The combustion of biomass holds a number of significant advantages over other types of renewable generation, chiefly its predictable and reliable nature. Indeed, one of the key failings of many renewable energy systems is their inability to act as base-load capacity, a factor which has to some extent restricted widespread penetration of renewables on the grounds of their inability to load follow and concerns over transmission system stability.
 

However, despite not sharing this disadvantage, the low calorific value of most biofuels when compared with conventional fossil fuels frequently restricts the application of biomass generation to sites where there is an ample supply of combustible material - typically wood processing and paper mills, sugar cane mills and the like.

Wood-waste biomass plant at Malchin, Germany (source: Siemens Power Generation)

This issue is compounded by a lack of any kind of established biomass market of a kind that typifies many other resources such as oil and gas. Consequently, attempts to drive wider biomass uptake in Europe inevitably need to overcome issues such as local resource availability, and developments will, by necessity, be limited in size to a few hundred megawatts, a far cry from Europe’s largest thermal station, the coal fired 4 GW Drax power plant in the UK’s north Yorkshire.

Nonetheless, a second and perhaps more important advantage biomass holds over other renewables is its capacity for use in combined heat and power (CHP).

The high thermal efficiency achievable in CHP allows biomass to effectively compete on economic terms with more conventional thermal generation, while smaller-sized plants are also ideally suited to the effective use of heat energy in applications which, for instance, require process steam or district heating. On this basis, biomass appears to tick all the boxes of a low-carbon, energy efficient and above all, economic, source of generation.

Collectively biomass includes a wide range of materials that are either digested or gasified to produce methane or biogas, or those that are burned directly such as wood chips and pellets.

Biomass fuels can be roughly divided into two types. First-generation fuels refer to biofuels made from sugar, starch, vegetable oil or animal fats ,which are converted into useable fuels using conventional technology such as anaerobic digestion or other micro-biological processes, or simply burned directly in boilers or engines.

Second-generation biofuels, which are currently attracting significant interest and investment in research and development, include lignocellulosic materials like wood and straw, which are processed using advanced techniques such as gasification to produce fuels. Liquid fuels include biodiesel and vegetable oil, and alcohols such as ethanol, which may use syngas production techniques and subsequent gas to liquids processes.

Agricultural products specifically grown as biofuels in Europe include flax and rape seed, short rotation coppice woods such as silver birch, and miscanthus grasses. Elsewhere, sugar cane and palm oil are widely produced in South-East Asia and South America.

Industrial and domestic waste streams can also prove significant sources of biomass materials such as straw, timber, manure, rice husks, sewage, and biodegradable waste. In Germany, for instance, biomass is classified from forest wood (Class A1) to highly polluted demolition wood (Class A4).

In the European Union (EU), bio-energy resources such as forestry and agriculture crops, biomass residues and wastes currently provide around five per cent of all energy and 65 per cent of total renewable energy. In 2006, the EU collectively consumed 70 million tonnes of oil equivalent (Mtoe) of biomass, of which 18 Mtoe was used in electricity generation and 50 Mtoe on heating.

Under the current policy objectives this should rise to a total of 195 Mtoe by 2020, of which 62 Mtoe will be used for electricity generation and 90 Mtoe on heat production. Overall, the European Commission (EC) estimates that Europe-wide a biomass production capacity of some 235 Mtoe is achievable in 2020, without environmental damage.

However, as demand for biomass increases both fuel availability and cost are growing issues, and seasonality can also be an important consideration for biomass supply as many agricultural products are harvested at the end of summer. Consensus opinion suggests that ten to 15 per cent of total energy production may be the limit for biomass in Europe.

Meanwhile, transport and storage of low bulk density materials adds to the cost of the fuel, particularly as most biomass materials have to be stored under shelter in order to reduce decomposition, mould formation and other potential problems.

Major technologies for utilizing biomass materials range from straightforward internal combustion engines burning primary fuels such as biogas, ethanol, biodiesel or vegetable oil, to sophisticated circulating or bubbling fluidized bed systems and gasification technologies. Frequently the technology chosen for an individual system is predicated by the available resources.

Liquid biofuels based on rape seed, jatropha and palm oil, or animal fats are ideal for reciprocating engines that can act as spinning and back-up reserve with a start-up time of less than ten minutes.

Liquid fuels can also be efficiently used in diesel engines with a power capacity of up to about 20 MW. Although highly acidic vegetable oils are not favoured by engine manufacturers because this can cause corrosion problems in the fuel injection system. Biogases derived from landfill sites or digesters are also ideally suited for use in gas engine gensets.

A total fuel efficiency exceeding 85 per cent in cogeneration applications and around 45 per cent in simple cycle can be achieved. Furthermore, the investment required for such distributed generation systems is about the same as for large-scale generators at around €700/kW ($942/kW).

Power stations based on reciprocating engines may have multiple units, which can even be located across a large geographical area and combined into ‘virtual’ power plants. In addition, an attractive option is the use of dual-fuel engines, which can switch to gas or liquid fossil fuels when renewable fuels are not available.

The world’s largest palm oil fired power plant, a 100 MW development in Italy, is currently under development by Finland’s Wärtsilä and is due for comissioning this year. In addition, two 18V32 generating sets from Wärtsilä have generated 16 MW of power in the ItalGreen CHP plant in Monopoli, Italy since 2004 and a third engine was installed in 2005 increasing output to 24 MW.

A further six gensets are to be installed in Monopoli under the latest development, while Wärtsilä has a total of six liquid biofuel projects and a total of 400 MW under development in the country. The ItalGreen II plant will mainly burn imported palm oil from South-East Asia.

Conventional grate firing technology to generate steam from burning biomass is used in less flexible developments, which lack the rapid load response needed for substantial spinning reserve.

Nevertheless, they are an attractive way of utilizing solid biomass, and waste and grate systems can burn fuels that have relatively high moisture contents without requiring any auxiliary fuel. Grate technology is typically employed in small-scale wood-based generators, and while overall thermal efficiency is generally good, steam conditions are such that electrical efficiency for smaller units tends to be lower than with other technologies.

Furthermore, while these steam-based systems have the advantage of relatively low fuel costs, higher development costs of around €3000/kW installed leaves the technology most frequently employed in countries such as Finland and Sweden with large forestry-based industries and a high demand for process steam. As a general rule, the lower calorific value of biomass fuels and their relatively high moisture content as compared to fossil fuels is such that any plant requires much larger mass flow characteristics and is consequently more expensive and time consuming to develop.

Fluid bed technology - bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) systems - in which solid fuels are suspended on turbulent jets of air during the combustion process, largely replaced grate-fired technology in Scandinavia during the 1980s as different types of biomass were introduced to the market.

CFB plant at Elimchheim, Germany (source: Foster Wheeler)

The first units were relatively small BFBs, mainly designed for generating district heat or process steam at industrial sites, such as paper mills, with the secondary objective of waste disposal for high moisture content materials not easily burned with conventional methods like grate boilers.

During the past 20 years the technology has evolved allowing a wide range of fuels from bark to demolition wood and now refuse derived fuel and municipal solid waste burning as fuels with increased risk of slagging, fouling, and corrosion in the boiler created additional demands for the boiler design.

CFB has also gradually superseded BFB with its greater fuel flexibility and the possibility of using fossil fuels. The technology also allows easier emissions control, especially the case of carbon monoxide and nitrogen oxides (NOx) emissions as it burns fuel at temperatures of 750-900 °C, well below the threshold where NOx form. Typically, CFB plants will range from around 30-50 MW in capacity.

A still more recent development is the gasification fluidized bed combustion system. However, a significantly higher capital cost has so far limited this technology to a modest range of applications.

Perhaps contrary to expectation, often the most economic way to use biomass is co-firing in coal fired power stations as the overall efficiencies for utility coal fired plants are greater than smaller-scale dedicated biomass plants. Co-firing of biomass fuels is also an effective way to achieve net reductions in carbon dioxide, sulphur dioxide and NOx emissions in a coal fired plant, whilst retaining reliable large-scale generation.

Alstom, for instance, has developed a system for biomass combustion within existing coal fired boilers, which involves dedicated burners. This offers a significant increase in the proportion of biomass that can be co-fired and the range of biomass materials that may be used.

The attractiveness of co-firing has not been lost on developers and it should be noted, for example, that a swathe of recent applications for new coal fired generation stations in the UK are universally capable of firing at least ten per cent biomass. Even Drax is to be upgraded in a $200 million deal with Siemens that will allow it to co-fire up to ten per cent from renewable sources by the end of 2009.

Just a few months ago the EC set out a comprehensive package of measures in its new Energy Policy for Europe designed to combat climate change and boost the security of energy supplies.

The core objective of the new policy is for Europe to reduce greenhouse gas emissions by 20 per cent by 2020, rising to 60-80 per cent reduction by 2050, and within this an ambitious and binding target to achieve 20 per cent of overall energy by 2020 from renewable energy was established. Although each member state will be allowed to choose the best renewable energy mix for its own circumstances, along with other technologies this measure is all but certain to demand a massive expansion of electricity generation from biomass and biofuels across the member states.

In addition, the EU is in the process of establishing a regulatory framework to encourage the use of biomass in CHP installations, together with development of its agricultural and forestry policy to promote biomass production and availability. Within the Common Agricultural Policy a review of an energy crop scheme is planned, while a forest action plan and EU Structural funds are expected to support biomass utilization. The Commission is also expected to approve a biomass action plan later this year through the Renewables Directive which is due to be formally adopted by 2009. As part of this process member states are expected to produce national action plans for biomass by 2010.

Alongside wider European legislation individual members states have also induced policies, which support biomass. In May the UK released its Energy White Paper setting out energy policy for the coming decades, which included a number of measures to support generation from biomass.

Announcements in the White Paper include the publication of a Biomass Strategy, as well as a progress report on the Strategy for Non-Food Crops and Uses and proposed legislation to band the renewables obligation. A key measure is the removal of a cap on the amount of co-firing with biomass that will qualify for support under existing schemes, opening the door for major thermal generators to increase the volumes of biomass used. Furthermore, a distributed generation report is to be published by the end of 2008 including simplification of energy market and licensing arrangements for localised energy and clearer export tariffs for micro-generators.

In Germany, as with many EU states, biomass generation is also supported under a renewable energy support law the Erneuerbare Energien-Gesetz. Under German legislation, a subsidy starting at €0.087/kWh in 2002 and reducing ten per cent for each subsequent year, will be paid for the next 20 years for power from new plants restricted to 20 MW. Meanwhile, in Italy all power producers are required to supply at least 2.4 per cent of their power to the grid using renewables such as biomass.

Andris Piebalgs, European Energy Commissioner, recently cited countries such as Finland, Sweden and Austria which “successfully use some of their wood supply for energy.” A key factor in these countries has been co-ordination between forest owners, energy, wood-processing, harvesting and logistical industries, and public authorities, says Piebalgs.

As with EU policy, further action is expected to address regulatory and market barriers to wider uptake of biomass technologies, although some observers still complain of a lack of financial incentives for farmers to grow biomass crops across the EU and poor co-ordination on biomass policy between government departments across a number of states. In addition, concerns have also been raised that biomass products are transported across international borders to regions with more favourable support regimes, effectively negating any potential carbon dioxide benefit.

Along with national policy measures and schemes such as the EU Emissions Trading Scheme (EU ETS) and the wider Kyoto Protocol, the many potential commercial advantages that biomass generation offers has ultimately prompted utilities to develop new biomass projects.

Swedish energy major Vattenfall, for instance, recently announced a major investment programme worth more than €4 billion to develop projects to increase annual renewable generation by the company by approximately ten TWh by 2016. Of this, biomass projects are expected to contribute 0.5 TWh.

Co-firing has also enjoyed a rising profile over recent years, in part supported by carbon credits available under certain regimes such as the UK. Even so, perhaps the most important commercial aspect of biomass, as with any resource, are available volumes and price, and thanks to the number of new plants, the availability of wood and wood waste is becoming increasingly restricted in Europe. Robert Giglio, director of marketing for Foster Wheeler’s Global Power Group, argues that the development of a biomass fuel market, similar to those of other energy resources, will develop over time bringing a more open and transparent market for biomass products for energy use. “Nordic countries will probably be more progressive in that area, they have a large source of biomass and they have traditionally depended on that as a energy source,” says Giglio.

Logistics of biomass transport, storage and seasonality are also important factors, as are the technical abilities to handle, prepare and utilize the fuel in an efficient, low emission manner and the available subsidies that go with renewable generation.

Along with purely commercial considerations some concerns have also been raised regarding the sustainability of certain biofuels. RWE npower, the UK subsidiary of the German giant, recently announced a decision not to proceed with the planned conversion of an oil fired power station in the UK to run on palm oil on environmental grounds, for instance.

Meanwhile, the EC is currently working on a system to discourage biofuel production, which creates more greenhouse gas emissions than it saves and the conversion of land with high biodiversity value to grow feedstocks for biofuels. Co-firing existing thermal plants with biomass has also faced criticism from a number of quarters, notably on the grounds that burning biomass in inefficient and elderly coal fired boilers is a distortion of the plant’s true nature and effectively a waste of a valuable biomass resource that could be better employed in a CHP facility.

Nonetheless, the use of biomass as an energy source is perhaps one of the most mature technologies used by man, beginning with the controlled use of fire, and there remains a big market for biomass plants in Europe. Consequently, as it has always been so, there will be new biomass fuels and new technologies with which to exploit them.