Direct Fired Biomass

According to the US Department of Energy, there is currently 40,000 MW of installed direct fired biomass capacity worldwide.  The majority of this capacity is in the pulp and paper industry in combined heat and power systems. 

Direct biomass combustion power plants in operation today essentially use the same steam Rankine cycle introduced into commercial use 100 years ago.  By burning biomass, pressurized steam is produced in a boiler and then expanded through a turbine to produce electricity.  Prior to combustion in the boiler, the biomass fuel may require some processing to improve the physical and chemical properties of the feedstock.  Furnaces used in the combustion of biomass include spreader stoker-fired, suspension-fired, fluidized bed, cyclone and pile burners.  Advanced technologies, such as integrated biomass gasification combined cycle and biomass pyrolysis, are currently under development.

Applications

Wood is the most common biomass fuel.  Other biomass fuels include agricultural residues, dried manure and sewage sludge, and dedicated fuel crops such as switchgrass and coppiced willow.  There are also many municipal waste burners installed throughout the world.  However, plants combusting municipal waste are becoming more difficult to permit due to environmental concerns regarding toxic air emissions. 

The capacity of biomass plants is usually less than 50 MW because of the dispersed nature of the feedstock and the large quantities of fuel required.  Furthermore, biomass plants will commonly have lower efficiencies compared to modern coal plants.  The lower efficiency is due to the lower heating value and higher moisture content of the biomass fuel compared to coal.  Additionally, biomass is typically more expensive and has a lower heating value than coal.  These factors limit use of biomass technology to applications of inexpensive or waste biomass sources.

An economical way to burn biomass is to co-fire it with coal.  Co-fired projects are usually implemented as retrofitted coal plants that have a biomass fuel system added.  Capital costs for co-firing systems are much lower than greenfield biomass plants.

In addition to electrical generation, there are many biomass plants that produce thermal energy for heating applications.  The replacement of coal in small district heating systems is one promising application in the COO. 

Resource Availability

In rural areas the agricultural economy can produce significant fuel resources that may be collected and burned in biomass plants.  These resources include fruit tree prunings, rice hulls, wheat straw, and other agricultural residues.  In urban areas, a biomass project might burn wood wastes such as construction debris, pallets, yard and tree trimmings, and railroad ties.  Generally, availability of sufficient quantities of biomass is not as large of a concern as delivering the biomass to the power plant at a reasonable price.

Environmental Impacts

Biomass plants have some of the same emissions issues as fossil fuel plants.  They emit carbon dioxide (CO₂), nitrogen oxides (NO_x), particulate matter and other substances that are byproducts of fuel contaminants.  Taking in to consideration the life cycle of the biomass (including regrowth), these emissions, particularly CO₂, have a much lower net impact on the environment than fossil fuels.  Another environmental benefit of burning biomass is that consumption of biomass reduces land-filling and other disposal processes that would otherwise waste an available feedstock.

Biomass Cofiring

An economical way to burn biomass is to cofire it with coal in existing plants.  Cofired projects are usually implemented by retrofitting a biomass fuel feed system to an existing coal plant, although greenfield facilities can also be readily designed to accept a variety of fuels. 

A major challenge to biomass power is that the dispersed nature of the feedstock and high transportation costs generally preclude plants larger than 50 MW. By comparison, coal power plants rely on the same basic power conversion technology but have much higher unit capacities, exceeding 1,000 MW. Due to their scale, modern coal plants are able to obtain higher efficiency at lower cost. Through cofiring, biomass can take advantage of this high efficiency at a more competitive cost than a stand-alone direct fired biomass plant.

Applications

There are several methods of biomass cofiring that could be employed for a project.  The most appropriate system is a function of the biomass fuel properties and the coal boiler technology.  Provided they were initially designed with some fuel flexibility, stoker and fluidized bed boilers generally require minimal modifications to accept biomass.  Simply mixing the fuel into the coal pile may be sufficient. 

Cyclone boilers and pulverized coal (PC) boilers (the most common in the utility industry) require smaller fuel size than stokers and fluidized beds and may necessitate additional processing of the biomass prior to combustion.  There are two basic approaches to cofiring in this case.  The first is to blend the fuels and feed them together to the coal processing equipment (crushers, pulverizers, etc.).  In a cyclone boiler, generally up to 10 percent of the coal heat input could be replaced with biomass using this method.  The smaller fuel particle size of a PC plant limits the fuel replacement to perhaps 3 percent.  Higher cofiring percentages (around 10 percent) in a PC unit can be accomplished by developing a separate biomass processing system at somewhat higher cost. 

Even at these limited cofiring rates, plant owners have raised numerous concerns about negative impacts of cofiring on plant operations. These include:

·          Negative impact on plant capacity

·          Negative impact on boiler performance

·          Ash contamination impacting ability to sell coal ash

·          Increased operation and maintenance costs

·          Limited potential to replace coal (generally accepted to be 10 percent on an energy basis)

·          Minimal nitrogen oxide reduction potential

·          Boiler fouling/slagging due to high alkali in biomass ash

·          Negative impacts on selective catalytic reduction air pollution control equipment (catalyst poisoning)

These concerns have been a major obstacle to more widespread biomass cofiring adoption. Most of these concerns can be addressed by using an external biomass gasifier to convert the energy of the solid biomass into a low energy gas ("syngas") to be fired in the boiler.  Using gasification technology, it is expected that 25 percent or more of the coal heat input could be displaced without significant operational problems.  Additionally, the syngas can be used as a reburn fuel to significantly reduce NO_x emissions.  The gasification system has a higher cost than the other cofiring approaches, but still a fraction of the cost of a new direct-fired plant. 

Coal and biomass cofiring may also be considered for new power plants. Designing the plant from the outset to accept a diverse fuel mix would allow the specifications for the boiler to incorporate the biomass fuel into the design, ensuring high efficiency with low operational and maintenance impacts.  Fluidized bed technology is often the preferred boiler technology as it has inherent fuel flexibility.  There are many fluidized bed units around the world that burn a wide variety of fuels, including biomass.  An example is the 240 MW CFB owned by Alholmens Kraft Oy in Finland, which burns a mix of wood, peat and lignite.  This unit was supplied by Kvaerner Pulping and was commissioned in 2001. 

Resource Availability

In rural areas the agricultural economy can produce significant fuel resources that may be collected and burned in cofiring power plants.  These resources include fruit tree prunings, rice hulls, wheat straw, and other agricultural residues.  In urban areas, a biomass project might burn wood wastes such as construction debris, pallets, yard and tree trimmings, and railroad ties.  Generally, availability of sufficient quantities of biomass is not as large of a concern as delivering the biomass to the power plant at a reasonable price.  In the case of cofiring, however, the capital costs of the plant are much lower than for direct fired biomass, allowing for greater flexibility in the price paid for the resource.

Environmental Impacts

As with direct fired biomass plants, the biomass fuel supply must be collected in a sustainable manner.  Assuming this is the case, cofiring biomass in a coal plant generally has overall positive environmental effects.  The clean biomass fuel typically reduces emissions of sulfur, carbon dioxide, nitrogen oxides and heavy metals, such as mercury.  Further, compared to other renewable resources, biomass cofiring directly offsets fossil fuel use. 

Critics are opposed to cofiring biomass with coal because they feel it is a form of “green washing” dirty coal plants.  They believe that biomass could be used to justify extended lives for coal plants.  For these reasons, they argue that the cofired biomass should not be counted as renewable.