Biomass Gasification By Thermo-Chemical Conversion PDF

Fuel Engineering Biomass Gasification Thermo-Chemical Conversion By Dr. M. Salman Haider Biomass Gasification by Thermo-Chemical Conversion Objectives To explore the Biomass Gasification by Thermo-Chemical Process with the help of chemical reactions ...

Fuel Engineering Biomass Gasification Thermo-Chemical Conversion By Dr. M. Salman Haider Biomass Gasification by Thermo-Chemical Conversion Objectives To explore the Biomass Gasification by Thermo-Chemical Process with the help of chemical reactions BIOMASS GASIFICATION Thermo-Chemical Conversion  Introduction Gasification is the process in which the supply of oxygen is controlled and the raw materials are converted into chemical product called producer gas/ syngas This process transfers most of the chemically bound energy of the solid fuel into the gas phase. The oxygen supply will be 20 to 40 percent of the stochiometric air requirement. In combustion process, raw materials are directly converted into heat energy In gasification raw materials are converted into an intermediate chemical product, which constitutes mainly of carbon monoxide and hydrogen.  Combustion Combustion: rapid oxidation of a fuel Complete combustion: total oxidation of fuel (adequate supply of oxygen needed) Air: 20.9% oxygen, 79% nitrogen and other Nitrogen: (a) reduces the combustion efficiency (b) forms NOx at high temperatures Carbon forms (a) CO2 (b) CO resulting in less heat production  Gasification To convert solid carbonaceous fuel (biomass and coal etc.) into a gaseous mixture i.e. H2, CO CO2 and CH4. Gasification Steps Solid Biomass or Coal Liquid  Gasification  Gasification means converting solid fuel into gaseous fuel by Thermo Chemical process.  Biomass gasification means incomplete combustion of biomass resulting in production of combustible gases, consisting of (mainly): Carbon monoxide, CO, Hydrogen, H2 Methane, CH4 The mixture is called Producer Gas  Gasification The equipment in which the gasification process takes place it is known as Gasifier. Gasifiers are used to convert the solid fuel materials into producer gas. The calorific value of producer gas is 950-1200 kcal /m3. (Natural Gas calorific value 37.5-43.0 MJ/m3) The main components of producer gas are : Carbon Monoxide (13-19%), Hydrogen (18-22%), Carbon Dioxide (9-12%), Nitrogen (45-55%), Methane (1-5%) Water Vapour (4%). It has the thermochemical conversion efficiency of about 70  Gasifier Updraft Gasifier Gasifier Thermo-Chemical Process Gasified Power Generation Systems Gasified Power Generation Systems  Thermo-Chemical Process The solid biomass fuel is subjected to a series of thermochemical processes like drying, pyrolysis, oxidation and reduction. The resultant product is producer gas. Drying Zone Heat transfer from the lower parts causes drying of feedstock occurs in this zone. Biomass + Heat ⇢ Loss of water  Thermo-Chemical Process Pyrolysis Zone At this zone feedstock starts pyrolysing. Large molecules such cellulose, hemi cellulose and lignin break down into medium size molecules and char. Biomass + Heat ⇢ Carbon + Gas (CO, CO2 ,H2 , H2O, CH4 , tar vapors) Pyrolysis: Pyrolysis is the application of heat to raw biomass, in an absence of air, so as to break it down into charcoal and various tar gasses and liquids. It is essentially the process of charring. *Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation.  Thermo-Chemical Process Oxidation Zone Air is introduced at this zone. Reaction with oxygen are highly exothermic and result in a sharp rise of the temperature. Actual combustion of tar and charcoal takes place producing Carbon Dioxide and Water Vapour and Nitrogen. Reduction Zone The reaction products of the oxidation zone (charcoal and hot gases) move downward into the reduction zone. Carbon Dioxide is reduced to form CO and Hydrogen. 3C+2H2O+2CO2 +2H2 + Heat ⇢ 5CO+4H2 +H2O  Pyrolysis Biomass begins to rapidly decompose with heat once its temperature rises above around 240°C. The biomass breaks down into a combination of solids, liquids and gasses. The solids that remain commonly; we call charcoal. The gasses and liquids that are released we collectively call tars. The gasses and liquids produced during lower temp pyrolysis are simply fragments of the original biomass that break off with heat. Pyrolysis is the application of heat to biomass in the absence of air/oxygen. The volatiles in the biomass are evaporated off as tar gases, and the fixed carbon-to- carbon chains are what remains— otherwise known as charcoal.  Pyrolysis Pyrolysis: heat from the combustion zone is transferred via radiation, conduction, and convective hot streams to the surrounding biomass where oxygen/air is not sufficient or absent. Due to the heat, pyrolysis occurs to form CO2, CO, CH4, C2H4, H2O, char (C), and other organic solids and liquids as primary tar Pyrolysis Apparatus  Reduction Reduction is the process of stripping oxygen atoms off combustion products of hydrocarbon (HC) molecules, to return the molecules to forms that can burn again. Reduction is the direct reverse process of combustion. Combustion is the combination of combustible gases with oxygen to release heat, producing water vapor and carbon dioxide as waste products. Reduction is the removal of oxygen from these waste products at high temperatures to produce combustible gases. Reduction in a gasifier is accomplished by passing carbon dioxide (CO2) or water vapor (H2O) across a bed of red-hot charcoal (C).  Reduction Reduction: after the above two steps, hot reactants react in situ with the biomass and each other via a series of reactions. Water-gas reaction: C+H2O+118.5kJ/mol→CO+H2 Methanation reaction: C+2H2→CH4+87.5kJ/mol Steam reforming methanation: CH4+H2O+206kJ/mol→CO+3H2 Boudouard reaction: C+CO2+159.9kJ/mol→2CO Reverse water-gas shift reaction: CO2+H2+41.2kJ/mol→H2O+CO  The Reduction Reactions It’s the Heart of Gasification  Limitation of Producer Gas The main weakness of gasification by oxygen/air is due to a large portion of inert nitrogen in the agent (79–80%), which makes the resulted syngas diluted. It can be roughly estimated that syngas from this type of gasification mainly contains around 30–60% of nitrogen and 10–15% of CO2 since its heating value is typically between 4 and 6 MJ/m3 (for comparison, HHV of H2 = 12.76 MJ/m3, CO = 12.63 MJ/m3, CH4 39.76 MJ/m3 and CH4 is commonly much less than CO and H2). Low quality syngas is the main disadvantage of this technique for applications which require high temperature and steady operation, such as internal combustion engine, metallurgy, and melting glass industries. Five Processes of Gasification  Applications of Producer Gas Electric power generation from few kW to MW, either for local consumption or for grid power To operate diesel engine on dual fuel mode with 80-85% diesel replacement To operate the gas engine on 100% mode To operate water pumps for irrigation purposes Applications of Producer Gas  Biomass Gasification by Thermo-Chemical Conversion Summary This lecture has described in detailed Biomass Gasification by Thermo- Chemical route. It details the various zones of the Biomass Gasification Units with the help of major chemical reactions occurring due to heating of the biomass.