Gasification Gas Cleaning Research Articles

Evaluating the Application of Bubble Wet Scrubber Systems for Gas Cleaning in Gasification

Evaluating the Application of Bubble Wet Scrubber Systems for Gas Cleaning in Gasification

Carbon Aerogel-Supported Nickel and Iron for Gasification Gas Cleaning. Part I: Ammonia Adsorption

Biomass gasification is a promising way to obtain “green energy”, but the gas composition makes it unsuitable for use in traditional technologies (i.e., IC engine). Gas purification over nickel and/or iron catalysts is an attractive alternative. Cellulose-based carbon aerogels (CAGs) have shown suitable physical chemical properties for use as catalyst supports. In this work, nickel and iron catalysts are supported on CAG made from cellulose microfibers. Microfibers were impregnated with (NH4)2SO4 to increase the mass yield. Carbonization was evaluated at different heating rates, maximum temperatures, and dwell times to generate CAGs. Resulting chars were characterized by N2 adsorption, X-ray diffraction (XRD), and Raman spectroscopy. The CAG with better properties (specific surface, pore size, thermal resistance) was impregnated with the metal precursor salt via incipient wetness and treated with H2. Catalysts were characterized by transmission electron microscopy (TEM), XRD, N2 adsorption, and inductively coupled plasma optical emission spectrometry (ICP-OES). Ammonia adsorption was studied over CAG and catalysts to estimate the thermodynamic parameters. The impregnation with ((NH4)2SO4 improves thermal resistance of the char obtained from carbonization. The catalysts exhibit higher adsorption capacity than CAG (without metal), indicating chemical interaction between ammonia and metals. The metal-ammonia interaction is stronger on Fe than on Ni catalyst, which is consistent with reported theoretical calculations.

Promoting effect of H2S on the performance of ZrO2 and La2O3-ZrO2 catalysts in biomass gasification gas clean-up

Use of the syngas produced by biomass gasification is rendered unviable because of impurity compounds, whose efficient removal is key to the feasibility of gasification concepts. Tars present in the biomass gasification gas can be efficiently oxidized by ZrO2 catalysts. H2S has been shown to increase the oxidation activity on ZrO2 and most notably on La2O3-ZrO2. Gasification gas clean-up experiments were performed at 600–800 °C to study the beneficial effect of H2S on naphthalene and toluene conversions. During time-on-stream with repeated doses of H2S on ZrO2, cumulative and strong H2S adsorption was observed as the conversion of toluene and naphthalene increased during the first H2S doses and stabilized towards the end. Therefore, a limited capacity of the catalyst to adsorb H2S was suggested, which is the origin of sulfur tolerance. Activity enhancement with H2S occurs most notably on La2O3-ZrO2 and towards naphthalene oxidation. XPS characterization of the used catalysts showed sulfur retention on the La2O3-ZrO2 but not on pure ZrO2, indicating that La2O3-ZrO2 holds more sulfur on its surface or binds it stronger. The surface properties of La2O3-ZrO2 were thus characterized with in situ DRIFTS using CO2 and CH3OH as probe molecules. The surface of La2O3-ZrO2 was suggested to have more low coordination sites than ZrO2, where sulfur is suggested to be bound and alter the reactivity of the surface oxygen.

Benzene steam reforming kinetics in biomass gasification gas cleaning

Benzene steam reforming kinetics was studied in conditions relevant to biomass gasification gas cleaning on Ni/Al2O3 catalyst i.e. temperature range 750–900°C and in presence of H2S. The benzene concentration was 600–3500ppm. The qualitative effect of the main gasification product gas compounds on the benzene steam reforming kinetics was studied. The first order kinetic model in respect to benzene was used to compare the effect of H2, CO and CO2. It was observed that especially H2 decelerated and CO2 accelerated the benzene steam reforming kinetics. With a gas mixture representing air gasification gas, the Langmuir-Hinshelwood type model taking account effect of H2 in denominator described the benzene decomposition well. However, the model did not describe the kinetics of benzene steam reforming for the gas representing O2/H2O gasification gas.

Toluene oxidation over ZrO2-based gasification gas clean-up catalysts: Part B. Kinetic modeling

Energy, liquid biofuels and chemicals can be produced from biomass via gasification followed by gas cleaning over ZrO2-based catalysts. When a small amount of oxygen is added, ZrO2-based catalysts can be utilized in tar decomposition reactions (tar molecules need to be removed before further use). Oxidation of toluene as tar model compound was studied by temperature-programmed experiments over ZrO2, Y2O3-ZrO2 and SiO2-ZrO2. In the first part of this study (Part A: Effect of oxygen on the formation of primary products), special attention was given to the effect of oxygen amount on the formation of synthesis gas components (CO and H2) from toluene. The data obtained in Part A was extended and subjected to kinetic modeling. As a result, the reaction mechanism of toluene oxidation over ZrO2-based catalysts was discovered. The first step in toluene oxidation is the adsorption of toluene as a benzyl species. Next, the surface benzyl species is oxidized, possibly via superoxide surface species (O2−), into four products in a single net primary reaction. Oxidation of CO and H2 are the secondary reactions in the model. Furthermore, the present study clearly demonstrates the strength of temperature programming as a transient technique for comprehensive mechanistic investigations, kinetic model development and parameter estimation.

Toluene oxidation over ZrO2-based gasification gas clean-up catalysts: Part A. Effect of oxygen and temperature on the product distribution

Biomass gasification combined with effective gas cleaning is an interesting way to produce energy or syngas for numerous applications. ZrO2-based catalysts have been proven to remove the undesired tar molecules when a convenient oxygen amount is added. The oxidation of toluene (a tar model compound) was addressed in this work by applying temperature-programmed surface reaction (TPSR) experiments with a continuous feed of toluene and oxygen. Complete toluene conversions were achieved over all zirconia-based catalysts (ZrO2, Y2O3-ZrO2 and SiO2-ZrO2) in toluene oxidation above 550°C (toluene WHSV 0.06–0.07 1/h). Toluene was oxidized into four products (CO2, H2O, CO and H2). The formation of incomplete oxidation products (CO and H2) suggests that these catalysts are able to convert undesired tar molecules also into valuable synthesis gas components. Increasing the oxygen amount increased the product ratio of CO2 and CO. Over pure ZrO2, the formation of CO and H2 was detected at 600°C even with the highest feed ratio of O2/TOL (≈3.5×theoretical toluene total oxidation ratio), while over the doped zirconias the formation of CO and H2 approached zero with increasing temperature and over-stoichiometric feeds. Separate CO oxidation experiments confirmed different but appreciable activities of different zirconia materials for that reaction. However, a very minor water-gas shift activity in the presence of oxygen was detected only over pure ZrO2 at the studied temperature range. The collected multiresponse reaction data was subsequently subjected to transient kinetic modeling established on hypotheses of surface reaction mechanisms (Part B: Kinetic modeling).

Tackling the challenges in modelling entrained-flow gasification of low-grade feedstock

Development of a new technology for conversion of residual biomass into a liquid fuel via pyrolysis–gasification–gas cleaning–synthesis is the overall objective of the on-going bioliq® project. The present paper gives an overview on research activities dedicated to mathematical modelling of entrained-flow gasification for conversion of biomass-based suspension fuels into a medium calorific (LCV around 15 MJ/kg) synthesis gas. The objective is to identify knowledge gaps that currently prohibit a knowledge-based mathematical description of reacting high-pressure multi-phase flows so as to model the bioliq® gasification reactor in particular and biomass conversion in entrained flow gasifiers in general. Substantial knowledge gaps for high pressure process conditions have been identified for atomization of high viscous liquids, gasification chemistry for biomass-based fuels, radiative heat transfer as well as slag formation mechanisms. The paper proposes an interdisciplinary research approach in a holistic manner to close these gaps.

Interaction of H2S with ZrO2 and its influence on reactivity of surface oxygen

ZrO2 catalysts can be efficiently applied to convert tars into less harmful compounds in biomass gasification gas clean-up, also in the presence of H2S. In fact, H2S has even been observed to enhance naphthalene (a model compound for tar) oxidation activity on ZrO2. Sulfur binding and its effect on the reactivity of surface oxygen species, including OH groups as well surface lattice oxygen, on ZrO2 was studied using temperature-programmed methods. At 30°C approx. half of the adsorbed H2S was physisorbed and the other part dissociated titrating terminal OH groups. This dissociated H2S was strongly adsorbed. Moreover, it was found that after treatment with H2S until 400°C half of the adsorbed species were irreversibly adsorbed. H2S adsorbed inducing desorption of water at two distinct temperatures (∼170 and 280°C) during temperature ramp. Thus, H2S adsorption at elevated temperatures produced surface sulfur by replacement of surface lattice oxygen at two types of minority sites (max. 5% of a monolayer). Therefore we suggest that multicoordinated OH groups and surface lattice oxygen at defective sites are involved. CO-TPR revealed increased reactivity of surface lattice oxygen on ZrO2 with increasing amount of sulfur on the surface. In addition, strong adsorption of carbonates was suppressed by the presence of sulfur. The observed H2S-induced enhancement of naphthalene oxidation in gasification gas clean-up is suggested to be caused by the increased reactivity of surface oxygen species in the vicinity of sulfur species on the surface.

Molecular level insights to the interaction of toluene with ZrO2-based biomass gasification gas clean-up catalysts

Gasification of biomass, followed by ZrO2-catalyzed hot gas clean-up at 600–900°C for the oxidation of impurities (such as tar), is an environmentally attractive way to produce heat and power or synthesis gas. The interaction of toluene (as a model compound for tar) with ZrO2-based gasification gas clean-up catalysts was studied by in situ DRIFTS and temperature-programmed gas-phase analysis. Toluene was found to interact in four ways with ZrO2 surfaces: forming molecularly adsorbed toluene, surface benzoate species, carbonaceous deposits and surface benzyl species. The adsorption of toluene in the absence of gas-phase oxygen at ambient temperature on ZrO2-based catalysts resulted in weakly adsorbed molecular toluene, while the adsorption of toluene at higher temperatures yielded carbonaceous deposits and surface benzoate species. Combined analysis of infrared and TP data showed that some of the benzoate species decomposed, producing benzene and carbon dioxide. Surface benzyl species, on the other hand, were detected on the surface of ZrO2-based catalysts only in the presence of gas-phase oxygen at a temperature where toluene and oxygen started to convert. Therefore, it is suggested that benzyl species are the active intermediates from toluene to carbon oxides during the gasification gas cleaning over ZrO2-based catalysts.

Influence of H2S on ZrO2-based gasification gas clean-up catalysts: MeOH temperature-programmed reaction study

Addition of H2S in the gasification gas stream has been found to improve naphthalene and ammonia conversion over ZrO2 and Y2O3-ZrO2 catalysts, whereas over SiO2-ZrO2 such effect has not been observed. The differences in the properties of the catalysts were studied by spectroscopic (MeOH-DRIFTS) and temperature-programmed methods (H2S-TPD and MeOH-TPSR). Methanol was also used to probe the changes after sulfidation of the catalysts. H2S adsorption sites were found to be different on the ZrO2 and Y2O3-ZrO2 catalysts compared with the ones on the SiO2-ZrO2 catalyst. Sulfur was also found to be more reactive on the ZrO2 and Y2O3-ZrO2 catalysts than on SiO2-ZrO2. It was suggested, that the positive effect of H2S on naphthalene conversion during gasification gas clean-up is connected with the reactions of adsorbed sulfur on their surface. In contrast, on SiO2-ZrO2 the lack of such an effect may be related to limited reactivity of sulfur on the surface.

Precious metal catalysts in the clean-up of biomass gasification gas part 2: Performance and sulfur tolerance of rhodium based catalysts

Rh, Pt, and Pt-Rh catalysts on modified commercial zirconia support (m-ZrO 2) were screened for the clean-up of gasification gas from tar, methane, and ammonia both in the absence and presence of H 2S while varying the Rh metal content from 0.5 to 5 w-%. Our goal was to optimize the composition of the Rh/m-ZrO 2 catalyst in view of the production of ultra clean gas applicable for liquid biofuels synthesis. In the presence of 100 ppm sulfur, increasing Rh concentration from 0.5 to 5 w-% did not greatly improve the activity of the catalyst. The bimetallic Pt/Rh/m-ZrO 2 catalyst was also less active than the 0.5 w-% Rh/m-ZrO 2 catalyst. Furthermore, the Rh/m-ZrO 2 catalyst regained its performance at the set point of 800 °C when the sulfur feed was turned off even after exposures to 500–1000 ppm sulfur. Our data allow us to suggest that in the presence of sulfur, the active sites responsible for the reforming reactions are poisoned, but less impact occurs on sites responsible for oxidation reactions. Furthermore, the screening experiments allow to suggest that the Rh/m-ZrO 2 catalyst could be applicable to hot gas cleaning in the presence of sulfur (> 50 ppm) at above 800 °C using a moderate gas hourly space velocity of approximately 3400 1/h. Since biomass gasification gas generally contains sulfur, the 0.5 w-% Rh/m-ZrO 2 catalyst could be a promising option for gasification gas clean-up applications at temperatures above 800 °C where it reduces tar to very low levels.

Acidic and basic surface sites of zirconia-based biomass gasification gas clean-up catalysts

The surface structures of ZrO 2, Y 2O 3–ZrO 2 and SiO 2–ZrO 2 catalysts were studied in relation to their basicity and acidity, and the relationship between surface structure and gasification gas clean-up activity was evaluated. The physical properties of the catalysts were characterized by BET, XRD and Raman spectroscopy. The basicity and acidity of the catalysts were measured by CO 2-TPD and NH 3-TPD, respectively. The types of active surface sites were determined by in situ DRIFTS experiments for the adsorption and desorption of CO 2. The order of activity of the catalysts in the decomposition of tar from synthetic gasification gas was ZrO 2 > Y 2O 3–ZrO 2 > SiO 2–ZrO 2. Acidity was observed to be an unfavorable characteristic for ZrO 2-based gas clean-up catalysts, whereas basicity seems to be essential. The redox properties of Y 2O 3–ZrO 2 enhance its overall oxidation activity and the lower performance of Y 2O 3–ZrO 2 in tar decomposition compared to ZrO 2 can be explained by its oxidation activity towards other gas compounds than tar.

The Effect of Sulfur on ZrO2-Based Biomass Gasification Gas Clean-Up Catalysts

The effect of sulfur on biomass gasification gas clean-up over ZrO2, Y2O3–ZrO2 and SiO2–ZrO2 catalysts was examined. Experiments were carried out at the temperature range of 600–900 °C with sulfur free and 100 ppm H2S containing simulated gasification gas feeds. A mixture of toluene and naphthalene was used as a tar model compound. Results revealed that the sulfur addition affected positively on the catalyst properties mainly at 600 and 700 °C: over Y2O3–ZrO2 and ZrO2 sulfur addition improved naphthalene and ammonia conversion. However, over SiO2–ZrO2 no clear effect with H2S addition was observed. The effect of sulfur addition on the catalyst properties was connected to the formation of SO2 from H2S when oxygen was available. The intensity of the sulfur effect increased with the Lewis basicity strength of the catalysts. This indicates that the sulfur adsorption has a role in generating new type of active sites and/or plays role in changing the redox properties of the zirconia. Since the biomass gasification gas contains usually significant amounts of H2S the sulfur tolerance of the zirconia based catalysts is a remarkable benefit.

Ammonia Decomposition on Tungsten-Based Catalysts in the Absence and Presence of Syngas

Synthesis gas produced from biomass gasification can serve as a starting point for producing electricity, low-to-medium energy fuels, and even hydrogen for fuel cells. The major barrier in commercialization of biomass gasification is the presence of impurities such as NH3, tars, and H2S in the gas products that are detrimental to downstream processes. This paper reports the results of a study of NH3 decomposition to N2 and H2 on tungsten-based catalysts, tungsten carbide (WC) and tungstated zirconia (WZ), for a gasification gas cleanup strategy that involves removal of tars first followed by NH3 decomposition. The effects of the presence of H2 and CO on the behavior of these catalysts are also reported. At the NH3 decomposition reaction conditions used in the present study (1 atm, 465−650 °C, 4000 ppm), both WC and WZ showed an induction period. The main reason for this induction period is hypothesized to be a restructuring of the catalyst surface by NH3 so that the surface is more favorable for reaction. Both WC and WZ displayed superior activity compared to a commercial Fe-based NH3 synthesis catalyst (Amomax-10). At 600 °C and in the presence of syngas, no conversion was observed on WC while ca. 20% and 10% conversions were observed on WZ and Amomax-10, respectively, at steady state for the reaction conditions used. This corresponds to intrinsic rates of 1.2 μmol/g cat./s and 0.53 μmol/g cat./s (0.021 μmol/m2 cat./s and 3.1 μmol/m2 cat./s), respectively, for WZ and Amomax-10.

Sulphur poisoning of nickel-based hot gas cleaning catalysts in synthetic gasification gas: II. Chemisorption of hydrogen sulphide

The effect of different components of gasification gas on sulphur poisoning of nickel catalysts were studied. In addition, the sulphur distribution and content of nickel catalyst beds were analysed to account the poisoning effect of sulphur on the activity of catalysts to decompose tar, ammonia and methane. The desorption behaviour of chemisorbed sulphur from the bed materials was monitored by temperature programmed hydrogenation (TPH). It was established that bulk nickel sulphide was active in decomposing ammonia in high-temperature gasification gas-cleaning conditions. The decomposing activity of methane was not affected by bulk nickel sulphide formation, but that of toluene was decreased. The activity of the catalyst regained rapidly when H 2S was removed from the gas. However, the conversion of ammonia was not regained at as high a level as before sulphur addition, most probably due to irreversible sulphur adsorption on the catalyst. The temperature increase could also be used to regenerate the catalyst performance especially in respect to methane and toluene. Sulphur adsorbed on nickel catalysts in different chemical states depends on the process conditions applied. At >900°C the sulphur adsorbed on the catalyst formed an irreversible monolayer on the catalyst surfaces, while at <900°C the adsorbed sulphur, probably composed of polysulphides (multilayer sulphur), was desorbed from the catalyst in sulphur-free hydrogen containing atmosphere. However, a monolayer of sulphur still remained on the catalyst after desorption. The enhanced effect of high total pressure on sulphur-poisoning of nickel catalysts could be accounted for the increased amount of sulphur, probably as a mode of polysulphides, adsorbed on the catalyst.

Coal gasification gas cleanup

Coal gasification integrated with combined cycle power generation (IGCC) is an alternative technology to utilize coal for electric power generation. IGCC technology has a higher efficiency and lower sulphur emissions than that of conventional power generation by coal combustion. Under gasification conditions, part of the coal S and N is converted to H2S and NH3, which have to be removed from the product gas before its combustion in the gas turbine. Sorbents can be used to capture H2S at the reactor temperature, or at a lower temperature after the product gas is partially cooled. Controlled combustion of coal gas can convert NH3 to N2. Equilibrium calculations provide valuable information on gas cleanup as a function of operating conditions of the selected gasifier technology and the type of sorbent being used.