Solutions for Major Issues arising in Biomass Energy Projects

This article makes an attempt at collating some of the most prominent issues associated with biomass technologies and provides plausible solutions in order to seek further promotion of these technologies. The solutions provided below are based on author’s understanding and experience in this field.

  1. Large Project Costs: The project costs are to a great extent comparable to these technologies which actually justify the cause. Also, people tend to ignore the fact, that most of these plants, if run at maximum capacity could generate a Plant Load Factor (PLF) of 80% and above. This figure is about 2-3 times higher than what its counterparts wind and solar energy based plants could provide. This however, comes at a cost – higher operational costs.
  2. Technologies have lower efficiencies: The solution to this problem, calls for innovativeness in the employment of these technologies. To give an example, one of the paper mill owners in India, had a brilliant idea to utilize his industrial waste to generate power and recover the waste heat to produce steam for his boilers. The power generated was way more than he required for captive utilization. With the rest, he melts scrap metal in an arc and generates additional revenue by selling it. Although such solutions are not possible in each case, one needs to possess the acumen to look around and innovate – the best means to improve the productivity with regards to these technologies.
  3. Technologies still lack maturity: One needs to look beyond what is directly visible. There is a humongous scope of employment of these technologies for decentralized power generation. With regards to scale, few companies have already begun conceptualizing ultra-mega scale power plants based on biomass resources. Power developers and critics need to take a leaf out of these experiences.
  4. Lack of funding options: The most essential aspect of any biomass energy project is the resource assessment. Investors if approached with a reliable resource assessment report could help regain their interest in such projects. Moreover, the project developers also need to look into community based ownership models, which have proven to be a great success, especially in rural areas. The project developer needs to not only assess the resource availability but also its alternative utilization means. It has been observed that if a project is designed by considering only 10-12% of the actual biomass to be available for power generation, it sustains without any hurdles.
  5. Non-Transparent Trade markets: Most countries still lack a common platform to the buyers and sellers of biomass resources. As a result of this, their price varies from vendor to vendor even when considering the same feedstock. Entrepreneurs need to come forward and look forward to exploiting this opportunity, which could not only bridge the big missing link in the resource supply chain but also could transform into a multi-billion dollar opportunity.
  6. High Risks / Low pay-backs: Biomass energy plants are plagued by numerous uncertainties including fuel price escalation and unreliable resource supply to name just a few. Project owners should consider other opportunities to increase their profit margins. One of these could very well include tying up with the power exchanges as is the case in India, which could offer better prices for the power that is sold at peak hour slots. The developer may also consider the option of merchant sale to agencies which are either in need of a consistent power supply and are presently relying on expensive back-up means (oil/coal) or are looking forward to purchase “green power” to cater to their Corporate Social Responsibility (CSR) initiatives.
  7. Resource Price escalation: A study of some of the successful biomass energy plants globally would result in the conclusion of the inevitability of having own resource base to cater to the plant requirements. This could be through captive forestry or energy plantations at waste lands or fallow lands surrounding the plant site. Although, this could escalate the initial project costs, it would prove to be a great cushion to the plants operational costs in the longer run. In cases where it is not possible to go for such an alternative, one must seek case-specific procurement models, consider help from local NGOs, civic bodies etc. and go for long-term contracts with the resource providers.

Contributed by Mr. Setu Goyal (TERI University, New Delhi) who can be reached at setu.goyal@gmail.com

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Biomass Combined Heat and Power (CHP) Systems

Combined Heat and Power (CHP) is the simultaneous generation of multiple forms of useful energy (usually mechanical and thermal) in a single, integrated system. In conventional electricity generation systems, about 35% of the energy potential contained in the fuel is converted on average into electricity, whilst the rest is lost as waste heat. CHP systems use both electricity and heat and therefore can achieve an efficiency of up to 90%.

CHP systems consist of a number of individual components—prime mover (heat engine), generator, heat recovery, and electrical interconnection—configured into an integrated whole. Prime movers for CHP units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells.

A typical CHP system provides:

  • Distributed generation of electrical and/or mechanical power.
  • Waste-heat recovery for heating, cooling, or process applications.
  • Seamless system integration for a variety of technologies, thermal applications, and fuel types.

The success of any biomass-fuelled CHP project is heavily dependent on the availability of a suitable biomass feedstock freely available in urban and rural areas.

Rural Resources Urban Resources
Forest residues Urban wood waste
Wood wastes Municipal solid wastes
Crop residues Agro-industrial wastes
Energy crops Food processing residues
Animal manure Sewage

Technology Options

Reciprocating or internal combustion engines (ICEs) are among the most widely used prime movers to power small electricity generators. Advantages include large variations in the size range available, fast start-up, good efficiencies under partial load efficiency, reliability, and long life.

Steam turbines are the most commonly employed prime movers for large power outputs. Steam at lower pressure is extracted from the steam turbine and used directly or is converted to other forms of thermal energy. System efficiencies can vary between 15 and 35% depending on the steam parameters.

Co-firing of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels. Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Most forms of biomass are suitable for co-firing.

Steam engines are also proven technology but suited mainly for constant speed operation in industrial environments. Steam engines are available in different sizes ranging from a few kW to more than 1 MWe.

A gas turbine system requires landfill gas, biogas, or a biomass gasifier to produce the gas for the turbine. This biogas must be carefully filtered of particulate matter to avoid damaging the blades of the gas turbine.  

Stirling engines utilize any source of heat provided that it is of sufficiently high temperature. A wide variety of heat sources can be used but the Stirling engine is particularly well-suited to biomass fuels. Stirling engines are available in the 0.5 to 150 kWe range and a number of companies are working on its further development.

A micro-turbine recovers part of the exhaust heat for preheating the combustion air and hence increases overall efficiency to around 20-30%. Several competing manufacturers are developing units in the 25-250kWe range. Advantages of micro-turbines include compact and light weight design, a fairly wide size range due to modularity, and low noise levels.

Fuel cells are electrochemical devices in which hydrogen-rich fuel produces heat and power. Hydrogen can be produced from a wide range of renewable and non-renewable sources. A future high temperature fuel cell burning biomass might be able to achieve greater than 50% efficiency.

Conclusions

CHP technologies are well suited for Clean Development Mechanism (CDM) and sustainable development projects, because they are, in general, socio-economically attractive and technologically mature and reliable. In developing countries, cogeneration can easily be integrated in many industries, especially agriculture and food-processing, taking advantage of the biomass residues of the production process. This has the dual benefits of lowering fuel costs and solving waste disposal issues.

 

Woody Biomass Utilization 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. Short-rotation 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.

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 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 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-fueled 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.

Biomass CHP

Biomass conversion technologies transform a variety of wastes into heat, electricity and biofuels by employing a host of strategies. Biomass fuels are typically used most efficiently and beneficially when generating both power and heat through a Combined Heat and Power (or Cogeneration) system. Combined Heat and Power (CHP) technologies are well suited for sustainable development projects, because they are, in general, socio-economically attractive and technologically mature and reliable.

In developing countries, cogeneration can easily be integrated in many industries, especially agriculture and food-processing, taking advantage of the biomass residues of the production process. This has the dual benefits of lowering fuel costs and solving waste disposal issues. Prime movers for CHP units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. The success of any biomass-fuelled CHP project is heavily dependent on the availability of a suitable biomass feedstock freely available in urban and rural areas.

Primary Biomass Conversion Technologies – Thermochemical

A wide range of technologies exists to convert the energy stored in biomass to more useful forms of energy. These technologies can be classified according to the principal energy carrier produced in the conversion process. Carriers are in the form of heat, gas, liquid and/or solid products, depending on the extent to which oxygen is admitted to the conversion process (usually as air). The three principal methods of thermo-chemical conversion corresponding to each of these energy carriers are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air.

Conventional combustion technologies raise steam through the combustion of biomass. This steam may then be expanded through a conventional turbo-alternator to produce electricity. A number of combustion technology variants have been developed. Underfeed stokers are suitable for small scale boilers up to 6 MWth. Grate type boilers are widely deployed. They have relatively low investment costs, low operating costs and good operation at partial loads. However, they can have higher NOx emissions and decreased efficiencies due to the requirement of excess air, and they have lower efficiencies.

Fluidized bed combustors (FBC), which use a bed of hot inert material such as sand, are a more recent development. Bubbling FBCs are generally used at 10-30 MWth capacity, while Circulating FBCs are more applicable at larger scales. Advantages of FBCs are that they can tolerate a wider range of poor quality fuel, while emitting lower NOx levels.

Gasification of biomass takes place in a restricted supply of oxygen and occurs through initial devolatilization of the biomass, combustion of the volatile material and char, and further reduction to produce a fuel gas rich in carbon monoxide and hydrogen. This combustible gas has a lower calorific value than natural gas but can still be used as fuel for boilers, for engines, and potentially for combustion turbines after cleaning the gas stream of tars and particulates. If gasifiers are ‘air blown’, atmospheric nitrogen dilutes the fuel gas to a level of 10-14 percent that of the calorific value of natural gas. Oxygen and steam blown gasifiers produce a gas with a somewhat higher calorific value. Pressurized gasifiers are under development to reduce the physical size of major equipment items.

A variety of gasification reactors have been developed over several decades. These include the smaller scale fixed bed updraft, downdraft and cross flow gasifiers, as well as fluidized bed gasifiers for larger applications. At the small scale, downdraft gasifiers are noted for their relatively low tar production, but are not suitable for fuels with low ash melting point (such as straw). They also require fuel moisture levels to be controlled within narrow levels.

Pyrolysis is the term given to the thermal degradation of wood in the absence of oxygen. It enables biomass to be converted to a combination of solid char, gas and a liquid bio-oil. Pyrolysis technologies are generally categorized as “fast” or “slow” according to the time taken for processing the feed into pyrolysis products. These products are generated in roughly equal proportions with slow pyrolysis. Using fast pyrolysis, bio-oil yield can be as high as 80 percent of the product on a dry fuel basis. Bio-oil can act as a liquid fuel or as a feedstock for chemical production. A range of bio-oil production processes are under development, including fluid bed reactors, ablative pyrolysis, entrained flow reactors, rotating cone reactors, and vacuum pyrolysis.

Refuse-Derived Fuel (RDF) from Municipal Solid Wastes

  • Solid waste is a growing problem in all countries, and a critical problem in almost all the cities of the developing world. Developed countries have in recent years reduced the environmental impact of solid waste through sanitary landfills and high-temperature incineration, as well as conserving natural resources and energy through increased recycling, but the volume of waste generated in developing countries is rising astronomically. Very few cities have adequate solid waste collection and disposal systems, and the accumulating waste threatens health, damages the environment, and detracts from the quality of life. Therefore, it is necessary to make use of all possible waste management technologies to arrest the degradation of environment and foster waste-to-energy technologies.
  • Urban areas of Asia produce about 760,000 tonnes of municipal solid waste (MSW) per day, or approximately 2.7 million m3 per day. In 2025, this figure will increase to 1.8 million tonnes of waste per day, or 5.2 million m3 per day. These estimates are conservative; the real values are probably more than double this amount. Most common method of disposing of wastes is to dump them in low-lying areas on the outskirts of towns which is very haphazard and unscientific. This has serious environmental impacts like water pollution, methane emissions, and soil degradation.
  • The advantages of the refuse-derived fuel plant type are focused mainly on the relatively higher energy content of the RDF fuel, which originates from the pre-combustion separation processing.
  • RDF plant employs mechanical processes to shred incoming MSW separating the non-combustibles in order to produce a high-energy fuel fraction and thus improved efficiency.
  • One of the most appealing aspects of RDF is that it can be employed as a supplementary fuel in conventional boilers. Furthermore, RDF’s energy content is around half that of UK’s industrial coals and nearly two thirds that of low grade US coal.
  • Pelletization scores over mass-burning, anaerobic digestion and composting because the pellets’ energy content is close to that of coal and can be substituted in local industry.
  • A number of widely employed industrial and utility-scale coal utilisation technologies have the potential to co-utilise RDF and coal, such as large-scale pulverised coal-fired power plant boilers, cement kilns, fluidised bed or stoker-fired boilers, coal gasification plant
  • Due to reduction in fuel particle size and reduction in non-combustible material, RDF fuels are more homogeneous and easier to burn than the MSW feedstock.
  • RDF has been successfully burned in a variety of stoker boilers and in suspension as a stand-alone fuel in bubbling and circulating fluidized bed combustion technology boilers. It needs lower excess air and hence works at better efficiency. Also, handling is easier since non-combustibles have been already removed.
  • In utilizing MSW through a pelletization process, additional tonnage of recyclables will be positively selected to take to that market, assisting the developing regions to work towards sustainable development goals, while positively selecting appropriate materials to mix with purchased high BTU materials in the production of the high BTU pellets, that can be used either to replace coal or coke in industrial processes.
  • RDF is a much more uniform fuel than MSW with regard to fuel particle sizing and heating value resulting in a more efficient combustion process. In addition, a majority of the non-combustible material is removed from the RDF before the fuel is fed into the boiler which reduces the size of both the fuel and ash handling systems. These fuel characteristics result in a RDF boiler system which is generally less expensive than a mass-burn system, thereby offsetting the cost of the RDF processing equipment.
  • It is much easier to transport and store fuel pellets to power plants or industries than raw MSW.
  • Advanced thermal technologies like gasification, pyrolysis, and depolymerization are unattractive to developing countries due to their prohibitive costs.
  • Technologies to control the release of air contaminants have improved substantially in the past decade and that, therefore, the releases from modern RDF plants are not high enough to have negative impacts on human health.
  • Air emissions from incinerators should be compared with air emissions from other methods of generating energy. When such comparisons are made, it is found that natural gas power plants are the cleanest way to generate energy but that emissions from waste incinerators are equivalent to or less than the emissions from a coal or oil power plant.
  • Incinerator proponents also assert that the health risks from exposure to the releases from a RDF power plant are substantially less than the risks from many other common activities.

Peat as Biomass Fuel

Upon drying, peat can be used as a fuel. It has industrial importance as a fuel in some countries, such as Ireland and Finland, where it is harvested on an industrial scale. In many countries, including Ireland and Scotland, where trees are often scarce, peat is traditionally used for cooking and domestic heating.

In Ireland, large-scale domestic and industrial peat usage is widespread. Specifically in the Republic of Ireland, a state-owned company called Bord na Móna is responsible for managing peat production. It produces milled peat which is used in power stations. It sells processed peat fuel in the form of peat briquettes which are used for domestic heating. These are oblong bars of densely compressed, dried and shredded peat. Briquettes are largely smokeless when burned in domestic fireplaces and as such are widely used in Irish towns and cities where burning non-smokeless coal is banned.

In Finland, peat (often mixed with wood at an average of 2.6%) is burned in order to produce heat and electricity. Peat provides approximately 6.2% of Finland’s annual energy production, second only to Ireland. Finland classifies peat as a slowly renewing biomass fuel.

Woody Biomass and Energy Conversion Efficiency

Every energy conversion system wastes a portion of its input energy. For biomass to electricity conversion systems, 50% or more of the energy input can be lost – even up to 90% for some small-scale and alternative technologies. However, the energy rejected from a conversion system can often be used productively for industrial or residential heating purposes in place of burning fuels separately for that purpose. When this is done the overall efficiency can jump to 75-80%. Most systems must reduce their electricity production somewhat to make cogeneration feasible.

Thermal applications are the most efficient conversion technology for turning woody biomass into energy and should be considered in the development of a national Renewable Portfolio Standard (RPS). Thermal applications for woody biomass can be up to 90% efficient, compared to 20% for electricity and 50-70% for bio-fuels. Thermal systems can be applied at multiple scales, and are often more economically viable, particularly in rural and remote areas, than electrical generation.

By not including thermal energy, one of the most efficient uses of woody biomass energy is put at a disadvantage to generating electricity and processing liquid bio-fuels. This runs counter to the goals of displacing fossil fuels, promoting energy efficiency, and minimizing carbon emissions.

Woody Biomass 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. Co-firing 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 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.


Woody Biomass Resources

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:

Biomass Cycle
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.

Importance of Waste-to-Energy Plants

Waste-to-energy plants offer two important benefits of environmentally safe waste management and disposal, as well as the generation of clean electric power. Waste-to-energy facilities produce clean, renewable energy through thermal, biochemical and physicochemical methods. The growing use of waste-to-energy as a method to dispose off solid and liquid wastes and generate power has greatly reduced environmental impacts of municipal solid waste management, including emissions of greenhouse gases.

Waste-to-energy conversion reduces greenhouse gas emissions in two ways. Electricity is generated which reduces the dependence on electrical production from power plants based on fossil fuels. The greenhouse gas emissions are significantly reduced by preventing methane emissions from landfills. Moreover, waste-to-energy plants are highly efficient in harnessing the untapped sources of energy from a variety of wastes.

An environmentally sound and techno-economically viable methodology to treat biodegradable waste is highly crucial for the sustainability of\ modern societies. A transition from conventional energy systems to one based on renewable resources is necessary to meet the ever-increasing demand for energy and to address environmental concerns.