Combustible gas from gasification, anaerobic digestion and pyrolysis

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Information about Combustible gas from gasification, anaerobic digestion and pyrolysis

Published on May 11, 2011

Author: Altawell


Combustible Gas From Gasification, Anaerobic Digestion & PyrolysisThere are two main methods which cover a wide area of biomass conversiontechnologies, thermo chemical conversion and bio chemical conversion. To obtainthe energy, the combustion factor is the key for both technologies. Hardwarebiomass conversion systems can be stationary or mobile. The hardware mobilesystems are usually used in rural areas supplying power for a small number ofhomes, such as in a village, or for powering small to medium size countrysidebusinesses. However, the principle for both stationary and mobile hardwarecombustion systems is similar.The combustion can be made either using a furnace or a boiler. A furnace (directcombustion) is one of the simplest methods used to obtain energy by burning thebiomass materials in a chamber to obtain heat in the form of released hot gases.A boiler for biomass can be used to transform the heat into steam, this steam is usedto turn the turbine to generate electricity.There are three different types of boilers:1. Pile Burners2. Stationary or Travelling Grate Combustors3. Fluidized-Bed Combustors‘Direct Firing’ can be divided into four different methods. These methods comeunder the titles of Pile Burner, Spreader Stoker, Fluidized Bed and Suspension.The other method is Gasification, which can be divided into five different sub-branches, i.e. Biological Gasification, Landfill Gas, Pyrolysis, Thermal Gasificationand Micro Scale Biomass. 1

Bomass technology conversions methods have been listed in Table 1, wheretechnologies, conversion process type, major biomass feedstock and energy / fuelproduced have been compared. Direct Combustion, gasification, pyrolysis andmethanol production all come under ‘thermo-chemical’ conversion process. On theother hand, anaerobic digestion and ethanol production come under ‘biochemical’conversion process type. Finally, biodiesel production comes under ‘chemical’conversion process. Table1 Biomass technology conversion methods and materials used to obtain fuel/energy Technology Conversion Method Biomass Materials Used Fuel/Energy Anaerobic Digestion Biochemical Animal Manure, Agriculture Medium Btu gas (Anaerobic) Waste, Landfills, (Methane) Wastewater Biodiesel Production Chemical Rapeseed, Soy Beans, Biodiesel Waste Vegetable Oil, Animal Fats Direct Combustion Thermo-chemical Wood, Agricultural Waste Heat, Steam Municipal Solid Waste, Electricity Residential Fuels Ethanol Production Biochemical Sugar or Starch Crops, Ethanol (aerobic) Wood Waste, Pulp Sludge, Grass Straw Gasification Thermo-chemical Wood, Agricultural Waste Low or Medium- Municipal Solid Waste Btu Producer Gas Methanol Production Thermo-chemical Wood, Agricultural Waste Methanol Municipal Solid WasteA number of uses can be made from biogas produced via anaerobic digestion orpyrolysis. These are:1. Fuel for internal combustion engines2. To produce heat for commercial and domestic needs3. As a transport fuelThe following are three different methods for obtaining gases, as a source of energy,from biomass materials.GasificationGasification is described as the process of converting the organic fraction of biomassat higher temperatures and with the presence of air, into a gas mixture with fuelvalue and more variation than the original solid biomass. This gas can be combustedto produce heat and steam, and can be used in internal combustion engines or gasturbines to produce electricity as well as mechanical energy. 2

Reportedly, the production of electricity via gas turbines combined with steam cyclesis the most effective and economical use of the gaseous product. Several biomassgasification processes have been developed (and/or under development) forelectricity generation that offer advantages over direct burning, such as higherefficiency and cleaner emissions. Many of the gasification systems are currently atthe demonstration stage, and the development of these efficient systems forelectricity production is essential: BIGCC (Biomass Integrated Gasification andCombined Cycle) and BIG-STIG (Biogas Integrated Gasification Steam Injected GasTurbine) plants can achieve efficiencies of 42–47%. Significant developments havebeen made over the past fifteen years in the field of biomass gasification, especiallyin the area of medium to large-scale electricity production. Gas cleaning to improvethe quality of gas is a crucial issue in both combustion and gasification systems, andrequires measures such as reduction of emissions and removing of particulates andtars.Air gasification net product can be expressed by summing up the partial reactions, asillustrated in the following equation (Susta, et. al., 2003):Carbohydrate matter (C6H10O5)+O2 CXHY+CLHMON+CO+H2+HeatAnaerobic digestionAnaerobic digestion is the decomposition of wet and green biomass through bacterialaction in the absence of air. Generally speaking, anaerobic digestion process ismade up of four main biological and chemical stages:1. Hydrolysis2. Acidogenesis3. Acetogenesis4. MethanogenesisIt usually has a mixed gas output of methane (CH4) and carbon dioxide (CO2), calledbiogas. Landfill gas is the result of the anaerobic digestion of municipal solid wasteburied in landfill sites. The methane gas produced in landfill sites eventually escapesinto the atmosphere. However, the gas can be extracted by inserting perforatedpipes into the landfill.There are a number of benefits related to anaerobic digestion; these can bedescribed under the environmental benefits, rather than on the technical orcommercial side. Anaerobic digestion decreases methane emissions and can providea good treatment system for organic waste and consequently can preventgroundwater contamination and reduce odour from the local environment associatedwith this waste. ‘The Government should review its current strategy for the anaerobic digestionsector. In doing so, we recommend that it considers practical and financialmechanisms for encouraging the expansion of the UK’s AD capacity, while ensuringthat new AD systems deliver the optimal balance between production of biogas andprevention of uncontrolled methane emissions.’ (Biomass Task Force. 2005). 3

PyrolysisIn a temperature ranging from 300 to 700 °C and with the absence of oxygen, thechemical decomposition of organic materials by heating is a process called pyrolysis.However, in most cases and in practical terms the presence of oxygen cannot beeliminated completely.The final outcome of the pyrolysis process is that the organic materials aretransformed into gases and leave a solid residue (coke) made up from carbon andash. Biomass gasification can also be integrated with fuel cells. Also, usingpyrolysis, a solid biomass can be liquefied ‘direct hydrothermal liquefaction’ (USDE,2005). One of the main benefits of flash pyrolysis is that fuel production has beenseparated from power generation. This type of method is still at the demonstrationstage. As the development is still in the early stages, like the rest of the bio-oilupgrading processes, there is still a need to neutralise negative aspects, such ascorrosivity and low heating value. In conjunction with the existing systems,pyrolysis can be used for large scale electricity production.Najib AltawellReferencesBiomass Task Force (2005) Biomass task force report to the government. Department of environment, food andrural affairs (Defra) publications, London.Gunaseelan V.N. (1997) Anaerobic digestion of biomass for methane production: a review. Elsevier Science,Biomass and Bioenergy, Volume 13, Number 1, pp. 83-114 (32).Livingston W. L. (2007) Biomass ash characteristics and behaviour in combustion,gasification and pyrolysis systems. Technology & Engineering, Doosan Babcock Energy.Susta M. R., Luby P. ,Mat S. B. (2003) Biomass Energy Utilization & Environment Protection - CommercialReality and Outlook 2.5.2009USDE (2005) Energy efficiency and renewable energy – biomass. 4

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