Optimising fuel flow in pulverised coal-fired boilers – how and why?

Poor pulverised fuel (PF) distribution to the burners in a coal-fired boiler reduces combustion efficiency, increases wear of equipment and emissions, and so is bad for the economics of a plant.

In her new report for the IEA Clean Coal Centre, Optimising fuel flow in pulverised coal and biomass-fired boilers, Dr Maggie Wiatros-Motyka explains that if coal flow to the burners is too high carbon-rich zones can form which leads to increased slagging and carbon monoxide (CO) emissions. If coal flow to the burners is too little, oxygen-rich zones can form which may increase emissions of nitrogen oxides (NOx). A burner which delivers pulverised fuel at too high a velocity causes increased erosion of the system and high carbon-in-ash levels and can also cause detachment of the flame within the boiler. Conversely, delivery at too low a velocity can cause fall out of particulates and create pipe blockages which lead to fires and explosions.

The first step is to make reliable and repeatable measurements. Then accurate control and optimisation of the fuel flow can be introduced.

Measuring particle fineness as well as fuel flow is dominated by traditional isokinetic sampling in the majority of power plants, despite the fact that this is the least accurate of currently available measurement systems. New, online fuel flow systems are based on a number of techniques including laser, white light, acoustic emission, microwave, electrostatic and mathematical cross-correlation. There are a number of factors which can affect the performance of fuel flow measurement systems such as instrument location, proximity to an orifice, flue gas temperature and velocity. Thus, when choosing the most suitable instrument for fuel flow and control, it is important to consider if the equipment can be incorporated into the existing coal pipe geometry, and if not, what changes are required.

Reducing coal particle size is important for optimisation so mill performance must be managed well. The mill settings can be adjusted in response to coal fineness. New, non-extractive online particle fineness measurement systems are accurate and less labour intensive and can provide real time results. Often, they can also determine coal velocity or coal mass flow. Various parts of the mill can be adjusted to improve performance, such as throat clearance, spring compression, alignment of classifier blades, primary air supply and mill temperature.

Biomass particles are more heterogeneous than coal and their flow fluctuates more so in this situation fuel flow measurement systems must be calibrated carefully. Frequent and careful calibration is required to obtain an absolute fuel flow mass measurement. Moreover, for dense biomass flows (> 2.5 kg/s of biomass) some fuel mass flow measurement systems may not be accurate.

All air flows in a power plant must be measured and controlled to achieve optimum combustion at the boiler and to avoid problems such as high furnace exit gas temperature, secondary combustion, overheating in the back-pass and slagging.

The measurement and control of all combustion air streams has improved. New systems range from advanced pitot tubes through electrostatic based systems to virtual and optical sensors. Additionally, low NOx burners now have the option of individual burner measurement systems.

Instruments for measuring carbon-in-ash, oxygen and CO have been developed further. These measurements are excellent indicators of fuel flow optimisation and can be used in online control of both excess air and coal flow to the individual burners.

Currently available, online, non-extractive carbon-in-ash analysers are much more accurate and less labour intensive than extractive systems. Developments in oxygen and CO measurements enable simultaneous analyses of these emissions as well as other flue gas components. The sampling location is extremely important as it determines the representativeness of the results.

Dr Maggie Wiatros-Motyka found that regardless of the chosen system and the scale of the optimisation, plant operators reported clear benefits of using flow optimisation equipment. The benefits included: improved efficiency, greater flexibility, lower NOx and CO emissions, reduced carbon-in-ash, lower overall operational cost and improved performance of various equipment such as PM control technologies, and significant reductions in safety hazards and increased fuel flexibility. Despite these results and being commercially available, the majority of power plants do not yet use these systems. However, as emission limits are tightened and economic pressures increase, interest in optimising fuel flow is expected to grow.

Optimising fuel flow in pulverised coal and biomass-fired boilers, CCC/263 by Dr Malgorzata (Maggie) Wiatros-Motyka, 61 pp, January 2016 is available from the IEA Clean Coal website,-CCC-263