The Thermal Decompositon of Biomass
Pyrolysis is the thermal decomposition of biomass occurring in the
absence of oxygen. It is the fundamental chemical reaction that is the
precursor of both the combustion and gasification processes and occurs
naturally in the first two seconds. The products of biomass pyrolysis
include biochar, bio-oil and gases including methane, hydrogen, carbon
monoxide, and carbon dioxide. Depending on the thermal environment and the
final temperature, pyrolysis will yield mainly biochar at low temperatures,
less than 450 degrees C, when the heating rate is quite slow, and mainly
gases at high temperatures, greater than 800 degrees C, with rapid heating
rates. At an intermediate temperature and under relatively high heating
rates, the main product is bio-oil.
Pyrolysis can be performed at relatively small scale and at remote locations
which enhance energy density of the biomass resource and reduce transport
and handling costs. Heat transfer is a critical area in pyrolysis as the
pyrolysis process is endothermic and sufficient heat transfer surface has to
be provided to meet process heat needs. Pyrolysis offers a flexible and
attractive way of converting solid biomass into an easily stored and
transported liquid, which can be successfully used for the production of
heat, power and chemicals.
Figure 1: Process conditions for pyrolysis of biomass
Feedstock for Pyrolysis
A wide range of biomass feedstocks can be used in pyrolysis processes. The
pyrolysis process is very dependent on the moisture content of the
feedstock, which should be around 10 percent. At higher moisture contents,
high levels of water are produced and at lower levels there is a risk that
the process only produces dust instead of oil. High-moisture waste streams,
such as sludge and meat processing wastes, require drying before subjecting
to pyrolysis.
The efficiency and nature of the pyrolysis process is dependent on the
particle size of feedstocks. Most of the pyrolysis technologies can only
process small particles to a maximum of 2 mm keeping in view the need for
rapid heat transfer through the particle. The demand for small particle size
means that the feedstock has to be size-reduced before being used for
pyrolysis.
Figure 2: A glance at feedstock availability and energy products from
biomass pyrolysis
Types of Pyrolysis
Pyrolysis processes can be categorized as slow pyrolysis or fast pyrolysis.
Fast pyrolysis is currently the most widely used pyrolysis system. Slow
pyrolysis takes several hours to complete and results in biochar as the main
product. On the other hand, fast pyrolysis yields 60 percent bio-oil and
takes seconds for complete pyrolysis. In addition, it gives 20 percent
biochar and 20 percent syngas. Fast pyrolysis processes include open-core
fixed bed pyrolysis, ablative fast pyrolysis, cyclonic fast pyrolysis, and
rotating core fast pyrolysis systems. The essential features of a fast
pyrolysis process are:
* Very high heating and heat transfer rates, which require a finely ground
feed.
* Carefully controlled reaction temperature of around 500oC in the vapour
phase.
* Residence time of pyrolysis vapours in the reactor less than one second.
* Quenching (rapid cooling) of the pyrolysis vapours to give the bio-oil
product.
Uses of Bio-Oil
Bio-oil is a dark brown liquid and has a similar composition to biomass. It
has a much higher density than woody materials which reduces storage and
transport costs. Bio-oil is not suitable for direct use in standard internal
combustion engines. Alternatively, the oil can be upgraded to either a
special engine fuel or through gasification processes to a syngas and then
bio-diesel. Bio-oil is particularly attractive for co-firing because it can
be more readily handled and burned than solid fuel and is cheaper to
transport and store. Co-firing of bio-oil has been demonstrated in 350 MW
gas fired power station in Holland, when one percent of the boiler output
was successfully replaced. It is in such applications that bio-oil can offer
major advantages over solid biomass and gasification due to the ease of
handling, storage and combustion in an existing power station when special
start-up procedures are not necessary. In addition, bio-oil is also a vital
source for a wide range of organic compounds and speciality chemicals.
Importance of Biochar
The growing concerns about climate change have brought biochar into
limelight. Combustion and decomposition of woody biomass and agricultural
residues results in the emission of a large amount of carbon dioxide.
Biochar can store this CO2 in the soil leading to reduction in GHGs emission
and enhancement of soil fertility. In addition to its potential for carbon
sequestration, biochar has several other advantages.
* Biochar can increase the available nutrients for plant growth, water
retention and reduce the amount of fertilizer by preventing the leaching of
nutrients out of the soil.
* Biochar reduces methane and nitrous oxide emissions from soil, thus
further reducing GHG emissions.
* Biochar can be utilized in many applications as a replacement for other
biomass energy systems.
* Biochar can be used as a soil amendment to increase plant growth yield.
Conclusions
Biomass pyrolysis has been attracting much attention due to its high
efficiency and good environmental performance characteristics. It also
provides an opportunity for the processing of agricultural residues, wood
wastes and municipal solid waste into clean energy. In addition, biochar
sequestration could make a big difference in the fossil fuel emissions
worldwide and act as a major player in the global carbon market with its
robust, clean and simple production technology.
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