ACCESSA was established to improve the general level of understanding of the capabilities of catalytic after treatment technology. In the last 50 years there have been significant technical improvements in this field evidenced by the advances made controlling emissions frommobile sources. Large and medium sources of air pollution, from combustion and analogous processes continue to make significant contributions to air pollution. The approach to limiting these emissions has been less coordinated and the regionalised or local approach has meant that some power plants have very low emissions whilst others contribute as much pollution today as they did in the 1970s.
Regulatory authorities in the EU, USA and Japan have been under pressure from engine and equipment manufacturers to harmonize worldwide emission standards, in order to streamline engine development and emission type approval/certification for different markets. This facilitates improved air quality whilst driving economies of scale and scope, and lowering the cost of emission control technology.
The following table provides a summary of Pollutants which can be treated catalytically from specific industrial applications and the technology options which can be applied.
Selective Catalytic Reduction
Selective catalytic reduction (SCR) of NOx by nitrogen compounds, such as ammonia or urea is well proven in industrial stationary applications. The technology has found wide application with hundreds of thousands of systems installed. In addition to common applications, coal-fired cogeneration plants and gas turbines, SCR applications also include plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, as well as municipal waste plants and incinerators. The list of fuels used in these applications includes industrial gases, natural gas, crude oil, light or heavy oil, and pulverized coal.
Within the SCR system, the exhaust gas flows through the catalyst, with an injection of ammonia or urea. The chemical reactions proceed rapidly and selectively, i.e. without undesired side reactions, as described by the following reactions:
4NO + 4NH3 + O2 → 4N2 + 6H2O
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O
The SCR process requires precise control of the injection rate of ammonia or urea due to the fact that incorrect dosing may result in unacceptably low NOx conversions and/or ammonia slip to the atmosphere. According to the dominant SCR reaction detailed above, the stoichiometric NH3/NOx ratio in the SCR system is about 1. Ratios higher than 1 significantly increase the ammonia slip. In practice, ratios between 0.9 and 1 are used, which minimize the ammonia slip while still providing satisfactory NOx conversions. Using urea (or aqueous ammonia), SCR system and catalyst suppliers such as Johnson Matthey and Haldor Topsoe achieve NOx reductions of 90%+ for diesel or gas engines. Urea is considered the ideal reducing agent as it can be shipped and stored easily and is colourless, odourless, nontoxic and bio-friendly.
The standard SCR System includes SCR catalyst, durable housing, mixing duct, injection system components and control panel or an optional fully integrated skid-mounted package. The skid-mount option includes a converter, injection system components, electronic controls, urea day tank, air compressor and freeze protection.
Oxidation catalysts owe their name to an ability to promote oxidation of several exhaust gas components by oxygen, which is present in ample quantities in diesel of natural gas exhausts. The catalysts are formulated to achieve maximum conversion of CO, HAPs, VOCs and HCs from stationary gas or diesel engines in power generation, process industries and other applications. Oxidation catalysts for gas turbines were pioneered in the 1970s. Since then, they have been installed in some of the most environmentally challenging applications, consistently providing greater than 90% destruction of CO, VOCs, formaldehyde and other toxic compounds. Today, firms deliver products which can be applied to diesel engines of all sizes and very large gas engines.
Considering gas turbine oxidation catalysts, some company’s catalysts are formulated with Platinum Group Metals (PGMs) to achieve maximum conversion of pollutants at turbine temperatures. They have an established durability of 10 or more years of continuous operation, during which catalytic performance can be easily maintained or restored through washing if necessary.
While regulations for formaldehyde emissions vary around the world, it is one of the more regulated compounds due to its toxicity and odour. It can be effectively oxidized and removed from exhaust emissions, such as from automotive and industrial sources. Catalytic technology serves as an ideal method of oxidizing formaldehyde. By comparison to state-of-the-art thermal oxidation technologies, the oxidation reaction occurs at much lower temperature in the presence of a catalyst. For example, in a thermal oxidation reaction, temperatures up to 850°C are required to thermally remove exhaust components of the formaldehyde process, while with a catalytic option formaldehyde can be effectively eliminated under 300°C. Depending on how the thermal oxidation unit is heated (i.e., fuel, electricity, etc.), the catalytic option can equate to lower, long-term running costs.
NOx, VOC and particulate abatement with catalytic filtration technologies
Catalytically active filters, capable of removing particulates, NOx, dioxins and volatile organic compounds (VOC's) in one step, is a relatively new technology option. VOC’s can be a variety of species such as CO, formaldehyde, toluene, benzene and styrene. Catalytically active filters are relevant when the flue gas contains particulates and the plant is equipped with a bag filter house. Catalyst-coated filters are expected to be widely used in many industries such as power plants, cement plants, glass plants, biomass fired boilers and waste incineration.
The great advantage of the catalytically active filters is that several pollutants can be removed in one single process unit, namely the bag filter house. This eliminates the investment cost of a separate reactor for SCR and oxidation reactions, and will in many cases reduce both CAPEX and OPEX. Catalytic filters can be tailor-made to fit almost any bag house, and can thus be used to replace existing (non-catalytic) filter bags. They can be supplied as both filter bags (length 3-10 m) and ceramic filter elements (length 3 m). The catalytic filter bags can be used in the temperature interval 180°C to 260°C. The ceramic filter elements (sometimes called filter candles) can work in the temperature range from 180°C to 400°C. Removal efficiencies in the catalytic filters can be just as high as for conventional SCR and oxidation catalysts. NOx removal efficiencies can for example be more than 90% with an ammonia slip lower than 10 mg/Nm3.
In many cases, catalytic filters are an attractive option as ammonia slip catalysts, if the plant applies the SNCR technology and needs to eliminate excess ammonia. In cases where SNCR is no longer sufficient to reach NOx and ammonia emission limits “SNCR boosters”, which increase overall NOx and ammonia removal efficiency to meet the permissible levels due to stricter legislation, the catalytic filters can also be used as “SNCR boosters”, which increase overall NOx and ammonia removal efficiency to meet the permissible levels.
Methane emissions from natural gas engines
Engines powered by natural gas fuel (e.g. LNG or CNG) are seen as a preferred option for current and future power generation on land and at sea. Naturally low in sulphur and a fuel that produces relatively low NOx during combustion, natural gas also claims significant advantages in efficiency as determined by CO emissions. However these claims are not credible unless significant reductions in so called “methane slip” are made.
Natural Gas is a cleaner fuel than any other hydrocarbon fuel, being composed chiefly of methane (with some higher hydrocarbons such as ethane and propane). Under normal “lean” operation, natural gas engines suffer a significant problem whereby unburned fuel emerges in the exhaust and enters the atmosphere at the rate of ~ 6 g/kWh. Though methane doesn’t contribute appreciably to tropospheric ozone, it is a potent greenhouse gas with GHG factor 35 times that of CO at equivalent emission rates. With the advent of shale gas in the US and the growth in natural gas fuelling infrastructure, the Global Warming Potential is slowly being recognised by policy makers, but continues to go unrecognised or is ignored by many key stakeholders. Great strides have been made by the engine OEMs, but it has become clear in recent years that the efficiency benefits of natural gas can only be truly realised with very low emissions of methane, ~ 0.3 g/kWh – and that is likely to require some treatment in the exhaust gas.
Policy makers and influencers have a crucial role to play in the development and deployment of a technical solution to this problem. These include establishing a timetable for emissions regulations that set a limit of methane emissions at 0.5 g/kWh. This creates the market certainly that the private sector needs to ensure timely investment. This endeavour could be supported to facilitate and ensure timely demonstrating and deployment of technical solutions.
With each passing year, technological improvements and the enhanced economic viability of using catalytic emission control technologies to control emissions to air from stationary sources become increasingly apparent. Technological breakthroughs are consistently being made due to heavy investments being placed by the sector in research and development.
Over the coming years, legislation will demand lower pollution levels to control emissions from stationary sources. These new regulations will require novel and improved catalyst technology. To meet these challenges ACCESSA members will invest heavily in R&D, so to maintain leading positions in this growing market. The overarching objective, as ever for the industry, will be to deliver increasingly effective pollution abatement technologies, which require decreasing amounts of catalyst volume at lower cost. Such a trajectory is already in evidence. For example, oxidation catalysts require smaller amounts of raw materials than ever before - with improved chemical performance.
Concern over the impact of air pollution drives stricter emission limits. As emission limits are tightened, “in process” modification of the combustion give way to exhaust gas after treatment. Exhaust gas after treatment is more effective and often required where significant emissions reduction is required.
Taking its signal from the market and in particular its regulatory aspect, technology providers invest to develop new and improved technical solutions that are compliant, reliable, durable and cost effective. Technology providers generally follow a trajectory whereby first technology is developed to a stage where it can be demonstrated, and in a “learning / improvement” cycle the technology is fine tuned to the application. When a reliable durable technology is developed significant economies of scale and scope, such as those offered by a wider market, can dramatically bring costs down. If a technology trajectory is supported in the market place it can bring forward highly efficient and cost effective technical solutions to help solve air pollution problems.
The overarching objective, as ever for the industry, will be to deliver increasingly effective pollution abatement technologies, which require decreasing amounts of catalyst volume at lower cost. Such a trajectory is already in evidence. For example, oxidation catalysts require smaller amount of raw materials than ever before - with improved chemical performance.