The energy mix is evolving towards a lower carbon, more sustainable future. This evolution is slow and is predicted to take many decades – even in the more mature economies in the developed world. In many parts of the developing world the combustion of carbon based fuels is predicted to grow (in some cases rapidly) over the next few decades. There is growing pressure to mitigate the impact this might have on Air Quality Coal Fired Boilers continue to make a significant contribution to our growing need for power. Often very large (of the order of hundreds of MW installed), their combustion processes also make significant quantities of acidic gasses SOx and NOx. SOx emissions relate directly to the sulphur content in the fuel and can be removed in a scrubber. NOx is a process pollutant and can be prevented/removed via a number of methods, but the most efficient is SCR. The installation of SCR Reactors on coal fired and co-fired boilers have now become routine in many parts of the world, particularly in North America, Western Europe and China.
Depending on plant requirements (new plant, retrofit or space availability), SCR catalysts have been installed in High-Dust, Low-Dust or Tail end configurations to meet NOx reduction requirements.
Durable plate type SCR catalysts are ideally suited for high-dust environments in Waste-To-Energy and Biomass-To-Energy power plants.
By changing the pitch of the catalyst low pressure drop and very low dust accumulation is achieved. Soot blowers can be installed, if required, to remove dust from the face of the catalyst and maintain maximum catalyst activity.
The following diagrams illustrate the different configuration options for SCR on a coal fired boiler.
A key concern of operators is ensuring that the emissions from their power plant are compliant with the requirements of their licence to operate. Catalysts are installed in layers and each has a definitive lifetime of compliant operation – determined by their design and the conditions they face during installation and operation.
The decay in performance can be modelled and used to predict when a catalyst layer should be replaced with new or regenerated catalyst.
After replacement activity can be monitored and modelled to appreciate the next timeframe for catalyst recharge / replacement. All this allows the operator to manage the maintenance cycle of the SCR reactor with that of the rest of the Power plant.
Growing concern over mercury emissions to air from power generation e.g. from coal and biomass has led to some local and national limits being imposed. One of the co benefits of SCR is that the mercury can be oxidised in the presence of a halide such as chlorine or bromine. The reaction mechanism is thought to follow an Eley Rideal type where:
- The mercury (Hg) adsorbs at the Vanadium (V) Oxide (O) active site creating a V-Hg-O site.
- Hydrogen Chloride (HCl) reacts with the adsorbed Hg generating the oxidised Hg which diffuses into the gas.
The mercury chloride is then removed downstream in the scrubber.
Emissions to air from Gas Turbines
In developed and developing economies there is an increased interest in gas fuelled power e.g. Gas Turbines. Gas power is generally cleaner than coal. With relatively little sulphur the main pollutants of concern are NOx and CO both of which can be removed using SCR and oxidation catalysts respectively.
Very low NOx emissions can be achieved in well-designed SCR reactors and with optimised ammonia dosing and mixing. Some tolerance can be designed into the system by installing ammonia slip catalysts. Recently systems have been designed that combine the oxidation and ammonia destruction function – leading to greater efficiencies.
The movement of gas from its point of extraction to its point of use requires a significant infrastructure including an extensive pipeline network. At various locations in the network the gas is compressed to keep in under pressure and moving to where it is required. These compressor stations are either gas turbines or internal combustion (IC) engines.
Stationary IC Engines
Large IC engines are used for power generation as well as in gas compression and in the oil and gas sector. They are also used in shipping. Typical emissions to air that can be controlled are NOx, CO, unburned Hydrocarbon (fuel) referred to as Volatile Organic compounds and particulate material. Three-way catalysts can remove NOx, CO and VOC from a gasoline engine exhaust. Here the exhaust gas chemistry is controlled to ensure a balance of oxidation with reduction.
Diesel engines, using lean burn combustion so there is a surplus of oxygen in the exhaust gas, require separate SCR and oxidation processes to reduce the NOx and oxidise the CO and VOCs. In clean exhaust gas particulate matter can be oxidised in a wall flow filter. For other cases e.g. exhaust gas from sulphur containing fuel such as HFO, other filtration methods such as applications of filter candles have been developed.
IC engine Gas compressors such as the Loudon compressor station in Clarksburg, and that in Sacramento have been running with catalytic after treatment since 2004. The emissions for four of these engines are reported in the table below. The emissions of NOx and CO meet with local permitting requirements.
With advanced catalyst design and cleaner exhaust gas, catalysts age and decay less rapidly and recharge periods extended lowering the total cost of operation.
Design and evaluation of a catalytic off-gas solution for a European Portland Cement plant
With more than 200 plants and 160 million tons of cement produced in 2014, the Portland cement industry in Europe continues to represent a significant portion of the global cement market. In fact, 4 of the top 10 global cement producers are based in Europe with more than 500-million-tonnes of collective, annual capacity.
Europe has maintained a strong manufacturing base in this area due to the economics of cement distribution and its technology leadership in the field. Furthermore, because up to 35% of the cost of cement is attributable to transportation costs, it is advantageous to have localized production and distribution.
Although European manufacturers are bound by EU-wide legislation, country-specific and region-specific administration and regulations can vary. Emissions limits for toxic pollutants in the case of cement production differ and, in some cases, are significantly more stringent than the overall EU limits. Furthermore, cement producers in particular regions or states within a country may be subject to additional local pressure to operate with the maximum available technologies, such as sites in the vicinity of outdoor recreation areas or local populations.
In Germany, for example, the Federal Ministry for Environment, Nature Conservation and Nuclear Safety has been a driver for emissions reduction in Europe through the enactment of the “Federal Pollution Control Act". Recently this legislation was revised with respect to the combustion of waste materials to include Portland cement plants. Being designated as “co-incinerators” of waste materials, these plants are subject to the emissions restrictions created for combustion plants, including restrictions for the emission of carbon monoxide.
In most cases, these regulations are applied with specific exceptions for the operation of cement plants given unavoidable emissions due to the carbon-bearing nature of the raw materials. However, this revised regulation presents a difficult problem for German cement manufacturers. There are few, if any, demonstrated technologies to deal with these emissions, and the technologies that are commercially available, such as thermal oxidation, are often not practical due to energy costs. One cement producer affected by the aforementioned legislation identified the high risk of inaction and engaged in a pilot testing program to identify a combined catalytic solution for the abatement of VOCs, carbon monoxide, nitrogen oxides and particulate matter. The problem of identifying such technology was not trivial; until now no conclusively demonstrated catalytic oxidation technologies have been demonstrated to withstand the process conditions.
In order to test the combined catalytic solution under realistic conditions, a mobile set-up was designed to utilize a slip stream and perform the tests using actual cement off-gas. The test unit was installed in the cement plant for the duration of the screening and continued operation over one year to include long-term testing. Subsequently, the testing was continued at another facility in the region to evaluate the effect of different effluent gases on the catalyst system. The flexible design of the unit allowed for variation of process conditions to test a combination of: dust-removal, selective catalytic removal (SCR) of NOx, as well as catalytic oxidation of the volatile carbon species and carbon monoxide (see figure).
The testing unit was then used to screen a combination of different SCR and oxidation catalysts – from monolith-type vanadium-tungsten-titanium (VWT) catalysts to combination ceramic filter cartridges – for their lifetime performance in NOx reduction and CO/VOC oxidation. These cartridges were loaded with an active SCR component based on iron zeolite. The oxidation unit was comprised of monolith-type catalysts chosen from a range of commercially available products, as well as, a new generation of air purification catalysts based on a platinum-loaded zeolite coating.
The results of the slipstream testing showed that, under the specific conditions, it is possible to operate a combination of SCR catalysts and oxidation catalysts together to achieve almost total oxidation of carbon monoxide. These results were obtained in the trial under an ideal temperature range between 350-400°C. In the test, both traditional monolith SCR catalysts and catalytic ceramic filter media were used to maintain low NH3 slip. Although a number of oxidation catalysts were tested, it was only with the combination of the Pt-modified zeolite catalyst that long term performance was realized.
The duration testing experiments (see figure 3) showed that the performance of the catalyst combination could be maintained over more than 2500h. This was further confirmed by sporadic testing after shutdowns in which the temperature dropped significantly, indicating the performance could be maintained without degradation or deactivation due to sulphate build up.
NOx, VOC and particulate abatement from refineries
Refinery NOx and VOC emissions primarily originate from combustion processes in heaters, crackers, FCC regenerators, reformers, gas turbines and boilers.
NOx emissions from refineries can be removed by a variety of methods. The primary methods are combustion modifications, burner operation and maintenance optimization, reburning, flue gas recirculation or use of low-NOx burners. When the primary methods are not sufficient, the secondary methods Selective Non-Catalytic Reduction (SNCR), or SCR are applied. The SCR method is widely acknowledged as the BAT, making it possible to achieve more than 95% NOx removal.
At some plants, both secondary methods SNCR and SCR are used. The advantage of this hybrid solution is that a high NOx removal degree and a low ammonia slip can be achieved, while the needed SCR catalyst volume can be minimized relative to a pure SCR installation.
Implementation of SCR DeNOx technology on refinery applications has been increasingly applied over the last two decades. Initially, the SCR DeNOx technology was implemented in the USA, but later refineries in Europe and Asia have also adopted the technology. Often the SCR systems are made as retrofits on existing combustion equipment. For new units however, space is often reserved for the SCR reactor already in the design phase.
The SCR reactor can be placed a variety of places downstream of the combustion unit. The main requirement is a flue gas temperature in the interval 180°C to 500°C, and optimally 350-420°C. For boilers, the SCR reactor is normally placed immediately after the boiler and economizer sections. For FCC units, heaters, crackers, reformers and gas turbines, the SCR reactor will typically be placed in between the heat exchangers of a waste heat recovery section, where there is sufficient space and a good temperature window.
SCR DeNOx catalysts installed in refinery applications typically have long lifetimes compared to other industries. Guaranteed lifetimes of 3 to 5 years are standard whereas actual lifetimes may very well be longer than 10 years. These catalysts are designed for a desired end of run activity, meaning that the initial catalyst activity will be higher than the required activity and then slowly decrease to the required activity after several years of operation.
The SCR DeNOx catalysts will experience a slow decline in performance during their lifetime for one or more of the following reasons: chemical poisoning, thermal sintering, fouling of catalyst surface, erosion or plugging of catalyst channels.
Particularly, SCR catalysts in reformer and ethylene cracker applications experience high deactivation due to chromium poisoning of the SCR catalysts. The catalyst poison chromium arises from the reformer tubes when these are heated to above 700°C.
The table below provides several examples of SCR DeNOx installations in reformers, FCC units, crackers, heaters, gas turbines and boilers at different refineries. NOx removal efficiencies are typically in the range of 75% to 97%. NOx and NH3 emissions are typically 2-100 ppmvd @3% O2 and 2-15 ppmvd @3% O2, respectively. An estimate for the total number of SCR installations in refinery applications is more than 1,000 SCR applications globally. The SCR DeNOx technology is thus a well proven technology for refinery applications.
Removal of Volatile Organic Compounds such as CO, propane, butane, and toluene may be done catalytically. The catalyst is essentially a SCR DeNOx catalyst with palladium (Pd) added. The removal efficiencies are different from each VOC species. But generally the catalyst can be designed for high removal efficiencies well above 90% of most VOC species. The lower alkanes methane and ethane are exceptions; they cannot be removed catalytically but require elimination by thermal oxidation.
VOC-oxidation catalysts are so far primarily used in gas turbines and a few reformer installations. An example is Celanese Clear Lake methanol plant in USA, see Table below.
VOC oxidation catalysts may be placed before the injection of ammonia and SCR DeNOx catalyst. Or the catalysts may be designed as one dual functionality catalyst, able to remove both VOC’s and NOx. The dual function catalysts have the obvious advantages of minimal space required, lower pressure drop and reduced SCR catalyst volume.