Exhaust gas stack

It is normally required to install an SO2 monitor in the exhaust gas stack. 

Exhaust gas stack and composition of exhaust gas leaving the stack... 

Continuous Emissions Monitoring. A key aspect of the new CAAA is the requirement that plants prove their continued compHance to new emissions limits by installing continuous emissions monitoring systems (CEMs). The CAAA imposes new requirements for monitoring NO, SO2, and CO2 levels in a plant s exhaust gas stream. Affected plants typically must gather data from stack monitoring systems, gas analyzers, and the plant s data acquisition system and provide the data in a format approved by the EPA and state regulators. CEM systems must be in place by November 1993 for boilers affected by Phase I of the CAAA, and byjanuary 1995 for plants impacted by Phase II. 

A samphng probe is placed at any location in the stack, and a grab sample is collected in an evacuated flask. This flask contains a solution of siilfiiric acid and hydrogen peroxide, which reacts with the NO. The volume and moisture content of the exhaust-gas stream must be determined for calculation of the total mass-emission rate. The sample is sent to a laboratoiy, where the concentration of nitrogen oxides, except nitrons oxide, is determined colorimetrically. 

As a result of the larger flues and the restric ted surface area per unit of gas passed, regenerators employed with this type of furnace exhibit much lower efficiency than would be reahzed with smaller flues. In view of the large amount of iron oxide contained in open-hearth exhaust gas and the alkah fume present in glass-tank stack gases, however, smaller checkerbrick dimensions are considered imprac tical. 

Application The zirconia oxygen sensor is widely used for combustion control processes and for air/fuel ratio regulation in internal combustion engines. The closed-end portion of the electrode tube is inserted into the exhaust gas stream. In the control of industrial combustion processes, no out stack sampling system is required. 

Enclosed flares are composed of multiple gas burner heads placed at ground level in a stacklike enclosure that is usually refractory or ceramic lined. Many flares are equipped with automatic damper controls that regulate the supply of combustion air, depending on temperature, which is monitored upstream of the mixing, but inside the stack. This class of flare is becoming the standard in the industry due to its ability to more effectively control emissions. Requirements on emissions includes carbon monoxide limits and minimal residence time and temperature. Exhaust gas temperatures may vary from 1000 to 2000°F. 

The supervisor turned on the switch for the exhaust gas scrubber in order to detoxify the MIC, but the scrubber had been closed for a maintenance inspection. The flare stack to detoxify the exhaust gas by combustion had also been removed for a maintenance inspection and was of no use. About 23t of MIC vapor tore out the fracture plate, broke the safety valve, and flew out of the exhaust-gas scrubber to a height of 33m for two hours without being detoxified by the scrubber or the flare stack. After erupting, the vapor crept downwind over the residents, supposedly because of the stabilized atmospheric conditions and because the vapor was heavier than air. 

There have been numerous discussions about this accident, which produced the most casualties in the history of industrial disasters. Some arguments revolve around the direct cause of the accident. As is generally known, many major accidents have been caused by combinations of small accidents. The accident in Bhopal also happened as the result of a combination of serious mistakes the mixing of water with MIC caused by neglecting to put the metal sheet in place to separate reactive components, and the failures in operation of the exhaust gas scrubber and the flare stack. Such cases are frequently found where a safety device is temporarily removed because the device is troublesome. It is necessary to educate people that the reliability of a safety device should be tested and that the failure of a safety device can lead to unexpectedly terrible results. 

An alternative is the use of an optical method to measure particulate concentrations and size distributions. This technique has the obvious advantage of having a negligible effect on the particulates since the equipment would be external to the exhaust system. An optical method also has the potential to be much simpler to use since it would eliminate the need for elaborate and cumbersome systems containing probes, stack samplers, flow development tunnels, filters, and heat exchangers. In addition, final data from an optical system could be immediately obtained electronically as opposed to weighing the various filters in a particle impactor by hand, and as such, the optical analyzer is a real time instrument capable of following exhaust gas fluctuations and other nonsteady effects. 

The ventilated air and gaseous waste from the controlled area are released through the stack after dealing with high efficiency particulate air filters. There are two methods of continuously monitoring the concentration in exhaust gas. One is to measure the gas directly with a detector inserted into the stack or duct and the other is to measure a portion sampled from the exhaust air with a dust monitor or a gas monitor. 

The exhaust gas flowrate fi-om a facility is ISOOscfin. All of the gas is vented through a small stack, which has an inlet area of 1.3 ft. The exhaust gas temperature is 350°F. What is the velocity of the gas through the stack inlet in ft/s. Assume standard conditions to be 70°F and 1 atm. Neglect the pressure drop across the stack. 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. It might also be mentioned at this time that the exhaust (cleaned gas) from an absorption air pollution control system is usually released to the atmosphere through a stack. To prevent condensation in and around the stack, the temperature of this exhaust gas should be above its dew point. A general rule of thumb is to ensure that the exhaust gas stream temperature is approximately 50°F above its dew point. 

The diverter valve operation is controlled by the waste heat recovery boiler (WHRB) and directs the exhaust gas to the WHRB boiler or out of the bypass stack to maintain the required steam pressure. The ABCO WHRB is rated at 30,000 Ib/hr and has two firing modes. In the turbine-firing mode, a 5.8MMBtu duct burner is available to supplement the turbine exhaust stream. In the direct-fire mode, used when the turbine is offline, the direct fresh-air Are at 41.5MMBtu is available. 

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