**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].

**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/Nm<sup>3</sup> 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 CaHbOcNdSe [6, 19–24]. **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


**Table 2.**

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

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.

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 *Estimating the Calorific Value and Potential of Electrical Energy Recovery of Organic… DOI: http://dx.doi.org/10.5772/intechopen.108232*


**Table 3.**

*Determined hypothetical chemical formula organic fraction of MSW.*

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.

ΔHocomb ¼ 5ð Þþ �393*:*509 6ð Þ� � �285*:*830 ð Þ¼� 733 2949*:*5 kJ*=*mol ¼ �18*:*21 MJ*=*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 CcHhOoNnSs. By following the pattern of the Buswell equation, the equation anaerobic decomposition of hypothetical predetermined formula can be expressed by Eq. (13) below:

$$\rm C\_{842}H\_{1411}O\_{641}N\_{27}S + 188.5\ H\_2O - -- - 412.25\ CO\_2 + 426.75\ CH\_4 + 27\ NH\_3 + H\_2S \tag{13}$$

Eq. (13) shows that, 1 (one) ton or 0.05 kg mol organic fraction can produce 19.239 kg mol or 431 m<sup>3</sup> CH4 (STP). With regard to the energy content of methane is 36 MJ/m<sup>3</sup> , 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 m<sup>3</sup> . 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 m<sup>3</sup> CH4 [13].

$$\mathbf{Lo} = \mathbf{C} \times \mathbf{BF} \times \mathbf{0.5} \times \mathbf{22.4} / 12 \times \mathbf{1000} \tag{14}$$

#### **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**.

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 ttable �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


*Estimating the Calorific Value and Potential of Electrical Energy Recovery of Organic… DOI: http://dx.doi.org/10.5772/intechopen.108232*

> **Table 4.** *Assessment*

 *empirical relationship*

 *between HHV and the ultimate analysis data.*


#### **Table 5.**

*Estimation of energy recovery through thermal conversion and bioconversion.*

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].

**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.
