The increasing demand for low grade fuels results from a combination of three factors:
1. The long term trend for the price of coal is upwards, despite major short-term fluctuations.
2. The reduction in low-cost disposal routes for many waste materials, such as coal processing wastes and sewage sludge.
3. Growing interest in the use of biomass for energy production.
‘Low grade fuels’ are materials that have an energy content that may be recovered by direct (eg combustion) or indirect (eg gasification) processes, but have an energy content significantly lower than that of normal fossil fuels (oil, gas and coal). The lower energy content may be due to low inherent potential in the organic material, or by the ‘dilution’ of the carbonaceous material by mineral matter and water. Also, the fuel may be low grade because it has high concentrations of pollutant precursors such as sulphur. Some low grade fuels, such as waste plastics, have an intrinsically high energy content but are most frequently encountered in a diluted form such as refuse-derived fuel.
Circulating fluidised bed combustion (CFBC ) is particularly suitable for low grade fuels, or mixtures of these materials with coal, unlike competing technologies such as pulverised coal fired plant. This arises from basic design factors such as the large amount of inert bed material in a CFBC which makes it possible to have considerable variation in fuel properties, or to change fuels online without significant disruption to the combustion process. The circulating solids improve heat transfer and make it possible to burn high energy content fuels as well while maintaining the combustion temperature at 850–900 °C. A low combustion temperature minimises fouling and slagging of heat surfaces as ash melting and softening points are generally much higher than combustion temperature in CFB. The low temperatures also make emission control also more straightforward. CFBs solids circulation provides a long residence time for fuel and limestone particles meaning high combustion efficiency and low sorbent consumption.
So CFBC has definite advantages for utilising low grade fuels, but still different fuels present challenges to the technology. The benefits of using these fuels must be weighed carefully against the plant design and operating practice modifications, the recoverable energy content of the fuel and the fuel cost. These sometimes conflicting requirements are summarised in the figure below.
Different manufacturers have tackled these issues to ensure the availability of reliable plant with considerable success. Dr Ian Barnes’ latest report for the IEA Clean Coal Centre Operating experience of low grade fuels in circulating fluidised bed combustion (CFBC) boilers sets out examples of the range of low value fuels, their reserves and properties, with particular emphasis on coal-derived materials, the issues for CFBC plant in utilising these fuels and selected examples of manufacturer and operator experience with purpose built, or modified CFBC plant. Finally an up-do-date global inventory of CFBC plants using a range of low value fuels is presented.
Dr Barnes commented: “Given their inherent flexibility, CFB-based plants seem to be the technology of choice for utilising low value fuels, either singly or in combination with coal”.
The report Operating experience of low grade fuels in circulating fluidised bed combustion (CFBC) boilers, CCC/253, 68 pp, June 2015 by Dr Ian Barnes is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports. Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration.