Читать книгу Handbook of Biomass Valorization for Industrial Applications - Группа авторов - Страница 55

Gasification is the thermal process that results in large quantities of gaseous products with lower char and ash formation. The major outcome of gasification of biomass is syn gas (CO/H₂) that can be utilized in Fischer Tropsch process to yield high value products. The reaction side products and contaminants present in biomass inhibits the catalyst and results in lower yields and selectivity of syn gas. Gasification is multi step endothermic process involving various sequential and parallel reactions. Gasification has high efficiency in chemical transform, low capital investment and distributed production. Generally, low molecular oxygenates produced during the gasification and enters the pore of the catalyst to produce gases such as CO₂, H₂, CH₄, CO, H₂O, and gaseous hydrocarbons [16, 28]. Routes to produce syn gas from biomass helps in the production of chemicals, biofuels, hydrogen and electricity. In general gasification consist of drying, pyrolysis, combustion and reduction [29]. The moisture in biomass is first removed by heating it at low temperature, then the temperature is raised to decompose the biomass to high molecular weight volatiles, tar and solid char in absence of oxygen. The products from the pyrolysis will be further oxidized to gaseous product, carbon monoxide, carbon dioxide and water. Then the reduction of gaseous products results in the production hydrogen in addition with CO, and CO₂. The formation of tar and solid char during the gasification reaction hampers the efficiency of turbine engine, hence various studies have been reported to clean up the product gas. The temperature range for the gasification is 800–900 °C at atmospheric pressure. The steam to carbon ratio varies from 0.8:1 to 1.5:1 during the gasification reaction to give hydrogen rich product gas. The tar content can be lowered by carrying out gasification in presence of catalysis that reduces the formation of tar by promoting the reforming, cracking, selective hydrogenation, and oxidation reaction [5]. The catalyst can be added in the step of gasification or can be used afterwards for the upgradation of the products, depending on their addition and role they are classified as in situ and ex situ catalytic gasification. The majorly employed catalysts are composed of alkali metal, transition metal and composite formulations. Among alkali metal catalysts sodium, potassium and caesium are found to be equally effective in lowering the combustion temperatures and increasing reaction temperatures. The highest activity of sodium is found in the range of 3 ⁎ 10^–4–1.5 ⁎ 10^–4 mol of alkali per gram of biomass. Different salts of potassium as KCl, CH₃COOK, K₂CO₃ were also found to be effective in improving the gasification efficiency. Vamvuka et al. [30, 31] further explored the activity of Li, Rb, Ca Na, and K containing catalysts in gasification of waste and trend obtained as follows: Li₂CO₃ > K₂CO₃ > CaCO₃ > Rb₂CO₃ > CaSO₄ > Cs₂CO₃ > Na₂CO₃. Calcined dolomite due its abundance, inexpensive and catalytic activity towards reducing the tar formation attracts the researcher. Various studies have been reported using dolomite as catalyst under batch as well flow conditions such as fixed and fluidized bed reactor. Calcined Dolomite also suffers the drawback of melting point which makes it unstable at higher temperature and doesn’t achieve tar conversion beyond 90–95%. The catalytic activity of noble metal (Rh/CeO₂) based catalyst lowered the gasification temperature to 600 C. RuO₂ showed good catalytic activity towards the production of hydrogen under supercritical water under conditions of 44 MPa, 450 °C and a residence time of 120 min [32, 33].