Catalytic ammonia removal from hot syngas

Syngas obtained from gasification of coal, biomass and their blends contains contaminants (H2S, HCN, NH3, HCl, tar) beside the main gas components (hydrogen, methane, carbon monoxide and carbon dioxide) which come from sulphur, fluorine, chlorine, nitrogen, alkaline content of feedstocks. These contaminants should be removed according to needs of last application of syngas. Elemental nitrogen content of solid fuels change between 0.03-6.5% [1]. Approximately 60-90% of nitrogen content of fuels converts into ammonia (NH3) in syngas produced by gasification process [2]. For example; ammonia concentration in syngas produced from gasification of lignite is about 2000–3000 ppm [3], when biomass is gasified ammonia concentration might be as high as 6000 ppm [4]. Ammonia causes severe environmental problems because of converting into NOx emissions when the syngas is burned in Integrated Gasification Combined Cycle (IGCC) systems. Ammonia is a contaminant cause activity loss of catalyst in case of catalytic processes like Fischer-Tropsch (FT) and NH3 content of syngas is expected to be less than 1 ppmv [2]. Ammonia removal technologies branch of two according to process temperature. Ammonia scrubbing is a low temperature process which is a proven technology [5]. Water or diluted sulfuric acid solution is used as washing agent. When the syngas is cooled sensible heat of the gas can not be used and the total thermal efficiency of the process decreases. Thermal efficiency loss of the process must be prevented by not cooling the syngas during ammonia removal. This could only be achieved via catalytic reactions. Selective Catalytic Oxidation (SCO) and decomposition of ammonia are two important catalytic routes. But they don't have commercial applications yet. The selective catalytic oxidation reactions take place in partial oxygen environment and between 300-550°C operation temperatures. In this process, hydrogen might oxidize together with ammonia according to process conditions and catalyst used. The hydrogen loss unfavourably effects the syngas composition and it is not desired for following catalytic reactions. On the other hand only nitrogen and hydrogen are produced with catalytic decomposition of ammonia reaction. In contrast of SCO; H2/CO ratio of the syngas increases due to the hydrogen production. This is an advantage for the processes which need high H2/CO ratio like Fischer-Tropsch (FT) and methanol production. In the scope of conducted PhD thesis, catalytic ammonia removal from hot syngas was studied. In the first stage of the thesis, the most convenient gas analysis technique of ammonia measurement in syngas was investigated. Ammonia is a very reactive gas and it can easily interact with inner surfaces of the pipelines. It can overlap with other gas components in the gas detectors. Four different measurement techniques were used to find the most convenient ammonia measurement technique and conduct the calibrations; thermal conductivity detector, nitrogen chemiluminescence detector, ultraviolet-visible detector and gas measurement tube with the principle of colour change by chemical interaction. After the decision of ammonia measurement technique, catalytic activities of some natural minerals with iron, calcium content and commercial catalysts were investigated. The tested natural minerals, limonite, dolomite and olivine, were provided from different regions of Turkey. Silicon carbide and silica were the other two minerals that were investigated. Three different commercial catalysts were used for the reactions. The first commercial catalyst is nickel oxide which is used for hydrogen production from natural gas. The other one is iron oxide pellet and the last one is alumina supported noble metal catalyst used for hydrogen production from heavy paraffinic hydrocarbons. All commercial catalysts were provided in pellet form and grinded to powder form before tests and characterizations. In the third stage of studies, zeolite supported iron and nickel catalysts were sythesized by wet impregnation method, characterized and tested for the ammonia removal activities. In this scope three different zeolite supports with different silica/alumina ratios and cyristal structures were studied (SiO2/Al2O3 ratios; 5.2, 38 and 280 ZY, Zβ and ZSM-5, respectively). Catalytic activities of all tested catalysts were compared on the same conditions. The zeolite supported catalyst which showed the best activity was chosen. It was Zβ supported iron catalyst (Fe/Zβ). Effects of different syngas components (H2, CO, CO2, CH4, H2O) and different contaminants (H2S, xylene) on the activity of Fe/Zβ were investigated parametrically. Thus, side reactions like Boudouard and water gas shift reactions which cause the loss of catalyst activity were investigated. The conducted studies showed that catalytic ammonia decomposition reaction starts after 400°C in the presence of simulated syngas and reaches the acceptable conversion values at around 700-800°C. To lighten the effect of alumina content of zeolite on catalytic activity, dealumination studies were conducted on ZY supported iron catalyst which was showed lower activity than Zβ supported iron catalyst. Hydrothermal dealumination method was preferred for the dealumination of ZY (SiO2/Al2O3=5.2) and the extraframework Al2O3 species were removed from the structure by acid wash. Two different hydrothermal process temperatures (350 and 750 °C) were tried. For the purpose of comparison, commercially dealuminated ultrastable ZY (USY; SiO2/Al2O3=80) was also used. Thus, ZY supports which have four different SiO2/Al2O3 ratios but the same crystalline structures were obtained. ZY supported iron catalysts were prepared with all ZY supports, characterized and tested. In the last stage of studies, selective catalytic oxidation reaction of ammonia on Fe/Zβ catalyst were investigated at 300-550°C temperature interval, different oxygen concentrations (2000-6000 ppmv) and in the presence of 10% hydrogen. Activities of Fe/Zβ catalyst both for two reactions (SCO and decomposition) were compared. The main findings obtained during the studies of the thesis were summarized below: • Four different measurement techniques were applied for ammonia measurement. They are; gas chromatography with thermal conductivity detector (GC-TCD), analyser with nitrogen chemiluminescence detector (NCD), gas measurement tubes and gas analyser with ultraviolet-visible detector (UV-VIS). Measurements were conducted for 4000 ppmv and 8000 ppmv NH3 (balance N2) calibration gases with GC-TCD equipped with Poropak-Q column under different carrier gases (N2, Ar, H2). Resulted NH3 peaks were not proper to evaluate the measurements. Although there are some NH3 measurement studies with GC-TCD raported in litreature, it is proved with this study that ammonia measurement on the order of ppm level is not suitable with GC-TCD. Pressurized or non-pressurized gas samples could be analysed with NCD analyser. The measurement principle of the instrument is combustion of nitrogenous organic compounds to convert into NO2 and measurement of resulted NO2. Ammonia measurement trials with NCD were started with the standard configuration of the instrument but were continued with Pt catalyst placed into combustion zone of the instrument. Calibration studies for different concentration levels with the Pt catalyst were completed for the highest limit of 800 ppmv ammonia measurement of the instrument. Following the calibration studies, measurments of tested gas samples were done. It was seen that hydrogen also oxidies together with ammonia gas in oxygen rich atmosphere and high concentration of water vapor occurs. Produced water vapor overlaps and inversely affects the measurement results of NCD. For this reason it is concluded that NCD is not a convenient tecnique for ammonia measurement in gasification atmosphere. In the scope of studies, application of gas measurement tubes which satisfies fast measurement and no calibration needed were investigated. Gas measurement tube results were confirmed with ammonia calibration gases and it is evaluated that gas measurement tubes might be used for ammonia measurements. But this technique can not be used for continuous measurement on pipelines. For continuous ammonia measurement on pipelines an industrial analysis instrument; UV-VIS process spectrophotometer was used. The ammonia gas diluted in reaction test system to different concentrations was fed to the instrument both in inert and hydrogen atmospheres and measurements were recorded at each 5 seconds. It was seen that changing ammonia concentrations were measured correctly. For this reason, UV-VIS process spectrophotometer was used in the scope of all tests. • Before developing an ammonia removal catalyst from syngas it is necessary to evaluate the current situation. For this reason, natural minerals (limonite, olivine, dolomite, silica) and some commercial catalysts (nickel oxide, iron oxide and noble metal) were investigated for their ammonia removal activities. It was seen that Limonite 1 including approximately 46% iron by weight was showed a very high activity than other minerals and FeO pellets at 700 °C. ~%65 ammonia conversion was observed at 700 °C and 30% H2 atmosphere. The commercial FeO catalyst includes 75% iron by weight on the other hand this ratio is 46% for Limonite 1. High reaction rate observed on unit iron atom of Limonite 1 was related to reducibility of iron in Limonite. Although noble metal catalyst showed closed activity to Fe/Zβ at 0-10% H2 concentrations, it lost its activity faster than Fe/Zβ when H2 concentration was increased. • The Zeolite Y (SiO2/Al2O3=5.2), Zeolite β (SiO2/Al2O3=38) and ZSM-5 (SiO2/Al2O3=280) zeolites having different pore diameters and acidities were preferred for the synthesis of zeolite supported ammonia removal catalysts. Iron and alternatively nickel as active metals were used on the zeolite supports. The nickel is known as effective on tar removal. For the investigation of catalytic activities of zeolites without active metals, tests were conducted at 700 and 800 °C in different hydrogen concentrations. Zeolite Y was seen to have a higher activity than other zeolites. Its reason was interpreted as Zeolite Y has more acid sites (SiO2/Al2O3=5,2) than others. Then 10% iron was impregnated on all zeolite samples via wet-impregnation method. Sythesized zeolite supported catalysts were characterized by using chemical and spectroscopic methods. The highest ammonia decomposition activity was observed on Fe/Zβ catalyst. The highest activity over Fe/HZβ catalyst might be due to the better hydrothermal stability of Zeolite β compared to Zeolite Y regarding their aluminum contents. The role of the zeolite was probably to maintain well dispersion of active iron clusters without considering support effect on N-H bond breaking. Therefore the best coordination environment for active iron species and improved iron cluster interactions can be ensured over zeolite Hβ framework. • Investigation of catalytic activity of nickel as active metal was also realized. Zeolite supported nickel catalysts were prepared via wet-impregnation method, characterized and tested. The ammonia conversion values changed around 20-30% at 700 °C and 30% H2 atmosphere for all three catalysts. The worst activity was observed on Ni/ZSM-5 catalyst which has highest silica content. Characterization studies showed that active phase for ammonia decomposition reaction on nickel catalyst might be NiO rather than Ni. This study showed that iron catalysts supported on acidic zeolites are more active than nickel loaded catalysts at both 700 and 800 °C and in hydrogen atmosphere. This shows that N-N bond breaking capability of iron on these zeolites is better than nickel. • In the scope of conducted studies, the possibility of increasing the activity of Y type zeolite by a modification application was also investigated. Thus, the effect of different SiO2/Al2O3 ratios of the same type of zeolites on ammonia decomposition reaction was discussed. The alumina content of Zeolite Y (SiO2/Al2O3=5.2) was changed by hydrothermal dealumination technique. Hydrothermal dealumination process was performed at 350 and 750 °C. The highest activity was observed on the Fe/DZY-350 catalyst (dealuminated at 350 °C) at 700 °C for all H2 concentrations. The activities of all catalysts supported on Y type zeolites were changed with the order of: Fe/DZY-350 > Fe/DZY-750 ~ Fe/USY > Fe/ZY. The results showed that the optimization of alumina content of zeolites might be provided by hydrothermal process. Thus, the catalysts which have optimum iron-alumina interaction can be prepared. • Fe/Zβ catalyst showed the highest activity on ammonia decomposition reaction at 700 °C. Therefore the effect of syngas components on catalytic activity was investigated on this catalyst. Although 97.3% NH3 conversion was obtained on Fe/Zβ catalyst in N2 atmosphere, when the syngas components were added to the reaction media ammonia conversion was seriously decreased. The decrease of catalyst activation in the presence of hydrogen was explained in the light of kinetically competing reactions; N-H bond breaking and N-N bond association. Inhibition effect of CO was interpreted as competing reactions of C-O and N-H bond breakings on the same active sites. When the CO and H2 were together in the reaction atmosphere, severe conversion loss (until 12.5%) and coking problem were observed. CH4 addition to the CO-H2-CO2 mixture caused to decrease the NH3 concentration. The possible reaction between CH4 and NH3 (produces HCN) might be the reason of this decrease. It was proved by measurement of CN- ion concentartion in the exit gas stream collected in deionized water (1,8 mg/L CN-). Finally, H2S in simulated syngas did not poison the active sites of the catalyst but produced elemental sulfur because of the decomposition of H2S into N2 and H2. This study also showed that the water vapor inversely effects the catalytic activity. • The syngas contains water vapor and CO2 and H2 in syngas might produce water vapor. So that SCO reaction was tried alternatively to NH3 decomposition reaction and results were compared. In the scope of thesis, Fe/Zβ catalyst activity on both catalytic decomposition and selective catalytic oxidation of ammonia were investigated in detail. It was seen that SCO activity of the catalyst changes with respect to temperature and O2 amount. Although comparatively high ammonia conversions were obtained on SCO reactions in N2 atmosphere, ammonia conversion decreased when there was hydrogen in the reaction media. Because combustible hydrogen gas inhibited the oxidation of ammonia. Its reason was thought as H2 and NH3 oxidation reactions were competed on active sites of the Fe/Zβ catalyst. On the other hand, despite the higher temperatures needed for decomposition reaction of ammonia, it is not as sensitive as to hydrogen concentration. • The catalyst developed for ammonia removal from syngas should not be effected from syngas components or should be effected minimally. The ammonia decomposition activity of Fe/Zβ catalyst decreases in the presence of water vapor. In the case of selective catalytic oxidation of ammonia, the hydrogen gas causes the decrease of the activity of Fe/Zβ catalyst.