Types of biomass

The global biomass resource is the vegetation on Earth’s surface. This is equivalent to around 220 billion dry tonnes, or 4500 EJ (4500 × 10¹⁸ J), of energy according to the World Energy Council. Between one- and two-thirds of this (the proportion depends on the means used to estimate the amount of carbon fixed annually) are regenerated each year by photosynthesis. At the beginning of the 21st century, as already noted, biomass equivalent to 50 EJ was being used each year to provide energy, mostly from wood fuel for heating and cooking. Estimates suggest that between 200 EJ and 500 EJ could eventually be utilized for power generation. With primary energy demand expected to reach between 600 EJ and 1000 EJ by 2050, biomass sources could, in principle at least, provide a significant proportion of total demand.

From the perspective of power generation biomass can be divided into two categories: biomass wastes and energy crops. Biomass wastes are the most readily available forms of biomass but their quantities are limited. Energy crops, grown on dedicated plantations, are more expensive than wastes but they are capable of being produced in much larger quantities as and where required. Location is important because biomass has a lower energy content than coal and cannot be transported cost effectively over great distances. It is normally considered to be uneconomical to transport it more than around 100 km, although some biomass is traded over much greater distances today, confounding conventional economics. If energy plantations are established close to a biomass power plant, transportation costs can be minimized.

Biomass wastes

Biomass wastes can be divided into four categories: urban waste, agricultural waste, livestock waste, and wood waste. Urban biomass waste is a special category, available in relatively small quantities. It usually comprises timber waste from construction sites and some organic household refuse together with wood and other material from urban gardens. Most of this is cycled through an urban refuse collection and processing infrastructure where the biomass waste must be separated from the other refuse if it is to be burned as fuel. (However, more urban waste can be burned in a specialized power-from-waste plant and this may make separation of biomass waste from household refuse uneconomical.) While separation is an expensive process there is often a fee available for disposing of the waste and this helps keep fuel costs low.

Some organic urban waste ends up in landfill sites, although the use of landfills is being reduced in most developed regions because of the growing need to recycle. When organic waste is buried in this way it can generate methane through anaerobic decomposition underground. This methane is both an extremely potent greenhouse gas and a potential hazard, but it can be collected relatively easily and then either flared or used in a power generation plant to provide electricity.

Agricultural wastes, often referred to as agricultural residues, are one of the most important sources of biomass today. These are available throughout the world and they include a number of very important biomass resources. Across Europe and North America there are enormous quantities of wheat and maize straw produced each year. These farming residues are valuable fuels but they are seasonal and therefore require storing if they are to provide a year-round supply for a power station. Sugarcane processing produces a waste called bagasse at the processing plant where it can easily be utilized to generate electricity. Further waste, called trash, is left in the field but this can be collected and used too. Rice produces straw in the fields and husks during processing—both potential fuels. The shells and husks from coconuts can be used to generate electricity as can waste from oil palms, while the periodic recycling of oil palms and rubber trees (plantation trees have a life of 20–30 years) can provide wood waste for power generation. Many other crops from all parts of the world produce stalk waste that can be utilized. Indeed, wherever crops are grown and harvested there is normally some residual material that can be used as a source of energy.

There is one important caveat to the use of agricultural wastes as power plant fuel. From the perspective of sustainability it is important that some biomass material is returned to the soil after a crop has been harvested if the soil is to retain its fertility. If all the biomass material is removed, artificial fertilizers must then be used. This is likely to be considered an unsatisfactory trade-off both environmentally and from an energy balance perspective.

Livestock residues are another special category of biomass. While there is probably the equivalent of around 20–40 EJ of livestock residue generated each year, most of this is in the form of dung, which has a very low energy content and is not a cost-effective fuel for power generation (though it is used for heating in some parts of the world). It is only where livestock is farmed intensively that it becomes economical to utilize the waste, and then only when the operation is being carried out on a sufficiently large scale.

Dairy and pig farms fall into this category and it can be cost effective to use a biomass digester to convert the animal effluent into a biogas containing methane that can be burned in a gas engine to generate power. It is often not cost effective for small farms to install their own digesters, but in Denmark there are schemes where the waste from a number of small farms is collected and then processed centrally. Sewage farms that treat human waste are another potential source of methane-rich gas. Meanwhile, poultry farm residues have been used in combustion plants in the United Kingdom and United States.

Wood waste comes from three sources: material that can beneficially be removed from natural and managed forests to improve the health of the plantation, residues left in a forest after trees have been logged, and the waste produced during the actual processing of wood in sawmills and paper manufacturing plants. Process plant waste is the cheapest and most economical to utilize. Many sawmills and most modern paper plants burn their waste, producing heat and electricity for use in the facility. Any surplus power may be sold. Residues left after logging are generally expensive to collect and transport, but they have been utilized in situations where the demand for biomass fuel is high. Similarly, the removal of dead trees and undergrowth from natural forests, while improving their health and reducing the risk of fire, is an expensive process that only becomes cost effective if the value of the fuel is high.

Fuelwood

Fuelwood is the wood that is used for heating and cooking in many parts of the world and that still makes up a significant proportion of primary energy consumption, particularly in poorer areas of the world. Such usage is expected to continue well into the 21st century. In sub-Saharan Africa, for example, these fuels will still account for 42% of primary energy consumption in 2035 according to the International Energy Agency.

The use of fuelwood is also important in Asia, although consumption in India and China, two major users in the past, is expected to fall over the next two decades. Latin America and the Caribbean also use significant quantities. Many regions also use large quantities of charcoal, which is produced using traditional methods from fuelwood. Total fuelwood consumption was around 18 EJ in 2005 according to the World Energy Council.

In the future some of this wood could be utilized for power generation. The amount that would be available depends on the sustainability of the forests from which it is taken, but it could represent a significant resource for future renewable generation.

Energy crops

The large-scale development of biomass power generation will depend on a supply of fuel from energy crops. Wastes from the sources outlined above can only provide fuel for a limited generating capacity. Large-scale bioenergy farming is already carried out for the production of biofuels; sugarcane is produced in Brazil and maize in the United States, both for ethanol production, while oil-seed crops are grown in Europe and palm oil in the tropics, both of which provide oils. These liquid fuels are primarily used for transportation applications with ethanol added to gasoline and bio-oils used as diesel substitutes. Crops of a different sort are required for power generation. Such crops are being grown on a small scale in some parts of Europe and the United States, but if a significant industry is to be developed, then much larger areas of land will need to be devoted to such production.

An energy crop that is suitable for a power plant fuel must be fast growing, provide a high annual yield, and be relatively easy to harvest. A number of species have been tested and two groups appear to show the most promise for this purpose: fast-growing trees and grasses. Both of these can be grown relatively easily with minimal use of fertilizers, which are expensive, and both demand relatively little management once established.

Among the tree species that appear to show promise are several species that have traditionally been coppiced as a source of fuel domestic. Willows and poplars thrive in Europe, Scandinavia, and parts of the United States and Canada, and both species can be coppiced.

Coppicing is still the preferred means of harvesting these trees as power plant fuel. Once plantations are established, the growth is cut to the ground every three to five years leaving a stump from which new growth will appear. In warmer, more southerly climates other species such as sweetgum, cottonwood, sycamore, and eucalyptus have also shown promise. Some of these must be harvested using more traditional wood-cutting techniques.

Since wood from a single plantation cannot be harvested every year, a crop industry will require plantations that can be harvested in rotation so that fuel is available all the time. Typical yields from wood plantations are 10–11 dry tonnes/ha/y for willow and 8–13 dry tonnes/ha/y for poplar as shown in Table 15.1. This compares with a yield of 2.5 dry tonnes/ha/y for forest biomass.

Wood harvesting requires specialist equipment and new types of machinery are being developed for the purpose. Wood, when harvested, contains around 50% moisture and should be dried before combustion to offer the best energy content. Once dried, wood can be burned directly without need for processing other than sawing. However, wood is sometimes ground to sawdust and formed into pellets using a binder. The resulting fuel is more expensive than untreated wood but does offer a standard product for combustion.

Grasses grow rapidly each year, providing an abundant supply of biomass that can be harvested annually in the autumn when the stalks are virtually dry. Under these conditions the water content of grasses is around 15%, much lower than for freshly harvested woods. Grasses that show promise as crops include switchgrass and miscanthus. The grasses will grow in a variety of habitats but are best suited to natural grasslands such as the U.S. prairies or grassy regions in other parts of the world. Annual yields for switchgrass are 8–14 dry tonnes/ha, as shown in Table 15.1.

The harvesting of grasses is more straightforward than for wood and can be carried out with harvesters and balers similar to those used for cereals. The energy density of loose grass is much lower than wood, so grass is generally formed into briquettes or pellets before transportation. In this form, grass will have around 95% of the energy density of wood.

Table 15.2 shows figures for the calorific value of a range of biomass fuels. As-harvested wood contains around 10 GJ/tonne, rising to 19 GJ/tonne once it has been dried. Cereal straw contains 15 GJ/tonne, while the energy content of switchgrass is 16 GJ/tonne and for miscanthus it is 16 GJ/tonne. Undried, coppiced willow has an energy content of 10 GJ/tonne typical of an undried wood. In comparison, bituminous coal generally contains 27–30 GJ/tonne and oil 42–45 GJ/tonne.

The combustion of grasses normally produces more ash than the combustion of wood. Against this, the content of trace elements such as chlorine, nitrogen, potassium, and sulfur are often lower, making emission control simpler.

Growing and harvesting a biomass combustion fuel is only the first stage of complex infrastructure that will be required to support the long-term and large-scale development of biomass for power generation. Since harvesting will often be seasonal, fuel must be stored to provide a year-round supply and transportation networks need to be established to keep power plants supplied. This will require extensive integration of the agriculture and energy industries.

The key issue to be solved, if development is to proceed, is how much land can be put aside for fuel production and how much must be retained for food production. Difficulties are already arising in the tropics where rain forests are being destroyed to make room for palm oil plantations. Balancing the two will be difficult, but there does appear to be areas of land not used or not suitable for food production that could provide combustion fuel. These include some northern forest regions.

The areas of land required to supply a biomass power plant are considerable. On the basis that one hectare of land can produce 10–12 tonnes of dry fuel each year, a 10 MW power plant would require around 7000 ha dedicated to supply it with fuel.

Biomass trade

The relatively low energy density of biomass as combustion fuel means that it is generally not considered economical to move it over long distances. In spite of this, there is a growing trade in biomass combustion fuel in the form of pellets, particularly in Europe. The driving force for the trade is the push toward renewable generation in Europe and the various subsidies available to support it. This includes support for biomass power generation in the form of feed-in-tariffs of tax relief that can make it cost effective to purchase wood pellet fuel for a biomass power plant.

The main sources for wood pellets in Europe are the United States and Canada with a growing supply from Russia. There is a nascent export industry in Australia and New Zealand, and South Africa also supplies pellet fuel to Europe. Meanwhile, Canada sends pellets to Japan, a country with few natural energy resources that must import all of its fossil fuel as well.

The size of the pellet industry remains small but it has the potential to grow if current incentives continue. There is also potential for a market similar to that in Europe opening in the United States. There are two primary sectors of the pellet market: the residential and the industrial. In the United States most pellets are consumed residentially, but in Europe the big market driver is industrial use by power utilities.

Biomass energy conversion technology

There are a number of ways of converting biomass into electricity but the most common is to burn the material in a furnace, raising steam that is used to drive a steam turbine, an approach analogous to the use of coal in a coal-fired power plant. The main alternative to this is biomass gasification in which the fuel is converted into a combustible gas that can be burned to provide heat. The development of coal gasification systems is expected to drive improvements in biomass gasification.

Biomass plants tend to be small compared to conventional coal-fired plants and less efficient. However, efficiency can be improved if they can provide heat as well as electricity in a combined heat and power plant, and this configuration is common, particularly in industries that use their own biomass waste for power generation. A further method that allows for more efficient use of biomass is co-firing. This involves adding a proportion of biomass to the coal in a coal-fired power plant. Large modern coal plants can operate at high efficiency and, when co-fired, biomass is converted into electricity with similar efficiency.

For some animal wastes it is also possible to generate a combustible gas using anaerobic digestion. This is the same process that occurs in landfill sites and generates methane. Digesters can be designed to generate methane from both animal and human wastes. The gas is then normally used to fire a gas engine to provide electric power.

Direct firing

The direct firing of biomass involves burning the fuel in an excess of air inside a furnace to generate heat (Figure 15.1). Aside from heat the primary products of the combustion reaction are carbon dioxide and a small quantity of ash. The heat is absorbed by a boiler placed above the main furnace chamber. Water flows through tubes within the boiler where it is heated and eventually boils, producing steam that is used to drive a steam turbine. Direct-firing technology was developed in the 19th century for coal combustion but has been adapted to other fuels including biomass. While the heat from a biomass furnace is normally used to generate steam in this way, it may be exploited directly in some industrial processes too.

The simplest type of direct-firing system has a fixed grate onto which the fuel is piled and burned in air that enters the furnace chamber from beneath the grate (underfire air). Further air (overfire air) is then added above the grate to complete the combustion process. This type of direct-firing system, called a pile burner, can burn wet and dirty fuel but its overall efficiency is only around 20% at best. The fixed grate makes it impossible to remove ash except when the furnace is shut down, so the plant cannot be operated continuously either, a further disadvantage for this design.

An improvement over the pile burner is the stoker combustor. This type of combustor allows fuel to be added continuously, either from above (overfeed) or from below (underfeed), and has a mechanism for continuously removing ash. The stoker grate was first developed for coal combustion in the 1920s and in the 1940s the Detroit Stoker Co. designed a stoker boiler for wood combustion.³

Fuel is distributed more evenly in a stoke grate than in a pile burner, allowing more efficient combustion. Air still enters the furnace from beneath the grate and this air flow cools the grate. The airflow determines that maximum temperature at which the grate and thus the furnace can operate and this in turn determines the maximum moisture content of the wood that can be burned, since the dampest wood will require the highest temperature if spontaneous combustion is to be maintained.

For an underfeed stoker combustor, fuel is added from beneath and the grate behaves like a slowly erupting volcano. Ash, when it is formed, falls down the flanks of the fuel pile and is removed from the sides. In an overfeed stoker combustor, the grate itself normally moves via some form of chain mechanism allowing fuel to be added from one side and ash removed from the other (Figure 15.2). This type of design is called a mass-feed stoker. A second type, called a spreader stoker, disperses finely divided fuel pneumatically across the whole surface of the grate with finer particles burning in the region above the grate. There are a number of other refinements to the stoker combustor such as an inclined and water-cooled grate. Even so, maximum overall efficiency is only 25%.

Most modern coal-fired power plants burn finely ground coal that is fed into the power plant furnace through a burner and then ignites in midair inside the furnace chamber, a process called suspended combustion. It is possible to burn biomass in this way but particle size must be carefully controlled and moisture content of the fuel should be below 15% (Figure 15.1). Suspended combustion of biomass, while it can provide a higher efficiency, is not widely used in dedicated biomass power plants. However, it does form the basis for co-firing, which is discussed at greater length in the following section.

The main alternative to the stoker combustor for modern direct-fired biomass plants is the fluidized bed combustor. This type of combustor can cope with fuels of widely differing type and quality, making it much more versatile than a stoker combustor. The fluidized bed contains a layer of a finely sized refractory material, such as sand, that is agitated by passing air through it under pressure so that it becomes entrained and behaves much like a fluid. Fuel is mixed with this refractory material where it burns (when the bed is at its operating temperature) to release heat as in a conventional furnace. Depending on the pressure of the air that is blown through the fluidized bed it will be a bubbling bed, behaving much like a boiling fluid, or a circulating bed in which the particles are entrained with the air and those that escape the boiler are subsequently captured in a cyclone filter and recycled. Fuel content within the bed in usually maintained at around 5%.

Fluidized beds can burn a wide range of biomass fuels with moisture content as high as 60%. For low-quality fuels the bubbling bed is preferred, whereas the circulating bed is better for high-quality fuels. Overall efficiency is again only 25% at best, similar to a stoker combustor.

Where the fuel quality is low, gas or coal can be used to raise the bed temperature sufficiently at startup for combustion to commence. An additional advantage of the fluidized bed is that material can be added to the bed to capture pollutants like sulfur that would otherwise result in atmospheric emissions.

Direct-fired biomass power plants typically have a generating capacity around 25–50 MW. This small size, combined with the relatively low combustion temperature in the furnace (biomass is more reactive than coal and so tends to burn at a lower temperature), are the two main reasons for these plants’ low efficiencies compared to coal plants where overall efficiencies above 40% are now common in new facilities.

Improvements are possible. Increasing the size of the typical plant to 100–300 MW would allow larger, more efficient steam turbines to be used, and several 300 MW are being planned in Europe. New small steam turbines that incorporate advanced design features currently found only in large coal plant turbines will also improve efficiency. Adding the ability to dry the biomass fuel prior to combustion can result in a significant increase in performance. With these changes, direct-fired biomass plants should be able to achieve 34% efficiency.

co-firing

Much more efficient conversion of biomass into electricity can be achieved quite simply and on a relatively large scale in another way—by the use of co-firing. Co-firing involves burning a proportion of biomass in place of some of the coal in a coal-fired power plant. Since most coal stations operate at much higher efficiencies than traditional direct-fired biomass plants, co-firing can take advantage of this to achieve conversion efficiency of 40% or more in a modern high-performance coal-fired facility.

There is another form of co-firing in which a predominantly biomass combustion plant uses a fossil fuel, normally natural gas, to both stabilize and supplement the biomass fuel. This technique will normally be used in a dedicated biomass plant to increase performance and flexibility.

Co-firing of the first type is attractive to coal plant operators because it allows them to burn biomass and therefore reduce their net carbon dioxide emissions with very little plant modification. Biomass co-firing can also reduce sulfur emissions because biomass contains virtually no sulfur. Since coal-fired plants can burn large quantities of biomass this also offers a means of establishing the biomass infrastructure needed to enable biomass power generation to develop into a large-scale industry.

The most efficient and common type of coal-fired power plant in operation is the pulverized coal (PC) plant that burns coal that has been ground to a fine powder. Plants of this design can burn up to 10% biomass with little modification to their plant. Biomass is simply mixed with the coal before it is delivered to the coal mills where the mixture is ground prior to injection into the combustion chamber. For a 1000 MW power plant this would be equivalent to a 100 MW biomass plant, but with much higher efficiency than a dedicated plant.

Simple co-firing of this type is limited by the type of biomass fuel that can be used and by the proportion of co-firing possible. To overcome both these limits another approach is to have a dedicated biomass fuel delivery line. Fuel from this line can either be mixed with the powdered coal before combustion or delivered to dedicated biomass burners in the furnace. The latter are usually located lower down the combustion chamber allowing a longer transit time to completely burn the biomass fuel. This will allow a plant to burn up to 15% biomass by heat content. Higher proportions are possible but generally require greater plant adaptation and consequently more expense.

Co-firing has become common in the United Kingdom where most plants burn 5–10% biomass and is becoming so in the United States with plants generally using 10% biomass. At this level, boiler efficiency can fall by up to 2%. Such plants will typically burn wood wastes and some will use wood pellets.

An alternative to conventional co-firing involves gasification of the biomass in a dedicated biomass gasifier attached to the coal plant. The combustible gas is then burned in the coal-fired furnace. This approach is more expensive than the simple co-mixing of fuels but it avoids some of the problems that can be associated with conventional co-firing and allows a greater proportion of biomass to be burned.

A third approach is called parallel co-firing. This involves having a separate biomass furnace, with the hot gases generated by combustion of biomass being mixed with the hot coal-flue gases. In a variation of this approach, the biomass has its own steam-raising boiler too with the steam flows blended before entering the steam turbine. Again this is more costly than conventional co-firing.

Conventional co-firing remains the favored approach but it does have its problems. Some wastes such as sawdust can block fuel feed systems and need to be avoided. Other biomass fuels such as grasses have a high alkali content that can cause problems in coal-fired boilers. Biomass is also more volatile than coal when burned so much of the combustion takes place higher in the combustion chamber and more overfire air may be necessary to ensure complete combustion. Additionally, the ash from a co-fired boiler has a different composition to that from a plant that burns only coal. Coal plant ash is often used in various ways by the building industry but the reuse of the ash from a plant that burns biomass has led to regulatory difficulties in the past.

Biomass gasification

Gasification involves the heating of biomass in a reducing atmosphere with the addition of water vapor to generate a gas that contains around 40% hydrogen and carbon monoxide with most of the remainder being nitrogen. The composition of a typical wood gas from a gasifier is shown in Table 15.3. Further reaction can convert the carbon monoxide into more hydrogen if required and the gas will usually be cleaned before combustion. Wood gas has a low calorific value between 4 MJ/m³ and 19 MJ/m³; by comparison the calorific value of natural gas is around 38 MJ/m³.

There are two types of gasifier used for biomass gasification: a fixed bed and a fluidized bed. The fixed bed is the simplest type, comprising a cylindrical vessel with a grate at the bottom, as shown in Figure 15.3. Biomass is introduced at the top and air from the bottom, and the heat for the gasification process is provided by the partial combustion of the biomass. Ash is then removed from the bottom of the gasifier. Fixed-bed gasifiers are relatively primitive and inefficient and are generally only used at capacities of 5 MW of less.

The alternative, a fluidized-bed gasifier, is similar to the fluidized-bed combustor described previously but is operated with a reduced supply of air so that combustion cannot go to completion (Figure 15.4). The heat content of the gas from both types of gasifier is often less than 6 MJ/m³. The low-quality gas from a fixed-bed gasifier has limited use and will normally be burned to raise steam to drive a steam turbine. The gas from a fluidized-bed gasifier can potentially be utilized in a gas turbine, a reciprocating engine, or a fuel cell too.

One of the potentially most interesting gasification configurations is the biomass-based integrated gasification combined cycle (IGCC) plant. Plants of this type have been developed for coal combustion and form one of the future options for carbon dioxide capture and storage from coal-fired power stations. In a biomass IGCC plant, biomass is gasified, cleaned, and burned in a gas turbine. The hot exhaust gases from the gas turbine are then used to raise steam to drive a supplementary steam turbine. With close integration of the plant components, an overall efficiency of 45% might be achieved. A further advantage of this type of plant is that since biomass is carbon neutral, no carbon dioxide capture is necessary.

Alternative ways of exploiting low-quality biogas include the use of an organic Rankine cycle turbine similar to that used to generate electricity from low-grade geothermal reservoirs or of a Stirling engine similar to that used in some solar thermal plants. If the mixture of carbon monoxide and hydrogen is converted into just hydrogen it could also be used in a fuel cell. Some high-temperature fuel cells can burn carbon monoxide as well as hydrogen, making this conversion potentially unnecessary.

As outlined earlier, another way of exploiting biomass gasification is to feed the output of the gasifier directly into the combustion chamber of a coal-fired power plant, a form of co-firing. This avoids many of the problems associated with utilizing a low-grade gas but is an expensive co-firing configuration.

Fuel handling

Fuel handling is important for an effective biomass power plant. Since most biomass fuels are seasonal, there must be facilities to store large quantities if power plants are to be supplied throughout the year. This is particularly significant for grasses that will all be harvested in the autumn. These will normally be formed into briquettes and then stored until needed, but if this is to be the primary source of fuel for a power plant, a store capable of holding one year’s supply will be needed.

Woods can be harvested at different times of the year and this can help with the fuel supply management. Depending on the harvesting techniques the wood will arrive at the power station or storage depot in the form of bundles of coppiced branches, chipped wood, or whole trees. Wood is easier to chip when green so this will normally be carried early in the harvesting process, probably before it reaches the power plant. The latter will generally need to be able to store several weeks’ supply of fuel under conditions where it is protected from the weather so that it does not get wet. Otherwise, the fuel will deteriorate and lose energy content.

The efficiency of a biomass power plant will depend on the moisture content of the fuel. The lower the moisture content, the higher the efficiency. For example, a reduction in moisture content from 50%, as harvested, to 10% after storage can result in boiler efficiency rising from 70% to 83%. It is important, therefore, to allow wood to dry before it is burned.

The form in which fuel is actually supplied to the power station will vary. Most often it will be chipped although the use of pellets is expanding. For plants that use suspension firing (e.g., in co-firing with coal) the chips must be further reduced in size using grinding equipment. There is also a novel approach in which whole trees are supplied to a specially designed power plant. The trees are dried for 30 days and then sections are delivered to the boiler that is designed especially for this form of fuel. Efficiencies of 34% have been predicted for a 150 MW plant of this type.⁴

Biomass digesters

Biomass will ferment naturally in the absence of air to produce a gas that is rich in methane. This is the process that occurs underground in landfill waste disposal sites and it can also be found in the lakes of large hydropower plants when there is a large quantity of biomass immersed beneath the waters. The same process can be harnessed in biomass digesters to produce a methane-rich gas from wastes. The technique is normally applied to agricultural animal wastes.

Biomass digestion is only cost effective for large farming operations, most usually on dairy or pig farms where the slurry produced by the animals must be treated to prevent it causing an environmental hazard. Digesters of differing sophistication are available depending on the size of the farm. For small farms the most suitable is usually a lagoon digester, essentially a pond (the lagoon) into which the slurry is placed. The lagoon is covered with an impermeable membrane cover that is used to collect the emitted gas. The slurry must contain less than 2% solids and the lagoon must usually be maintained above 30 °C, which limits the application to warmer climates since it is not economic to heat a lagoon digester.

A more sophisticated system is the tank digester (Figure 15.5). Slurry is loaded into a tank that is fitted with a stirring mechanism to mix the contents evenly. The tank can be heated to keep the fermentation at the optimum temperature. Tank digesters can handle slurries with 3–10% solids. For slurries with higher solids content a plug flow digester is preferable. This has three elements: a mixing tank, digester tank, and settling tank. The slurry is first fed into the mixing tank from where it enters the digester tank, which contains heating pipes to maintain the ideal temperature. The material moves slowly across the digester tank, with fermentation proceeding to completion in about 20 days as it crosses the tank. After 20 days it passes into a settling tank where the remaining solid material is removed and can be used as fertilizer.

The gas from an anaerobic digester has a heating content of 22 MJ/m³, suitable to be burned in a reciprocating engine to generate electricity and heat. However, the capital cost of such systems is high and can only be supported when there is a large quantity of waste to ferment. Similar systems can be used to treat municipal sewage waste and they form an effective means of both rendering it harmless and producing a valuable by-product.

Most biomass digester-based power generation plants are relatively small with capacities of 100 kW or less. Landfill gas sites, which can produce large volumes of methane, can sometimes support gas engines with generating capacities of 20–30 MW.

Liquid fuels

There are a range of liquid biofuels that are manufactured from biomass sources. These include bio-alcohols made from the fermentation of crops rich in starch or sugar and biodiesel, which is derived from oil-producing crops.

One of the most important liquid biofuels today is ethanol, which produced in Brazil from sugarcane and in the United States from maize. In both cases the fuel is blended with gasoline so that up to 10% of vehicle fuel may be bioderived. There is a growing ethanol production industry in Europe too where other crops such as wheat may be used as the feedstuff. Some biomethanol is also produced.

Europe is the main region in which biodiesel is produced and used. The fuel is derived from crops such as sunflowers or oil-seed rape. It can also be produced from animal wastes and some palm oil is imported into Europe for this use. As with ethanol, biodiesel is blended with diesel fuel for transportation. A European Biofuel directive from 2003 called for 5% of biofuel to be blended with diesel by 2010. By 2012 the actual proportion was 4.5%. A further directive intended that this should rise to 10% by 2020, but a recent policy change means that food-derived biofuel, which accounts for most of that produced, will be limited to 5%. This change reflects the still unresolved issue of how to balance fuel and food production.

Ethanol and biodiesel are considered first-generation biofuels. Scientists are currently trying to develop second-generation fuels that are made from nonfood sources such as cellulose and algae. If these processes can be developed effectively, then the impact of liquid biofuel production on food production should be much smaller.

While most of the liquid biofuel manufactured is being used for transportation fuel, both types of biofuel can also be used to generate electric power. The most suitable generators are reciprocating engines, but gas turbines can also burn biofuels and they can be used to fire a boiler to raise steam.

Cost of biomass power generation

A biomass-fired power station is technically similar to a coal-fired power plant and the economics of the two are based on similar principles. In both cases the cost of the electricity generated depends on two factors: the cost of building the plant and the cost of operating the plant. The first of these is usually dominated by the cost of the actual installation, although the cost of any loan required to finance the project can also have a significant impact. The second depends mostly on the cost of the fuel.