682 Process Modification

There are a number of techniques that may be employed to change the existing process in such a way as to reduce NOx emissions. These methods are often more radical and expensive and are not often used except under somewhat unusual circumstances. These must be analyzed on a case-by-case method to see if they are viable.

6.8.2.1 Reduced Production

If the mass of NOx emitted from a plant is too high, an alternative is to reduce the firing rate, which means a corresponding reduction in production. The reduction in NOx is proportional to the reduction in firing rate as less fuel is burned and therefore less NOx is formed. However, this is generally not a preferred alternative for obvious reasons as less of the product being made is available to sell. Depending on the costs to reduce NOx, this may be the most economic alternative in some cases.

In boilers, reducing the firing rate reduces the overall temperature inside the boiler, which reduces the thermal NOx formation [57]. This technique is known as derating and is not desirable if the boiler is capacity limited, but in certain limited applications it may be a viable alternative.

6.8.2.2 Electrical Heating

One process modification that is sometimes used to minimize or eliminate NOx emissions is to replace some or all of the fossil-fuel-fired energy with electrical energy. The electrical energy produces no NOx emissions at the point of use and moves the emissions to the power plant. In general, the resulting NOx emissions at the power plant are often lower than at an industrial site because of the strict limits imposed on the plant and the various methods employed to minimize NOx that are often more cost effective on a unit mass basis because of the economies of scale.

There are a number of potential problems with this method. The first is that the economics are usually very unfavorable when replacing fossil fuels with electrical energy. In most hydrocarbon and petrochemical processes, the fuel used in the heaters is a by-product that is available at little or no cost. On the other hand, electrical energy is often much more expensive than even purchased fossil fuels like natural gas or oil. Besides the higher operating costs, there would be substantial capital costs involved in converting some or all of the existing fossil-energy heating to electricity. Besides the removal of the existing burners, there would be the cost of the new electrical heaters and often large costs of installing electrical substations that would be required for all of the additional power. In many parts of the country, large additional sources of electricity are not readily available so a new source of electricity may need to be built at the plant, such as a cogeneration facility. However, although the electrical costs may be reduced in that scenario because the transmission losses are much lower, the NOx emissions are now at different locations at the site and little may then be gained in reducing overall NOx emissions for the plant. It is likely in the future that regulations will consider the net NOx generated during the production of a product and would include the NOx formed in the generation of electricity. This will make replacement of fossil energy with electricity less attractive as most of the power generated in the United States is by fossil-fuel-fired power plants.

6.8.2.3 Improved Thermal Efficiency

By making a heating process more efficient, less fuel needs to be burned for a given unit of production. Since the firing rate is directly proportional to the NOx emissions, less fuel used equals less NOx produced. There are many ways to improve the efficiency of a process. A few representative examples will be given. One is to repair the refractory and air-infiltration leaks on an existing heater. This is often relatively inexpensive and saves fuel while reducing NOx. Another is to add heat recovery to a heating process that does not have it currently. The heat recovery could be in several forms. One method is to preheat the incoming combustion air. As previously discussed, this can increase NOx emissions due to the higher flame temperatures if it is not done properly. Another method is to add a convection section on to a heater that does not presently have it. This has other operational benefits as well and is often a good choice. A more drastic method of increasing the thermal efficiency of a heating process is to replace an old existing heater with a new, more modern design. This may make sense if the existing heater is very old, requires high maintenance, and is not easily repairable or upgradable.

6.8.2.4 Product Switching

Another radical process modification that can reduce NOx is to switch the product being produced to one that requires less energy to process. In a process heater, this would involve replacing the existing process fluid with one that requires less energy to heat. For example, the heavier crude oils require more energy to process than do lighter, purer crudes so less energy would be needed to process the latter. Less energy consumption means less NOx generated. However, this is obviously not an option in most cases and is only considered under extreme circumstances. In the above example, purer or "sweeter" crudes are much more expensive raw materials than less pure or more "sour" crudes. Therefore, the savings in energy may be more than offset by the higher raw material costs.

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