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Platinum Metals Review UK ISSN 0032-1400 PLATINUM METALS REVIEW A quarterly survey of research on the platinum metals and of developments in their application in industry VOL. 31 OCTOBER 1987 NO. 4 Contents Emission Control for Gas Turbines I 62 Johnson Matthey Metals Loans Scheme 171 Platinum Ternary Alloys 172 Direct Methanol Fuel Cells 173 Fabricating Platinum Disc Microelectrodes 181 An Exchange of Ideas on Catalysis I 82 Corrosion in Nitric Acid Plants 185 The Chemistry of the Platinum Group Metals 186 Weldability Test for Thin Iridium Sheet 193 Exhaust Gas Pollution Control 194 High Temperature Gas Thermometry and the Platinum Metals 196 Abstracts 208 New Patents 215 Index to Volume 31 220 Communications should be addressed to The Editor, Platinum Metals Review Johnson Matthey Public Limited Company, Hatton Garden, London ECl N 8EE Emission Control for Gas Turbines PLATINUM-RHODIUM CATALYSTS FOR CARBON MONOXIDE AND HYDROCARBON REMOVAL By H. J. Jung and E. R. Becker Johnson Matthey, Catalytic Systems Division Johnson Matthey are reporting successful commercialisation of oxidation catalystsfor the clean-up of gas turbine exhaust. The reduction of carbon monoxide and hydrocarbons is achieved with a platinum-rhodium catalyst which minimises the formation of sulphur trioxide. The catalyst system, supported on an energy saving, low pressure drop metal monolith, has been used continuously for five years on large industrial gas turbines. The catalytic control of atmospheric pol- bines in a large stoichiometric excess of oxygen lutants from automobiles has been firmly consists of two parts. The first level of control established in the U.S.A. and Japan, and is is practised by injecting water or steam into the increasingly being adopted by European and combustion zone to lower the combustion tem- other industrial countries. Nitrogen oxides, perature. This “wet firing” allows nitrogen carbon monoxide and hydrocarbons in car oxide levels to be reduced by 50 per cent, from exhaust gas are reduced by controlling the 100 ppm to 50 ppm. The second step is the air : fuel ratio and by simultaneous conversion selective catalytic reaction of ammonia with the of all three pollutants in either a dual bed remaining nitrogen oxide, a process known as catalytic converter or in a single bed “three- Selective Catalytic Reduction (SCR). The way” catalytic converter ( I). The accumulated ammonia is injected into the flue gas and the benefits from this control technology (2) have mixture is then passed over a base metal oxide not been paralleled in the control of emissions catalyst, typically supported on alumina or from stationary sources. Over half of the man- titania (4). The SCR method can achieve made carbon monoxide, hydrocarbons and nitrogen oxide levels below 10 ppm, and it is nitrogen oxides reaching the atmosphere is used in Japan, California and West Germany. emitted from stationary sources including While the waterheam injection minimises power station boilers, industrial boilers, sta- nitrogen oxides formation, it increases the tionary internal combustion engines and gas amount of carbon monoxide and hydrocarbons turbines (3). Strict emission limits for control- in the combustion products which then require ling nitrogen oxides, carbon monoxide, hydro- flue gas treatment to lower the emissions. The carbons and sulphur oxides have been enacted well-established catalytic oxidation technology by many states in the U.S.A. and in West used for automotive emission control can be ex- Germany. tended to control carbon monoxide and hydro- With the exception of rich-burn internal carbon emissions from gas turbines. Johnson combustion engines, all stationary combustion Matthey, a leader in automotive catalyst tech- is carried out using a large excess of oxygen, in nology, has developed oxidation catalyst and order to maximise fuel efficiency. This pre- reactor technology for this purpose, the tech- cludes a direct extension of automotive three- nology being introduced into the market in way catalyst technology to stationary sources. 1982. It is currently available as a stand-alone The control of nitrogen oxides from gas tur- product or in conjunction with Johnson Platinum Metals Rev., 1987, 31, (4), 162-170 162 Fig. 1 A Johnson Matthey emission control reactor is located in the centre of this picture, between the gas turbine and the stack. This reactor contains several catalyst panels measuring 10 feet by 14 feet, and purifies 4 million cubic feet of flue gas per hour. The steam from this plant is pumped underground to assist in the recovery of viscous oil Matthey SCR catalyst and control systems as superheater and 260OC at the inlet of the econo- shown in Figure I. This paper describes the miser. When duct burners are used to make oxidation catalyst and the reactor technology. more steam, the temperature at the inlet to the superheater can be as high as 65oOC. Reactor Design Each application has a unique set of emission Automotive oxidation reactors are required to objectives which the catalyst and reactor are treat relatively small volumes of gas over custom designed to achieve. The main design approximately 2,000 hours of operation at tem- considerations which most oxidation reactors peratures varying from ambient, during start- have in common are: up, to occasional excursions up to 1,0ooOC. In High reactor productivity to minimise contrast, gas turbine exhaust reactors treat volume and cost. much larger quantities of gas, for example, 7.5 Low pressure drop to maximise useful work million cubic feet per hour (SCFH) for a 20 from the expanding gas. MW gas turbine. The catalyst temperature is Low sulphur dioxide conversion to sulphur controlled between 250 and 65ooC, and the trioxide to minimise sulphate particulate catalyst is required to operate without replace- emissions. ment for more than 20,000 hours. A typical gas Continuous operation exceeding 20,000 turbine exhaust heat recovery system, where hours. combustion gases are expanded through a tur- Some co-generation installations require fre- bine to produce electricity, is shown in Figure quent start-up and shut-down features, for 2. Before they go to the stack the exhaust gases which thermal shock resistance is required. pass through a superheater, boiler and econo- Proper relationship to the SCR reactor to miser, where heat is recovered in the form of meet overall emission control objectives, that low and high pressure steam. The exhaust tem- is to minimise the reconversion of ammonia perature is normally 470°C at the inlet to the to nitrogen oxides. The interaction of the Platinum Metals Rev., 1987, 31, (4) 163 Fig. 2 The typical features of a gas turbine exhaust heat recovery system are shown. Although most oxidation Heat Recovery reactors have a number of Boiler common design considera- tions, each application has a unique set of emission objec- tives which catalyst and re- - 470 actor are designed to achieve Gas in - f'4: - -- 262 - Li- -1 - - - ~ 160 Steam out 239 Tcmperat ure Profile Water in SCR reactor with the oxidation reactor has and saturated hydrocarbons, while minimising been reported elsewhere (5). the oxidation of sulphur dioxide. The productivity of a reactor is measured by the hourly gas volume which each unit volume Metal Honeycomb Substrate of reactor can clean up to the specified emission Industrial emission control reactors must level. This is referred to as the gas hourly space result in only a low pressure drop at high flow velocity (GHSV). The productivity of oxidation rates, since minimising co-generation operating reactors equipped with noble metal catalysts costs depends upon minimising the pressure varies from 50,000 to I 50,000 gas volumes per drop. It is generally assumed by turbine hour, per volume of reactor. The oxidation re- operators that every 4 inches of water gauge actor consists of four components: backpressure causes a 0.4 to I. 5 per cent loss of 111 A panel of catalyst blocks which fits into power output. The thin walls of metal substrate the reactor housing, sized to minimise the allow a minimum pressure drop for a given pressure drop of the flue gas through the amount of catalyst coating. Thus the combined reactor. thickness of support and catalyst coating is less [21 A metal honeycomb substrate which pro- than the wall thickness of a ceramic substrate of vides the low pressure drop unit cell of the equivalent surface area. catalyst panel, and forms the interfacial Johnson Matthey's metal honeycomb tech- area between the gas and the catalyst. This nology was originally developed for automotive area is relevant when the reactor is operated emission control in the 1970s (6). The high in the mass transfer limited region of the temperature-resistant ferritic stainless steel temperature range. monoliths and the catalyst coating technology [3l A high surface area refractory oxide coating developed then have now been adapted and which provides the internal surface area for modified to suit the large industrial catalyst the dispersion of the catalyst ingredients. blocks used in SCR and oxidation reactors. 141 The active catalyst material. These blocks measure 2 ft square by 3.5 inches The active catalyst material used in the deep and constitute one cubic foot of effective Johnson Matthey reactors described here is a reactor volume. In addition to the low pressure combination of platinum and rhodium. This drop characteristics, the metal substrate offers catalyst provides high carbon monoxide conver- significant advantages over ceramic substrate in sion and high conversion of both unsaturated situations where thermal shock is experienced. Plarinum Metals Rev., 1987, 31, (4) 164 Fig. 3 The use of heat resistant metal substrates provides low pressure drop characteristics and superior resistance to ther- mal shock. A common foil con- figuration is shown here A number of metal substrate designs are available. The most common is a stack of alternate layers of flat and corrugated foil strips, Figure 3.
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