Woody Biomass Utilization for Power Generation – An Overview
Salman Zafar
Renewable Energy Advisor
Biomass power is the largest source of renewable energy as well as a vital part of the waste
management infrastructure. An increasing global awareness about environmental issues is
acting as the driving force behind the use of alternative and renewable sources of energy. A
greater emphasis is being laid on the promotion of bioenergy in the industrialized as well as
developing world to counter environmental issues.
Biomass may be used for energy production at different scales, including large-scale
power generation, CHP, or small-scale thermal heating projects at governmental, educational
or other institutions. Biomass comes from both human and natural activities and incorporates
by-products from the timber industry, agricultural crops, forestry residues, household wastes,
and wood. The resources range from corn kernels to corn stalks, from soybean and canola
oils to animal fats, from prairie grasses to hardwoods, and even include algae. The largest
source of energy from wood is pulping liquor or black liquor, a waste product from the pulp
and paper industry.
Woody biomass is the most important renewable energy source if proper management
of vegetation is ensured. The main benefits of woody biomass are as follows:

Uniform distribution over the world’s surface, in contrast to finite sources of energy.

Less capital-intensive conversion technologies employed for exploiting the energy
potential.

Attractive opportunity for local, regional and national energy self-sufficiency.

Techno-economically viable alternative to fast-depleting fossil fuel reserves.

Reduction in GHGs emissions.

Provide opportunities to local farmers, entrepreneurs and rural population in making
use of its sustainable development potential.
The United States is currently the largest producer of electricity from biomass having more
than half of the world's installed capacity. Biomass represents 1.5% of the total electricity
supply compared to 0.1% for wind and solar combined. More than 7800 MW of power is
produced in biomass power plants installed at more than 350 locations in the U.S., which
represent about 1% of the total electricity generation capacity. According to the International
Energy Agency, approximately 11% of the energy is derived from biomass throughout the
world.
Biomass Resources
Biomass processing systems constitute a significant portion of the capital investment and
operating costs of a biomass conversion facility depending on the type of biomass to be
processed as well as the feedstock preparation requirements. Its main constituents are systems
for biomass storage, handling, conveying, size reduction, cleaning, drying, and feeding.
Harvesting biomass crops, collecting biomass residues, and storing and transporting biomass
resources are critical elements in the biomass resource supply chain.
All processing of biomass yields by-products and waste streams collectively called
residues, which have significant energy potential. A wide range of biomass resources are
available for transformation into energy in natural forests, rural areas and urban centres.
Some of the sources have been discussed in the following paragraphs:
Figure 1: A host of natural and human activities contributes to the biomass feedstock
1. Pulp and paper industry residues
The largest source of energy from wood is the waste product from the pulp and paper
industry called black liquor. Logging and processing operations generate vast amounts of
biomass residues. Wood processing produces sawdust and a collection of bark, branches and
leaves/needles. A paper mill, which consumes vast amount of electricity, utilizes the pulp
residues to create energy for in-house usage.
2. Forest residues
Forest harvesting is a major source of biomass for energy. Harvesting may occur as thinning
in young stands, or cutting in older stands for timber or pulp that also yields tops and
branches usable for bioenergy. Harvesting operations usually remove only 25 to 50 percent of
the volume, leaving the residues available as biomass for energy. Stands damaged by insects,
disease or fire are additional sources of biomass. Forest residues normally have low density
and fuel values that keep transport costs high, and so it is economical to reduce the biomass
density in the forest itself.
3. Agricultural or crop residues
Agriculture crop residues include corn stover (stalks and leaves), wheat straw, rice straw, nut
hulls etc. Corn stover is a major source for bioenergy applications due to the huge areas
dedicated to corn cultivation worldwide.
4. Urban wood waste
Such waste consists of lawn and tree trimmings, whole tree trunks, wood pallets and any
other construction and demolition wastes made from lumber. The rejected woody material
can be collected after a construction or demolition project and turned into mulch, compost or
used to fuel bioenergy plants.
5. Energy crops
Dedicated energy crops are another source of woody biomass for energy. These crops are
fast-growing plants, trees or other herbaceous biomass which are harvested specifically for
energy production. Rapidly-growing, pest-tolerant, site and soil-specific crops have been
identified by making use of bioengineering. For example, operational yield in the northern
hemisphere is 10-15 tonnes/ha annually. A typical 20 MW steam cycle power station using
energy crops would require a land area of around 8,000 ha to supply energy on rotation.
Herbaceous energy crops are harvested annually after taking two to three years to
reach full productivity. These include grasses such as switchgrass, elephant grass, bamboo,
sweet sorghum, wheatgrass etc.
Short rotation woody crops are fast growing hardwood trees harvested within five to
eight years after planting. These include poplar, willow, silver maple, cottonwood, green ash,
black walnut, sweetgum, and sycamore.
Industrial crops are grown to produce specific industrial chemicals or materials, e.g.
kenaf and straws for fiber, and castor for ricinoleic acid. Agricultural crops include
cornstarch and corn oil; soybean oil and meal; wheat starch, other vegetable oils etc. Aquatic
resources such as algae, giant kelp, seaweed, and microflora also contribute to bioenergy
feedstock.
Thermo-chemical Conversion Technologies
There are many ways to generate electricity from biomass using thermo-chemical pathway.
These include directly-fired or conventional steam approach, co-firing, pyrolysis and
gasification.
1. Direct Fired or Conventional Steam Boiler
Most of the woody biomass-to-energy plants use direct-fired system or conventional steam
boiler, whereby biomass feedstock is directly burned to produce steam leading to generation
of electricity. In a direct-fired system, biomass is fed from the bottom of the boiler and air is
supplied at the base. Hot combustion gases are passed through a heat exchanger in which
water is boiled to create steam.
Biomass is dried, sized into smaller pieces and then pelletized or briquetted before firing.
Pelletization is a process of reducing the bulk volume of biomass feedstock by mechanical
means to improve handling and combustion characteristics of biomass. Wood pellets are
normally produced from dry industrial wood waste, as e.g. shavings, sawdust and sander dust.
Pelletization results in:
1. Concentration of energy in the biomass feedstock.
2. Easy handling, reduced transportation cost and hassle-free storage.
3. Low-moisture fuel with good burning characteristics.
4. Well-defined, good quality fuel for commercial and domestic use.
The processed biomass is added to a furnace or a boiler to generate heat which is then run
through a turbine which drives an electrical generator. The heat generated by the exothermic
process of combustion to power the generator can also be used to regulate temperature of the
plant and other buildings, making the whole process much more efficient. Cogeneration of
heat and electricity provides an economical option, particularly at sawmills or other sites
where a source of biomass waste is already available. For example, wood waste is used to
produce both electricity and steam at paper mills.
2. Co-firing
Co-firing is the simplest way to use biomass with energy systems based on fossil fuels. Small
portions (upto 15%) of woody and herbaceous biomass such as poplar, willow and switch
grass can be used as fuel in an existing coal power plant. Like coal, biomass is placed into the
boilers and burned in such systems. The only cost associated with upgrading the system is
incurred in buying a boiler capable of burning both the fuels, which is a more cost-effective
than building a new plant.
The environmental benefits of adding biomass to coal includes decrease in nitrogen
and sulphur oxides which are responsible for causing smog, acid rain and ozone pollution. In
addition, relatively lower amount of carbon dioxide is released into the atmospheres. Cofiring provides a good platform for transition to more viable and sustainable renewable
energy practices.
3. Pyrolysis
Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored
and transportable fuel, which can be successfully used for the production of heat, power and
chemicals. In pyrolysis, biomass is subjected to high temperatures in the absence of oxygen
resulting in the production of pyrolysis oil (or bio-oil), char or syngas which can then be used
to generate electricity. The process transforms the biomass into high quality fuel without
creating ash or energy directly.
Wood residues, forest residues and bagasse are important short term feed materials for
pyrolysis being aplenty, low-cost and good energy source. Straw and agro residues are
important in the longer term; however straw has high ash content which might cause
problems in pyrolysis. Sewage sludge is a significant resource that requires new disposal
methods and can be pyrolysed to give liquids.
Pyrolysis 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.
4. Biomass gasification
Gasification processes convert biomass into combustible gases that ideally contain all the
energy originally present in the biomass. In practice, conversion efficiencies ranging from
60% to 90% are achieved. Gasification processes can be either direct (using air or oxygen to
generate heat through exothermic reactions) or indirect (transferring heat to the reactor from
the outside). The gas can be burned to produce industrial or residential heat, to run engines
for mechanical or electrical power, or to make synthetic fuels.
Biomass gasifiers are of two kinds - updraft and downdraft. In an updraft unit,
biomass is fed in the top of the reactor and air is injected into the bottom of the fuel bed. The
efficiency of updraft gasifiers ranges from 80 to 90 per cent on account of efficient countercurrent heat exchange between the rising gases and descending solids. However, the tars
produced by updraft gasifiers imply that the gas must be cooled before it can be used in
internal combustion engines. Thus, in practical operation, updraft units are used for direct
heat applications while downdraft ones are employed for operating internal combustion
engines.
Figure 2: Schematic of updraft and downdraft gasifiers
Large scale applications of gasifiers include comprehensive versions of the small scale
updraft and downdraft technologies, and fluidized bed technologies. The superior heat and
mass transfer of fluidized beds leads to relatively uniform temperatures throughout the bed,
better fuel moisture utilization, and faster rate of reaction, resulting in higher throughput
capabilities.
Woody Biomass and Sustainability
Harvesting practices remove only a small portion of branches and tops leaving sufficient
biomass to conserve organic matter and nutrients. Moreover, the ash obtained after
combustion of biomass compensates for nutrient losses by fertilizing the soil periodically in
natural forests as well as fields. The impact of forest biomass utilization on the ecology and
biodiversity has been found to be insignificant. Infact, forest residues are environmentally
beneficial because of their potential to replace fossil fuels as an energy source.
Plantation of energy crops on abandoned agricultural land will lead to an increase in
species diversity. The creation of structurally and species diverse forests helps in reducing the
impacts of insects, diseases and weeds. Similarly the artificial creation of diversity is
essential when genetically modified or genetically identical species are being planted. Shortrotation crops give higher yields than forests so smaller tracts are needed to produce biomass
which results in the reduction of area under intensive forest management. An intelligent
approach in forest management will go a long way in the realization of sustainability goals.
Improvements in agricultural practices promises to increased biomass yields,
reductions in cultivation costs, and improved environmental quality. Extensive research in the
fields of plant genetics, analytical techniques, remote sensing and geographic information
systems (GIS) will immensely help in increasing the energy potential of biomass feedstock.
Bioenergy systems offer significant possibilities for reducing greenhouse gas
emissions due to their immense potential to replace fossil fuels in energy production.
Biomass reduces emissions and enhances carbon sequestration since short-rotation crops or
forests established on abandoned agricultural land accumulate carbon in the soil. Bioenergy
usually provides an irreversible mitigation effect by reducing carbon dioxide at source, but it
may emit more carbon per unit of energy than fossil fuels unless biomass fuels are produced
unsustainably.
Conclusions
Biomass can play a major role in reducing the reliance on fossil fuels by making use of
thermo-chemical conversion technologies. In addition, the increased utilization of biomassbased fuels will be instrumental in safeguarding the environment, generation of new job
opportunities, sustainable development and health improvements in rural areas. The
development of efficient biomass handling technology, improvement of agro-forestry systems
and establishment of small and large-scale biomass-based power plants can play a major role
in rural development. Biomass energy could also aid in modernizing the agricultural
economy. A large amount of energy is expended in the cultivation and processing of crops
like sugarcane, coconut, and rice which can met by utilizing energy-rich residues for
electricity production. The integration of biomass-fuelled gasifiers in coal-fired power
stations would be advantageous in terms of improved flexibility in response to fluctuations in
biomass availability and lower investment costs. The growth of the bioenergy industry can
also be achieved by laying more stress on green power marketing.