With coal remaining a primary source of fuel, coal mines and coal-fired power plants continue to require efficient processing tools to maintain quality output. From the early stages of mining coal ore to the final combustion stages at the power plant, on-line quality controls must be put in place to ensure the stability of production.
At all stages in the production process, it is helpful to know general chemical composition of the coal material being used. On the mining front, there can be a strong variability in the ore composition due to geological circumstances. From the power plant perspective, the variability may also come from choosing a multitude of suppliers. At either end, process managers and engineers can gain from knowing the precise composition of the coal.
A current popular method for on-line analysis of coal ore is prompt gamma neutron activation analysis (PGNAA). With this technique, the sample is irradiated with a continuous neutron beam. The neutrons are absorbed by each of the elements within the sample, which then emit gamma rays at characteristic energies. The gamma rays are then directed towards a gamma ray spectrometer where the peaks are identified. The energies where the peaks are found signify the constituent elements within the sample, and the magnitudes of the peaks reveal the concentrations of each component. The response time for measurements is on the order of 1 minute .
While this analysis tool can be installed on a conveyor belt to continuously analyze coal samples, it has its drawbacks. The biggest drawback is the requirement of maintaining a nuclear isotope source to provide the neutrons. This draws extra costs in handling, training, meeting regulatory requirements, and replenishing the source. Furthermore, there stands the question of employee safety and shielding from the radiation. Finally, the PGNAA on-line analysis machine is quite bulky (3200 kg) and requires an involved calibration procedure. [2,3]
A long-standing pulsed laser technique that is promising for many applications in coal processing is light-induced breakdown spectroscopy (LIBS). All elements radiate characteristic frequencies of light when excited to high enough temperatures. LIBS exploits this by focusing an energetic laser pulse into the sample to be investigated. For solid targets, the laser pulse ablates a small amount of material from the target surface. The ablated material is heated to high enough temperatures to ionize and form localized plasma from the target constituents. Immediately following the plasma formation, a continuum of light frequencies is radiated from the plasma. Shortly after this phase, the plasma begins to cool and the characteristic emission lines from the target's constituent elements become visible. This light is collected and analyzed with a spectrometer to reveal the chemical make-up of the target. 
Commercial LIBS systems can be purchased that are faster (~30 seconds) and more lightweight (250 kg) than their PGNAA counterparts. Laser Detect Systems has developed commercial machines specifically for coal analysis and installation on coal mine conveyor belt systems. Preliminary studies on their effectiveness suggest that they provide an attractive alternative to traditional PGNAA methods. [2,5]
On-line analysis of coal ore at mines allows process engineers to determine the proper direction to take in mining operations. Coal compositions can vary with mine location and depth. Prompt analysis of compositions reveals whether chosen mining directions are maintaining steady quality, or whether they are moving towards unfavorable compositions.
Currently, PGNAA techniques dominate the on-line analysis of mined coal ore. With the drawbacks to this technique and the promising outlook of LIBS systems, studies have been performed to determine the feasibility of replacing PGNAA analyzers with LIBS analyzers. [2,5]
In 2007 at the Optimum Colliery mine in South Africa, Laser Detect Systems installed and tested their LIBS machine on a conveyor belt system in-line with a the PGNAA analysis machine that had been in operation for several years. Samples were tested on the on-line LIBS analyzer, the on-line PGNAA analyzer, and offline in a laboratory setting. Comparisons between the LIBS and PGNAA measurements revealed an average standard error between the two of only 0.32%. Both techniques were found to be slightly less accurate than laboratory tests, but off-line analysis is inherently much slower. [2,5]
By switching to LIBS systems for on-line coal composition monitoring, coal mines can maintain the accuracy compositional analysis, while lowering costs and risks associated with maintaining radioactive sources for PGNAA analysis.
All coal that reaches coal-fired power plants contains a small portion of inorganic material which cannot be economically removed before combustion. Depending on the source and the procedures used to purify the material at the source mine, up to 40% by weight may be unwanted inorganics. When put in the furnace at the power plants, these inorganic materials can plaster the walls of the furnace and tubing, an unwanted effect known as slagging. Slagging can lead to inefficiencies in the combustion process, degradation of the chambers, and ultimately to significant internal damage. Buildup of deposits can also require machine downtime to clean the inside of chambers. Traditionally, preparation to protect against this is laboratory analysis of the coal ash. This analysis time can be on the order of days for power plants, and therefore cannot be considered if real-time slagging risks are to be investigated. 
There is potential to use LIBS prior to the furnace in power plants to help predict when slagging is likely. In combination with artificial intelligence techniques, the measured slagging potential can provide information to boiler operators concerning the coal composition and the likelihood of slagging. This information can be crucial for smooth operations, allowing operators to take on mitigation measurements to lower the slagging potential. These measurements include blending with purer coals, rerouting to other furnaces, adjustment of the boiler temperature, or flat-out rejection of the coal. 
As with all technology, there comes a time when a new discoveries and techniques outdate the current standard. In the coal industry, this is manifesting itself with PGNAA techniques becoming outdated by LIBS analysis. LIBS offers benefits when compared to its PGNAA counterpart in that it is faster, less bulky, and does not require maintenance of a radioactive source. That it can be used at several points in the production process makes the transition to LIBS an attractive one.
© David Nicholson. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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