Biomass co-combustion is known as the ‘partial substitution of coal (or other fossil fuel)’ with biomass in one process. Typical co-combustion power plant capacity is in the electrical output range of 50 MW to 700 MW in Denmark, Belgium, Poland, UK, etc. Co-combustion can be applied in existing coal equipped with pulverized coal firing systems or fluidized bed combustion systems or a gas fired power station where the fraction of biomass is up to 20% of the total fuel weight or energy consumption (VGB, 2008). Biomass co-combustion offers renewable energy with the lowest capital cost.

There are six types of biomass co-combustion methods as listed in Figure 4.1 (Livingston etal., 2011). Number 1 is the milling of biomass (pellets) through modified coal mills, number 2 is the pre-mixing of the biomass with the coal, then the mixed fuel is milled and fired through the existing coal firing system, number 3 is the direct injection of pre-milled biomass into the pulverized coal running piping, number 4 is the direct injection of pre-milled biomass into modified coal burners or directly into the furnace, number 5 is the direct injection of the pre-milled biomass through dedicated biomass burners and number 6 is the biomass gasification gas, which is burned with coal in the boiler.

In summary, it is possible to distinguish the application of biomass in coal-fired power plants with three different biomass co-combustion concepts, which are shown as following (EUBIA, 2007; VGB, 2008; Tillman et al., 2000; Brouwer et al., 1995; Swanekamp etal., 1995; Surmen et al., 2003; Hunt et al., 1997) [2]

steam generation. The steam is used in the power plant together with the main steam. Parallel co-combustion is very popular in the pulp and paper industries as dedicated biomass boilers are used for the utilization of bark and waste wood. These industries economize and increase their energy efficiency by using the bio-residues and by-products from their main focus, the production of paper.