji średniokalorycznego gazu przydatnego w systemach energetycznych i do różnych procesów syntezy chemicznej.
1. Introduction
Waste is generated by every activity in a modern society. Increasing volumes of municipal solid waste (MSW) pose disposal problems as landfilling becomes difficult and expensive. The conversion of MSW to electrical energy can conserve more valuable fuels and lessen any harmful impact on environment. The production of transportation fuels (gasoline, methanol) or hydrogen from MSW through gasification and Fischer-Tropsch synthesis also becomes an attractive option both economically and environmentally. Among factors that recently changed the outlook for the production of energy and fuels from MSW are [1]—
• Increasing space constrains for landfilling of wastes • High and ever-rising prices of crude oil and natural gas • Public opposition to new waste incinerators
• Evidence that the environmental benefits of fuel production from MSW could be substantial compared with incineration
• Recent advances in vehicle technology that will lead to environmental benefits for methanol and hydrogen vehicles while maintaining cost competitiveness with petroleum-fuelled vehicles, even when oil prices are relatively low.
In this paper “typical” physical and chemical properties of MSW are presented and followed by a brief description of main conversion processes. A non-stoichiometric equilibrium model is developed to predict the performance of an oxygen-blown MSW gasifier. The influence of temperature, pressure and oxygen ratio on the chemical composition and heating value of the product gas and its possible applications are described.
2. Physical and chemical properties of MSW
MSW is a complex mixture of common household trash, with paper and yard wastes providing the largest fractions. A “typical” composition by material of MSW in the USA is presented in Table 1.
Table 1. Typical composition of MSW by material [2].
Material wt.% vol.%
Pulp and paperboard 37.5 37.0
Glass 6.7 2.3
Ferrous metals 6.3 8.8
Aluminium 1.4 3.1
Plastics 8.3 18.3
Rubber and leather 2.4 5.8
Textiles 2.8 5.4
Wood 6.3 5.9
Food wastes 6.7 2.7
Yard wastes 17.9 9.2
MSW, compared with coal, is extremely heterogeneous in chemical composition and particle size distribution. Its moisture content and heating value may vary over wide ranges. Thus, thermal treatment systems for MSW must be designed to cope with this large variability in fuel properties. “Typical” chemical compositions and heating values for MSW and bituminous coal are shown in Table 2.
Table 2. Chemical analyses and heating values for MSW and bituminous coal [2].
Component MSW wt.% Bituminous coal wt.% C 25 70 H 3 5 O 20 5 N 0.5 1.5 S 0.2 1–5 Cl 0.2–0.6 0.005–0.6 Moisture 25 5 Ash 25 10 Heating value, MJ/kg 11 28
3. Thermo-chemical conversion of MSW
Incineration, gasification and pyrolysis are common processes used for the thermochemical conversion of MSW [3–18]. The flowchart in Figure 1 shows these processes together with the intermediate energy carriers and the final products.
Fig. 1. Processes, intermediate energy carriers and final products from the thermochemical conversion of MSW.
Gasification has received increasing attention in the past two decades because of the growing demand for clean gaseous fuels and chemical feedstocks. Essentially, gasification
is an upgrading process that converts MSW into a gaseous form by partial oxidation at high temperatures, typically in the range 800–1000ºC. The product gas can be readily purified and used directly as a fuel (fuel gas) or as a feedstock for the synthesis of other fuels and chemicals (synthesis gas or syngas). Low heating-value gas (4–8 MJ/m3), produced in air-blown gasifiers, is used as a fuel in boilers and gas turbines. Medium heating-value gas (10–18 MJ/m3), produced in oxygen- and steam-blown gasifiers, can be used as a fuel or as a feedstock for chemical synthesis (synthetic gasoline and diesel oil, methanol, and ammonia) and the production of hydrogen for fuel cells. High heating-value gas (>21 MJ/m3) is used as a substitute for natural gas.
A promising concept is gasification of MSW (or co-gasification with coal or biomass) in the integrated gasification combined cycle (IGCC), where product gas is converted to electricity in gas turbines with a high overall conversion efficiency. IGCC systems can achieve net thermal efficiencies in the range of 40–50% and up to 60% when combined with fuel cells. An important advantage of IGCC technology is that the volume of gas generated in the system is far smaller than in combustion-incineration systems, which allows for more compact and less-costly gas-cleaning equipment. Emissions of particulates, NOx and SO2 from IGCC units are expected to meet, and possibly better, all current pollutant emission standards.
Syngas produced from MSW can also be used for the synthesis of methanol and the production of hydrogen. Both products are considered to be the “clean” transportation fuels of the future.
4. The model
From a chemical point of view, a MSW gasifier can be viewd as a two-phase reactor where a number of hetero- and homogeneous reactions take place during a given residence time. The performance of a real gasifier depends upon chemical equilibrium constraints and kinetic factors such as reaction rates and rates of heat and mass transfer. The actual composition of products in a real process and the equilibrium composition, therefore, may be different. In spite of this difference, equilibrium models are valuable because they can provide useful information about trends and thermodynamic limits. This information can be utilized in process design and control, especially in the case of high-temperature processes characterized by rapid chemical kinetics.
In the present work, a non-stoichiometric equilibrium model [19] is used to predict the performance of an oxygen-blown MSW gasifier. The approach used allows handling of multiple feed streams of unknown molecular structure. The feed is considered to be an element abundance vector determined from the ultimate analysis of the MSW (Table 3). It is assumed that the combustible matter of the feed is composed of six elements—C, H, O, N, S and Cl. Minerals and trace elements in ash are considered to be chemically inert. Any catalytic effect of the minerals and trace elements is assumed to be negligible.
Table 3. Ultimate analysis of MSW (wet basis) used in the equilibrium model. Element
wt.%
C
40.0
H
6.3
O
32.2
N
0.9
S
0.1
Cl
1.7
Moisture
9.0
Ash
9.8
The total free energy (Gibbs function) for the considered system as a function of temperature, and pressure is
(
)
∑
==
N i i in
P
T
G
1,
µ
(1)Table 4. Species considered in the equilibrium model. Group Chemical formula
(1) C(g), CH, CH2, CH3, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, C4H8, C4H10 (2) H, H2, O, O2, CO, CO2, OH, H2O, H2O2, HCO, HNO, HO2
(3) N, N2, NCO, NH, NH2, NH3, N2O, NO, NO2, CN, HCN, HCNO (4) S(g), S2(g), SO, SO2, SO3, COS, CS, CS2, HS, H2S
(5) Cl, Cl2, COCl, COCl2, HCl (6) C(s), S(s)
The exercise is to minimize G for fixed T and P subject to mass-balance constraints
k N i i i k
n
b
a
=
∑
=1 , k = 1, 2, . . . M (2)0
≥
i
n
(3)The initial calculations took 51 gaseous species and 2 solid species into account. These species are listed in Table 4. Subsequent calculations eliminated all the species for which molar fractions were less than 10–10. The FactSage™ 5.3 software was used to run the calculations.
5. Results
Figure 2 shows predictions of the equilibrium composition of five major gaseous species as functions of temperature when MSW is gasified at 0.1 MPa with an oxygen ratio, a, of 0.2. The predictions correspond to species in wet gas. Hydrogen and carbon monoxide concentrations increase as temperature increases, whereas concentrations of H2O, CO2 and CH4 decrease. At temperatures above 800ºC, the concentration of CH4 drops below 1%, while the H2 concentration reaches its maximum of 45.4% (at 850ºC).
0 10 20 30 40 50 60 300 500 700 900 1100 Temperature, oC M o la r fr a c ti o n , % H2O CO2 H2 CO CH4
Fig. 2. Gas composition as a function of temperature at 0.1 MPa and a = 0.2.
Figure 3 shows the effect of pressure on the predicted carbon distribution for a = 0.2. Concentrations of solid carbon, CO2 and CH4 increase with increasing pressure, whereas that of CO decreases. However, the influence of pressure diminishes significantly at temperatures above 1000ºC.
0 10 20 30 40 50 60 300 500 700 900 1100 Tem perature, oC M o la r fr a c ti o n , % C(s) CO2 CH4 CO O.1 MPa 2.5 MPa 7.5 MPa
Fig. 3. Effect of pressure on carbon distribution at a = 0.2.
The predicted concentrations of major nitrogen, sulfur and chlorine species are shown in Figures 4 and 5. N2, H2S and HCl are the main nitrogen-, sulphur- and chlorine-bearing species produced in oxygen-blown gasification. Nitrogen in MSW is liberated as N2 and only a small quantity of waste-nitrogen is converted to NH3 and HCN.
0,001 0,01 0,1 1 10 100 1000 0,0 0,2 0,4 0,6 0,8 1,0 Oxyge n r atio M ol a r fr a c ti on, ppm H2S CO S HS S2 SO 0,001 0,01 0,1 1 10 100 1000 10000 0,0 0,2 0,4 0,6 0,8 1,0 Oxyge n r atio M o la r fr a c ti o n , p p m NH3 HCN N2 HNC O HCl Cl
Fig. 4. Concentrations of sulfur species at 900ºC and 0.1 MPa
.
Fig. 5. Concentrations of nitrogen- and chlorine-bearing species at 900ºC and 0.1 MPa.
Figure 6 shows the predicted conversion of carbon as a function of oxygen ratio. At low temperatures a full conversion of carbon can only be achieved at oxygen ratios much higher than those required for oxidation of carbon to CO. At a = 0.2, the full conversion of carbon is predicted at 750ºC (see Figure 3) in the case of wet feed and at 850ºC in the case of dry feed. Water contained in MSW takes part in two important reactions—namely, the carbon-steam reaction and the CO shift reaction. Both these reactions promote carbon conversion and help to produce gas with higher H2 concentrations.
20 40 60 80 100 0,0 0,2 0,4 0,6 0,8 1,0 Oxyge n r atio C o n v e rs io n , % 300o C 900o C 600 o 5 6 7 8 9 10 11 12 300 500 700 900 1100 Tem perature, oC L H V , M J /m 3
Fig. 6. Effect of air ratio on carbon conversion at 0.1 MPa (solid line, waste as received; dashed line, dry waste).
Fig. 7. Effect of temperature on heating value at 0.1 MPa and a = 0.2 (solid line, waste as received; dashed line, dry waste).
The heating value of a product gas is a measure of its quality for energy-recovery applications. Figure 7 shows the lower heating value (LHV) of the product gas as a function of temperature. The LHV of wet gas first increases with temperature, and then, above 800ºC, levels off. At higher temperatures, the LHV of gas produced from dry MSW is higher because of higher CO content.
The quality of gas for chemical and metallurgical industries can be estimated by the sum of H2 and CO contents and H2/CO ratio. A gas with a high levels of H2 and CO (>80%) has a strong reducing potential and can be used in some metallurgical processes (e.g., for direct reduction of iron ores). A high H2/CO ratio (>1.7) indicates a gas useful for chemical syntheses (e.g., methanol production). These two measures of gas quality are shown in Figures 8 and 9 as a function of oxygen ratio. Both the sum of H2 and CO and the H2/CO ratio decrease as more oxygen is supplied to the system. Gasification of dry MSW at a > 0.1 produces gas with less than the maximum H2 + CO and a H2/CO ratio that is ~25–50% less than the maximum.
0 20 40 60 80 100 0,0 0,2 0,4 0,6 0,8 1,0 Oxyg e n r atio H2 + C O , % 0,6 0,8 1,0 1,2 1,4 1,6 0,0 0,2 0,4 0,6 0,8 1,0 Oxygen ratio H2 /C O
Fig. 8. Effect of air ratio on H2 + CO content at 900ºC and 0.1 MPa (solid line, waste as received; dashed line, dry waste).
Fig. 9. Effect of air ratio on H2 to CO ratio at 900ºC and 0.1 MPa (solid line, waste as received; dashed line, dry waste).
6. Conclusions
Despite its limitations, equilibrium modelling is a useful tool for the analysis of the MSW gasification process. It allows one to follow changes in composition of the product gas as a function of operating conditions and assess its quality. As the predictions of the model show, atmospheric oxygen-blown gasification of MSW can produce a medium-heating value gas suitable for both power generation and chemical synthesis.
Acknowledgement
This paper is published with the permission of Mintek. I thank my colleague, Paul den Hoed, for his comments on a draft of the paper.
Notation
a oxygen ratio, defined as the ratio of actual oxygen flow to oxygen flow required for complete combustion
ak,i coefficient in the element-species matrix representing species k containing element
i
bk number of moles of the k-th element in the system
G Gibbs free energy, J
M number of elements present in the system
N number of species considered
P absolute pressure, Pa
T temperature, K
µi chemical potential of species i
References
[1] E. D. Larson, E. Worrel and J. S. Chen. Clean fuels from municipal solid waste for fuel cell buses in metropolitan areas. Resources, Conservation and Recycling, 17 (1996), 273– 298.
[2] L. A. Ruth. Energy from municipal solid waste: A comparison with coal combustion technology. Progr. Energy Combust. Sci., 24 (1998), 545–564.
[3] WM. Randal Seeker. Waste combustion. 23rd Symp. (Int.) on Combustion, The Combustion Institute, (1990), 867–885.
[4] C. R. McGowin and G. A. Wiltsee. Strategic analysis of biomass and waste fuels for electric power generation. Biomass and Bioenergy, 10 (1996), 167–175.
[5] J. L. Easterly and M. Burnham. Overview of biomass and waste fuel resources for power production. Biomass and Bioenergy, 10 (1996), 79–92.
[6]M. Morris and L. Waldheim. Energy recovery from solid waste fuels using advanced gasification technology. Waste Management, 18 (1998), 557–564.
[7] T. J. Min, K. Yoshikawa and K. Murakami. Distributed gasification and power generation from solid wastes. Energy, 30 (2005), 2219–2228.
[8] T. Malkow. Novel and innovative pyrolysis and gasification technologies for energy efficient and environmentally sound MSW disposal. Waste Management, 24 (2004), 53–79. [9] J. D. Murphy and E. McKeogh. Technical, economic and environmental analysis of energy production from municipal solid waste. Renewable Energy, 29 (2004), 1043–1057. [10] V. Belgiorno, G. De Feo, C. Della Roca and R. M. A. Napoli. Energy from gasification of solid wastes. Waste Management, 23 (2003), 1–15.
[11] L. Bébar, P. Martinák, J. Hájek. P. Stehlík, Z. Hajný and J. Oral. Waste to energy in the field of thermal processing of waste. Applied Thermal Eng., 22 (2002), 897–906. [12] C. Borgianni, P. De Filippis, F. Pochetti and M. Paolucci. Gasification process of wastes containing PVC. Fuel, 81 (2002), 1827–1833.
[13] A. Björklund, M. Melaina and G. Keoleian. Hydrogen as a transportation fuel produced from thermal gasification of municipal solid waste: an examination of two integrated technologies. Int. J. Hydrogen Energy, 26 (2001), 1209–1221.
[14] P. H. Wallman, C. B. Thorsness and J. D. Winter. Hydrogen production from wastes. Energy, 23 (1998), 271–278.
[15] L. Bébar, P. Stehlík, L. Havlen and J. Oral. Analysis of using gasification and incineration for thermal processing of wastes. Applied Thermal Eng., 25 (2005), 1045– 1055.
[16] J. I. Na, S. J. Pank, Y. K. Kim, J. G. Lee and J. H. Kim. Characteristics of oxygen-blown gasification for combustible waste in a fixed-bed gasifier. Applied Energy, 75 (2003), 275–285.
[17] M. Rovatti, A. Converti, M. Bisi and G. Ferraiolo. Pyrolysis of refuse derived fuel: Kinetic modelling from product composition. J. Hazardous Materials, 36 (1994), 19–33. [18] P. De Filippis, C. Borgianni, M. Paolucci and F. Pochetti. Prediction of syngas quality for two-stage gasification of selected waste feedstocks. Waste Management, 24 (2004), 633–639.
[19] W. R. Smith and R. W. Missen. Chemical reaction equilibrium analysis: theory and algorithms. Wiley, New York, 1982.
