By fully metering combustion air and fuel flows, refineries increase efficiencies, improve safety, and reduce emissions for the industrial boilers, fired heaters, regenerative thermal oxidizers (RTOs), and burners used in the petroleum refinement processes in the oil and gas industry. Each of these applications has a set of common problems that must be overcome in order to achieve the desired result of improved combustion efficiency, thereby minimizing unburnt carbon, reducing excess air and eliminating harmful emissions to the extent possible. An accurate measurement of combustion airflow and fuel flow is the first step to achieving these goals. This discussion will focus on the Combustion Airflow Measurements.
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The aforementioned problems are common to each application and are as follows:
- Limited Availability of Straight Run – the larger duct sizes typically associated with combustion airflow result in fewer diameters of straight run upstream and downstream of the measurement device. Often the airflow measurement is taken at the fan intake, inside of a silencer, or immediately downstream of a fan or heater discharge. The technology chosen must be able to handle the resulting velocity profile stratification common to short runs of duct.
- Broad Velocity Turndown Requirements – typical range of operation of a boiler, burner, fired heater, or RTO starts at 10% to 25% of full load and varies thru the full capacity of the unit. There is also the consideration of purge air to ensure safe operation at startup per NFPA86. The purge flow is at a greatly reduced flow compared to the full scale of the flow measurement system. The broad operational range of combustion airflow often results in very low velocities particularly at minimum load. The technology chosen must be accurate throughout this broad flow range.
- Particulate laden flow streams - Often the combustion air is contaminated with some sort of dust or other particulate from the plant environment that is pulled in through the FD fan. In other cases soot or fly ash may contaminate the combustion air when either regenerative air preheaters or leaky tubular air preheaters air used. This type of contamination can also occur when flue gas recirculation is employed as an emissions control technique. The type and amount of particulate is dependent on the fuel being used. In each of these cases where the air is particulate laden, the sensing ports on a DP measurement device are subject to plugging and the RTDs on a thermal mass system are subject to coating. An effective high pressure auto-purge system is the only proven method to keep the sensing ports clean and maintain the accuracy of the measurement system.
- High Temperatures – Combustion air to fired heaters is often measured at elevated temperatures. These combustion air ducts are typically refractory lined. The measurement technology chosen must be able to work in these elevated temperatures and be able to work with refractory lined ducts.
Find out more about how the use of Air Monitor products can help achieve your efficiency, safety, and emissions goals by contacting one of our experts.