Off gases from the cracking process are sent to regenerative thermal oxidizers (RTOs) and waste heat boilers for destruction, to generate energy, and to meet air quality standards. To get RTOs and boilers to operate at peak efficiency, operators look for accurate combustion air, fuel flow, and tail gas flow measurement devices to optimize their performance. 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, tail gas flow and fuel flow is the first step to achieving these goals. This discussion will focus on the combustion airflow and tail gas flow 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 and tail gas flow 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 tail gas is measured prior to entering the RTO or Boiler and is typically in the vicinity of a modulating valve, tee or elbow. The technology chosen must be able to handle the resulting velocity profile stratification common to short runs of duct and sometime damper modulation as well.
- Broad Velocity Turndown Requirements – typical range of operation of a boiler 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. Tail gas flow brings it own set of contamination problems. Tail gas is very nasty and will plug or foul conventional measurements quickly. The type and amount of particulate is dependent on the fuel being used in combustion air applications. Tail gas can always be counted on as one of the dirtiest and most difficult gases to measure. In each of these cases where the air/gas is particulate laden, the sensing ports on a DP type of measurement 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.
- Tail Gas density variation – The composition of tail gas is dependent on the type of oil used in the cracking process. Densities and subject to vary and target fuel / air ratios will also vary.
Find out more about how the use of Air Monitor products can help achieve your control, efficiency, safety, and emissions goals by contacting one of our experts.