INTRODUCTION. The management of municipal solid waste (MSW) is one of the most important problems especially for developing countries.

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2 INTRODUCTION The management of municipal solid waste (MSW) is one of the most important problems especially for developing countries. Incineration is a popular method of dealing with waste. This scheme is also known as waste-to to-energy (WTE) conversion. However, due to the environmental problems that go hand- in-hand with incineration, waste incinerators are required to install sophisticated exhaust gas cleaning equipment. Depending on the regulations of the country, this gas cleaning equipment can be large and expensive Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 1

3 INTRODUCTION Gasification is another kind of WTE conversion that is very attractive. It is a solution for both MSW management and the need to find new energy resources. Furthermore, it also has been proven to be more friendly to the environment. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 2

4 INTRODUCTION Because waste is inherently non-homogeneous material, the composition of MSW varies significantly and depends on many factors, such as location, local policy, origin of the waste, and etc. For techno-economical economical evaluation, actual construction of a gasifier for the purpose of evaluation, is not always feasible and economically sound because experimentation usually involves much greater time, effort, and cost. Thus, a model for that system is more useful. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 3

5 INTRODUCTION In this study, the equilibrium model based on equilibrium constant was developed. To improve the model, the equilibrium constants were multiplied by the coefficients determined from the comparison of the predicted results with the experimental results from other works. After modifying the model, data on MSW of Thailand was used for the simulation to study the effects of moisture content (MC) on the composition of the producer gas, on the reaction temperature, and on the calorific value. Finally, the second law efficiency was estimated for the solid waste with 20%, 25%, and 30% moisture content. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 4

6 THE MODEL Feedstock is defined as CH x O y N z. The global gasification reaction can be written as follows: CH O N + wh O + m(o +3.76N ) = n H + n CO + n CO + x y z H 2 CO CO z n H O + n CH + ( m)N HO 2 CH 4 2 The left side of Eq.. (1) are defined at 25 o C Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 5

7 THE MODEL: Mass balance To find the five unknown species of the producer gas, five equations were required. Carbon Balance: f 1 = 0 = n CO + n CO + n 2 CH Hydrogen Balance: f = 0 = 2n + 2n + 4n - x - 2w 2 H H O CH Oxygen Balance: f 3 = 0 = n CO + 2n CO + n 2 H2O - w - 2m - y Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 6

8 THE MODEL: Thermodynamic equilibrium For the remaining two equations, they were obtained from the equilibrium constant of the reactions occurring in the gasification zone Water-Gas Shift Reaction: CO+H2O =CO 2+H2 Methane Reaction: C+2H 2 =CH4 The thermodynamic equilibrium was assumed for all chemical reactions in the gasification zone. All gases were assumed to be ideal and all reactions form at pressure 1 atm. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 7

9 THE MODEL: Thermodynamic equilibrium The equilibrium constant for water-gas shift reaction: (n ) (n ) ν P 1 i o i CO H O i ν i CO i 2 H2 K= (x) = P (n ) (n ) f 4= 0 = K 1(n CO)(n H2O) - (n CO )(n 2 H ) 2 The equilibrium constant for methane reaction: ν P total 2 i o 2 i P (n H ) i ν i CH i 4 K = (x ) = (n ) (n ) f= 0 = K(n )-(n )(n ) H CH total 2 4 K 1 and K 2 are equilibrium constant of the water-gas shift reaction and methane reaction, respectively. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 8 2 2

10 THE MODEL: Thermodynamic equilibrium For finding the equilibrium constants, Gibbs free energy was used, as shown: o ΔG T o o lnk = - ΔG = νδg RT T i f,t,i i o o 2 c 3 d 4 e Δg f,t = h f - a Tln(T) - b T - ( )T - ( )T + ( ) + f + g T 2 3 2T Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 9

11 THE MODEL: Thermodynamic equilibrium Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 10

12 THE MODEL: Energy balance The temperature of the gasification zone needs to be calculated in order to calculate the equilibrium constants. For this reason, the energy balance or enthalpy balance was performed. Energy Balance o ( o h ) f,j = ni h f,i + ΔhT,i j = react i = prod Δh = T T 298 C (T)dT p C (T)=a + bt + ct + dt p 2 3 Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 11

13 THE MODEL: Energy balance The enthalpy of formation for solid fuel h =LHV+ n (h ) o o f,fuel k f k k=prod Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 12

14 THE MODEL: Calculation procedure Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 13

15 VALIDATION AND MODIFIED MODEL: Validation The model developed in this study was tested by comparing the calculation results with the data from the literature. Nine experimental results form Jayah et al. [13] were used to compare with simulation results of this model. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 14

16 VALIDATION AND MODIFIED MODEL: Validation From the comparison, the predicted results generally agree with experimental data, except CH 4. The interesting points in the comparisons are the amount of H 2 and CH 4. This model predicted higher amounts of H 2, but the predicted amounts of CH 4 lower than all experimental data. Predicted over value of H 2 and predicted under value of CH 4 were often found in literatures. In calibrating the model of Jayah et al [13], predicted methane was adjusted in such a way that it was equal to the amount of methane measured in the product gas. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer15

17 VALIDATION AND MODIFIED MODEL: The modified model To modify the model in this study, a coefficient, 11.28, was used to multiply with K 2 in the calculation procedure. This coefficient came from the average value of ratio of CH 4 in experiment obtained from 9 experiment cases of Jayah et al s s work including one case from Zainal et al s s work and CH 4 calculated from the model. For the CO concentration, a value 0.91 was defined to be the coefficient for modifying K 1. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 16

18 VALIDATION AND MODIFIED MODEL: The modified model The modified model was used to simulate and compare with the previous cases again. Moreover, the modified model was also compared with SynGas model and the experimental data at 1073 K, 10% MC form Altafini et al. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 17

19 VALIDATION AND MODIFIED MODEL: The modified model Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 18

20 THE EFFECT OF MOISTURE CONTENT The MSW from developing countries normally has high level of moisture when compared to those from developed countries. In Thailand, the main composition of MSW is food, thus it mainly consists of moisture. Thus, the effect of moisture content in the waste is an interesting aspect in the case that the waste will be used to produce synthetic gas. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 19

21 THE EFFECT OF MOISTURE CONTENT Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 20

22 THE EFFECT OF MOISTURE CONTENT In this study, all noncombustible and recyclable materials except paper were excluded. Paper was included because it can produce high levels of H 2, CO, and CH 4. The chemical formula of this solid waste, based on a single atom of carbon, is defined as CH O N To study the effect of moisture content of the waste, the amount of oxygen was fixed at 0.4 of the stoichiometic requirement Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 21

23 THE EFFECT OF MOISTURE CONTENT Fig.1. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 22

24 THE EFFECT OF MOISTURE CONTENT When MC increases from 0% to 40%. H 2 gradually increases from 16.04% to 20.11% Inverse relationship for CO, it decreases from 25.01% to 12.01%. CH 4 has a very low percentage in the producer gas, though it showed an increase from 0.134% to 1.86%. N 2 in the producer gas which was almost constant with only a slight change from 52.80% to 50.25%. CO 2 increases from 6.00% to 15.74%. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 23

25 THE EFFECT OF MOISTURE CONTENT Fig.2. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 24

26 SECOND LAW EFFICIENCY Exergy efficiency or second law efficiency is defined as: Ε prod η ex = E feed + E medium E medium medium is the exergy of the gasifying medium, which in this study can be neglected because it is the air at atmospheric conditions. The exergy can be divided into two major components shown below that are chemical exergy, E ch and physical exergy,, E ph. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 25

27 SECOND LAW EFFICIENCY The specific chemical exergy of ideal mixture gas, in kj/kmol kmol, can be calculated by: ε = x ε + RT x lnx ch,m i ch,i o i i i i The specific physical exergy of each gas species can be calculated by ε ph=(h - h o) - T o(s - s o) Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 26

28 SECOND LAW EFFICIENCY The exergy of solid fuel with mass ratio 2.67< (O/C)<0.667, expressed in kj/kg ε solid = ϕdry [LHV + mwh fg ] ϕ dry = H H N ( ) C C C O C Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 27

29 SECOND LAW EFFICIENCY The simulation was performed at temperature equal to 1073K. MC is a very important factor. Therefore, waste should be suitably dried when it will be used for gasification purposes. Solar drying of waste before gasification process should be accompanied with waste segregation and proper management. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 28

30 CONCLUSION The coefficients for correcting the equilibrium constant of water-gas shift reaction and methane reaction were used to improve the model. After developing the model, it was then employed to simulate the gasification of Thailand MSW to further study the effect of moisture content. The results showed that the mole fraction of H 2 gradually increases and CO decreases, when MC increases. CH 4, which has a very low percentage in the producer gas increases, N 2 slightly decreases and CO 2 increases with increasing MC. The reaction temperature and the calorific value decrease when MC increases. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 29

31 CONCLUSION Finally, evaluating the effect of MC at 20%, 25%, and 30% on the second law efficiency showed that at constant temperature, the second law efficiency decreases. The amount of required air increases when MC increases obviously to maintain the required reaction temperature. However, increasing the amount of air causes many disadvantages. This finding just further proved that waste segregation and solar drying of waste are necessary steps for effective gasification of MSW. Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 30

32 THANK YOU Thermodynamic Equilibrium Model and Second Law Analysis of a Downdraft Waste Gasificer 31