The Effect of Temperature and Addition of Cao to Hydrogen Production From Pattukku Coal Char Gasification

Hydrogen is an environment-friendly fuel and has a high caloric value. Hydrogen as a molecule is not found in nature, but it is found in compounds with other elements. Besides catalytic steam reforming of natural gas, hydrogen can also be produced from thermochemical processes such as combustion, pyrolysis, and gasification. The process of gasification using steam as gasification agent can increase the yield of H2 in the gas products. The objectives of this research are to study the influence of temperature and the addition of CaO on H2 production. This research was conducted in an up-draft reactor for 60 minutes with three different temperatures; i.e. 600, 700, and 800 oC and ratio of CaO:char of 0 and 0.5. Based on this study, the rise of temperature will improve the yield of H2 and CO2 in the gas products. At gasification temperature of 800 oC, the yield of H2 and CO2 is maximum. Moreover, the addition of CaO can improve the char conversion and reduce the concentration of CO2 in the gas products.


INTRODUCTION
As fossil fuel reserves depleted, the development of alternative energy becomes very important to prevent energy crises (Liu et al., 2010). Hydrogen is one of the most promising alternative energy sources. Hydrogen has a high heating value and is also called clean energy. Hydrogen does not exist freely in nature but is generally present in the form of compounds with other elements such as carbon in methane (CH4), coal and petroleum (Moulijn et al., 2013;Florin & Harris, 2007).
Hydrogen can be made in several methods, including gasification. Gasification is a thermal process for converting solid materials containing carbon into fuel (Basu, 2010). Reactions in the gasification can be divided into five types: boudouard reaction, water-gas reaction, methanation reaction, shift conversion, and steam reforming. The equation of the reaction is as follows (Bell et al., 2011) In the process of gasification, coal or char is reacted with a gasification agent such as air, oxygen, steam, or CO2. The composition of the gas formed depends on the operating conditions and the gasification agent. Air is a source of cheap raw materials. However, its use as a gasification agent will produce gas that has a low heating value of about 4-7 MJ/Nm 3 . This is because N2 from the air will reduce the concentration of the gas (Basu, 2010;Bell et al., 2011). Oxygen can be used as the gasification agent replaces the air to eliminate the influence of N2 in the gas products. The gas produced will be dominated by the CO and CO2 as well as have a high thermal value i.e. approx. 12-28 MJ/Nm 3 (Basu, 2010). The use of air or oxygen produces gas with low H2 concentrations (Li et al., 2014).
The gasification process using steam will produce gas that has a high H2 content and CO (Bell et al., 2011). The reaction between CO gas with H2O (Eq. (4)) is exothermic so that it can become a provider of energy in the gasification process. The reaction between CO gas and H2O also produces CO2 gas that can reduce the concentration of H2 in the product gas. To increase the concentration of hydrogen in the product gas, it is recommended the use of CO2 adsorbent (Balasubramanian et al., 1999). The CO2 adsorption reaction equation by CaO is as follows: CaO+CO2CaCO3 ΔH o rx = -178.3 kJ/mol (6) The use of a calcium-based adsorbent such as Ca (OH)2 and CaCO3 have been widely used in gasification processes Guoxin & Hao, 2009). Other than as an absorber, CaO can also significantly increase the reaction conversion (Sobah et al., 2013;Murakami et al., 2015) and alter the direction of equilibrium (Guoxin et al., 2008).
Limestone is a mineral rock that is widely available in Indonesia. The largest component found in limestone is calcium carbonate (CaCO3) which is above 92%. The high content of calcium compounds and the relatively cheap causes these rocks have the potential to serve as the raw material for CO2 absorbers in the gasification process. Before being used as a CO2 absorbing agent, limestone is calcined first to convert CaCO3 to CaO.
To study the potential utilization of limestone as the absorber material, conducted research that aims to find out the influence of the temperature and the addition of CaO against yield of hydrogen on gasification of Pattukku coal char.

Raw Materials and Research Apparatus
The raw materials used are Pattukku coal obtained from the district of Bone South Sulawesi. This coal is low-rank coal with high sulfur content. The results of proximate analysis of raw materials can be seen in Table 1. Other raw materials used are N2 gas (95%) obtained from U.P. Sumber Agung Sukses (SAS) Yogyakarta, steam, and CaO.

Calcination Process
Two hundred of crushed limestone is placed on a ceramic plate then put into the furnace. The furnace is then turned on and the furnace temperature is set to 900 o C and the process was held for two hours. Calcined limestone is then analyzed its CaO concentration using EDX-8000 (Energy Dispersive X-ray Fluorescence Spectrometer). The results of the CaO content analysis are presented in Table 2.

Pyrolysis
The pyrolysis process is carried out in a tubular reactor (3.5 cm diameter and 55 cm high) at 450 o C and 1 atm pressure. A schematic diagram of the pyrolysis equipment is shown in Figure 1.
Fifty grams of Pattukku coal with a diameter of 3.35-4 mm are fed into the reactor. The air inside the reactor was removed by flowing the nitrogen from the bottom of the reactor for 15 minutes. The reactor is then heated using an electric heater (furnace) until it reaches a temperature of 450 o C. After the operating temperature is reached, the temperature controller is switched on and the isothermal process is run for 60 minutes. Char then analyzed using ultimate analysis. The results of the ultimate analysis are presented in Table 3.

Gasification
The reactor used in the gasification is an up-draft reactor with a height of 45 cm and a diameter of 3.5 cm. A schematic diagram of the pyrolysis equipment is shown in Figure 2.  One Hundred grams of char is fed into the reactor. The nitrogen was flowed from under the reactor to remove the air present in the reactor. The reactor was heated by using an electric heater to raise the temperature of the reactor. After the desired reaction temperature was reached, steam was passed from under the reactor to be reacted with the char present in the reactor. This process takes place at a constant temperature (isothermal) for 60 minutes. The non-condensed gas product was collected continuously in a tank containing water after it passed through the condenser. After the process is complete, samples were taken using syringe to analyze the gas composition using gas chromatography (GC) Shimadzu GC 2010 while the volume of gas produced was calculated based on the amount of water coming out from the gas reservoir.
To study the effect of temperature and CaO addition on hydrogen gas production, an experiment conducted with temperature variation i.e. 600, 700 and 800 o C) and ratio of CaO: char i.e. 0 and 0.5.

Method of Analysis
As much as 200 μL gas sample was injected into a port injector that has been preheated to a temperature of 40 ° C. The samples are then flowed to a packed column of 60 o C by the carrier gas. In the columns, the constituent components of the gas will be separated. One by one the gas component exits the column toward the thermal conductivity detector (TCD). The detection result is then recorded by the recorder (chromatogram). The number of peaks present in the chromatogram shows the amount of gas components present in the sample while each of the peak area represents the composition or quantity of the gas component. The value of each gas composition is obtained by comparing the peak area of the sample gas component with the standard peak gas area. The gas components analyzed include CH4, CO, H2, and CO2.

Calculation of Yield Gas and Carbon Conversion
Yield gas for each gram of char is obtained by dividing the total gas volume (mL) produced after the gasification process with the initial mass of sample (g).
The carbon conversion is calculated by the equation: where nCH4, nCO, nCO2 are the mol of each gas component, whereas nC is mol C in the initial sample.

Effect of Temperature
The composition of each component present in the gas product and the yiels of gas at various temperatures is presented in Table 4. While for the compositions of each component is the gas product as shown in Figure 3. Table 4 shows that the gas yield increases with increasing temperature. The gas yield at 600 o C was 29.3 mL/g and rising to double that at a temperature of 700 o C, (33.6 mL/g). Significant increase occurred at temperatures of 800 o C i.e. 137.1 mL/g or about six times compared to the yield of gas obtained at a temperature of 600 o C. Conversion of carbon also increased. The carbon conversion rises by 1.5 times when the temperature is raised from 600 to 700 o C and becomes doubled when the temperature rises from 600 to 800 o C. The same trend is also shown from the results of research conducted by . The results of both researchers showed that the conversion has increased by 1,3 times when the reaction temperature is increased from 500 to 600 o C. The increase in gas yield and carbon conversion are caused at high temperatures, the kinetic energy of the particles will increase and the collision energy between the particles will be greater so that the reaction will take place more quickly. Increased carbon conversion is also caused by the reaction between carbon (C) and steam (H2O) is an endothermic reaction. Under the Le Chatelier principle, when the temperature system is raised, the response of the system will lower the temperature. As a result, the reaction will shift toward a reaction that absorbs heat (endotherms). Thus, when the temperature is increased, the reaction conversion will increase so that the gas yield will increase. At high reaction temperatures and in the presence of steam will lead to greater reactivity of the char so that the gas that will form more and more .  Figure 3 it shows that the higher the temperature, the higher the yield of H2 and CO. The rise in temperature from 600 to 800 o C resulted in the yield of CO gas rising from 21 mL/g to 24 mL/g, while the yield of H2 increased from 1 mL/g to 104 mL/g. This is because the reaction of H2 and CO formation is an endothermic reaction so that with higher temperature, the reaction will lead to the formation of the product (Luo et al., 2009;Wei et al., 2007;. The increasing trend of composition and the yield of H2 in the gasification process has also been reported by . In their study, pine bark gasification is carried out with temperature variations of 500, 600, and 700 o C. From the results of this study obtained that the composition and the yield of H2 rise along with rising temperatures. Figure 3 also shows an increase in CO yield, but the increase is not significant compared to an increase of CO2. This is because some CO reacts with H2O to form CO2 and H2 in the gas phase based on Equation (1) (Luo et al., 2009). Figure 4 shows the effect of temperature on the yield of CH4. Figure 4 shows the yield of CH4 is very small compared to the other gas components. This is because the reaction between char and hydrogen to form methane is very slow (Walker et al., 1959). The yield of CH4 is also seen to decrease. This decrease is due to the reaction of methane decomposition by steam (equation 5) is an endothermic reaction, where if the temperature of the system is raised, the reaction will shift toward the formation of the product.  Table 5 shows that the yield of gas increased significantly. Comparing the yield in tables 4 and 5 shows that at a temperature of 600 o C the yield increased about 38.6% (from 29.3 to 47.7 mL/g). At a temperature of 700 o C, the yield increased about 52.6% (from 62.9 to 96.0 mL/g), whereas at 800 °C the yield increased significantly by approximately 146.2% (from 166.4 to 409.6 mL/g). Table 5 also shows an increase in char conversion. Conversion of char increased about 1.5 times at 600 °C (from 2.02 to 3.1%), 1.7 times at 700 °C (from 2.95 to 5.2%) and 1.9 times at temperature 800 o C (from 4.43 to 8.2%).  Figure 5 shows the effect of temperature on the yield of each component at various temperatures with the addition of CaO. Comparing the results in Figure 3 and Figure 5 shows that each gas (other than CO2) increases with the addition of CaO. At temperatures of 800 o C, CH4, CO, and H2 increased about 441.8%, 239.8% and 183.2% respectively while CO2 decreased about 7.4%.

Effect of CaO Addition
The increase in reaction conversion due to CaO addition can be explained by the following water-gas reaction mechanisms as follows: C+2H2OCO2+2H2 ΔH o rx = 100 kJ/mol (9) The addition of CaO in the gasification process will absorb the CO2 formed so that the partial pressure of the product will drop and the system will shift the direction of the reaction to the formation of the product. The carbonation reaction (equation 6) is an exothermic reaction which will cause an increase in temperature inside the reactor. Rising temperatures will cause the endothermic reactions to take place more rapidly and encourage reactions toward the formation of the product so that the conversion of the reaction will increase and also will increase the yield of H2. The addition of CaO will increase the reaction rate and absorb CO2 so that the yield of each gas increases (Sobah et al., 2013;Murakami et al., 2015). The same results were also shown by some previous researchers Guan et al., 2007;Wei, 2008;Wang et al., 2006). In the C/H2O/CaO gasification system, the carbonation reaction is a key step, not only for CO2 adsorption but also for the enthalphy balance in the gasifier (Wang et al., 2006). The relationship between the pressure equilibrium of CO2 (Peq, CO2) Gas 600 deg C 700 deg C 800 deg C and the temperature in the carbonation process can be expressed in terms of the following empirical equations (Abanades et al., 2003): log 10 ( ) = 7,079 − 8308/ ( ) where Peq is the pressure equilibrium of CO2 (atm) and T is temperature (K). From Equation (9) we can graph the relationship of Peq versus T as follows ). Figure 6. Partial pressure of CO2 as a function of temperature Table 4 shows that at 600 °C, the concentration of CO2 is 24.2%. When the gasifier operating pressure is 1 atm, the partial pressure of CO2 at that temperature is 0.24 atm. Based on these results, the CO2 partial pressure exceeds the CO2 equilibrium pressure at 600 °C so that when gasification with CaO addition is carried out at this temperature there will be CO2 adsorption.

CONCLUSION
Hydrogen is one of the alternative sources of energy that are environmentally friendly. The production of hydrogen through the gasification process can be increased by absorbing CO2 present in the product gas. From the results of this study can be concluded that: 1. Rising reaction temperature will increase reaction conversion and the yield of H2. A significant yield of H2 occurs at a temperature of 800 °C. 2. The use of CaO can increase the conversion of the reaction and decrease the concentration of CO2 thus increasing the concentration of H2 in the gas product.