This document discusses three methods for obtaining combustible gas from biomass materials: gasification, anaerobic digestion, and pyrolysis. Gasification involves converting biomass into a gas mixture at high temperatures with air. Anaerobic digestion is the decomposition of wet biomass through bacterial action without air to produce biogas. Pyrolysis thermally decomposes biomass absent of oxygen to produce gas and a solid residue.

1. Combustible Gas
From
Gasification, Anaerobic Digestion & Pyrolysis
There are two main methods which cover a wide area of biomass conversion
technologies, thermo chemical conversion and bio chemical conversion. To obtain
the energy, the combustion factor is the key for both technologies. Hardware
biomass conversion systems can be stationary or mobile. The hardware mobile
systems are usually used in rural areas supplying power for a small number of
homes, such as in a village, or for powering small to medium size countryside
businesses. However, the principle for both stationary and mobile hardware
combustion systems is similar.
The combustion can be made either using a furnace or a boiler. A furnace (direct
combustion) is one of the simplest methods used to obtain energy by burning the
biomass 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 used
to turn the turbine to generate electricity.
There are three different types of boilers:
1. Pile Burners
2. Stationary or Travelling Grate Combustors
3. Fluidized-Bed Combustors
‘Direct Firing’ can be divided into four different methods. These methods come
under 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 Gasification
and Micro Scale Biomass.
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2. Bomass technology conversions methods have been listed in Table 1, where
technologies, conversion process type, major biomass feedstock and energy / fuel
produced have been compared. Direct Combustion, gasification, pyrolysis and
methanol production all come under ‘thermo-chemical’ conversion process. On the
other 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 Waste
A number of uses can be made from biogas produced via anaerobic digestion or
pyrolysis. These are:
1. Fuel for internal combustion engines
2. To produce heat for commercial and domestic needs
3. As a transport fuel
The following are three different methods for obtaining gases, as a source of energy,
from biomass materials.
Gasification
Gasification is described as the process of converting the organic fraction of biomass
at higher temperatures and with the presence of air, into a gas mixture with fuel
value and more variation than the original solid biomass. This gas can be combusted
to produce heat and steam, and can be used in internal combustion engines or gas
turbines to produce electricity as well as mechanical energy.
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3. Reportedly, the production of electricity via gas turbines combined with steam cycles
is the most effective and economical use of the gaseous product. Several biomass
gasification processes have been developed (and/or under development) for
electricity generation that offer advantages over direct burning, such as higher
efficiency and cleaner emissions. Many of the gasification systems are currently at
the demonstration stage, and the development of these efficient systems for
electricity production is essential: BIGCC (Biomass Integrated Gasification and
Combined Cycle) and BIG-STIG (Biogas Integrated Gasification Steam Injected Gas
Turbine) plants can achieve efficiencies of 42–47%. Significant developments have
been made over the past fifteen years in the field of biomass gasification, especially
in the area of medium to large-scale electricity production. Gas cleaning to improve
the quality of gas is a crucial issue in both combustion and gasification systems, and
requires measures such as reduction of emissions and removing of particulates and
tars.
Air gasification net product can be expressed by summing up the partial reactions, as
illustrated in the following equation (Susta, et. al., 2003):
Carbohydrate matter (C6H10O5)+O2 CXHY+CLHMON+CO+H2+Heat
Anaerobic digestion
Anaerobic digestion is the decomposition of wet and green biomass through bacterial
action in the absence of air. Generally speaking, anaerobic digestion process is
made up of four main biological and chemical stages:
1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis
It usually has a mixed gas output of methane (CH4) and carbon dioxide (CO2), called
biogas. Landfill gas is the result of the anaerobic digestion of municipal solid waste
buried in landfill sites. The methane gas produced in landfill sites eventually escapes
into the atmosphere. However, the gas can be extracted by inserting perforated
pipes into the landfill.
There are a number of benefits related to anaerobic digestion; these can be
described under the environmental benefits, rather than on the technical or
commercial side. Anaerobic digestion decreases methane emissions and can provide
a good treatment system for organic waste and consequently can prevent
groundwater contamination and reduce odour from the local environment associated
with this waste.
‘The Government should review its current strategy for the anaerobic digestion
sector. In doing so, we recommend that it considers practical and financial
mechanisms for encouraging the expansion of the UK’s AD capacity, while ensuring
that new AD systems deliver the optimal balance between production of biogas and
prevention of uncontrolled methane emissions.’ (Biomass Task Force. 2005).
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4. Pyrolysis
In a temperature ranging from 300 to 700 °C and with the absence of oxygen, the
chemical decomposition of organic materials by heating is a process called pyrolysis.
However, in most cases and in practical terms the presence of oxygen cannot be
eliminated completely.
The final outcome of the pyrolysis process is that the organic materials are
transformed into gases and leave a solid residue (coke) made up from carbon and
ash. Biomass gasification can also be integrated with fuel cells. Also, using
pyrolysis, a solid biomass can be liquefied ‘direct hydrothermal liquefaction’ (USDE,
2005). One of the main benefits of flash pyrolysis is that fuel production has been
separated from power generation. This type of method is still at the demonstration
stage. As the development is still in the early stages, like the rest of the bio-oil
upgrading processes, there is still a need to neutralise negative aspects, such as
corrosivity and low heating value. In conjunction with the existing systems,
pyrolysis can be used for large scale electricity production.
Najib Altawell
References
Biomass Task Force (2005) Biomass task force report to the government. Department of environment, food and
rural 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 - Commercial
Reality and Outlook
http://www.powergeneration.siemens.com/NR/rdonlyres/FDD06929-8B80-49FA-B20B-A80294CDDBFC/0/4_Biomasse_Energy.pdf
2.5.2009
USDE (2005) Energy efficiency and renewable energy – biomass. http://www1.eere.energy.gov/biomass/pyrolysis.html#thermal
4.12.2007
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