Application of CFD in NOx reduction
A study of parameters affecting NOx formation in an ageing boiler unit confirmed the measures needed to meet emissions targets
Shahebaz Mulla and G Raman
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Most refineries processing crude oil employ a large number of combustion units, including fired heaters for heating process fluids and boilers for steam generation. Refinery off-gases and natural gas are the fuels of choice for combustion. Combustion is a process in which fuel reacts with oxygen to produce carbon dioxide and water. It also results in a visible flame and useful heat. The adiabatic flame temperature is about 1650°C, which results in the formation of NOx.
The term NOx is used in general to describe a group of compounds of oxides of nitrogen, such as NO, NO2, N2O2 and others. NO2 is widely used in combustion calculations as a representative of NOx. Three types of NOx are formed during combustion: thermal NOx, fuel NOx and prompt NOx. Thermal NOx is formed when N2 and O2 combine at high temperatures. Nitrogen from fuel combines with oxygen to form fuel NOx and prompt NOx. Figure 1 shows the various sources of NOx that are emitted to the atmosphere.
It is necessary to reduce NOx because it is a greenhouse gas. It produces ozone (O3) and forms acid rain. The US government passed the Clean Air Act in the 1970s to reduce air pollution and set air quality standards. The Environmental Protection Agency (EPA) and state pollution control agencies, such as the Texas Commission of Environmental Quality (TECQ), enforce the emissions limits. Over the last 40 years, there have been a number of additions and revisions, which have set stricter emission standards for air pollutants, including NOx. Currently, for the state of Texas, TECQ NOx emission limits for old boilers with a duty of more than 40 MMBtu/hr are set at of 0.06 lb/MMBtu (HHV basis).
The formation of NOx increases with a higher combustion temperature. Fuel gas containing nitrogen produces more fuel NOx. Air also has a linear relationship with NOx formation. In theory, NOx can be significantly reduced if stiochiometric air is supplied for combustion and all oxygen reacts with fuel to form carbon dioxide and water. In practice, some nitrogen reacts with oxygen to form NOx. This results in incomplete combustion, formation of carbon monoxide and loss of useful heat. Hence, it is necessary to provide excess air for combustion. In natural draft systems, it is common to provide 15–20% excess air. In a preheated, forced draft system, the excess air is about 10–15%.
NOx reduction methods1,2
NOx reduction methods can be broadly classified into two groups: pre-combustion and post-combustion.
These methods apply various techniques to reduce NOx formation during the combustion process:
• Low NOx burners One of the most popular methods is installing low NOx burners. Staged air, staged fuel, external flue gas recirculation and ultra-low NOx burners with internal flue gas recirculation are commonly used. Each type works on a different principle and is best suited to a certain combustion system in achieving reduced NOx. Most of these burners help in reducing flame temperatures and residence time, thereby reducing thermal NOx.
Selecting a particular type of low NOx burner depends on several factors, such as type of fired equipment, type of draft system (forced or natural), air preheat or no air preheat, gas or liquid fuel. A combustion specialist can best advise on the selection of a suitable type of burner
• Flue gas recirculation External flue gas recirculation (FGR) reduces NOx by recirculating part of the flue gas back into the combustion zone. FGR reduces the flame temperature by diluting the oxygen content of the combustion air and distributing the heat of combustion into a larger amount of flue gas. It is recommended that less than 30% of flue gas generated should be recirculated. FGR not only reduces NOx, but also reduces the boiler heat duty. To be able to produce the same amount of steam before flue gas injection, the boiler has to fire more fuel. Therefore, the burners have to be designed for higher firing rates. FGR is frequently used along with low NOx burners in systems where air preheat is present
• Water or steam injection Water or steam injection can lower NOx levels, but result in a decrease in thermal efficiency of 3–4% and a decrease in the capacity of the boiler. It is advised to use water/steam injection for NOx reduction only under special operating conditions.
These methods reduce NOx after it has formed rather than controlling it during the combustion process. Selective catalytic reduction (SCR) is a method in which a mixture of flue gas and ammonia passes through a catalyst at a specific temperature (230–510°C) to achieve NOx reduction to N2 and water vapour. Three types of catalysts are used in SCR, depending on the flue gas temperature. Low-temperature, platinum-based catalysts are very effective in an ideal temperature range of 260–340°C. Medium-temperature, vanadium-titanium-based catalyst has an ideal temperature range of 290–400°C. For high flue gas temperatures, a zeolite-based catalyst, with an ideal operating temperature range of 450–510°C, can be used.
Selective non-catalytic reduction (SNCR) is another method, in which ammonia or urea is injected directly into the firebox or duct containing the flue gas at a high temperature. SNCR requires a temperature of about 870–1370°C. NOx reduction of 75% can be achieved with SNCR, whereas with SCR a reduction of 90% can be achieved.
There are other methods available and more information on them can be found in EPA1 and American Petroleum Institute (API)2 publi-cations. Two NOx reduction methods are often combined to meet the stricter NOx emissions results, which applies to the case reported here. A comparison of NOx emissions from a typical boiler burner in various circumstances is shown in Table 1.
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