Notations

DSS	Diluted Soluble Solid
MW	Molecular Weight
NVDS	Non Volatile Dissolved Solid
P	Pressure
T	Temperature
TDVS	Total Dissolved Volatile Solid
TS	Total Solid
TVA	Total Volatile Acid = result of microbial breakdown of pretreated products and are a direct precursor to methane production
TVS	Total Volatile Solid

Gasification is a process that converts carbonaceous materials, such as coal or biomass, into gas mixture of carbon monoxide, hydrogen, methane, and other gas by reacting raw material at certain condition which can involve or not involve other reactant. The resulting gas mixture is called synthesis gas or syngas and is itself a fuel. Gasification is a very efficient method for extracting energy from many different types of organic materials, and also has applications as a clean waste disposal technique.

The advantage of gasification is the syngas utilization which is more efficient than direct combustion of the original fuel; more of the energy contained in the fuel is extracted. Syngas may be burned directly in internal combustion engines (ICE), used to produce methanol and hydrogen, or converted via Fischer-Tropsch process into synthetic fuel. Gasification of fossil fuels is currently widely used on industrial scales to generate electricity. In this work, gasification is distinguished into two kind based on the operating temperature: conventional gasification (higher temperature) and biogasification (lower temperature).

Conventional gasification process is conducted in relatively extreme operating condition (T > 7000°C, P > 1 atm), which consumes more capital and operating cost. Biogasification has a rather mild operating condition and therefore consumes less cost. But on the other hand, the products are not diversified as much as conventional gasification and have slower reaction rate per feed mass unit. This work aimed to give perspective about biogasification process especially the one that uses lignite coal as feed.

Conventional Gasification

In conventional gasification, coal undergoes several consecutive processes:

Figure 1. Illustration of pyrolysis process in conventional gasification.

Figure 2. Illustration of the gasification process in conventional gasification.

1. The heating or drying process. Moisture content of the coal will be removed by this process.
2. The pyrolysis (or devolatilization) process occurs as the carbonaceous particle heats up. Volatiles are released and char is produced. The process depends on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.
3. The combustion process occurs as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions. Letting C represent a carbon-containing organic compound, the basic reaction here is C + 0.5 O₂ -> CO
4. The gasification process occurs as the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen, via the reaction C + H₂O -> H₂ + CO
5. CO from gasification process reacts with steam to yield carbon dioxide and hydrogen gas until equilibrium is reached. The equilibrium reaction is CO + H₂O = CO₂ + H₂

There are three types of contacting methods between coal and other reactants in conventional gasification: fixed bed, fluidized bed, entrained flow. Further information about these contacting methods can be seen in this article (article in Bahasa Indonesia).

Biogasification

Biogasification process (Fig. 3) is a highly complex microbial process. Although many microorganisms can be involved in these fermentative reactions, this work only talks about bacteria fermentation because of following reasons:

1. Bacteria cell has the second smallest size amongst microorganism (the first was virus, which has been known does not grow on coal) so they can penetrate better into micropores of the coal than other microorganism.
2. Bacteria is the microorganism that has been studied thoroughly as biocatalyst of biogasification process.

Figure 3. Schematic diagram of biogasification process. (A) hidrolytic process (B) acidogenic process (C) methanogenic process

In this process, organic compounds of coal are being degraded in three consecutive and parallel reactions, which are: hidrolytic, acidogenic, and methanogenic, which are explained below.

1. Hidrolytic bacteria, which are involve in the first step, degrade complex organic to simple organics, etc. These bacteria are usually obligate anaerobes in genera such as Bacteroides, Bifidobacterium, Eubacterium, etc. One that often used is Escherichia coli.
2. Acidogenic bacteria degrade the simple organics further. These bacteria are usually anaerobes too. An example of this group is Methanobacterium omelianskii.
3. Methanogenic bacteria are the one that produce methane and carbon dioxide. They are unicellular, Gram-variable, strict anaerobes that do not form endospores. Several species of methanogenic bacteria have been isolated, studied in pure culture. Some of notable species are Methanobacterium formicicum, M. bryantii, Methanobrevibacter ruminantium, Methanococcus halophillus.
   The microorganisms for the reaction can be obtained from anaerobic waste water treatment, preferably the one that treats coal slurry.

Assumption, Data, and Additional Information

1. The bases of calculation is 100 tones of coal
2. Physical properties (molecular weight, heat capacity, and density) of coal are taken by simulation (Aspen Hysys 3.2 ®) and calculation from available data. The values of the physical properties are displayed in Table 1.

Table 1. Properties of coal

Properties of coal	Value
Heat capacity (MJ/ton.K)	4.1816
Molecular weight	8.053643
Density (kg/m3)	1350

3. Conventional gasification syngas component properties, which are: mean % yield (mol/mol dry coal) and heat of combustion, are obtained from Klass and Geankoplis, respectively. The values are displayed in Table 2.
4. Raw gas compositions of the product gas from biogasification process are obtained by using data from Leuschner and calculating the mass balance. The process scheme is displayed in Fig. 4.

Figure 4. Schematic diagram of a biogasification plant

The detail of data and assumption that was taken to calculate the mass and energy balance from Figure 4 is mentioned below.

Pretreatment

1. Adding of water (solvent and steam) 10 times of lignite mass flow.
2. Adding of NaOH (0.17 b/b slurry) and H2O2 (1.8 b/b slurry).
3. Preheating to 250°C and 45 bar.
4. Retention time in reactor: 80 s.
5. Pretreatment Reactor Yield (DSS/feed) = 56.25%.
6. TDVS = 43.75 % of feed = 77.78 % of DSS.

Biogasification

1. There are two methods to fermented solubilized lignite: anaerobic packed column, and anaerobic halophilic in salty cavern.
2. Anaerobic packed column is chosen because:
   • has shorter retention times than salty cavern (24 hours to 16 days) to achieve same rate of methane production (1.2×108 SCFD)
   • has smaller reactor volume than salty cavern (2.3×107 gal to 3.68×108 gal)
   • Economic analysis by Leuschner shows that using anaerobic halophilic microorganism will cost more in capital, operation, and maintenance.
3. Operation condition of anaerobic digester: temperature 350C in atmospheric pressure.
4. TDVS is converted 100%.
5. Conversion of TDVS into CH4 = 40% of TDVS = 31.112 % of diluted soluble solid or 17.5 % from dry raw coal
6. Conversion of TDVS into TVA = 20% of TDVS = 15.556 of diluted soluble solid.
7. The rest of TDVS is converted into CO2.
8. Energy consumption is defined as energy that needed to conduct the highest operating temperature for each process.
9. Our parameter of comparison is amount of energy production and percentage of energy consumption per energy production.

Results

The result which is displayed in Table 3 showed us that biogasification process has the lowest % energy (consumption/production) but also has the lowest amount of energy production. This fact indicates that biogasification process is the most efficient process in energy usage but produce the least energy per unit mass of feed coal.

Conclusion

From the result we can conclude that biogasification is a prospective industry from energy efficiency point of view. The disadvantage of the process, which is small amount of energy production, can be reconsidered since the solid phase still can be utilized as feed for other processes like combustion reaction to produce additional energy. The calculation of this combustion process can not be done because of the lack of information in such process. It is suggested to do further research in biogasification of coal before establish it in large scale industry.

References

1. Klass, Donald L., 1998. Biomass for Renewable Energy, Fuels, and Chemicals. Academic Press.
2. Leuschner, A. P., Mark J.L., and Annette S. M., Biological Methane Production from Texas Lignite. Bioprocessing and Biotreatment of Coal, 109-130
3. Sasongko, D., 2006. Coal Utilization and Processing. Lecture Note of Department of Chemical Engineering, Faculty of Industrial Technology, Institut Teknologi Bandung.
4. Setiadi, Tjandra and Retno G. D., 2003. Pengelolaan Limbah Industri. Lecture Note of Department of Chemical Engineering, Faculty of Industrial Technology, Institut Teknologi Bandung.
5. Speight, J.G., 1994. The Chemistry and Technology of Coal. Marcell Dekker, Inc.