Co-firing of biomass and wastes

Co-firing means firing materials along with coal or other fuels in the existing or modified boiler firing systems. Materials such as biomass and waste are solid fuels in their own right, but of low calorific value, of variable composition, and requiring special low-intensity combustion conditions. If mixed with coal, the average calorific value is increased, enabling it to be burned in conventional combustors and to have more average fuel properties. If coal is mixed with material such as a typical biomass, then the combustion process can be considered to be increasingly CO 2 neutral. If coal is mixed with refuse or other waste materials, its combustion provides a route to the disposal of unwanted materials with the advantageous production of energy.

Biomass co-firing

There have been extensive research to increase the use of biomass, and one of the easiest ways of introducing this technology is by co-firing with coal in existing coal installations. A similar approach can also be taken with the disposal of waste, especially domestic refuse. The combustion of biomass therefore also parallels that of coal in many respects. The first step involves devolatilization similar to that of coal.

Namely: Biomass volatiles + char

In this reaction volatiles have a composition controlled by the biomass composition, the heating rate, and the final temperature. A typical biomass will have cellulose linked via oxygen bands; hence the high oxygen content in biomass fuels. Many biomass materials contain mainly carbon and some hydrocarbons; consequently, the composition of the volatiles is dominated by the production of CO, some hydrocarbons, and a small amount of tar. The char itself has reactivity very similar to coal char and has enhanced catalytic reactivity at low temperatures due to the metallic content of Na and K. For the mixtures of biomass and coal, the initial components devolailize and their chars burn independently. The volatiles mix, however, and the calorific value of this mixture and resultant flame temperature is very much a function of the composition of the components; the combined flame temperature is fed back to the initial chemical steps and consequently there is a very significant synergistic interaction. The subsequent rates of burn out of the chars are determined by their individual reactivity (and combined temperature) and generally biomass chars burn out more rapidly than coal chars.

Waste co-firing

The types of wastes suitable for co-firing include biomass waste, municipal solid waste, and automotive tires. The comparison is variable but has a calorific value between 10-20 MJ/kg. When the waste is taken from a controlled source, e.g., waste paper, with a constant calorific value, then the process of co-firing is simplified. In the other extreme, relatively small amounts of certain toxic wastes can be co-fired with coal in cement manufacture. The cement contains the trace elements in a generally satisfactory way although great care has to be used to ensure that it meets the required environmental emission standards.