Bill Livingston of Doosan Babcock provides an overview of the technical and other issues associated with co-firing of biomass materials in large coal fired power stations, and discusses the more advanced direct injection co-firing systems that can enable higher co-firing ratios.

Bill Livingston, Doosan Babcock, UK

The latest predictions from the International Energy Agency (IEA) and others indicate that by 2030 the worldwide demand for coal will be almost double current levels, and that close to 50 per cent of global power production will be from coal fired generation1. The market demand will increasingly be for high efficiency, clean coal power generation, with biomass co-firing capability and, in the longer term, with the capability to capture and store carbon dioxide.

The principal driver for the increasing demand for the capability to co-fire biomass materials in new and existing coal boiler plants is that co-firing is regarded as representing a very attractive option for biomass utilization and for the delivery of renewable energy, in terms of the capital investment requirement, security of supply, power generation efficiency and generation cost.

Biomass screw feeder used with a direct injection system
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This is recognized, for instance, by IEA Bioenergy (2006), in the EC Biomass Action Plan (2005), and by a number of European Union member states and other governments, which have introduced specific policy instruments to encourage energy recovery from biomass and, in some cases, co-firing activities at both existing and future coal fired power plants.

Available biomass materials in Northern Europe

The principal solid biomass materials that have been available in Northern Europe in the quantities relevant to co-firing in large coal power plants have included:

  • Surplus cereal straws and other baleable, dry agricultural residues
  • Forestry and sawmill/pulp and paper industry residues
  • The solid residues of the large-scale processing of agricultural materials, such as vegetable oils, grains, coffee and cocoa
  • Energy crops, which include short rotation coppice wood and other perennial crops that are grown specifically as fuels.

Technological options for co-firing biomass

There are a number of basic technical options available for the direct co-firing of solid biomass materials in large pulverized coal fired power stations:

  1. Pre-mixing the biomass with the coal, normally on the main coal conveyors, the feeding of the mixed fuel into the coal bunkers and the further processing of the mixed fuel through the existing coal milling and firing equipment.
  2. The modification of one or more of the existing coal mills to mill the biomass material on its own, and the firing of the milled material through the existing pulverized coal pipework and burners.
  3. The installation of new, dedicated biomass milling equipment, either on or off site, and the pneumatic injection of the milled fuel into the existing coal firing system, or through new, dedicated biomass burners.

The indirect or parallel options for biomass co-firing, i.e. those that involve the installation of a separate biomass gasifier or biomass boiler, have been implemented in a small number of cases in Europe, but there has not, as yet, been a great deal of interest in the wider use of these techniques.

Co-firing by pre-mixing and co-milling

To date, the majority of the biomass co-firing activity in Europe has been by pre-mixing the biomass with the coal in the coal handling system, and processing the mixed fuel through the installed coal mills and firing equipment. The maximum achievable co-milling ratio, and hence the level of co-firing, without significant mill throughput constraints, is limited and depends on the design of the coal mill and the nature of the biomass material. Co-firing ratios of up to 5-10 per cent on a heat input basis can normally be applied on a commercial basis.

Having established biomass co-firing activities using the pre-mixing approach, a number of the stations have embarked on projects that involve the direct injection co-firing of pre-milled biomass materials. These systems involve by-passing the installed coal mills, and can provide higher co-firing ratios. The commercial experience to date with these systems provides the technical basis for the development of advanced co-firing systems for future projects.

Direct injection biomass co-firing systems

As stated above, the majority of the biomass co-firing systems that are currently in service in Europe and elsewhere are associated with retrofit applications on existing coal fired power plants. This has clearly influenced the decisions about the technical approach to co-firing, in terms of the specification of the fuels fired, the maximum co-firing ratio achieved, and the means of introduction of the biomass to the boiler.

For new build applications, there may be some scope to be more ambitious in terms of the range of biomass fuels fired and the maximum co-firing ratio that can be achieved, although it should be recognized that these issues might be related.

The following technical comments should be considered relevant to the development of advanced biomass co-firing systems for both retrofit and new build projects:

  • Advanced biomass co-firing systems will be designed to permit high levels of co-firing (10-50 per cent, on a heat input basis) with increased fuel flexibility and minimum impacts on boiler plant performance, availability and integrity.
  • In the longer term, the advanced biomass co-firing systems will be compatible with the advanced pulverized coal boiler tech nologies that are currently under development, i.e. supercritical and ultra supercritical boiler plants, with carbon capture and storage capabilities.
  • For some applications, the systems will be required to co-fire both subsidized and unsubsidized biomass materials, i.e. they should be capable of co-firing secondary recovered fuels and waste-derived fuels in addition to energy crop and other clean biomass materials.
  • It is anticipated that the major technical concerns with advanced co-firing at high co-firing ratios will be associated with the behaviour of the inorganic components of the biomass and the potential impact on the technical and environmental performance and the integrity of the boiler plants.

There are a number of options for advanced systems including direct injection, indirect and parallel co-firing. The main industrial interest at the moment is on systems involving the direct injection co-firing of pre-milled biomass materials into large pulverized coal fired boilers.

All of the relevant technical approaches to direct injection co-firing involve the installation of dedicated systems for the milling of the biomass to a size that will combust efficiently in a pulverized fuel flame, and all involve the pneumatic conveying of the milled biomass from the handling/milling facilities to the boilers. The requirements of the handling, milling and pneumatic conveying systems will inevitably place some constraints on the biomass quality, particularly in regard to the moisture content, the particle size distribution and the level of tramp material.

There are three basic direct injection co-firing options for pre-milled biomass. Firstly, the direct injection of the pre-milled biomass into the furnace, with no flame stabilization and no additional combustion air. Secondly, the firing of the pre-milled biomass through dedicated biomass burners, with the associated combustion air supplies, and thirdly, the injection of the pre-milled biomass into the pulverized coal pipework or at the burners, and co-firing with the pulverized coal through the existing burners.

Direct injection into furnace

The first option involves direct injection through the furnace wall and is relatively cheap and simple to install although, in retrofit applications, it does involve the installation of new, albeit relatively small, furnace penetrations in the appropriate locations. There are no significant modifications to the boiler draft plant. There is no stable flame and the biomass particles depend on the heat and excess oxygen already available in the furnace for combustion.

This approach has been applied successfully at relatively low co-firing ratio in downshot fired furnaces, designed for the combustion of coals with low volatile matter contents. It is not, however, regarded as being an elegant solution to direct injection co-firing, and the application in more conventional wall or corner-fired furnaces is considered to be limited.

Dedicated biomass burners

The installation of dedicated burners for the pre-milled biomass may have some attractions, however there are a number of problems to be resolved. They include the requirement to retain the full load capability of the boiler on coal firing, i.e. the biomass burners are additional to the coal burners, and additional locations for the biomass burners, generally within the burner belt, have to be identified. This involves the installation of a number of significant additional furnace penetrations. In practice, the limited availability of suitable locations for the biomass burners may present problems, particularly in retrofit applications.

In addition, a secondary air supply for the biomass burners is required, i.e. there are significant modifications required to the boiler combustion air supply systems. The impacts of co-firing on the performance of the pulverized coal combustion system and on the furnace performance also need to be assessed in detail. The flue gas flows in large multi-burner furnaces are highly striated in nature, and the impact of concentrated flue gas streams comprising the combustion products of biomass materials may present problems within the furnace, in the boiler convective section and in the flue gas cleaning systems.

Furthermore, the dedicated biomass burners are based either on conventional pulverized coal burners or on cyclone burners and these have not been extensively demonstrated commercially for this type of application. Finally, the requirement for additional dedicated biomass burners with the associated combustion air supplies and burner management systems means that there are significant interfaces, both mechanical and control, with the coal firing system and the boiler.

Overall, therefore, the direct co-firing options involving the installation of dedicated biomass burners are considered to be relatively complex and expensive to install. There are a small number of co-firing systems in Europe based on the installation of dedicated biomass burners, although it is fair to say that the accumulated plant experience to date is not extensive, and this experience has not been overly encouraging.

However one exception to the rule is the Hasselby heat and power station, near Stockholm, Sweden, where Doosan Babcock converted the coal milling and firing system to the milling and firing of wood pellets in the mid 1990s. No significant modifications to the installed coal burners were required, and the converted system has been in successful commercial operation since that time. In general terms, therefore, this experience, and some of the experience elsewhere, indicates that dedicated burners for pre-milled biomass have been demonstrated at industrial scale, although it is clear that further longer-term operational experience, particularly in co-firing applications, is required.

Injection into the coal firing system

The final option involves the injection of the milled biomass into the pulverized coal firing system downstream of the coal mills, i.e. into the coal pipework, or directly into the coal burners, suitably modified for co-firing. In both cases, additional biomass conveying air and heat, in the form of the biomass fuel, are introduced to the mill group. It is generally preferable that the mill primary air and coal flow rates should be reduced accordingly, to maintain both the coal mills and burners within their normal operating envelopes. For safety and other reasons, the co-firing system should also be arranged such that the biomass is at all times being co-fired into a stable pulverized coal flame.

Reviewing biomass feed pipework
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This option is relatively inexpensive and simple to implement, however there are significant interfaces with the coal mill and combustion control system, which have to be carefully managed. It should be noted that these systems essentially make use of the turndown capabilities of the coal mill, both in terms of the coal and primary air flows to accommodate the conveying and firing of the biomass. This can place restrictions on the maximum biomass and the conveying air flow rates into the mill group. The principal options for the location of the biomass injection point are directly into the burner with appropriate modification, into the mill outlet pipework local to the mill outlet and into the pulverized coal pipework just upstream of the burner

The first option involves significant modification of the coal burners. It will be necessary to adopt this approach with some biomass materials, where there is concern about the potential for the blockage of the coal pipework system, and particularly of splitters, riffle boxes, and of the coal burners. One example of a burner modification for this type of application is at Studstrupvaerket in Denmark, where chopped straw is co-fired through the core air tubes of four Doosan Babcock Mark III low NOx burners. The pulverized coal is fired through the primary air annulus, as normal.

In this case, the biomass material is carried along four independent pneumatic conveying lines, at a rate of five tonnes per hour per line, from the straw handling plant to the boiler.

The biomass is then injected directly through the core air tubes of the burners. Significant modification of the coal burners was required, including relocation of both the oil lance and the flame scanner to clear the core air tube for the biomass injection.

This approach has the disadvantages that it inevitably involves some interference with performance of the tertiary air swirlers, and it means that both the oil lance and the flame scanner can be in non-ideal positions. The engineers were forced down that particular route because of the requirement to provide a clear passage down the core air tube for the relatively large straw particles to avoid blockages.

In general terms, however, the experience at Studstrupvaerket has been positive, with limited negative impacts on the combustion process, the NOx emission levels, or the furnace ash deposition2. This approach may have some attractions, particularly for pre-milled biomass prepared from baled materials, which are difficult to mill to small particle sizes, and for materials that have a tendency to block pneumatic conveying pipework.

The principal alternative to the approach taken at Studstrupvaerket is to introduce the biomass into the fuel pipework upstream of the coal burners. In this case, the pulverized coal/biomass mixture is carried forward along the pulverized coal pipework, through the burner and then enters the combustor via the primary air annulus, as normal. This type of approach is, in principle, applicable to all burner designs.

In general, two potential locations for the introduction of the biomass into the pulverized coal pipework are apparent. Firstly, the introduction of the biomass into the pulverized coal pipework in the pipework just upstream of the non-return valves local to the furnace, i.e. downstream of the pulverised coal splitters, if any. Secondly, the introduction of the biomass into the mill outlet pipework, just downstream of the product dampers, and upstream of the pulverized coal splitters, if any.

The first of these options has a number of advantages. Firstly, the point of introduction of the biomass and the shut-off valve, in general, will be readily accessible from the burner galleries for inspection and maintenance. Secondly, the potential process risks associated with the introduction of a significant quantity of biomass into the coal pipework are minimized, by having the shortest possible length of pipework carrying the mixed fuel stream, and avoiding any splitters in the coal pipes. And finally, the introduction point for the biomass is well away from the coal mill, and therefore the potential impact of mill incidents or mill vibration on the biomass conveying and injection system is reduced.

The principal disadvantage of this approach, however, is that biomass feeding and conveying systems to each burner are required, which is relatively expensive. In some cases, the routing of the biomass pipework through the congested region local to the boiler front, and the arrangements for supporting the biomass pipes, can also become overly complex and expensive.

For most applications, the second approach may be preferred. This approach is generally much easier to engineer and install, and has the advantage that the number of biomass pneumatic conveying pipes are reduced. The mixed biomass/pulverized coal stream is then carried forward to the burners, via any splitters in the pulverised coal pipework.

The main disadvantages of this system are that the injection point for the biomass is closer to the coal mill, i.e. there are greater risks of interference with the pulverized coal transport system, and particularly at the pulverised coal splitters, and the impact of any mill incident on the biomass conveying system may be greater.

In all cases, the introduction point of the biomass to the pulverized coal pipework is fitted with an actuated biomass isolation valve to allow rapid isolation of the biomass system from coal firing system.

Overall, it is clear that there are a number of viable options for the direct injection co-firing of pre-milled biomass into conventional pulverized coal firing systems. The preferred technical option for any particular application will depend on the type of biomass to be co-fired, on the desired co-firing ratio and on a number of site-specific factors. A number of these direct firing systems are currently in commercial operation, however it is fair to say that to date the operational experience of these systems has generally been encouraging.

Elevating co-firing ratios

Overall the application to date of direct injection biomass co-firing has been at co-firing ratios up to between 10-15 per cent of the total heat input to the boiler. The principal constraints have been:

  • The availability of suitable biomass supplies to the stations
  • The limitations of the fuel storage and handling systems at the stations
  • The technical concerns about the risks of negative impacts on boiler performance and integrity with particular biomass fuels at elevated co-firing ratios

A number of direct injection co-firing systems, however, have been installed, generally to single or a small number of individual mill groups, and biomass co-firing ratios of up to around 50 per cent on a heat input basis to individual mill groups have been successfully applied.

Biomass injection point at a UK power plant
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It is possible to replicate this approach to more than one mill group, providing the potential for increased biomass co-firing ratios on a boiler unit basis. In the first instance it may be more appropriate to install a simple system to connect a single biomass metering and pneumatic conveying system to a second mill group on the same boiler unit. This has been done successfully in the UK, to reduce the dependency of the utilization of the direct injection biomass co-firing system on the availability of a single coal mill. This is much simpler to engineer for a system based on direct injection to the coal pipework.

For new build applications, it is possible to conceive of a system where all of the mill groups in a boiler have the capability to co-fire biomass, providing a total co-firing capability of up to 50 per cent to the boiler. For retrofit applications the maximum co-firing ratio may be limited by site-specific factors and plant constraints.

For new build applications, where the desire is to achieve higher co-firing ratios, there will be a requirement to co-fire through a number of mill groups, and the implications on the turndown capability of the mills and the boiler will need careful consideration. Co-firing systems, which have automatic control of the biomass feed rate, are also currently being developed in Europe, however the experience to date with these systems is limited.

Why direct injection makes sense

For both new build and retrofit applications, the direct injection systems have a number of key advantages over the alternative systems:

  • There are no requirements for significant modification of the boiler draught plant, secondary air ductwork, etc
  • The boiler and the mills are started up on coal as normal and the biomass co-firing is not introduced until all of the combustion and boiler systems are functioning properly
  • If there are problems with the biomass system on one mill group the biomass co-firing system can be turned off and isolated rapidly and the boiler load can be picked up on coa l firing on that mill as normal
  • If there are problems with the coal mill, e.g. coal feeder problems, a fire in the mill, the biomass co-firing system can be switched off and isolated quickly, allowing the operator to deal with the problems on coal-firing as normal
  • The biomass control system only communicates with the mill controls, i.e. it is an add-on to the normal boiler/mill/burner controls and the appropriate safety interlocks are in place
  • The biomass can be co-fired with the coal through the coal burners at up to 50 per cent heat input, and the biomass com bustion is always supported by a stable coal flame. The technical risks of significant negative impacts on the combustion efficiency, burnout, flame shape & furnace heat transfer factors are much lower than for co-firing of the biomass through dedicated burners
  • The products of the combustion of the biomass are always well mixed with those from coal. This means that the risks associated with the striated flows in furnaces and boilers producing localized deposition and corrosion effects because of the concentration of the products from biomass combustion are minimized

Overall it is clear that direct injection co-firing is a technically robust and cost-effective approach to the co-firing of pre-milled biomass in large pulverized coal boiler, at elevated co-firing ratios and for both retrofit and new build applications. This is based on successful commercial experience, albeit in a small number of applications, at power plants throughout Europe.

1 IEA World Energy Outlook 2007

2 Overgaard P, Sander B, Junker H, Friborg K & Larsen O H: Two years operational experience and further development of full-scale co-firing of straw, 2nd World Conference on Biomass for Energy, Industry & Climate Protection, Rome, Italy, 2004

The article is based on the paper ‘Advanced biomass co-firing systems’ presented at COAL-GEN Europe, 1-3 July 2008 in Warsaw, Poland.