Various types of waste can be used as secondary energy sources.
Animal manure can be a major source of greenhouse gases. Wet slurry from housed livestock stored in bulk decomposes anaerobically (which means decomposition in the absence of air) releasing methane - a more potent greenhouse gas than carbon dioxide. Farmers are therefore encouraged to invest in controlled anaerobic digestion (see Section 4.6.3) which does not result in such emissions.
Another source is poultry litter, a mixture of chicken droppings, straw, wood shavings etc., which has an energy content in the range 9–15 GJ t−1, enabling its direct combustion for electricity generation.
Household waste is mostly collected as municipal solid waste (MSW) with an energy content of about 9 GJ per tonne. The average household in an industrialised country generates more than a tonne of solid waste per year.
In continental Europe and elsewhere, refuse incineration with heat recovery, or energy-from-waste (EfW), is an important part of waste management. The heat generated is used directly for district heating or power production.
Commercial and industrial wastes of organic origin can be used as fuel. The UK generates about 36 Mt of such specialised wastes each year, of which about two thirds are combustible, and include:
- commercial food processing waste
- fats (used oil/meat off cuts)
- incinerated hospital waste
You have now looked briefly at some of the products that can be used as secondary biomass energy sources. Now we need to look at how biomass is processed.