Cofiring

Co-firing is the combustion of two (or more) different types of materials at the same time. One of the advantages of co-firing is that an existing plant can be used to burn a new fuel, which may be cheaper or more environmentally friendly. For example, biomass is sometimes co-fired in existing coal plants instead of new biomass plants. Another example is that biomass primary fuel fractions can be cofired with waste-derived fuels in biomass plants leading to an environmentally friendly destruction of waste fractions and cost-effective heat and power production. Co-firing can also be used to improve the combustion of fuels with low energy content. For example, landfill gas contains a large amount of carbon dioxide, which is non-combustible. If the landfill gas is burned without removing the carbon dioxide, the equipment may not perform properly or emissions of pollutants may increase. Co-firing it with natural gas increases the heat content of the fuel and improves combustion and equipment performance. As long as the electricity or heat produced with the biomass and landfill gas was otherwise going to be produced with non-renewable fuels, the benefits are essentially equivalent whether they are cofired or combusted alone. Also, co-firing can be used to lower the emission of some pollutants. For example, co-firing biomass with coal results in less sulfur emissions than burning coal by itself. Co-firing is the combustion of two (or more) different types of materials at the same time. One of the advantages of co-firing is that an existing plant can be used to burn a new fuel, which may be cheaper or more environmentally friendly. For example, biomass is sometimes co-fired in existing coal plants instead of new biomass plants. Another example is that biomass primary fuel fractions can be cofired with waste-derived fuels in biomass plants leading to an environmentally friendly destruction of waste fractions and cost-effective heat and power production. Co-firing can also be used to improve the combustion of fuels with low energy content. For example, landfill gas contains a large amount of carbon dioxide, which is non-combustible. If the landfill gas is burned without removing the carbon dioxide, the equipment may not perform properly or emissions of pollutants may increase. Co-firing it with natural gas increases the heat content of the fuel and improves combustion and equipment performance. As long as the electricity or heat produced with the biomass and landfill gas was otherwise going to be produced with non-renewable fuels, the benefits are essentially equivalent whether they are cofired or combusted alone. Also, co-firing can be used to lower the emission of some pollutants. For example, co-firing biomass with coal results in less sulfur emissions than burning coal by itself. Co-firing (also referred to as complementary firing or co-combustion) is the combustion of two different fuels in the same combustion system. Fuels can be solid fuels, liquid fuels or gaseous, and its source either fossil or renewable. Therefore, use of heavy fuel oil assisting coal power stations may technically be considered co-firing. However, the term co-firing is used in the present technological framework to designate combined combustion of two (or more) fuels sustained in time, as a normal daily practice. The interest in co-firing and the use of this term sprung up in the 1980s in the U.S. and Europe, and referred specifically to the use of waste solid residues (paper, plastic, solvents, tars, etc.) or biomass in coal power stations that were designed only for the combustion of coal, and attempted, because of the existence of those new opportunity fuels, to carry out a combined combustion in order to increase benefit margins. This interest in co-firing has grown in the last decade mainly due to increasing social concern about global warming and greenhouse gas (GHG) emissions. The consequences of this concern are new policies on energy and the environment aimed at reducing emissions. Co-firing is regarded as a great opportunity for replacing coal (solid fossil fuel) used for power generation with renewable fuels (biomass) with lower costs and a direct decrease in greenhouse gas emissions. During the last few decades research has provided very diverse solutions for co-firing biomass in coal power stations with a limited impact on efficiency, operation and lifespan. In the present context the definition of co-firing could be: The use together of two (or more) fuels, the primary being fossil and the secondary from another source (renewable or residual), in a boiler originally designed for fossil fuel, either using the original combustion system or additional devices. The concept of co-firing is quite simple. It consists in the use of two or more fuels inside the same combustion device. It is applicable to all kind of combustion systems traditionally used for power generation (pulverized fuel, fluidized bed combustion and grate firing). Co-firing in cement kilns is already a quite widespread solution for valorization of waste materials mostly, as well as for biomass. The iron industry (blast furnace) and domestic sector (coal stoves) are also sectors where co-firing could be implemented. The use of a secondary fuel (biomass or waste) replacing a share of the original fossil fuel may require trivial changes in the facility, or a complete retrofitting with important reforms. Modifications will depend on the characteristics of the fuels, the original combustion technology, the plant layout and the type and location of auxiliary systems. The percentage of original fuel replaced, also known as co-firing rate (either expressed in mass or in energy basis) is furthermore a definitive parameter limiting the technical solutions valid for a specific plant. The co-firing systems, according to the current state of the art and the future perspectives, can be classified into direct and indirect co-firing technologies. The former refer to those systems where combustion of both fuels takes place at the same combustion device or into the same boiler simultaneously. The secondary fuel (biomass, waste) may be either mixed with coal before the combustion starts or fed by a separate device, e.g. specific biomass burners. Indirect co-firing, on the contrary, separates the combustion of both solid fuels, though Combustion Gasses may be mixed afterwards. Direct biomass co-firing systems entail advantages of simplicity and economics. However direct co-firing systems are also more sensitive to variations in fuel quality and heterogeneity. Additionally other problems limit the rate of secondary fuel replacing the original fossil fuels. In example ash deposition (fouling and slagging) and corrosion usually increase with the use of biomass and wastes replacing coal, what may shorten the lifespan of diverse devices in contact with Combustion Gasses like superheaters, heat exchangers, selective catalytic reduction (SCR), etc. Direct co-firing systems include next technological solutions: Indirect co-firing systems imply usually more complex and expensive solutions, but they reduce usually problems related with corrosion, fouling, slagging, etc. This, a priori, allows co-firing rates larger than direct systems, that is, larger percentages of coal substituted by biomass or waste. In addition, indirect co-firing systems are in general better for fuel mixtures where secondary fuel may include potential contaminants like heavy metals or other dangerous inorganic compounds.