Biomass Co-firing With Coal as an Emissions Reduction Strategy
30 states (& the District of Columbia) have mandated, or enforceable, renewable portfolio standards (RPS). Another seven states have voluntary goals for renewable energy. (See the Renewable Portfolio Standards
page for more information.)
Coal consuming utilities and industrials are exploring biomass as an option for RPS compliance. Biomass is expected to be the largest source of renewable energy ~ comprising 6.4% of the targeted 15% RES by 2020 according to JP Morgan.
Biomass co-firing has the potential to reduce emissions from coal-fueled generation, without substantially increasing costs or infrastructure investments. Research has demonstrated that, when implemented at relatively low biomass to coal ratios, there are significant reductions in energy consumption, and solid waste generation, as well as reduced emissions. However, the nature and chemical makeup of biomass fuels can lead to significant cost increases, maintenance problems, boiler slagging and fouling issues, increased boiler corrosion, and decreased efficiency if biomass use is not very closely managed.
- What is co-firing with biomass?
- Why use biomass co-firing?
- Who can use biomass co-firing?
- What is biomass?
- Some challenges to biomass co-firing
- Some benefits to biomass co-firing
- Sources & Links
What is Co-firing with biomass?
There are three types of co-firing in use around the globe
Direct Co-firing – is the simplest of the three and the most common option chosen. In this version of co-firing, more than one type of fuel in a furnace/boiler at the same time. The same, or separate feed systems, mills, and burners can be used depending on the fuel characteristics
Indirect co-firing – biomass is converted from a solid fuel to a gaseous form before firing. The gas is then burned in the same furnace/boiler as the coal
Separate biomass boiler – a separate biomass boiler is added to the overall system to bolster the steam capacity of an existing coal boiler.
Why use biomass co-firing?
The primary reason for co-firing coal with biomass is as a means of reducing the potential environmental impacts associated with the combustion of fossil fuels.
Who can use biomass co-firing?
Given that co-firing requires large amounts of biomass fuels, co-firing typically works best with large coal-fueled utilities that have materials handling capability already on site. However, other industrial users – cement plants, heating plants, etc. – could also make use of biomass co-firing.
What is biomass?
"Biomass” includes any natural (biological), renewable fuels, such as wood (or wood wastes), agricultural residues, food wastes, and industrial wastes. Biomass is derived from living or recently living organisms. Although fossil fuels are also derived from plant or living organisms, the primary difference in make up (as it relates to GHG emissions) is one of time scale. Fossil fuels removed carbon from the atmosphere many years ago, while biomass has the potential to remove carbon from the atmosphere as it grows today or in the very near future.
As with other renewable energy sources, biomass cannot compete on an economic footing with coal (or other fossil energies) due to low thermal efficiency, high cost, variable impacts on boiler and milling equipment, and high technical risk
biomass typically has low bulk energy density, is wet and is strongly hydrophilic and therefore, requires a great deal of fuel handling technology compared to its heating contribution
While fuel costs may be low, transportation, preparation, and handling costs for biomass can rapidly exceed total fuel costs for other fossil options.
Potential for increased corrosion rates in boilers due to higher alkali levels in biomass fuel
Biomass fuels can have as much as 50% moisture – moisture in a boiler will reduce efficiencies
As ash fusion temperatures for most biomass fuels are far less than coal ash fusion temperatures (as low as 750 Celsius vs. over 1,000 Celsius for coal) there is a strong possibility that the rate and extent of boiler slagging will increase
Negative impacts on fly ash usability as ASTM specifications require that fly ash be derived wholly from coal combustion. Initial tests indicate a need for updating this specification as biomass-containing fly ashes behave in a manner similar to coal only fly ashes.
European tests indicate that co-firing with biomass can have a significant negative impact on SCR catalysts (deactivation) if not managed properly
Biomass can use the pre-existing infrastructure investments for fossil fuels
Actual fuel costs can often be very low – even free – as biomass producers are often happy to be rid of what they consider to be "waste.”
Biomass costs compare very favorably to other renewable options
The addition of biomass to a coal-fueled boiler is not likely to, or will, at worst, have only a minimal negative impact on generation efficiency (depending on fuel preparation measures taken)
With proper fuel selection and management, overall emissions of SOX, NOX, and mercury, as well as and net GHG emissions can be reduced. Biomass is typically nearly free of sulfur and mercury, so these emissions are reduced in proportion to its use (NOX emissions may fall or rise slightly depending on boiler operation)
Biomass is considered to be "carbon-neutral,” so its use can have a proportional reduction in GHG emission.
Co-firing coal with biomass is a potentially valuable tool to help decrease greenhouse gas and other emissions in coal-fueled boilers. Use of biomass at low to moderate biomass to coal ratios appears to produce the best performance enhancements and can result in overall life-cycle energy consumption reductions, as well as reduced solid waste generation. However, there are many potential obstacles to biomass use that can decrease efficiencies, as well as increase costs, maintenance (corrosion, slagging, etc.), and boiler down time if biomass use is not managed very carefully.
Sources and Links:
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