Mike Andrews, MD, OEN Enterprises
South African coals generally, and all Gondwana coals in particular, have organic materials in them (i.e. macerals) which do not occur at all in the well-known northern hemisphere coalfields.
SA coals have some reactive materials (vitrinite macerals) like the northern coalfields but also have very high proportions of less-reactive or non-reactive organic materials (inertinite macerals). The coals in the northern countries and other parts of the world are composed predominantly of vitrinitic reactive materials (80-90%) which burn at the usual 1200 to 1400degC. South Africa have an average of 20-25% of those reactive materials but also have 75-80% of the inertinite materials which burn at between 1400 and 1700degC. The reason for the different proportions is in the origin of the coals – namely, climate and the conditions in which the original vegetable matter accumulated.
Problems with burning South African coals
Most coal-fired boilers, furnaces and kilns currently in use in South Africa were designed and built to burn the standard northern-hemisphere coals. When good-quality coal was plentiful, the undesirable characteristics of SA coals were generally not too serious in most applications, but as the export markets expanded, the high- quality SA coals have been allocated to that sector leaving only the low-quality coals for local users.
The main problems with burning low-quality SA coals are:
$11. High-temperature combustion with coal-beds, flames and fireballs reaching temperatures up to 1750degC. Depending on the process, this can result in burnt grates, thermal damage to boiler tubes and refractory walls as well as off-spec product.
$12. Difficulty in igniting the fuel and maintaining the flame which can result in flame-outs.
$13. Delayed or extended combustion. This phenomenon can result in a wide range of problems such as unbalanced fireballs, extended flames, un-burnt fuel and off-spec product…
$14. Increased clinker formation associated with high-temperature combustion and high ash content.
$15. Increased NOX emissions. The formation of NOX from the combination of nitrogen in the air and excess oxygen in the combustion gases is temperature-related and these gaseous emissions increase proportionately with temperature.
In order to optimize any combustion process, the process must be monitored. Some essential parameters that need to be measured are excess oxygen, Combustibles, Combustion temperature, Flue gas exit temperature, Coal sizing, handling and bed profiles, Pulverised Fuel particle size and flow and Coal quality.
South Africa coal users have very little choice but to use inferior coals that are often not suitable for achieving optimal results in specific applications which includes amongst others power and steam boilers, grate-fired boilers and furnaces, waste furnaces and rotary kilns.
Optimisation of coal-based combustion processes can no longer be achieved through the simple application of manufacturer’s operating instructions that were based on burning high quality coal. Operators need to understand the unique combustion characteristics of South African coals and combine this with on-line monitoring that will enable them use specific boiler control strategies in order to optimise their processes.