15 January 2009
The solution to this is the development of rapid response combustion analysers that measure both oxygen and combustibles at the same point. One example is the Servomex Servotough Fluegas (2700) combustion gas analyser that uses a combination of a patented zirconium oxide cell for measuring oxygen and a 'thick film' catalytic sensor for combustibles (CO equivalents, COe).
Designed for high temperature operation up to 1750°C, the Fluegas is suitable for a range of applications in extreme heated environments including process heaters, utility boilers, thermal crackers and incinerators – and therefore for use within the extreme environments with the combustion process in power stations.
Zirconia cell technology measures oxygen on a ‘wet’ basis, meaning that there is no need to condition the sample gas first. This avoids added complexities associated with a sample conditioning system and means that the readings do not have to be converted from dry-basis to wet-basis, which can sometimes be a cause of inaccurate readings. Meanwhile a thick film calorimeter that monitors carbon monoxide levels gives a precision of +/-25ppm, with sufficient sensitivity and speed of response to enable the combustion process to be continuously fine-tuned. Both sensors are installed in the same heated, insulated sensor head based in single low-flow extractive type probe is used through which oxygen is extracted through the process wall.
Both of these technologies are highly reliable, non-depleting and require no need for a sampling system to condition the gases. Response times (T90) are better than 20 seconds for O2 and 30 seconds for combustibles, which means that the process can be closely controlled and 'breakthrough' be detected extremely rapidly. As a result, the plant can be routinely operated much closer to the optimum air-fuel ratio.
In addition to the physical environmental benefits, further economic advantages are created for operators in countries such as the USA where mechanisms exist for trading of NOx credits. If significant reductions in NOx emissions can be achieved, it is possible for the resulting NOx credits to be more valuable than the fuel savings. Even if trading NOx credits is not available to an operator, the reduced loading on any NOx cleanup equipment installed on the plant can also reduce the running costs. Finally for plants that occasionally run fuel-rich and therefore suffer from increased particulate levels, the improved combustion control will reduce maintenance costs.
One further, literal by-product of the optimised process is the potential to produce better quality ash by-product for resale brick and tarmac companies. The optimised oxygen mix helps regulate the CO level which, if too high, makes the process inefficient by having a detrimental effect on the NOx and makes the resulting ash also too black to use.
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