Calorific value: basis 6000 kcal/kg NCV, minimum 5850 NCV† Hardgrove Grindability Index: typical, and not to be used for determining whether or not a shipment complies with theSpecification‡ Fusibility of ash (DT) [ash fusion temperature - initial deformation]
Commercial testing for coal quality and behaviour during combustion continues at power plants aspart of daily operations, but results are rarely published. Public research continues but more in thefield of understanding the effects of cofiring biomass or waste with coal. Research into thecombustion behaviour of coal and fly ash from blending different coals is still published by somegroups but the findings are usually for either small-scale furnaces or associated with technologies suchas gasification or oxyfuel based environments. These papers are therefore atypical of the currentoperating fleet of power stations worldwide.Nevertheless, work has been done since the 1990s when various studies examined the effects ofchanges in ash deposition, corrosion, moisture, and stack emissions during a period when blendingand switching coal was becoming increasingly necessary. The IEA CCC studied the effects ofswitching to cheaper coals in power stations (Carpenter, 1998) in a literature study on large-scaleoperating plants. Much of the study by Carpenter (1998) focused on switching to low rank coals suchas lignite, which is an unusual measure for most power station operators. However, much of theanalysis also discussed the issues of switching from a 100% bituminous coal to a blend of, orcomplete switch to, subbituminous coals in the USA in the 1990s.It is relevant to look at the experiences in the 1990s during a time when power stations in the USAwere being subjected to stricter regulations on SO2emissions, forcing some utilities to considerblending lower sulphur coals. This section draws on the experience of power stations in the USA thattested subbituminous coals from the PRB of Montana and Wyoming. Although some generalisationscan be made about the behaviour of introducing new coals, each unique combination of coal and unitdesign is individually evaluated to confirm the acceptability of the coal and performance of thestation. Even if a blended coal product closely resembles the design coal specification, the blend maynot burn in the same way. Blending or switching to imported coals of a lower rank, notablysubbituminous coals, is more complex than just heating values and moisture, although they arefundamental to the impact assessment on boiler and power station performance. This section of thereport examines just a few aspects of introducing new coals that must be considered by power stationoperators and fuel procurement officers, these include: ash deposition, moisture content, chlorinerelated corrosion, and sulphur emissions.