182 Process Modification

Process modification refers to either replacing the incoming feed materials to either the process or the combustion system with some other alternative, or to modifying the end product in some way. Ramachandran [90] refers to the former as raw material substitution. An example of a fuel change would be to change from a high-sulfur coal to a low-sulfur coal to reduce SOx emissions. In that example, the fuel type remains the same but a different grade or composition is being used. A more radical example would be to change from fossil-fuel heating to electrical heating. In that case, the emissions are moved from the industrial plant location to the power generation facility. Changing fuels is usually referred to as fuel switching. A variation of this technique is to switch part of the fuel, part or all of the time. For example, if the NOx emissions from a combustion process are slightly over a new regulatory limit using oil fuel, then replacing some of that oil fuel with natural gas can reduce the emissions to below the new limit. This has the advantage of using as much of the cheaper fuel as possible. The potential disadvantage is that the combustion system must work on multiple fuels. However, this is not a significant problem today as there are standard burners available for multifuel firing.

In the case of the oxidizer, the combustion air can be replaced with high-purity oxygen.

Some even consider emissions trading and early equipment retirement under the category of operation changes to reduce pollutant emissions [91]. These are not considered in this book as they do not deal with technology but rather with management strategy and would normally only be viable options for a plant looking to reduce production. In the extreme case, it is possible to shut down an entire plant to sell the emissions credits if their value exceeds the profits that could be generated from selling the product.

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