Biomass for Energy
Biomass can be defined as any material which is, or is derived directly or indirectly from, plant matter, animal matter, fungi or algae. This includes wood, straw, energy crops, sewage sludge, waste organic materials and animal litter. Barry Caslin has more information
Biomass can be defined as any material which is, or is derived directly or indirectly from, plant matter, animal matter, fungi or algae. This includes wood, straw, energy crops, sewage sludge, waste organic materials and animal litter.
Wood fuels can take the form of logs, chipped wood, shredded wood or pellets. Logs are commonly used in small-scale systems (less than 50kW thermal output), which are manually loaded on a daily basis. Chipped or shredded wood is generally used for automated systems from 50kW up to 1MW and beyond. Pellets have a much higher energy density than other wood fuels, and are commonly used in smaller automated systems or where space is restricted.
Energy crops can be divided into several categories: short-rotation energy crops; grass-energy crops; and, other agricultural energy crops (which may be grown for biogas production or to make transport fuels, and are not discussed further in this guide).
Sewage sludge, waste organic materials and animal litter usually require specific energy conversion and fuel-handling equipment. The type of equipment will be determined predominantly by whether the fuel is wet or dry. Wet fuels are usually unsuited to combustion or gasification as it takes energy to dry the fuel before these processes can take place. It must be noted that there are environmental and regulatory constraints associated with the use of waste as a biomass fuel. These constraints may significantly increase the costs associated with the installation and maintenance of a waste- fuelled biomass plant.
Biomass has been successfully used for heating for many decades and is a proven technology. Biomass heating systems can be used for space heating, hot water production, steam production, or a combination of these uses.
While there are a number of different types of biomass boilers, the key elements of biomass systems are the same. Fuel is fed to the grate mechanically, where it undergoes combustion to produce energy. Biomass combustion takes place in the following four stages, which can occur simultaneously:
- Warming and drying
- Combustion of gases
All fuel supply and specification is vitally important to the effective operation and success of a biomass installation. Certain biomass boilers are designed to operate on a range of fuel types and specifications, but the most efficient systems require a particular grade of fuel.
Biomass fuels are made up of a number of different characteristics but the most important variables to consider are: Moisture content as a percentage of the weight; Particle size or dimensions; and Contaminants.
The production of heat by a biomass installation is now supported by the Support Scheme for Renewable Heat (SSRH). The SSRH provides a subsidy payment based on the amount of heat produced by the boiler that goes in to the process or building being served.
Biomass combined heat and power
Combined heat and power (CHP) is the simultaneous generation of usable heat and power in a single process. CHP units recover the steam and hot water produced in generating electricity for further use in industrial processes or community and space heating. Large scale (>1MW) biomass CHP units typically use conventional, super- heated steam turbines to generate electricity. Alternatively, a gas turbine can be used with a biomass gasification plant. At a smaller scale, CHP technology based on renewable fuels is still being developed and is not wholly proven in Ireland.
Emissions standards and sustainability
For details about limits on biomass emissions contact the Sustainable Energy Authority of Ireland (SEAI) on 01-248 4982 or email@example.com. The UK’s Renewable Heat Incentive (RHI) scheme ensures that emission limits are observed. Flue gas emission standards were introduced for the UK’s RHI eligibility from September 23, 2013, and proof of compliance is now required on application to the RHI. The UK standards are:
- NOx limits of 150g/GJ energy
- particulates less than 30g/GJ.
While challenging at first for some biomass systems, especially those operating on straw or miscanthus fuel, it appears that most models of boiler can meet regulatory limits. Check carefully with your supplier that emissions compliance has been met for your intended fuel.