Bioenergy is gaining importance among the public, policy formulators and decision-makers due to high cost and insecurity related to fossil fuels, coupled with concerns associated with nuclear energy. An increasing global awareness about environmental issues is acting as the driving force behind the use of alternative and renewable sources of energy. The industrialized as well as the developing world is laying greater emphasis on the promotion of bioenergy to counter environmental issues keeping in view the provisions of the Kyoto Protocol.

The United States is currently the largest producer of electricity from biomass, with over 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 power is the largest source of renewable energy as well as an important part of the waste management infrastructure. 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, raw material from forests, household wastes, and wood. Biomass 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 processes of the pulp, paper, and paperboard industry.

The principal advantage of woody biomass is its low greenhouse gas emission characteristic. Biomass absorbs an equal amount of carbon in growing as it releases when consumed as a fuel, therefore it is not a contributor to global warming. Presence of low amount of sulphur than coal ensures less SO2 on combustion. Woody biomass is the most important renewable energy source if proper management of vegetation is ensured. The main advantages 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, national and international energy self-sufficiency.
• Techno-economic viable alternative to fast-depleting fossil fuel reserves,
• Reduction in GHGs emissions.
• Opportunities for local farmers, entrepreneurs and rural population as a means of sustainable development.

An Overview of Biomass Resources
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

Pulp and paper industry residues

The largest source of energy from wood is pulping or black liquor, a waste product from the pulp and paper. Pulping is the separation and breaking down of the lignin fibers of a plant in order to suspend the cellulose fibers to create paper. Logging and processing operations generate vast amounts of biomass residues. Wood processing produces sawdust and a collection of bark, branches and leaves/needles. In general, paper mills utilize their pulp residue to create energy for in-house usage, which consumes a vast amount of electricity in order to run.

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. Stands damaged by insects, disease or fire are additional sources of biomass. Harvesting operations usually remove only 25 to 50 percent of the volume, leaving the residues available as biomass for energy. Forest residues normally have low density and fuel values that keep transport costs high, and so it is highly economical to reduce the biomass density in the forest itself.

Agricultural or Crop Residues

Agriculture crop residues include corn stover (stalks and leaves), wheat straw, rice straw and processing residues such as nut hulls. Corn stover is a major source for bioenergy applications due to the huge areas dedicated to corn cultivation worldwide.

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.

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 use. Rapidly-growing, pest-tolerant, site and soil specific crops have been identified by making the best 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, miscanthus (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, corn oil, soybean oil, 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.

Direct Fired or Conventional Steam Boiler

Most of the woody biomass-to-energy plants use a direct-fired system or a 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.
In case of direct-firing, biomass is dried, sized into smaller pieces and then pelletized or briquetted. 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:

• Concentration of energy in the biomass feedstock.
• Easy handling, reduced transportation cost and hassle-free storage.
• Low-moisture fuel with good burning characteristics.
• 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.

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 that than building a new plant.
There environmental benefits of adding biomass to coal includes decreases 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.

Pyrolysis

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.  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 like petroleum to generate electricity. This 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 low lignin but 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.

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 counter-current 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.

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.

Conclusions

Biomass can play a major role in helping reduce reliance on fossil fuels by making use of efficient thermo-chemical conversion technologies. In addition, the increased utilization of biomass-based 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 systems, 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 b advantageous in terms of improved flexibility in response to fluctuations in biomass availability and lower investment costs.

The growth of the bioenergy industry can be achieved by laying more stress on green power marketing. Consumers should be given the choice to purchase power from renewable resources, such as biomass. Additionally, electricity consumers may be motivated to cover the incremental cost of renewable energy which will result in a greater level of investment in renewable energy technologies.