NOx & SOx as Pieces of an Overall Emissions Control Puzzle

Eco Power Solutions operates two of its systems - one on gas-fired boiler and the other for coal-fired - at their technology center in Louisville, Ky. Photo courtesy of Eco Power Solutions.
Eco Power Solutions operates two of its systems – one on gas-fired boiler and the other for coal-fired – at their technology center in Louisville, Ky. Photo courtesy of Eco Power Solutions.

By Kevin Crapsey, Vice President of Corporate Strategy and Development, Eco Power Solutions

Since the passage of the Clean Air Act, fossil-fired power generators have fueled a robust market for technologies to address nitrogen oxides (NOx) and sulfur oxides (SOx) emissions. The industry estimated in 2009 that it had spent $75 billion to comply with the Clean Air Interstate Rule alone. A number of different technologies have succeeded in addressing one or both of these pollutants, including flue gas desulfurizers (FGDs), selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) and sorbents (materials used to absorb gases and liquids).

While this market addressed the needs of power generators in the early 1990s, it may be time to consider the needs of 2013 and beyond. As the Electric Power Research Institute (EPRI) noted under its Integrated Environmental Controls program (Program 75), “fossil fuel-burning power plants also need lower-cost and better-performing sorbents and technologies than those currently available.”

One of key reasons to review emissions control options is that NOx and SOx are not the only pollutants that are regulated. Gas and coal power generation plants must address emissions to remain competitive due to additional regulations by the Environmental Protection Agency (EPA). The introductions of the Mercury and Air Toxics Standards (MATS) and upgrades to the Maximum Achievable Control Technology (MACT) rules need to be tied into NOx and SOx control strategies.

Traditional emission control technologies only capture one or two pollutants at a time, meaning that addressing the full complement of regulated emissions requires capital expenses for multiple technologies and excess space to install. The lack of technological options to comply with EPA regulations has resulted in uncertainty among power plant operators about the ability to meet deadlines and deliver cost-effective electricity.

The emission control industry, however, has introduced some advanced systems that may solve this problem. Multi-pollutant emissions control systems have been documented capturing NOx and SOx – as well as many other regulated emissions – with one system, less costs and, generally, less space. Also, many of these systems capture greater amounts of NOx and SOx than traditional technologies, can be modified to accommodate predicted future emissions restrictions and can be installed in comparably short amounts of time.

Current NOx and SOx Control Options

Traditional flue gas desulfurizer (FGD) systems are built for the express purpose of removing SOx from the exhaust flue gases of fossil-fuel power plants and sometimes from the emissions of other SOx emitting industrial processes. These systems generally employ five removal methods:

  • Wet scrubbing that uses alkaline sorbent or seawater to scrub the flue gas;
  • The spray-dry scrubbing that uses similar sorbent slurries;
  • Wet sulfuric acid process that recovers sulfur in the form of sulfuric acid;
  • SNOX Flue gas desulfurization that removes sulfur dioxide, nitrogen oxides and particulates from flue gases;
  • And dry sorbent injection systems.

FGDs employ two stages: one to remove fly ash and the other to remove SOx. In wet scrubbing systems, the flue gas passes through a fly ash removal device, either an electrostatic precipitator or a wet scrubber, and then into the SOx-absorber. In dry injection or spray drying operations, the SOx reacted first with the sorbent, and then the flue gas passes through a particulate control device.

These devices have been applied to combustion units firing coal and oil from 5MW to 1500MW. Approximately 85 percent of FGDs units installed in the United States are wet scrubbers, 12 percent are spray dry systems (similar to dry injection systems) and 3 percent are dry injection systems. On average, web scrubbers achieve the highest SOx removal rates – above 90 percent – and dry injection systems achieve the lowest – below 80 percent. The choice between the two is often dependent on costs and the nature of the power generation facility.

With selective catalytic reduction (SCR) systems, flue gas is first treated with a reactant, which oxidizes it, and then is absorbed into a catalyst. These catalysts are manufactured from various ceramic materials, which are used as carriers, and active catalytic components that are either oxides of base metals, like vanadium or tungsten, zeolites or precious metals. Naturally, each catalyst component that makes up an individual catalyst has its advantages and disadvantages, so component selection is often tailored to the nature of each facility and mix of gases that will be absorbed. Commercial SCR systems generally reduce the level of NOx by 70 to 95 percent and can be found on large utility boilers, industrial boilers and municipal solid waste boilers.

These systems have their limitations. SCR systems have been documented as sensitive to contamination and plugging during normal, and abnormal, operations. The pores of the catalyst are easily plugged by a variety of compounds present in ordinary flue gas and certain pollutants can render the system ineffective at NOx reduction, or cause oxidation of ammonia present (forming more NOx). They require tuning to perform properly, which can be time consuming and cost restrictive and have a period during their start-up cycles where exhaust temperatures are too cool for NOx reduction to occur. This can lead to unchecked emissions and fines from government offices.

When speaking specifically about coal-fired power plants, SCR systems have operational difficulty with binding of the catalyst by fly ash. Because of these issues, SCR catalysts have a limited operational lifetime of 16 to 40 thousand hours in coal-fired power plants, depending on the flue gas composition, and up to 80 thousand hours in cleaner gas-fired power plants.

In selective non-catalytic reduction (SNCR) systems, a reagent is injected into the furnace to react with the heated flue gas and convert the present NOx into nitrogen and water, which can then be captured and stored. Though in theory SNCR systems can achieve roughly 90 percent removal rates, practical constraints like required minimum temperatures, time and mixing often lead to results ranging from 30 to 50 percent. Though SCR systems have been documented as more effective in NOx removal, SNCR systems are often favored due to their lowers cost since they do not use a catalyst.

A typical SNCR system consists of reagent storage, multi-level reagent-injection and control instrumentation equipment. The SNCR reagent storage and handling systems are similar to those for SCR systems, but because of the higher stoichiometric ratios, both ammonia and urea SNCR processes require three or four times as much reagent as SCR systems to achieve competitive NOx reductions.

Advanced Emissions Control Technology

With multi-pollutant emissions control technologies, a single system is able to remove multiple pollutants from flue gas before they are released into the atmosphere. Currently, the emissions targeted include NOx, SOx, mercury and other heavy metals, halogens and particulate matter. A considerable advantage of a multi-pollutant strategy is lower capital investment when compared with investing in several different technologies to address each pollutant. Likewise, the installation of a single unit is faster and requires less downtime. This is especially relevant due quickly approaching deadlines. Also, multi-pollutant technologies generally require a smaller footprint since their processes are encapsulated in one system.

Given the advantages of multi-pollutant emissions control technologies, what are the barriers to larger scale deployment in the power sector?

The primary barrier is that traditional emissions control technologies seem like a safe bet because they have been used by the industry for years. The owners of fossil-fired power plants know the capital investment, and operations and maintenance (O&M), costs of older technologies and operators can more easily anticipate problems that may arise.

However, the commercial readiness of multi-pollutant emissions control systems has been overlooked.

Eco Power Solutions operates two of its systems – one on gas-fired boiler and the other for coal-fired – at their technology center in Louisville, Kentucky. Visiting engineers, investors and public officials can see, in real time, the performance of the systems in real world tests. As demonstrated during at technical tour at Coal-Gen 2012, the technology scrubbed the flue gas from the coal-fired boiler and removed 98.6 percent on NOx, 99.9 percent of SOx and captured 75.5 percent of CO2.

Its system injects ozone and hydrogen peroxide into the flue gas stream to convert the criteria pollutants, which include NOx, SOx, mercury, acid gases and particulate matter, into an oxidized, water soluble compound. The flue gas then passes through a series of high pressure fogging arrays that inject a water mist into the gas stream that creates a condensable acidic mist. The water mist cools the flue gas and the pollutants are condensed out from the flue stream once the temperature drops below the acid dew point. The waste stream is then treated, neutralized and disposed of safely. The remaining water can them be treated and recycled back into the process to conserve water.

There are also several other technologies taking a multi-pollutant approach.

Lextran Ltd reduces SOx, NOx and mercury. Like SCR systems, Lextran’s absorbs pollutants through a catalyst, in this case an organic substance in an emulsion form, in a wet scrubber environment process. The Lextran catalyst contains an active sulfur-oxygen functional group, having properties that enhance the oxidation reactions of SOx, and NOx into SO4 and NO3 anions.

After initial oxidation the catalyst is released and recycled back into the process, leaving the pollutants in a chemical form amenable enough to become commercially beneficial by-products. They first must be neutralized by ammonia, KOH, or other basic reagents to control the type of by-products, which could include ammonium nitrate, ammonium sulfate, potassium sulfate and potassium nitrate.

Hamon Research Cottrell’s ReACT system captures NOx, SOx and mercury while reducing particulates. With this system, a moving bed absorber provides contact between flue gas and activated coke pellets, where SO2, SO3, NOx and mercury are absorbed onto the carbon surfaces. Ammonia is then injected upstream in order to promote the SO2 and NOx reactions as the moving bed acts as a particulate collection step. The impingement of the flue gas on the activated coke pellets provides “polishing” control of particulate.

Cleaned flue gas moves to the smoke stack for discharge with little or no plume compared to SDA, CDS, or WFGD processes. Activated coke from the absorber is then processed in a regenerator vessel that completes the reduction of NOx to N2, and drives off SOx in a concentrated sulfur rich gas stream. Absorbed mercury is retained in the activated coke in a region of the regenerator where temperature conditions allow the mercury to accumulate.

Regenerated activated coke is screened to remove fines and returned to the absorber while the sulfur rich gas is processed using conventional technology to produce salable sulfuric acid. This process is best suited for utilities burning low sulfur coals, such as PRB coals, and for sites where water use, water treatment or discharge are issues.

Two Options for the Industry Moving Forward

Power generators should understand that NOx and SOx are parts of a holistic emissions control strategy. MATS and MACT are only hints of what is to come. Most industry experts are expecting some form of carbon regulation and even stronger restrictions on the emission of heavy metals, acid gases and particulate matter.

The industry faces two options: continued reliance on the technologies that are meeting the needs of yesterday’s regulations or investing in advanced emissions control technologies that meet the needs of today and tomorrow. Eco Power Solutions, Lextran Ltd and Hamon Research Cottrell’s technologies have already been short-listed by the EPA as systems that “offer(s) the potential of reduced compliance costs and improved overall environmental performance.”

These technologies represent the forefront of innovation in the fossil-power generation industry and provide a cost-effective option to continue operating aging plants that still have significant service time available. Likewise, it is an option for co-ops and municipal utilities that lack the resources to invest in a series of traditional technologies yet do not have the option of shutting down their power generation facilities if they cannot comply.

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