Combustion is the most common way of converting solid biomass fuels to energy. Worldwide, it already provides over 90% of the energy generated from biomass, a significant part of which in the form of traditional uses for cooking and heating. Biomass of different forms can also be used to produce power (and heat) in small-scale distributed generation facilities used for rural electrification, in industrial scale applications, as well as in larger scale electricity generation and district heating plants. Two technologically mature and cost-attractive options involve burning biomass in standalone units or co-firing it with fossil fuels in standard thermal power plants. Biomass is an interesting option for electricity and heat production in parts of the world where supplies of residues from agriculture or the forest products industry are abundant. It can significantly lower GHG emissions and local air pollutants from power generation, contribute to improved energy security and general jobs and income in rural areas.
Combustion is the most common way of converting solid biomass fuels to energy. Worldwide, it already provides over 90% of the energy generated from biomass, a significant part of which in the form of traditional uses for cooking and heating. This is mostly the case in developing countries, where biomass combustion provides basic energy for cooking and heating of rural households and for process heat in a variety of traditional industries in developing countries. However, many of these traditional applications are relatively inefficient and be characterised by high indoor air pollution and unsustainable use of forests.
Biomass of different forms can also be used to produce power (and heat) in small-scale distributed generation facilities used for rural electrification, in industrial scale applications, as well as in larger scale electricity generation and district heating plants. Several feedstock and conversion technology combinations are available to produce power and combined heat and power (CHP) from biomass. Two technologically mature and cost-attractive options involve burning biomass in standalone units or co-firing it with fossil fuels in standard thermal power plants.
Standalone biomass combustion
Standalone biomass combustion can be done using different types of feedstock, sizes of applications and conversion routes.
a) Biomass-based generators: Vegetable oils, such as jatropha, can replace diesel in diesel generators to produce electricity for off-grid applications or independent mini-grids.
b) Biomass-based power plants: The heat produced by direct biomass combustion in a boiler can be used to generate electricity via a steam turbine or engine. The electrical efficiency of the steam cycle is not high but it is currently the cheapest and most reliable route to produce power from biomass in stand alone applications (IEA Bioenergy, 2009).
c) Biomass-based cogeneration (CHP) plants: Co-generation is the process of producing two useful forms of energy, normally electricity and heat, from the same fuel source. Co-generation significantly increases the overall efficiency of a power plant (and hence its competitiveness) if there is an economic application for its waste heat (IEA Bioenergy, 2009). In the case a good match can be found between heat production and its demand, combined heat and power (CHP) plants, also called cogeneration plants, can have overall (thermal + electric) efficiencies in the range of 80-90%. The process of using the heat from biomass combustion for industrial processes (e.g. for drying of products such as tiles), is well established in some industries, e.g., pulp and paper, sugar mills, and palm oil mills.)
d) Waste-to-energy plants based on Municipal Solid Waste (MSW): Municipal solid waste (MSW) is a very diverse and usually heavily contaminated feedstock, requiring robust technologies and strict controls over emissions, increasing the costs of waste-to-energy facilities, leading to MSW remaining a largely unexploited energy resource despite its significant potential in most countries (IEA Bioenergy, 2009).
Biomass co-firing (or co-combustion) involves “supplementing existing fossil-based (mostly pulverised coal) power plants with biomass feedstock” (IEA Bioenergy, 2009). The biomass fuels usually considered range from woody to grassy and straw-derived materials and include both residues and energy crops. The fuel properties of biomass differ significantly from those of coal and also vary considerably between different types of biomass. Properties of biomass which differ from those of coal are ash contents, a generally high moisture content, potentially high chlorine content, relatively low heating value, and low bulk density. These properties affect design, operation, and performance of co-firing systems (IEA Bioenergy, Task 32, 2002). There are three types of biomass co-firing:
a) Direct co-firing: The biomass is burnt directly in the existing coal furnace. Direct co-firing can be done either by pre-mixed the raw solid biomass (generally in granular, pelletised or dust form), with the coal in the coal handling system or by the milling it and directly injecting it into the pulverised coal firing system.
b) Indirect co-firing: The biomass is first gasified before the resulting syngas is combusted in the coal furnace; and
c) Parallel co-firing: The biomass is burnt in separate boilers, with “utilisation of the steam produced within the main coal power station steam circuits” (IEA Bioenergy, 2009).
Indirect and parallel co-firing options are designed to avoid biomass-related contamination issues, but have proven much more expensive than the direct co-firing approach as additional infrastructure is needed. Parallel co-firing units are mostly used in pulp and paper industrial power plants (IEA Bioenergy, 2009).
An advantage of biomass co-ﬁring is that it reduces greenhouse gas (GHG) emissions from coal-ﬁred power and enables power generation from biomass with the high efficiency achieved in modern, large-size coal-ﬁred power plants, which is much higher than the efficiency of dedicated, 100% biomass power plants. The total energy efficiency can be increased even further if biomass co-ﬁ ring takes place in combined heat and power (CHP) plants. The other advantage of biomass co-ﬁ ring is that the incremental investment for burning biomass in coal-ﬁ red plants is signiﬁcantly lower than the cost of dedicated biomass power. At present, co-ﬁring projects in coal-ﬁred power plants exceed the biomass capacity of dedicated biomass plants. A further advantage is that only a relatively small incremental investment is required to adapt existing coal-fired power plants into co-fired plants (IRENA, 2013).
Biomass combustion in small-scale application is gaining increasing attention as a means for rural electrification in developing country areas where extension of national grid would be too costly. The technology used for vegetable oil-based power production (e.g. diesel generators) is very well-known and requires little or no adaptation.
For industrial applications, direct co-firing in large-scale modern coal plants is today the most cost effective use of biomass for power generation. This technology only requires minor investment to adapt handling and feeding equipment without noticeably affecting boiler efficiency, provided the biomass is not too wet and has been pre-milled to a suitable size. Furthermore, electric efficiencies for the biomass-portion range from 35% to 45%, which is generally higher than the efficiency of biomass dedicated plants (IEA, 2007). A range of liquid biomass materials (e.g. vegetable oil, tallow) is also co-fired in existing plants on a commercial basis, however at a scale much lower than for the solid biomass. The biomass co-firing ratio is mainly controlled by the availability of biomass and is usually limited to around 5-10% on a heat input basis (IEA Bioenergy, 2009).
The most cost-effective biomass-to-energy applications are those relatively large scale (30-100 MWe), and using low cost feedstocks which are available in large volumes, such as agricultural residues (e.g. bagasse), or wood residues and black liquor from the pulp and paper industry. However, in a fragmented biomass supply market, the cost of purchasing large quantities of biomass may increase sharply as the distance to suppliers (and thereby logistical cost) increases. At the same time, an increasing number of viable smaller scale plants (5-10 MWe) using other types of residues are emerging throughout Europe and North America (IEA Bioenergy, 2009). In both cases, the biggest challenge is provision of a constant stream of biomass feedstock. Video 1 is an illustration of a biomass installation in the United Kingdom.
IEA Bioenergy (2009) sums up the critical issues in biomass logistics as:
- “The specific properties of biomass: low energy density, often requiring drying and densification; seasonal availability and problematic storage requiring further pre-treatment.
- Factors limiting the supply: availability and appropriateness of mechanized equipment; and inadequate infrastructure to access conversion facilities and markets.”
The main solutions to these issues, according to the IEA Bioenergy (2009) are “the development of advanced densification and other pre-treatment technologies, diversifying procurement geographically and in terms of biomass types, and the optimisation of fuel supply chains from field to plant gate (including the development of specialized harvesting and handling equipment), leading to lowest delivered costs” (IEA Bioenergy, 2009).
The sustainability of biomass-based technologies including biomass combustion depends on the current source of existing fossil fuel reserves and their reliability on one the hand and the risks involved with securing sufficient supplies of biomass over a long term, on the other hand (OECD/ IEA, 2007).
Local availability of large quantities of cheap biomass makes biomass co-ﬁring more economically attractive. If local sources are insufficient, high energy-density, pre-treated biomass (e.g. wood pellets) can be used. In these cases, long-distance transportation and logistics (e.g. an inland harbour) play an important role in the economic viability. In developing countries, the use of waste streams from agriculture and forestry may also create additional value and job opportunities while contributing to ruraldevelopment (IRENA, 2013).
Status of the technology
Both biomass-only combustion as well as biomass co-firing on small or large scale can be considered fully commercial and can be integrated with existing infrastructure. Over the past decade, direct co-firing has been successfully demonstrated with many technology options and with a wide range of biomass feedstocks (wood and herbaceous biomass, crop residues, and energy crops).
For industrial scale applications, in spite of the significant progress achieved in co-firing over the last decade, biomass properties pose several challenges to coal plants that may affect their operation and lifetime. On the other hand, “the desire to burn uncommon fuels, improve efficiencies, reduce costs, and decrease emission levels continuously results in improved technologies being developed” (IEA Bioenergy, Task 32, 2002).
Figure 1 gives an overview over the development status of a number of biomass combustion (and gasification) technologies
Market penetration in different regions / countries and future market potential
Biomass is an interesting option for electricity and heat production in parts of the world where supplies of residues from agriculture or the forest products industry are abundant. But the rapid development of second-generation liquid biofuel technologies to produce transport fuels could create competition for feedstocks between the two uses (IEA 2010).
Biomass combustion already provides around 12% of global energy requirements, including use for traditional cooking and heating. In 2006 biomass-based power and heat plants consumed a feedstock volume equivalent to 3.5 EJ, which represents a mere 7% of the global biomass used for energy purposes (IEA 2008). Consumption in the OECD countries accounted for 82% of this volume.
Worldwide, the installed capacity for biomass-based power generation was about 45 GW in 2006, with an estimated electricity production of some 239 TWh (IEA 2008). According to the IEA Bioenergy (2009), this power production occurs mostly in:
• “Co-firing plants for those countries with coal plants;
• Combustion-based CHP plants for countries that possess district heating systems (Nordic countries in Europe), large pulp and paper or food industries (e.g. Brazil, USA). At present, some 230 power and combined heat and power (CHP) plants use co-ﬁring, mostly in northern Europe and the United States (Platts, 2011b), with a capacity of 50-700 MWe. Co-ﬁ ring in CHP plants is currently the most competitive option to exploit the biomass energy potential for both electricity and heat production. Biomass feedstock’s include forestry and agriculture residues, animal manure, waste and dedicated energy crops.
• MSW incineration plants, although a large potential is still untapped;
• Stand-alone power plants where large amounts of residues are available (e.g. sugar-cane bagasse in Brazil);
• Anaerobic digestion units (e.g. in Germany) and landfill gas units (e.g. in the UK), as a result of increasingly strict environmental regulations on waste disposal and landfills at EU level”.
In the EU, 55 TWh of biomass-based electricity were produced in 2004, mainly based on wood residues and MSW. Finland prodiced 12% of its power consumption from biomass and wastes. In the United States some 85% of all wood process wastes (other than forest residues) are used for power generation (IEA Bioenergy, 2009).
At the same time, a proliferation of smaller-scale biomass-to-power or CHP projects has been ongoing in both developed countries and emerging economies. In these countries, biomass-based co-generation is well established in a number of agro-industries. China, Brazil, Latin America, Thailand, and India are all increasingly employing biomass power alongside other renewable resources (IEA 2007). In Asia, Indonesia, Thailand and Taiwan peat, wood chips, bark, vegetable oil and sludge are being directly co-fired with coal in industrial plants (IEA Bioenergy Task 32a, 2010). On the other hand, the CDM has supported the development of hundreds of biomass-based power generation projects of small and medium size (>35 MW) across the developing world, often using agricultural residues as main feedstock. The vast majority of these projects are located in Asia (>70%), followed by Latin America and only a few in Africa (IGES, 2010). Video 2 illustrates a biomass to electricity project in India.
Further expansion of biomass combustion and co-generation will be limited by availability of cheap feedstock, which depends on local collection ability and logistics and development of energy densification technologies for imported biomass.
Similarly to liquid biofuels for transport, biomass combustion and co-firing can have significant positive effects on economy, people and environment or represent a threat to all these aspects if implemented unsustainably and inequitably. However, in contrast to first generation biofuels, biomass combustion and co-firing today generally do not compete with food production, as they rely mostly on agricultural or wood residues.
Social development opportunities
- Increased income and jobs in the agriculture and forestry sectors, which now supply part of the feedstock used in power and heat production (agricultural and forest residues)
- Job creation in the industrial sector for designing, building and operating the plants.
- Increasing inclusion in the economic system: well-organized farmers unions can gain access to energy markets.
- Increasing energy security and saving foreign currency by reducing the dependence on imported fossil feedstock, such as coal.
- Diverting part of expenses for imported fossil fuels to farmers supplying the biomass feedstock;
- Diversifying the industrial sector;
- Supporting rural electrification with all its developmental benefits.
- Reduced GHG emissions from the power sector. Many agricultural and forest residues can be assumed to be carbon neutral, which leads to significant attributable GHG emission reductions.
- Reduced NOX and SOX emissions compared to coal combustion. NOx emissions can be further reduced by implementing primary and secondary emission reduction measures.
The most important consideration when collecting biomass residues for energy use is to not to exceed the biological requirements of the soil (part of residues must be left on the field and on the forest floor to return vital nutrients to the soil). On a macro level, competition between traditional forestry-based sectors (e.g. fiber board and pulp & paper) can develop as increasing amount of woody biomass is combusted or co-fired in power generating facilities.
Determining the costs for biomass combustion and biomass co-firing involved analysing several components, in particular investment costs, operations and maintenance, and fuel. The actual costs can very significantly however depending upon the specific site to be used, the existing installation (in the case of co-firing) and the cost of the biomass itself. The fuel cost itself is the most important factor in additional costs for co-firing in fact (IRENA, 2013).
Investment costs: dedicated Biomass power plants cost approximately $760-900/kW (IPCC, 2011) whereas the cost of retrofitting an existing coal-fired power plant ranges from $300-700/kW for direct co-firing (IPCC 2011; IEA 2012; IRENA 2012). Indirect co-firing investment costs are approximately 10 times greater at around $3000-4000/kW (ECN, 2012b).
Operation and maintenance costs: for co-firing O&M costs are similar to coal-fired plants. For direct co-firing they typically average 2.5-3.5% of capital costs (IRENA, 2012) and approximately 5% for indirect co-firing (ECN, 2012b). As the biomass-to-coal ration increase or the quality of the biomass used decreases, the O&M costs rise.
Biomass fuel cost: this includes both the cost of the feedstock and the cost of transportation, preparation and handling. Feedstock costs vary greatly with the biomass origin (e.g. dedicated cultivation or agriculture and forestry waste), type and composition (i.e. energy and moisture content). A recent IRENA study provides feedstock cost data for a range of locally available biomass resources in the United States, Europe, Brazil and India (IRENA, 2012). These costs range from $0-11/MWh for bagasse in Brazil and India to $6-22/MWh for agricultural residues in the United States and Europe. For large-scale co-ﬁ ring, operators have to turn to inherently more expensive dedicated energy crops (e.g. short rotation coppices) or international biomass trade if the regional infrastructure allows for this option. Particularly over long distances, the transportation costs depend to a large extent on the energy density (i.e. heating value) of the biomass fuel. Biomass pelletisation is a way to signiﬁcantly increase the heat value per volume of biomass. Over the last four years10, prices of industrial pellets ﬂuctuated between € 24-30/MWh, which is around € 12/MWh more than the cost of coal (Hawkins Wright, 2011). Taking both components into account, large-scale biomass co-ﬁ ring would typically exceed the cost of coal. In Germany, DENA (2011) assumes a premium of € 12/MWhe on top of coal, with future premiums ranging between € 0-20/MWhe in 2030, depending on coal prices. However, this price differential can be overcome if the price of CO2 emission allowances is sufficiently high.
[This information is kindly provided by the UNEP Risoe Centre Carbon Markets Group]
Project developers of biomass projects in the CDM pipeline apply a variety of different CDM methodologies due to a variety of different feedstock and sectors where biomass projects are applied. Methodologies include ACM6 “Consolidated methodology for electricity generation from biomass residues”, ACM3 “Emissions reduction through partial substitution of fossil fuels with alternative fuels or less carbon intensive fuels in cement manufacture”, AMS-I.A.: Electricity generation by the user and AM36 “Fuel switch from fossil fuels to biomass residues in heat generation equipment”.
CDM projects based on biomass represent 13.6% of all CDM projects in the pipeline. Biomass projects have been the main driving force of CDM project development in many developing countries where agriculture is the main industry and agricultural wastes are abundant. Of the 277 registered projects, 168 are small-scale projects.
IEA (2007): International Energy Agency, Energy Technology Essentials, ETE03, Biomass for power generation and CHP
IEA, 2010. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.
IEA Bioenergy Task 32 (2002): Biomass Combustion and Co-firing: An Overview, available online on http://www.ieabcc.nl/overview.html
IEA Bioenergy Task 32 (2005): Co-firing database, available online on http://www.ieabcc.nl/database/cofiring.php
IEA Bioenergy (2009): Bioenergy – a Sustainable and Reliable Energy Source, available online on http://www.ieabioenergy.com/LibItem.aspx?id=6479
IGES (2010): IGES CDM project database, available online on http://www.iges.or.jp/en/cdm/report_cdm.html
IEA/ IRENA (2013). Biomass Co-firing Technology Brief. IEA/ IRENA
OECD/ IEA (2007). Good Practice Guidelines, Bioenergy Project Development and Biomass Supply, OECD/IEA, Paris, France.