Biomass EnergyDefinition
If we look at some of the definitions for co-firing, then usually most of the statements will refer to it as a process of combustion of two types of fuels. This kind of combustion, which is the burning of different materials at the same time, is related mainly to a biomass being burned with a fossil fuel. The most common fossil fuel used in co-firing is coal.
In simple terms, co-firing is a method of supplementing coal in a coal-fired boiler with a different type of fuel, such as biomass materials*.
Method
Presently, there are two different types of co-firing, direct and indirect.
For direct co-firing, two different methods have been developed. The first method is blending the biomass and coal in the fuel handling system and feeding the blend to the boiler. The second method is a separate fuel handling and separate special burners for the biomass, which thus have no impact to the conventional coal delivery system.
On the other hand, indirect co-firing is a process concept, which is based on thermal conversion of biomass or waste to gaseous or liquid fuel and the co-firing of these converted fuels together with the main fuel.
Three different types of indirect co-firing exist. These are "indirect co-combustion with pre-gasification", "indirect co-combustion in gas-fired power plants", and "parallel co-combustion (steam side coupling)".
Co-Firing Benefits
Co-firing can reduce the emission of a number of gases. It has already been established that these gases pollute the environment and can cause global warming. Co-firing, therefore, can be beneficial in a number of ways. The most important benefit we obtain from the co-firing system is CO2 reduction, usually associated with global warming. In addition to the above, we can benefit in the reduction of NOx as well as reduction in flame temperature. Also, co-firing with biomass can reduce the emission of SOx, due to the lower sulphur content in biomass materials.
Others important beneficial factors can be in cost saving, as a variety of biomass materials are much cheaper than fossil fuels and under no threat of exhausting the reserve, such as with fossil fuels. For this reason, co-firing can increase sustainability of energy supplies from power production, as well as producing less by-product than burning coal on its own (clearly that depend on the type of biomass being used in the co-firing system).
Finally, co-firing improves combustion due to the higher volatile content of biomass. In addition, increasing the use of biomass materials as a fuel may help in the creation of new jobs.
Challenges for Co-firing
There are a number of factors which can affect co-firing in one way or another. Some of these factors are purely technical, others are non-technical.
Many of these challenges do not apply to fossil fuels, and that is what makes co-firing with biomass a more challenging area, trying to establish itself in a more competitive environment where a number of systems and fuels are being tested and marketed at the present time. Co-firing, therefore, will need more support and more time to smooth out its various problems before it can compete commercially with fossil fuels.
Factors which may give rise to problems are usually related to fuel preparation, storage, delivery and fuel flexibility (quality and quantity) [2]. Also, ash deposition (increased need for soot blowing, more intensive cleaning of heat transfer surfaces in revisions may be required etc.), remains a problem [3].
In addition to the above, other issues such as pollutant formation, increased corrosion rates of high temperature components, the number of bed material changes per day increases (in Fluidised Bed combustion), fly ash utilization (un-burnt carbon, contamination) [4], higher in-house power consumption, difficulty for complete combustion as well as the difficulties in mixing coal and biomass in the boiler, remain [5].
Fouling and corrosion of the boiler (alkalis, chlorine) are other negative aspects of co-firing. Finally and as part of the technical challenges, the negative impact on flue gas cleaning (SCR DeNOx).
Concerning non-technical factors, these may range from economic aspects (lack of financial incentives, uncertain fuel prices, open market) to legislative aspects (utilization of fly ash in cement, determining green share, emission legislation), as well as the public perception of co-firing of biomass/waste.
Conclusion
Co-firing is one of the methods used in power generating companies as a way to reduce unwanted gaseous emissions, which can be the cause of global warming, as well as a health risk to humans, plus polluting the environment.
In addition to the above, co-firing is one of the methods used to reduce the dependency on fossil fuels.
Generally speaking, co-firing can be used to reduce the cost of energy production, as the use of biomass materials for generating energy tend to be cheaper than the use of fossil fuels.
References
1. Perry M., Rosillo-Calle F. "Co-firing Report - United Kingdom" International Energy Agency (IEA) Bioenergy Task40 - Imperial College 2006.
2. "Biomass Energy Data Book" US Dept. of Energy, Energy Efficiency and Renewable Energy, 2006.
3. "Biomass Research" NREL National Renewable Energy Laboratory, USA, 2006.
4. "Biomass Management - Fuel Removal and Mulching" - Hills Emergency Forum, Biomass Management Working Paper, 2007.
5. "Engineering Report" Guidance document on biomass co-firing on coal fired power stations " DTI Projet 324-2, 2007
*Co-firing should not be confused with multiple fuel boilers. Multiple fuel boilers are designed to burn a wide range of fuels. Co-firing on the other hand is carried out in a boiler specifically designed to burn only a specific type of coal [1].
Najib Altawell
N. Altawell © 2008
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