Regulations continue to reduce acceptable levels for air pollutants such as NOx. There are four general strategies for reducing combustion-generated pollutants: pre-treatment, process modification, combustion modification, and post-treatment. In pre-treatment, the fuel, oxidizer, and/or raw materials are modified in some way. An example of pre-treatment to reduce NOx would be to replace the air for combustion with pure oxygen which removes the nitrogen typically used to form NOx. Another example would be switching to a less polluting fuel. An example of process modification is to improve the thermal efficiency of a process which reduces the fuel consumed per unit of production and also proportionately reduces emissions. Some examples of combustion modification include flameless combustion and ultra low NOx burners (ULNBs). Selective catalytic reduction (SCR) is an example of post-treatment where NOx is removed from the combustion products before they exit the exhaust stack.
Pre-treatment may not be an economical alternative because modifying the incoming oxidizer, fuel, or raw materials can be very expensive. For example, replacing the combustion air with pure oxygen can increase operating costs significantly. For process modifications, improving the thermal efficiency of a process is typically a good strategy because operating costs are reduced while simultaneously reducing pollution. Assuming this has already been done, the question then is what is the next best strategy if emissions are still too high?
California probably has the most stringent pollution regulations in the world. There was a time when the only way to meet the NOx regulations was with SCRs. Today there are some next generation ULNBs that can meet those regulations as well. Numerous studies have compared the cost of NOx removal using ULNBs compared to SCRs. The latter are often higher than for ULNBs per unit mass of NOx removed. In general, it is usually much less expensive to prevent NOx formation than to remove NOx that has been formed because it is in such low concentrations. However, in some cases ULNBs may not be able to get low enough emissions so SCRs may be an effective alternative.
There are many factors to consider when comparing ULNBs and SCRs such as capital and operating costs, operating windows, other pollutants that may be formed, space requirements, maintenance, and range of operation. This paper will consider those factors and present some case studies using these technologies. As might be expected, in some cases ULNBs may be preferred and in other cases SCRs may be preferred.
