Essay: Gasification

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It is the thermochemical conversion of organic matter at high temperatures with partial oxidation. Energy in organic matter produces combustible gases (CO, CH4 and H2) and minor products (char, water and condensable matter). The first step, called Pyrolysis, decomposes organic matter by heat to produce gaseous and volatile matter and char, which is non-volatile, containing high carbon content. The second step is when hot char reacts with the gasses (mainly CO2 and H2O) to form producer gas (CO, H2 and CH4). The produced gas is mixed with pollutants and thus has to be cleaned. This clean producer gas, when mixed with air, can be used in gas turbines, gas engines, gasoline or diesel engine. Its heating value is 4 — 20 MJ/m3. The heating value depends on carbon and hydrogen content of biomass, properties of gasifier and type of gasifier agent or oxidant (for example, air, pure O2, steam or mixture of these gases) (Figure 6).
Figure 4 Schematic of a 1 MW rice husk gasification & power generation system (XiuLi Yin, et al., 2002)
This schematic is further explained into its small system components in figure down, till end process.
Figure 5 Basic process steps of a biomass gasification plant (NGUYEN, 2014)
Figure 6 Design and operation of a CFB gasication and power generation system for rice husk(XiuLi Yin, et al., 2002)
This figure above shows the compositions of different producer gases at different temperature and gas heating value during gasification of rice husk. The optimum gas composition in the sample lies in a temperature range of 750 and 800 °C. however increasing the temperature increases the efficiency but imposes a high risk of fire in the engine.
Out of all its system components Gas cleaner, gas engines and waste water treatment are most crucial for environment and health hazard because gas like carbon monoxide when exposed in the plant due to corrosion, erosion caused by particulate (ash, char), alkali metals, tars, H2S and HCl, etc. can cause fatality in the surrounding.
Waste water system components
Figure 7 Aeration tank where contaminated water with ash, char and tars is first discharged(XiuLi Yin, et al., 2002)
Aeration -tank where contaminated water with ash, char and tars is first discharged and COD of water is reduced from as mush as 3.6 g/m3to a range between 300 and 1800 mg /m3. Then it flows to precipitation tank where heavy particulates settle down. Absorbing tank has husk ash to absorbers contaminants and to bring down at 100 mg /m3. At the end filters remove remaining ash and now water can be discharged openly or can be further treated for safe drinking. The gas cleaning process requires 7 to 8 t h−1 of water if not treated properly would create many environmental and human health problems and the whole process will not be green.
2.2 Direct Combustion
A direct combustion system has two main components — the biomass-fixed boiler (that produces steam) and the steam turbine (that generates electricity). The two common types of …

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