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