Industries where emissions control is critical include electric utilities, oil and gas, chemical processing, iron and steel, paper, food, mining, metals and cement. But emission control concerns are not limited to these industries. Systems to control and reduce emissions are required for any industrial process that produces sulphur and nitrogen dioxides (popularly referred to as SOx and NOx), the major causes of acid rain, as well as airborne particulates and volatile organic compounds (VOCs).
On the horizon are new regulations designed to limit mercury emissions in flue gas. These rules will also apply to industrial facilities, including trash burners and industrial boilers, even if they only generate process steam. More stringent controls on particulates will also require new investments in emissions technologies.
NOx and SOx reduction
The U.S. Clean Air Act, the Clean Water Act, the pending Casper regulations and proposed Environmental Protection Agency (EPA) limits on greenhouse gases are driving the development of improved emissions control technologies. Casper regulations will further reduce NOx and SOx emissions limits in the Northeast and in certain Midwestern states such as Texas and Illinois. In addition to these government measures, green initiatives by corporations also emphasize emission reductions.
The list that follows touches upon the primary emission reduction methods used in industry:
• Optimized process control is central to reducing NOx emissions from coal-fired power plants. Oxygen is injected into the boiler to improve combustion and prevent pockets of NOx from being created. A secondary technology, selective non-catalytic reduction, or SNCR, injects urea or ammonia into the boilers, further reducing NOx emissions by up to 20 percent. New low-NOx burners have also been introduced that allow a cooler, more complete burn.
• The most successful NOx reduction technology—at 90 percent—has been selective catalytic reduction, or SCR. This capital-intensive technology, which is viable only for large coal-fired plants, involves very large reactors and again injects ammonia into the flow. Automated systems measure NOx levels before and after the reduction process, enabling operators to fine-tune the process.
• Distributed control systems manage the complex processes involved in balancing boilers, injecting air and adjusting dampers to optimize combustion, measure emissions and control heat levels within the boiler to prevent the build-up of slag. By tightening process controls, operators can decrease the amount of raw materials and energy used while reducing waste.
• Scrubbers, using either dry or wet processes, use automated systems that regulate water flows, monitor pH levels and spray lime or apply a slurry of limestone to remove 95 percent or more of sulphur dioxide. A by-product of the scrubber process is calcium sulphate, which is then used to make wallboard.
• At the stack, the air from the process is passed over a rack of sensors that measure oxygen, carbon dioxide and carbon monoxide levels, as well as sulphur dioxide, sulphur trioxide and nitrogen oxide content.
• Most air pollution control devices (APCDs) come as pre-built OEM packages that include PLC-based automation systems that tend to operate independently of the central DCS system. As new regulations drive greater investment in these APCDs, these systems will need to work together in a more holistic fashion. Achieving this goal will likely require additional automation integration.
Alternative fuel strategies
Although coal has traditionally made up 50 percent of the fuel source for American electric utility plants, many operators have begun building natural gas-fired plants that do not produce nitrogen or sulphur dioxide. Others are using flexible fuel processes, replacing up to 25 percent of their coal fuel with natural gas. Still others are blending coal from different regions, mixing high-sulphur but lower cost Appalachian coal with low-sulphur but higher cost Western coal to reduce the amount of sulphur dioxide their processes have to remove.
Each of these alternative fuel strategies, however, can complicate process control and require additional steps and systems to optimize combustion and reduce emissions. Blending highand low-sulphur coal, for example, can create a slag layer in boilers that requires installing a soot-blowing system to break up and remove it.
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