AMMONIA RECYCLING AND DESTRUCTION IN A CFB GASIFIER

Transcription

1 May 2004 ECN-RX AMMONIA RECYCLING AND DESTRUCTION IN A CFB GASIFIER Presented at The 2 nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection in Rome, Italy, May 2004 L.P.L.M. Rabou S.V.B. van Paasen

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3 AMMONIA RECYCLING AND DESTRUCTION IN A CFB GASIFIER L.P.L.M. Rabou and S.V.B. van Paasen Energy research Centre of the Netherlands P.O. Box 1, 1755 ZG Petten, The Netherlands rabou@ecn.nl, vanpaasen@ecn.nl ABSTRACT: Biomass contains nitrogen that is converted to NO x on combustion. Conversion of biomass into producer gas by thermal gasification offers an opportunity to reduce NO x -emissions by NH 3 removal from the producer gas with water. Wastewater will have to be treated before it can be drained. Recovered NH 3 may have value as base chemical, but usually will have to be disposed off as waste. Recycling within the gasification process is an option to dispose off NH 3 waste, provided NH 3 is broken down under gasification conditions. We present experimental data on NH 3 destruction in a CFB gasifier at temperatures from 780 C to 860 C, when NH 3 is injected into the gasification air. Within that temperature range, the fraction of NH 3 that is broken down increases from less than 50% to nearly 100%. Clearly, the presence of O 2 at the injection point leads to a much higher rate of NH 3 destruction than observed for purely thermal cracking. Although measurement of the NO x concentration in producer gas is disturbed by other components, the observed signal is low enough to conclude that hardly any NH 3 is converted to NO x. Recycling of NH 3 has negligible effect on the system efficiency or producer gas composition. However, it offers an elegant way to dispose off a waste stream at low costs and without harm to the environment. Keywords: circulating fluidised bed (CFB), emission reduction, gas cleaning 1 INTRODUCTION Biomass plays the key role in the European program to increase the energy production from renewable sources. However, if fossil fuels are replaced by biomass, the higher nitrogen content of biomass may lead to increased emission of NO x. Combustion of biomass converts part of the fuel-n into NO x. Gasification of biomass in a CFB (Circulating Fluidised Bed) gasifier converts roughly 50% of nitrogen in biomass into NH 3 [1]. A small fraction ends up in HCN or tar compounds, the remainder in N 2. The NH 3 yield increases with pressure but hardly changes with operating temperature. Combustion of NH 3 containing producer gas yields NO x, like direct combustion of biomass. Thermal destruction of NH 3 in producer gas to N 2 and H 2, prior to combustion, requires temperatures far above 1000 C. In the presence of catalysts, temperatures as low as 600 C may suffice [2]. Even carbon deposited on reactor walls or char formed in the gasification process may act as catalysts [3]. However, because of deactivation by carbon, sulphur or other contaminants, catalysts usually perform less well in practice than in real producer gas obtained by gasification of biomass [4]. Biomass gasification, followed by NH 3 removal from producer gas with the use of a water scrubber, offers a way to prevent conversion of biomass nitrogen into NO x. For economic reasons, the size of an installation will have to be at least 1 MW th. The NH 3 removed from the producer gas has to be disposed off in an environmentally friendly way. Disposal into surface water or emission into the atmosphere is not an option, as they are considered equivalent to emission into the atmosphere after 100% conversion of NH 3 into NO 2. In large installations, NH 3 may be recovered as salt that can be used in fertilizer production. More generally, combustion under well-controlled conditions, with N 2 and H 2 O as combustion products, would be the best solution. Here we consider whether a CFB gasifier itself may create the required conditions. 2 TEST CONFIGURATION At ECN (Energy research Centre of the Netherlands), a 500 kw th CFB gasifier is available for experiments (see Figure 1). The gasifier operates slightly above atmospheric pressure at temperatures between 750 C and 900 C. Tests have been performed with more than 20 different types of fuel. Wood pellets with 12% moisture are used as standard low-n fuel. Chicken manure is a typical example of a high-n fuel. Primary air, usually preheated, is supplied through a ECN-RX

4 distribution plate at the bottom. Secondary air can be added above the fuel entry point. Bed material and fuel particles that are swept out of the gasifier are separated from the producer gas by a cyclone and returned to the gasifier through a seal pot. Solids in the seal pot are fluidised by (a mixture of) N 2 or air to make them flow to the gasifier. Secondary Biomas air s Primary air Producer gas to cooler and Seal pot N 2 / air Figure 1: ECN 500 kw th CFB biomass gasifier. In recent years, equipment has been added to realise an installation for integral tests with gas cleaning devices and prime movers. The integral test installation used consists of the CFB gasifier, a gas cooler, a cyclone for ash and dust removal, two wet scrubbers, a wet ESP (ElectroStatic Precipitator), a booster and a gas engine (see Figure 2). The most recent extension, the OLGA system for tar removal, is presented in another contribution to the conference [5]. The installation can easily be adapted to provide a test facility for alternative gas cleaning devices or applications. Results for operation of a similar test configuration have been presented at the previous conference [6, 7]. Producer gas Cyclon Dus Figure 2: Test configuration at ECN, downstream of 500 kw th CFB biomass gasifier. Dry producer gas contains on volume base 15% CO, 16% CO 2, 11% H 2, 4% CH 4, 2% other hydrocarbons and 52% N 2 plus Ar. It has a lower heating value of about 6 MJ/m n 3. The NH 3 content is proportional to the nitrogen content of the fuel used. The present experiments have been performed with fuel containing less than 0.1% to 0.9% nitrogen (on mass base for dry and ash-free material), delivering producer gas with 250 ppm to 4000 ppm NH 3. The first scrubber cools and dries the gas, removes some dust and part of the NH 3 and tar. The ESP collects any remaining dust, water droplets and tar aerosols by charging and collecting them in a strong electric field. Downstream of the ESP, producer gas still contains tar vapour (mainly naphtalene and indene), but no condensable tar. Hence, the second scrubber is protected against tar pollution. The systems and techniques used for treatment of wastewater from the first scrubber and ESP fall outside the scope of the present article. Tar recovered from the wastewater can be returned to the gasifier and used as secondary fuel [8]. Air + NH 3 ES 1 st Tar 2 nd Boost Gas engine NH 3 free producer gas Stripper Scrubber Air Producer gas + NH Figure 3: System of water scrubber and stripper to remove and recover NH 3. ECN-RX

5 The second scrubber has been designed to reduce the NH 3 content by 98%. Traditionally, acid is added to scrubber water to bind NH 3, and base to stripper water to release NH 3. The system shown in Figure 3 is meant to work without added chemicals. To reach that goal, scrubber water is continuously recycled in a stripper, where NH 3 is removed from the water by hot moist air. Dissolution of CO 2 acidifies the scrubber water. The higher temperature in the stripper and low CO 2 concentration in air shift the equilibrium concentrations and drive both CO 2 and NH 3 out of the water. A heat exchanger is used to transfer heat from water flowing to the scrubber to water flowing to the stripper. Downstream of the stripper, the air is cooled to condense water vapour. Waste air from the stripper can be used in the gasifier if NH 3 is broken down to N 2 and H 2. Some H 2 may react to H 2 O, but any N x O y reaction products that might form initially should later on be broken down in the reducing atmosphere and not leave the gasifier. If stripper air is to be consumed in the gasifier, the flow rate should be about half the producer gas flow rate. The actual design leaves room for a second independent stripper. As a consequence, the gas to liquid ratio in the stripper may differ by a factor 4 from the gas to liquid ratio in the scrubber. 3 EXPERIMENTAL PROCEDURES The aim of the experiment is to show that NH 3 removed from the producer gas can be disposed off by recycling to the gasifier. For practical reasons, air loaded with NH 3 is not actually taken from the stripper to the gasifier. Instead, a constant flow of NH 3 from a gas bottle is added to the primary air or to seal pot air. The NH 3 flow rate is set to give a start concentration in producer gas well above the background level obtained from the fuel, but representative for actual conditions when biomass with high nitrogen content would be used. The air flow rate is kept constant by a mass flow controller. The air is heated to 300 C. No secondary air is used. The biomass feed rate is calculated from the weight loss of the fuel bunker. The average temperature in the gasifier is calculated from measurements at six points distributed over the height of the gasifier. Usually, a temperature gradient of about 20 C exists between the lower part (combustion zone) and upper part (freeboard) of the gasifier. If required, the temperature is raised (c.q. lowered) by a decrease (c.q. increase) of the fuel feed rate. The composition of the gas is continuously monitored for CO, CO 2, H 2, CH 4, O 2, NO and NO 2. Calibration of the NO and NO 2 detectors with synthetic producer gas of known composition and with a gas mixture of 99.95% N 2 and 0.05% NH 3 showed 20 ppm signals. Hence, NO or NO 2 formation can be reliably detected only if their concentrations rise significantly above the 20 ppm level. The concentrations of N 2, C 2 H 4, C 2 H 6, C 6 H 6 and C 7 H 8 are measured at five minutes intervals. The Ar and H 2 O concentrations are calculated from the mass balance. The dry gas flow rate is calculated from the N 2 concentration and mass balance. The maximum NH 3 concentration in producer gas due to NH 3 addition (i.e. if all added NH 3 leaves the gasifier unaffected) is calculated from the weight loss of the gas bottle. The NH 3 concentrations in producer gas at the exit of the gasifier, entrance and exit of the scrubber and in air at the exit of the stripper are determined off-line from the amount of NH 3 caught in acid washing liquid through which a known volume of gas has passed. Each measurement has been performed at least twice. The background NH 3 concentration is averaged over measurements taken before and after NH 3 addition. 4 RESULTS 4.1 AMMONIA SCRUBBING The performance of the NH 3 scrubber and stripper has been evaluated at various gas to liquid ratios and water and air temperatures. The producer gas was at ambient temperature and contained close to 1000 ppm NH 3. Within the range of conditions applied, the system performs according to design. The scrubber reduces the NH 3 concentration in producer gas by 95% to 98%. The stripper does not remove ECN-RX

6 all NH 3 from the water, but within a few hours equilibrium is reached in which the volume of NH 3 removed by the scrubber from producer gas equals the volume of NH 3 released by the stripper to air. A more thorough discussion of the system design and operating conditions, together with more extensive results will be published later. 4.2 AMMONIA DESTRUCTION Experiments have been performed at several temperatures. The background NH 3 concentration in the producer gas varied from 250 ppm to 4250 ppm (with clean wood for fuel and chicken manure, respectively). The amounts of added NH 3 would at least double the NH 3 concentrations in producer gas if none of the added NH 3 were destroyed. The experimental conditions and results for NH 3 addition to primary air are summarized in Table I. Table I: Destruction of NH 3 added to primary air as function of the operating temperature of the CFB gasifier. Temperature [ C] Background [ppm] Added [ppm] Destruction [%] The results shown in Table I show a clear trend towards higher NH 3 destruction with increasing temperature. Two additional experiments have been performed in which NH 3 was added to air in the seal pot (see Figure 1). In both cases, the temperature in the seal pot was 850 C. The fraction of added NH 3 that was destroyed increased from 79% for a gasifier temperature of 790 C to 92% for a gasifier temperature of 850 C. The result at 850 C confirms the observed trend. The result at 790 C shows how even a short residence time at 850 C can have a pronounced effect. Nearly total destruction of added NH 3 can be realized around 850 C. The required temperature is significantly below the onset value for thermal destruction [2]. The observed effect differs markedly from the effect of secondary air [9]. The difference is due to the absence of competing species, i.e. fuel-lean conditions, when NH 3 and air enter the hot gasifier bed [10]. 5 CONCLUSION Recycling of NH 3 with air to the gasifier offers an elegant way to dispose off a waste stream of the producer gas cleaning that is required to reduce NO x emission when biomass producer gas is fired in boilers or applied in combustion engines. The required temperature of 850 C lies within the usual operating temperature range of CFB gasifiers. REFERENCES [1] J. Leppälahti and T. Koljonen: Nitrogen evolution from coal, peat and wood during gasification. Fuel Processing Technology 43 (1995) [2] H. Depner and A. Jess: Kinetics of nickel-catalyzed purification of tarry fuel gases from gasification and pyrolysis of solid fuel. Fuel 78 (1999) [3] W. Wang, N. Padban, Z. Ye, A. Andersson and I. Bjerle: Kinetics of ammonia decomposition in hot gas cleaning. Ind. Eng. Chem. Res. 38 (1999) [4] W. Mojtahedi, M. Härkonen, T. Maunula, M. Luoma, J. Abbasian, J. Wangerow and M. Ylitalo: Catalytic decomposition of ammonia. Report AAA-Lieki-L95-1-Pt1, [5] H. Boerrigter, S.V.B. van Paasen, P.C.A. Bergman, J.W. Könemann, R. Emmen: OLGA tar removal in biomass CHP plants; pilot-plant commissioning and test programme results. Contribution V2A.30 to 2 nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection, Rome [6] L.P.L.M. Rabou, C.M. van der Meijden, L.M. Brenneisen, E. de Kant, H. Klein Teeselink and R. Wubbe: Gas engine operation on fuel gas from CFB biomass gasifier. Proceedings of the 12 th European Conference and ECN-RX

7 Technology Exhibition on Biomass for Energy, Industry and Climate Protection, Amsterdam, June 2002, [7] J.P.A. Neeft, C.M. van der Meijden, L.P.L.M. Rabou, H. Klein Teeselink, L.M. Brenneisen, F.B. van der Ploeg and L. Dinkelbach: Physical removal of tar aerosols from biomass producer gases by ESP and RPS. Proceedings of the 12 th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection, Amsterdam, June 2002, [8] L.P.L.M. Rabou: Biomass tar recycling and destruction in a CFB gasifier. Submitted to Fuel. [9] J. Leppälahti and E. Kurkela: Behaviour of nitrogen compounds and tars in fluidized bed air gasification of peat. Fuel 70 (1991) [10] Ø. Skreiberg, P. Kilpinen and P. Glarborg: Ammonia chemistry below 1400 K under fuel-rich conditions in a flow reactor. Combustion and Flame 136 (2004) ACKNOWLEDGMENTS The installation and work reported have been realised with financial support from the EWAB programme of the Netherlands Organisation for Energy and the Environment (Novem) and from the Agency for Research in Sustainable Energy (SDE). ECN-RX