Co-combustion in large-scale power plants can lead to an overall saving of fuels in comparison to independent fossil — and biomass-fired plants. Also, it can increase the fuel flexibility and reduce investment cost. Comparing with coal, biomass is a renewable energy source, which is considered as a CO2-neutral fuel with lower emissions of SO2, NOx, heavy metals. NOx emissions could be reduced by biomass, which has a low nitrogen and high volatile content. (EUBIA, 2007; Zhang et al., 2010; Fu et al., 2009; Daniele et al., 2007). The co-combustion of coal and biomass has many advantages which can be described as follows (EUBIA, 2007; VGB, 2008):

1. Reducing greenhouse gases emission — biomass is considered as a ‘carbon neutral’ fuel in that the CO2 emitted during biomass combustion is equal to that absorbed during the biomass growing. When biomass displaces a fossil fuel, a net reduction in greenhouse gas emissions is achieved.

2. Reducing local air pollutant emissions — burning biomass instead of fossil fuel results in lower emissions of SO2 and NOx.

3. Increasing electrical efficiency — the electrical efficiency of co-combustion power plant is higher than the traditional biomass plant, which has a small scale.

4. Ensuring security of supply — there exists a wide range of usable biomass fuels. Varying qualities and quantities of fuels can be partially compensated by adjusting the co-combustion rate.

5. Reducing cost — co-combustion presents the opportunity to use the existing fossil-fuel fired power plant infrastructure, which can be modified for co-combustion relatively easily. An optimum thermal biomass blending ratio of biomass co-combustion is 10% (on an energy basis) (Munir et al., 2010). Addition of biomass to a coal-fired boiler does not impact or at worst only slightly decreases the overall generation efficiency of a coal-fired power plant. Compared with other renewable options, biomass co-firing represents the most cost-effective means of renewable power generation in many cases (Belosevic et al., 2010; Baxter et al., 2005; Hein etal., 1998).

Meanwhile, co-combustion of coal and biomass has some disadvantages shown as follows (VGB, 2008):

1. Preprocess — A fuel handling system is designed for a particular water content, size distribution, dust etc. With co-combustion of biomass it is necessary to adapt the existing or even build a new combustion system for that fuel.

2. Corrosion — Higher corrosion risk due to increased HCl formation in case of substitution of fuels with higher chlorine content (sewage sludge, some cereals). Many biomass fuels contain large amounts of alkalines, especially potassium, which may aggravate the fouling problems (Baxter et al., 1993; Bakker et al., 1997; Robinsin et al., 2001a, b; Dunaway et al., 2003; Lokare etal., 2003).

3. A SCR DeNOx catalyst can be blocked by ash particles or deactivated by potassium, chlorine, and in case of sewage sludge also poisoned by some heavy metals and metalloids (As, Zn).

4. Operating costs are typically higher for biomass than for coal. The most sensitive factor is the fuel cost. Even if the fuel is nominally free at the point of its generation (as many residues are), its transportation, preparation and on-site handling typically increase its effective cost per unit energy such that it rivals and sometimes exceeds that of coal.

For the utilization of the ash in the cement and concrete industry, the concentrations of alkali metals, P2O5, SO3, Cl and unburned carbon in the ash are the critical parameters. It was found that the ashing temperature should be selected according to the biomasses proportion, when the biomass fraction is raised, the ash fusing temperature of blends decreases generally, and biomass with high P and K content proportion should not exceed 10% in co-firing (Dong et al., 2010).