Incentives have made co-firing fossil fuels with biomass a more attractive prospect for many power plant operators. The latest research uncovers the answers to some common co-firing issues.
The Power Technology team, E.ON UK
Electricity generators in the UK now carry out significant volumes of biomass co-firing which provides a cost effective means of increasing renewables capacity while utilizing the high thermal efficiencies of conventional, large coal fired boilers.
The economic viability of co-firing at existing power plants is currently supported in the UK by the Renewables Obligation on electricity suppliers and the impact of the Climate Change Levy on industrial and commercial energy users. Income contributions from these two environmental protection measures serve to help balance a biomass fuel price that is higher than that of coal.
However, that still leaves operators having to overcome a range of operational and technical challenges which vary according to fuel type, plant specifications and commercial requirements.
Power Technology, the engineering and scientific consultancy unit of E.ON UK, is among the leading developers of technologies for co-firing of biomass. It has identified answers to biomass issues covering fuel sourcing, supply, transportation, handling, storage, milling, combustion, ash disposal, plant integrity, health and safety, and the environment.
Power Technology’s work spans feasibility studies through to full-scale power station trials and has involved extensive use of its advanced 1 MW combustion test facility (CTF). This is currently Europe’s only test bed capable of firing 100 per cent biomass fuel, as well as co-firing with a range of other fuels at varying blend ratios.
The experience gained has enabled Power Technology to assist a cross-section of conventional fossil fuel fired plants to improve their income, with minimal plant modification, through the use of renewable fuel offering environmental benefits.
For example, on a 500 MW unit, displacing coal with one per cent by mass of carbon dioxide (CO2) neutral biomass cuts fossil fuel related CO2 emissions by up to three tonnes every hour. Emissions of nitrogen oxide (NOx) and sulphur dioxide (SO2) are also reduced.
Figure 1. Power Technology has researched issues such as fuel sourcing, supply, transportation, handling and storage
Power Technology has conducted many feasibility studies and trials with a variety of liquid and solid biomass materials from which a portfolio of fuels, with potential for use on conventional plant was identified. This includes wood pellets, cereal co-product, olive residue, straw, palm oil, meat and bone meal, as well as tallow. Work also included blending biomass fuels, which can be complicated by the incompatibility of liquid fuels and the volatility of solid biomass fuels.
Adapting a typical coal fired unit to accommodate biomass co-firing begins with addressing health and safety issues and constructing a safety case for the project. The major considerations arising with co-firing are the management of dust related issues, and the higher reactivity of the fuels, which can increase the fire and explosion risk unless proper controls are in place. In some instances there have been initial trials using Power Technology’s CTF to demonstrate that effective handling, combustion quality and emission compliance can be achieved with the selected fuel.
This is usually followed by feasibility studies and then controlled tests on a single mill. The final stage is usually a full unit trial to demonstrate satisfactory technical and environmental performance to support the power station’s application to the Environment Agency for permission to burn biomass on a commercial basis. Generally, the operational implications of co-firing biomass are significant and are often not fully understood.
Matching the fuels
Existing installations at power stations have been highly optimized and the introduction of fuels outside the usual specification can cause substantial operational issues. As a result, biomass fuels must be closely matched with individual plant designs for optimum performance, particularly when co-milling. It is often the ability to mill the material that dictates the maximum blend concentration of biomass when mixed with coal. If this limits plant flexibility or availability, co-firing can have an adverse impact on the trading of the business’s electricity.
The volatile matter content and other constituents of some biomass fuels vary considerably, making it difficult to maintain consistency in blending and combustion optimization. Consequently, it is important to have purchase specifications agreed with the supplier. Ensuring the suitability of biomass can extend to checking that it will also meet solid and gaseous emissions limits.
The problems of co-milling, including blending and potential high maintenance requirements, can be reduced by the addition of dedicated milling and handling plant. Specially designed for use with biomass, these systems allow increased throughput and offer the possibility of burning a wider range of biomass fuels. When adopting a dedicated system for injecting biomass into the boiler, there are several choices, including the use of existing burners, injecting the milled product into the plant’s pulverised fuel system or locating new burners in unconventional positions.
Figure 2. The many different biomass fuels available each present their own handling and maintenance issues for the plant operator
Consideration has to be given as to how the thermodynamics and aerodynamics within the furnace will be altered by the modifications. Corrosion and erosion of the steam pendants is a concern with burners situated high in the furnace and deposition of unburnt fuel from recirculation zones is a factor where burners are situated low in the furnace.
The volatile matter content of biomass is considerably higher than that of coal, limiting the ratio at which biomass can be blended with some coal types as a result of safety considerations. This problem is compounded by the fact that many stations use low sulphur coals that have a higher volatile matter content than other fuels. Blending these with biomass produces a highly volatile fuel mix that is more susceptible to accidental ignition.
A common problem with biomass is the increased incidence of fires in milling equipment which are a potential source of ignition for pressure excursions. As a result Power Technology has produced guidelines for blending biomass fuels with coal.
Volatility and hazards
The dust and associated hazards which can arise from transporting, handling, storing and blending biomass can generally be managed by a number of standard steps. These include good housekeeping, adherence to established health and safety procedures, installation of dust suppression systems, and use of personal protective equipment.
As many fossil fired power stations sell their fly ash to the construction and cement industries, consideration has to be given to how the ash composition may be changed when co-firing biomass fuels. Ahead of test trials, there can be concerns over the level of alkali metals, phosphates and carbon that might be present in co-fired ash. In reality, the overall impact of co-firing has been to slightly reduce the quantities of most trace elements. While carbon-in-ash remains a site specific issue largely dependent upon plant milling, power plant operators have generally managed to maintain their fly ash markets.
Figure 3. The introduction of biomass causes operational issues for highly optimized power plants
The long-term impacts on plant of co-firing biomass were examined by Power Technology’s material engineering teams using advanced metallurgical and analysis techniques, and computational fluid dynamics.
Laboratory and in-situ testing identified the materials most suitable and those at risk from specific biomass materials. For example, analysis carried out on tube failures in a chicken litter fired plant identified corrosion as a major damage mechanism. Investigations were carried out, pinpointing high risk areas thereby allowing maintenance costs to be optimized and outages reduced.
For long-term use of biomass, dedicated storage facilities are recommended. Biomass fuels generate more dust than coal and the material is hygroscopic. Therefore, it has proved advisable to store the material in dedicated enclosures, limiting moisture content and preventing airborne dust being scattered.
Co-firing is among the most popular forms of alternative generation technologies, despite the complexity of the legislation governing its use as a renewable energy source. However, the changing nature of the legislation has created a level of uncertainty within the market which in turn has slowed down the rate of investment. Not withstanding this, there continues to remain a considerable opportunity for increased rates of renewable generation from co-firing in the UK.
Power Technology’s experience and technology development confirms that co-milling is practical, environmentally beneficial and is making a real contribution to government renewable targets.
Co-firing, despite the issues involved, is being demonstrated by plant operators to be a technically and commercially attractive proposition. It is also encouraging generators to continue to assess future biomass fuel and technology options, including opportunities for using energy crops and schemes for achieving direct injection of biomass.
Best Practice Brochure: Co-Firing of Biomass (Main Report): Report No. COAL R287 DTI/Pub URN 05/1160 May 2005