EVC7: Biomass co-firing for heat and power

Definition

Biomass co-firing stands for adding biomass as a partial substitute fuel in high efficiency coal boilers. Coal and biomass are co-combusted in boilers that have been designed to burn coal. For this purpose, the existing coal power plant has to be partly reconstructed and retrofitted. Co-firing is an option to convert biomass to electricity, in an efficient and clean way, and to simultaneously reduce GHG emissions of the power plant.

Co-firing processes

Over the past few years, the process of co-firing has seen a rapid development. Many plants have been retrofitted for demonstration purposes and others have been newly-built to allow for biomass co-firing.

There are three different concepts for co-firing biomass in coal boilers.

• Direct co-firing – The biomass and the coal are burned in the same furnace. The mills for the grinding of the fuel and the burners may be the same as or be separate from the coal feeding. This depends on the biomass used and its fuel properties. This concept is most commonly used, because it is the easiest to implement and most cost-effective. However, the biomass energy fraction is typically limited to below 10-20 %. For high blend rate and for best performance, pre-treated fuels (e.g. white or black pellets) are used.
• Indirect co-firing – In this concept, the solid biomass is converted to a clean fuel gas, using a biomass gasifier. The gas can be burnt in the same furnace as the coal. For this reason, it is also possible to use biomass, which, for example is difficult to grind. The gas can be cleaned und filtered before use, to remove impurities. This principle is less used compared to direct co-firing. 
• Parallel co-firing – It is also possible to install a completely separate biomass boiler in addition to the conventional boiler. In this case, the biomass boiler provides energy to the feed water or generates steam at low temperature, while the conventional boiler tops up the superheat.

Motivation

Biomass co-firing is a good method to minimize greenhouse gases, because this process reduce net CO₂, CH₄, SO_X and often NO_X emissions, compared to coal combustion. The existing coal power plants can be retrofitted quickly and cost-effectively. The boiler efficiency drops only little or not at all, after adjustment to the new fuel mixture. Also, a big advantage is the existing network of coal power plants.

Challenges

Biomass co-firing also faces some challenges, like fuel supply, -handling and -storage. There is also a potential increase in corrosion and some ash deposition issues. If not carefully designed, co-firing involves some risks, like increased plant outages, possible interference with the operation of burner, furnace, boiler etc. In general, biomass is more expensive than coal. Even if biomass residues are used, which are available for free, these must be transported and pre-treated.