Estimating the Calorific Value and Potential of Electrical Energy Recovery of Organic Fraction of Municipal Solid Waste through Empirically Equations and Theoretically Way

Abstract

This chapter aimed to estimate the calorific value and potential of electrical energy that can be generated from the organic fraction of municipal solid waste through mathematical models, bioconversion, and thermochemical approach. The calorific values were calculated using the empirical relationship between higher heating value and ultimate analysis data, stoichiometric manner, and thermochemistry concept. The potential of electrical energy that can be produced was calculated based on literature data on the specified power plant. The result showed that the calorific value and potential of electrical energy recovery of the thermochemical approach are higher than others.

1. Introduction

The increase in population in urban areas results in an increase in the amount of municipal solid waste (MSW) generated. AusAID [1] reported that a total of 38.5 million tons of solid waste were generated annually by the 232 million inhabitants in Indonesia (450 gm per person per day), of the which, 21.2 million tons contributed by the inhabitants of the island of Java. The 26 biggest Cities in Indonesia inhabit a totally 40.1 million people, generating in total an estimated 14.1 million tons per year (about 1 kg per person per day). In this country, municipal waste is composed of 62% of mainly organic waste, 14% plastics, 9% paper, 2% glass, 2% rubber and leather, 2% metals, and 13% of other waste types [1].

These physical characteristics of solid waste are indicated as a potential source of biomass mainly their bioorganic contents. Biomass can be considered as the solar energy stored in chemical bonds of organic material [2]. If the bond between carbon and hydrogen, and oxygen is broken down through decomposition, combustion, or decomposition process for these materials, the chemical energy stored or potential energy will be released [2]. Therefore, its energy content will be influenced by the elementary composition.

To determine the potential energy stored in the biomass, traditionally by direct measurement or theoretical approach. Direct measurements can be carried out by an experiment using a bomb calorimeter [3] and the theoretical can be determined from their elementary content such as carbon, hydrogen, oxygen, nitrogen, and sulfur [4]. In this, to analyze the potential of energy recovery required adequate biomass characteristics, especially in elemental compositions [4, 5]. In relation to the utilization of solid waste as an energy source, the investigation of their chemical elemental characteristics is beneficial to the suitable choice of energy conversion technologies including bioconversion, incineration, or thermochemical conversion processes.

However, there is lack of data associated with the chemical elemental characteristics of solid waste in Indonesia. Based on their elemental chemical characteristics, the energy content of solid waste fuel can be calculated from the heat of combustion. In the combusting, fuel will release its energy potential. The energy released or the heat of fuel combustion is heat when a fuel undergoes complete combustion with oxygen under standard conditions. It can be calculated as the difference between the heat of the formation of the products and reactants. In this, a chemical equation is required. The heat of the formation of the products and reactants can be obtained through a thermochemical table [4]. This change of the enthalpy approach is a thermochemical conversion process based. Estimation of the potential energy from organic waste fuel can also be determined by using the Buswell equation. Buswell [6] suggested a general equation for the anaerobic bioconversion process of biodegradable organic matter. This second way either requires stoichiometric equations for the bioconversion process or the biodegradability of waste. The biodegradability of organic solid waste can be expected from its volatile solid and lignin content [6].

Besides based on the theoretical way, the elementary characteristics data can also be used to estimate the calorific value or energy content based on their empirical relationship. Researchers in several countries have carried out extensive research to determine the empirical relation between the elemental and calorific value of biomass and solid waste fuel [4, 7, 8, 9, 10, 11, 12]. The energy content of the estimation results can then be used to estimate the quantity of energy that can be recovered on fuel. Therefore, both through direct measurement, stoichiometric approach, as well as the empirical relationship can be used as a basis for estimating energy recovery from organic solid waste.

This paper aims to estimate energy content and recovery potential organic component of MSW by using experiment, theoretical, and empirical approaches. The energy recovery potential is determined by the combustion and anaerobic digestion process.

2. Methodology

There were four steps used. First, ultimate analysis data and calorific value of the organic fraction of MSW (OFMSW) were collected based on literature data [12, 13]. Second, the hypothetical chemical formula was calculated from their ultimate analysis data as a basis for estimating the energy released when thermal conversion and bioconversion processes were applied. Third, correlations between higher heating value (HHV) and ultimate analysis data were developed. Fourth, comparing the potential energy recovered from experimental, stoichiometric manner, and empirical models developed.

2.1 Characteristics of waste

The characteristics of OFMSW data such as ultimate analysis included carbon (C), hydrogen (H), nitrogen (N), oxygen (O), sulfur (S) (mass percentages on a dry basis), and water content (H2O, mass percentages as discarded), calorific value, volatile solid (VS), and lignin content were used. As reported in [12], the organic fraction of MSW was collected from a landfill site in Indonesia. The content of carbon (C) was determined by the standard American Standard for Testing Materials ASTM) D 5373, hydrogen (H) by the standard ASTM D 5373, nitrogen (N) by the standard ASTM D 5373, oxygen (O) by standard ASTM D 3176, and sulfur (S) by standard ASTM D 4239. These elemental analysis data were performed using CHNS-O Analyzers (Perkin-Elmer 2400 Series) at the mineral and coal technology laboratory, Bandung Indonesia [12]. The higher heating value (HHV) of the sample was determined based on the standard ASTM D 5865 by using Bomb Calorimeter [12]. The lignin content was analyzed at Food and Nutrient Laboratory Gadjah Mada of University Yogyakarta [13].

2.2 Hypothetical development chemical formula

The molecular formula is the actual whole number ratio between the elements. The quantity of each element was divided by its molar mass to give the number of hypothetical moles of each element. Furthermore, these mole ratios were used to determine the molecular formula. The molecular formula was used too in determining the principal reactions that occur during combustion and bioconversion.

2.3 Empirical correlations

Correlations between calorific value with percent carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S) were used to estimate calorific value. In this case, correlations equations were developed from previously available empirical equations (Table 1).

Eq.	Models	Sample	Ref.
(1)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=-1.494+0.474\mathrm{C}-0.803\mathrm{H}+0.034\mathrm{O}+0.982\mathrm{N}$	OFMSW	[12]
(2)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=-1.309+0.475\mathrm{C}-0.796\mathrm{H}+0.031\mathrm{O}+1.008\mathrm{N}-1.395\mathrm{S}$	OFMSW	[12]
(3)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=-1.3675+0.3137\mathrm{C}+0:7009\mathrm{H}+0.0318\mathrm{O}$	Biomass	[10]
(4)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=0.399\mathrm{C}+1.4\mathrm{H}-0.139\mathrm{O}+\mathrm{0,105}\mathrm{S}$	MSW	[14]
(5)	$\mathrm{HHV}\left(\text{kcal}/\mathrm{kg}\right)=81\mathrm{C}+342.5\left(\mathrm{H}-1/8\mathrm{O}\right)+22.5\mathrm{S}$	MSW and RDF	[7]
(6)	$\mathrm{HHV}\left(\text{kcal}/\mathrm{kg}\right)=81\left(\mathrm{C}-3/8\mathrm{O}\right)+171/8\mathrm{O}+342.5\left(\mathrm{H}-1/16\mathrm{O}\right)+25\mathrm{S}$	MSW and RDF	[7]
(7)	$\mathrm{HHV}\left(\text{kcal}/\mathrm{kg}\right)=81\left(\mathrm{C}-3/8\mathrm{O}\right)+342.5\mathrm{H}+22.55\mathrm{S}+171/4\mathrm{O}$	MSW and RDF	[7]
(8)	$\mathrm{HHV}\left(\mathrm{Mj}/\mathrm{kg}\right)=0.327\mathrm{C}+1.241\mathrm{H}-0.089\mathrm{O}-0.26\mathrm{N}+0.074\mathrm{S}$	MSW	[4]
(9)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=\left(35.160\mathrm{C}+116.225\mathrm{H}-11.090\mathrm{O}+6.280\mathrm{N}+10.465\mathrm{S}\right)\times {10}^{-2}$	Fossil fuel	[15]
(10)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=\left(340.39\mathrm{C}+1320.83\mathrm{H}+68.30\mathrm{S}-15.28\mathrm{ash}-118.5\left(\mathrm{O}+\mathrm{N}\right)\right)\times {10}^{-3}$	Coal	[16]

Table 1.

Correlation models from any research.

Note: RDF = refuse-derived fuel.

The fulfillment of model performances was assessed by four statistical criteria, namely: sample paired test, coefficient of correlation, average absolute error (AAE), and average bias error (ABE). Sample paired test and coefficient of correlation were determined by SPSS 17 version. The symmetrical relationships were used to determine the correlation between the calorific value of the calculation results of the model with the measurement. The coefficient correlations were used to determine strong or weak relationships by the following criteria: 0.00–0.199 very weak, 0.20–0.399 weak, 0.40–0.599 sufficient, 0.60–0.799 strong, and very strong 0.80–1.00.

The average absolute error can be expressed as follow:

$$\mathit{AAE}=\frac{1}{n}\sum _{i=1}^n\left|\frac{\mathit{Ya}-\mathit{Yp}}{\mathit{Ya}}\right|\times 100\%$$E11

Where AAE is the mean percentage of absolute error, Ya and Yp are the actual and expected values. n represents the number of data points. The sum of squared errors can be given by the following equation:

$$\mathit{ABE}=\frac{1}{n}\sum _{i=1}^n\frac{\mathit{Ya}-\mathit{Yp}}{\mathit{Ya}}\times 100\%$$E12

Where ABE is the mean percentage of bias error.

3. Results and discussion

3.1 Data analysis ultimate and calorific value

Data ultimate analysis of the organic fraction of MSW that is used in this chapter can be seen in Table 2. The database showed a variation content of C, H, N, and S. The waste contained 38.95% carbon. This value has a good agreement with the values reported in the literature. The average percentage of hydrogen is 5.44%. Compared with other studies, the value of hydrogen is higher than has been reported in [9, 17]. Table 2 also listed that the average percentage of available nitrogen is nearly 1.47%. The content of nitrogen was reported higher than the value reported by [9, 10, 17] and the average percentage of sulfur is 0.12%. This value is relatively low compared to the results presented in [6].

Sample	Elements composition (%)					Ash (%)	Calorific value (MJ/kg)
	C	H	N	S	O
1	39.37	5.71	1.87	0.18	37.63	15.24	15.42
2	42.31	5.98	1.69	0.13	38.40	11.49	16.64
3	24.63	3.96	1.04	0.13	29.56	40.68	8.91
4	25.81	4.48	1.11	0.13	34.82	33.65	9.84
5	39.65	5.70	1.87	0.19	39.62	12.97	15.8
6	42.89	6.06	1.60	0.11	39.27	9.97	17.00
7	39.99	6.07	1.69	0.10	39.89	12.26	15.77
8	40.27	6.04	1.71	0.09	39.44	12.45	15.82
9	39.22	5.86	1.75	0.11	39.50	13.56	15.43
10	40.15	5.99	1.80	0.14	40.06	11.86	15.92
11	40.34	6.07	1.74	0.11	40.59	11.15	15.90
12	38.94	5.79	1.73	0.14	30.43	14.97	15.19
13	41.47	5.74	0.74	0.11	42.48	9.46	15.76
14	40.14	5.59	1.02	0.15	40.99	12.11	15.26
15	38.68	5.38	0.74	0.02	40.39	14.79	14.53
16	39.89	5.52	1.12	0.17	40.12	13.18	15.27
17	40.57	5.68	0.74	0.11	41.71	11.19	15.58
18	40.29	5.58	1.08	0.17	41.16	11.72	15.56
19	40.11	4.85	1.83	0.12	42.43	10.66	16.80
20	41.16	4.93	1.68	0.09	42.33	9.81	16.99
21	39.45	4.86	1.66	0.11	41.75	12.17	16.60
22	39.66	4.87	1.65	0.13	41.43	12.26	16.52
23	39.19	4.86	1.7	0.11	41.97	12.17	16.48
24	40.51	4.91	1.78	0.11	42.26	10.43	16.92
Average	38.95	5.44	1.47	0.12	39.51	14.18	15.41
Deviation Standard	4.34	0.58	0.40	0.04	3.42	7.32	1.98

Table 2.

Ultimate and calorific value organic fraction of MSW (dry basis).

Table 2 also presented the calorific value of waste. The calorific value (CV) of a material indicates the energy content or the heat released when it is burnt in the presence of air. CV can be measured as energy content per unit mass or volume; kJ/kg for solids, MJ/L for liquid, or MJ/Nm³ for gas fuel. CV of fuel can be expressed in two forms: gross CV (GCV), or a higher heating value (HHV), and net CV (NCV), or lower heating value (LHV) [10]. HHV is the total energy content that is released when a fuel is burned in air, including the latent heat contained in the water vapor, and therefore represents the maximum amount of potential energy that can be charged from a source of fuel. The actual amount of energy that can be collected will vary depending on the form of fuel and conversion technologies used [6]. In this, the calorific value in Table 2 is the HHV of waste. The average HHV was found 15.41 MJ/kg with a standard deviation of 1.98. It also observed that the calorific value is lower than the calorific value of the lignocellulosic as well as cellulosic biomass. In [18], it was reported that the cellulosic biomass has an average calorific value of 17.73 MJ/kg and for lignocellulosic materials 26.7 MJ/kg.

3.2 Hypothetical chemical formula for organic component of MSW

The chemical formula of OFMSW can be approximated as a hypothetical compound of the form C_aH_bO_cN_dS_e [6, 19, 20, 21, 22, 23, 24]. Table 3 shows the determination of the hypothetical chemical formula of OFMSW using the average value of ultimate data. By using S as a base, then, the empirical chemical formula is C₈₄₂H₁₄₁₁O₆₄₁N₂₇S. However, in view of sulfur and nitrogen is relatively small components of it, if the nitrogen and sulfur are removed, the molecular structure of the waste is very close to the cellulose (C₆H₁₀O₅). On the other hand, in [22], it was reported that the structure of mixed food and plant wastes can be approximated by the molecular composition (C₆H₁₀O₄). The molecular compositions are useful to determine fundamentals reaction during thermal conversion or bioconversion process. Therefore, if the structure varies significantly then the range enthalpy of combustion will also vary, thus, increasing the uncertainty associated with the quantity of energy that can be recovered from the waste stream. Conversely, if the structure appears to be fairly stable, i.e. not deviating from the mean greatly, then the enthalpy of formation is likely to be fairly constant [24]. It supported the claim that the compound C₆H₁₀O₅ can be used to approximate the chemical structure of OFMSW in Bali. The standard enthalpies of formation and combustion can be used to deduce an approximate heating value of C₆H₁₀O₅, as well as the heating value, can be estimated from the biogas generated when the anaerobic decomposition was applied.

Items	Elemental data (dry basis)
	Carbon (C)	Hydrogen (H)	Oxygen (O)	Nitrogen (N)	Sulfur (S)
Composition (%)	38.95	5.44	39.51	1.47	0.12
Atomic weight (kg/kmol)	12.00	1.00	16.00	14.00	32.00
mol	3.25	5.44	2.47	0.11	0.00
mol ratio (S basis)	842.06	1410.60	640.70	27.29	1
mol ratio (C basis)	6	10.05	4.57	0.20	0.01

Table 3.

Determined hypothetical chemical formula organic fraction of MSW.

The estimated heat of the combustion reaction can be performed using the thermochemical table. The use of thermochemical data tables from textbooks that are generally located on the back page is a general way. For example, the large amounts of standard heat of formation data can be found in Perry’s Chemical Engineering Handbook or Handbook of Chemistry and Physics. Based on these data, the enthalpy change of the reaction can be predicted in various chemical reactions. The standard heat of the formation of a compound is the enthalpy change associated with the formation of 1 mol of a compound from its elements in their standard state at temperatures of 25°C and a pressure of 1 bar. The standard enthalpy of formation for H₂O (l) and CO₂ (g) is successively −285.830 kJ/mol and − 393.509 kJ/mol, while cellulose 733 kJ/mol. The standard enthalpy change for the combustion of the hypothetical chemical formula can be determined by finding the difference between the standard enthalpy of the formation of products and reactants. In the case of cellulose, the difference is.

$$∆{\mathrm{H}}_{\text{ocomb}}=5\left(-393.509\right)+6\left(-285.830\right)-\left(-733\right)=-2949.5\mathrm{kJ}/\mathrm{mol}=-18.21\mathrm{MJ}/\mathrm{kg}.$$

Estimating the heating value through the bioconversion process can be performed using the Buswell equation. Buswell in 1952 found an equation for estimating the products of the anaerobic decomposition of organic materials with the general chemical composition C_cH_hO_oN_nS_s. By following the pattern of the Buswell equation, the equation anaerobic decomposition of hypothetical predetermined formula can be expressed by Eq. (13) below:

$${\mathrm{C}}_{842}{\mathrm{H}}_{1411}{\mathrm{O}}_{641}{\mathrm{N}}_{27}\mathrm{S}+188.5{\mathrm{H}}_2\mathrm{O}----412.25\mathrm{C}{\mathrm{O}}_2+426.75\mathrm{C}{\mathrm{H}}_4+27\mathrm{N}{\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{S}$$E13

Eq. (13) shows that, 1 (one) ton or 0.05 kg mol organic fraction can produce 19.239 kg mol or 431 m³ CH₄ (STP). With regard to the energy content of methane is 36 MJ/m³, then, 1 ton (1000 kg) organic fraction of MSW has the potential of energy recovery 15.516 MJ/kg. These results assumed the substrate is perfectly biodegradable. The result is also not much different from the average calorific value obtained from the measurement results (15.41 MJ/kg) as mentioned in the previous section. With reference to the data reported by [13], the average biodegradability of the organic waste component in their study area was 0.16. Based on the data biodegradability, hence, net CH₄ generated from the equation Buswell is 68.96 m³. Thus, 1 ton (1000 kg) organic fraction of MSW has the potential of energy recovery only 2.482 MJ/kg.

Another possible way to estimate methane production is by using the carbon content of the organic fraction of MSW as shown in Eq. (14). Carbon content (C) expressed with gC. BF is the biodegradability of organic waste component, 0.5 is the value set for the volumetric ratio of methane to the total landfill gas, 22.4/12 is the conversion factor C into a gas at 1 atm and 25°C, expressed as L CH₄/gC. Lo expressed as m³ CH₄ [13].

$$\mathrm{Lo}=\mathrm{C}\times \mathrm{BF}\times 0.5\times 22.4/12\times 1000$$E14

3.3 Predicting HHV from ultimate analysis data

Several empirical correlations have been developed to estimate the HHV of various biomass and fossil fuels using ultimate analysis data. The correlations were generally derived from the equation for coal fuel developed by Dulong [9]. According to the concept that the fuel is essentially organic matter that has potential energy because of the carbon-hydrogen and carbon–oxygen bonds, the HHV relationship with the ultimate analysis data can also be adopted on other fuels, including solid waste HHV is a function of carbon, hydrogen, oxygen, and nitrogen have been developed for fuel garbage [6]. The development of empirical models based on multiple linear regression using the content (in% dry weight) of carbon, hydrogen, oxygen, nitrogen, and sulfur as the independent variable and the Higher Heating Value (HHV) as the dependent variable will help in determining the contribution each element in the ultimate analysis data to predict the HHV of waste. The assessment of developed models by several researchers (Table 1) was conducted in this section based on the results calculation of developed models obtained from the experiment. The results summary is given in Table 4.

Eq.	Models	Correlation	t-test	Error
				AAE	ABE
(1)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=-1.494+0.474\mathrm{C}-0.803\mathrm{H}+0.034\mathrm{O}+0.982\mathrm{N}$	0.997	0.501	0.932	0.182
(2)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=-1.309+0.475\mathrm{C}-0.796\mathrm{H}+0.031\mathrm{O}+1.008\mathrm{N}-1.395\mathrm{S}$	0.997	0.689	0.890	0.125
(3)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=-1.3675+0.3137\mathrm{C}+0:7009\mathrm{H}+0.0318\mathrm{O}$	0.919	0.004	6.052	−3.728
(4)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=0.399\mathrm{C}+1.4\mathrm{H}-0.139\mathrm{O}+\mathrm{0,105}\mathrm{S}$	0.816	0.000	15.172	−15.172
(5)	$\mathrm{HHV}\left(\text{kcal}/\mathrm{kg}\right)=81\mathrm{C}+342.5\left(\mathrm{H}-1/8\mathrm{O}\right)+22.5\mathrm{S}$	0.748	0.000	9.876	9.406
(6)	$\mathrm{HHV}\left(\text{kcal}/\mathrm{kg}\right)=81\left(\mathrm{C}-3/8\mathrm{O}\right)+171/8\mathrm{O}+342.5\left(\mathrm{H}-1/16\mathrm{O}\right)+25\mathrm{S}$	0.791	0.045	8.623	−4.055
(7)	$\mathrm{HHV}\left(\text{kcal}/\mathrm{kg}\right)=81\left(\mathrm{C}-3/8\mathrm{O}\right)+342.5\mathrm{H}+22.55\mathrm{S}+171/4\mathrm{O}$	0.881	0.000	50.438	−50.438
(8)	$\mathrm{HHV}\left(\mathrm{MJ}/\mathrm{kg}\right)=0.327\mathrm{C}+1.241\mathrm{H}-0.089\mathrm{O}-0.26\mathrm{N}+0074\mathrm{S}$	0.813	0.460	6.949	−1.695
(9)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=\left(35.160\mathrm{C}+116.225\mathrm{H}-11.090\mathrm{O}+6.280\mathrm{N}+10.465\mathrm{S}\right)\times {10}^{-2}$	0.831	0.171	7.103	−2.558
(10)	$\mathrm{HHV}\left(\mathrm{kJ}/\mathrm{kg}\right)=\left(340.39\mathrm{C}+1320.83\mathrm{H}+68.30\mathrm{S}-15.28\mathrm{ash}-118.5\left(\mathrm{O}+\mathrm{N}\right)\right)\times {10}^{-3}$	0.807	0.683	6.742	−1.093

Table 4.

Assessment empirical relationship between HHV and the ultimate analysis data.

According to the t-test value with 5% significance level, Table 3 shows that there is no significant difference in calorific value between the model’s calculation with obtained from the experiment because the value of the t-test is within the range of t-table −2.069 to 2.069. This implies that there were no measurement errors in bomb calorimeter operation in this ultimate analysis. Based on the correlation coefficient, only two equations (Eqs. (5) and (6) do not show a very strong correlation between the results of calculations with the experiment. Otherwise, based on the average value of absolute and bias error, only Eq. (1) and (2) are both less than 5%. However, in determining the energy that can be collected from the organic fraction of MSW, the estimation of Eq. (1), (2), (8), (9), and (10) are used due to their errors less than 10%.

3.4 Estimation of energy recovery

Based on estimation calorific values obtained from empirical relationships elected, thermochemical and biochemical processes through stoichiometric equations, carbon content, and direct measurement, the potential electrical energy recovery was determined. The results are summarized in Table 5. The value is obtained by assuming that the heat generated from the flue gases of combustion can be utilized to generate steam from the boiler. In this context, the boiler is an integral part of the conversion system. However, the efficiency of boiler performance varies according to the energy source. According to [6], the variables that determine the efficiency of the boiler included the energy content of the fuel, moisture content, flue gas temperature, and the inner physical design of the boiler. In this chapter, the efficiency boiler is set to 70% by adopting the value in [6] for mass burning. Similarly, from the same literature, the efficiency factor of steam turbines and electrical generators data were also used to estimate the electric power generated. Besides that, the facility consumption and loss using the data available in [6]. In the system of bioconversion, the production of methane gas from the process was considered to drive the gas turbine. The efficiency gas turbine (regenerative type) and the power generator was assumed 24% and 90% [6].

Approach	Potential of energy (kJ/kg)	Parameters			Gross electrical energy generated (kWh/kg)*
		Boiler efficiency (%)	Turbine efficiency (%)	Generator efficiency (%)
Empirical model (1)	15390	70	29	90	0.78
Empirical model (2)	15400	70	29	90	0.78
Empirical model (8)	15592	70	29	90	0.79
Empirical model (9)	15736	70	29	90	0.80
Empirical model (10)	15515	70	29	90	0.79
Thermochemical	18206	70	29	90	0.92
Bioconversion (Buswell)	2482	—	24	90	0.15
Carbon content (IPCC)	2199		24	90	0.13
Bomb calorimeter	15410	70	29	90	0.78

Table 5.

Estimation of energy recovery through thermal conversion and bioconversion.

*

1 kWh = 3600 kJ

Table 5 expressed, the thermochemical approach giving the highest value. In this case, the approach was assumed as a complete combustion process. In addition, the calculation in this approach was based on the assumption that the molecular formula of waste is cellulose or without the involvement of S and N. Thus, based on the electrical energy that can be generated, the thermal conversion process tends to be more advantageous than the bioconversion process. Energy potential generated from the thermal conversion process reaches more than five times the bioconversion process. This is possible because the biodegradability of waste is too small. However, it is important to note that the calculation of the energy obtained through measurement using a bomb calorimeter should be closest to the real conditions. The others were estimation approaches. The estimation closest to the measurement results is of equations developed from the same sample.

4. Conclusions

Increased attention on MSW to energy has resulted in the increasing need for the characteristics and potential of this material. In Indonesia, this attention is more focused on the organic fraction of MSW or their biomass components. Various methods can be used to determine the energy potential and the amount of electrical energy that can be generated from the organic fraction of MSW. These methods can be developed through an empirical relationship between elementary constituents of waste by energy content, theoretical approach, or through measurement using a bomb calorimeter. This chapter concluded that each approach provides different results of energy recovery potential. The thermochemical process has given the highest calorific value (18.21 MJ/kg) and electric energy recovery (0.92 kWh/kg).