BIOMASS-BASED POWER GENERATION

Biomass is a fuel derived from plants and animals. The material classified as biomass can include wood and wood derivatives, a variety of agricultural and animal wastes, a part of urban waste, and some industrial wastes and by-products.

In the past biomass was the most important source of energy for virtually all regions of the globe with societies relying almost exclusively on fuels such as wood and charcoal for heating and cooking. The Industrial Revolution introduced new forms of energy including coal, oil, and eventually electricity, and these have taken the place of biomass across the developed world. However, biomass remains an important fuel in underdeveloped regions. According to the World Energy Council’s Survey of Energy Resources, biomass still supplies 50 EJ of primary energy each year or around 10% of global primary energy consumption. Most of this is used for heating and cooking.

Although biomass can be burned like coal in a furnace to provide heat for steam generation, the use of the fuel for power generation has not yet been widely adopted. Where biomass power generation plants do exist, they are often linked to industries that can supply them with fuel. So, for example, the wood- processing industry, which produces a variety of wood wastes such as sawdust, has a long history of biomass power generation, as does the paper industry. Agricultural wastes can also provide fuel for a local facility. In recent years global warming has raised the profile of biomass as a potential renewable source of energy, and with this interest, its use has begun to expand beyond the use of wastes to include specially grown biomass crops supported by a growing trade in biomass fuel.

Even with this new interest, the size of the biomass power generation industry remains limited by the availability of fuel. Wastes, though commonly avail- able, can only provide a limited global capacity, and if the technology is to expand significantly, large plantations of fuel crops will be required. Such plantations are beginning to appear, but in many cases thecompete with agriculture for land normally used for food production and this can lead to opposition. In a world where many people still do not have sufficient food for their everyday needs the environmental credentials of biomass crops have yet to be clearly established.

Meanwhile, global biomass power generation today consists of a relatively small number of power plants that burn biomass exclusively and some coal- fired power plants that burn a small amount of biomass with coal to reduce their net greenhouse gas emissions. Many of these plants of both types burn wastes and most of them are to be found in either the United States or Europe. The actual size of global biomass power-generating capacity is difficult to esti- mate because many of the plants are small. According to one report, the size of the global capacity of dedicated biomass plants in 2013 was 59 GW1 or 1.2% of global generating capacity. Around half of this capacity is located in Europe. There are indications that biomass-generating capacity growth is accelerating in response to global warming, but overall capacity remains small compared to solar- and wind-powered generation, the two major renewable resources in use today. Predictions for future growth vary widely but capacity could reach between 80 GW and 120 GW by 2020. Potentially, however, the industry could become much larger if biomass resources that have so far remained untapped were brought into use.

There are a number of different ways of converting biomass into energy. The simplest and most widespread is to burn the fuel in a furnace and use the heat produced to generate steam that drives a steam turbine. Most existing plants of this type are small and relatively inefficient but technologies such as biomass gasification can improve efficiency significantly, as can an increase in plant size. It is also possible to mix a proportion of biomass fuel with coal and burn it in a coal-fired power plant, a process called co-firing.

Another approach is to produce liquid biomass fuels either by fermentation of crops such as sugarcane or from oil-bearing crops such as sunflowers and rape seed. In principle, such fuels can be used in piston engine or gas turbine–based power generation systems. However, most of these liquid fuels are being used for transportation and that is likely to remain their main application in the near future.

Biomass is considered to be greenhouse gas neutral. This is because while the combustion of biomass will generate carbon dioxide just as would the combustion of a fossil fuel, when that biomass is regrown, it will reabsorb the same amount of carbon dioxide as it released during combustion. Thus, the growing, burning, and the regrowing of biomass simply cycles carbon dioxide between the atmosphere and biosphere.

From a renewable energy perspective biomass has a number of advantages compared to other renewable sources. One of the most important is that biomass generation, based on a combustion power plant, does not rely on an intermittent source of energy and so it can be controlled to provide power when it is needed,just like a conventional fossil fuel–fired power plant. Second, because biomass power generation is based on combustion technology, utilities are familiar with it and therefore comfortable with adopting it.

The main problem with biomass is that under most circumstances the economics of power generation are not favorable. Biomass combustion plants have similar costs to conventional fossil fuel–fired plants of the same type, but because they tend to be less efficient, overall production costs are higher. This is not a problem where there is a ready source of cheap fuel available, which is why industries like wood and paper have adopted it widely. When bio- mass fuel must be purchased the technology is often not competitive, although costs are improving. However, it can become more favorable when there are subsidies for renewable energy production and it is in regions where these are available that the technology is beginning to gain ground. The International Energy Agency has predicted that the use of biomass for electricity generation could double between 2010 and 2020 and that by 2035 it could be four times higher than in 2010, based on current conditions within the electricity industry. With more aggressive promotion of renewable energy, usage could conceivably rise significantly higher.

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