Processes to upgrade low rank coals normally attempt to overcome their main shortcomings, namely high moisture content and, as a direct result, low calorific value. Merely drying these coals is not a solution because (a) they will reabsorb moisture during handling and stockpiling and (b) it will create dust problems. The answers therefore lie in modifying the pore structure of the coals as part of the drying process. Some solutions that are being tried include:
· Hot briquetting,
· The Upgraded Brown Coal process (UBC) whereby the coal is immersed in recycled
oil and heated to dry. Most of the oil is recycled, but the coal pores are sealed in the process, preventing moisture from entering,
· Sequential processes of drying, heating to generate tar from the coal, followed by
absorption of the tar to seal the pores.
These measures will reduce transport costs and reduce the required handling and milling capacities at the power plant. A valuable side-effect may be a reduction in the coal’s propensity to spontaneous combustion.
BLENDING OF LOW RANK COALS
Blending with higher rank coals may enable what are sometimes cheaper coals to be utilised in power plants not designed specifically for the unblended coals, thereby reducing the capital costs of the plant as well as the fuel costs.
However, blending also offers greater opportunities to overcome inherent problems of some higher rank coals, which may include:
· High nitrogen content which may exceed legal limits
· High ash content, resulting in difficult stack cleanup and high ash disposal costs,
· Abrasive minerals causing pulveriser wear and boiler tube erosion,
· Reflective ash limiting radiative heat transfer in the furnace,
· High resistivity ash lowering electrostatic precipitator efficiency.
Based on these considerations there are many scenarios for blending low rank and high rank coals to mutual advantage.
CONCLUSION
Many Indonesian coals have negative properties, some of which are related to their rank being lower than many Australian and other overseas coals. Depending on power plant design features, these properties will limit the plant performance and reduce the value of the coals.
Some of these limitations include:
· Low CV giving high freight costs,
· Self-heating and spontaneous combustion,
· Low CV needing high coal handling capacity,
· Low HGI together with low CV requiring higher mill capacity ratings,
· High moisture requiring greater air–heater capacity and causing mill fires,
· Slagging and fouling associated with their ash chemistry,
· Larger more expensive boilers.
Indonesian coals are typically superior to many Australian and other overseas coals in the following areas:
· Their low ash content has environmental advantages for stack cleanup and ash
disposal,
· They are highly reactive and give high burnout efficiency,
· The mineral matter has low abrasivity, giving low mill wear and boiler tube erosion,
· Some have very low sulphur contents,
· The ash has favourable electrostatic precipitation characteristics,
· They have medium to low nitrogen contents, satisfying any legal nitrogen limits,
· They produce low NOx levels.
Blending will enable many of these characteristics to complement the different characteristics of competitor coals.