Article pubs.acs.org/IECR
Application of CaO Sorbent for the Implementation and Characterization of an in Situ Desulfurization Steam-Blown Bubbling Fluidized-Bed Test Rig for Biomass Gasification Moritz Husmann,*,† Thomas Kienberger,‡ Christian Zuber,§ Wiebren de Jong,∥ and Christoph Hochenauer† †
Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25b, 8010 Graz, Austria Chair of Energy Network Technology, University of Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria § Agnion Highterm Research GmbH, Conrad von Hötzendorfstrasse 103a, 8010 Graz, Austria ∥ Department of Process & Energy, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands ‡
S Supporting Information *
ABSTRACT: In this work, a test rig is presented that is suitable for investigations on the equilibrium of high-temperature in situ desulfurization sorbents under real process conditions of a fluidized-bed gasifier. Gasification was performed using wood pellets as fuel with addition of a defined amount of CS2 to raise the content of sulfur without significantly changing the composition of permanent gases. The conversion of CS2 to H2S under gasification conditions ensures a high sulfur content in the produced gas in the range of 1200 ppmv, representing a synthesis gas derived from gasification of low-grade residual biomass. This type of gas is suitable for investigating the equilibrium of the desulfurization reaction under real process conditions, as the initial sulfur content is high enough to observe a desulfurization effect upon addition of potentially suitable sorbents. Furthermore, the low ash content of wood pellets ensures long-term stable gasification conditions. Investigations using lime as a sulfur sorbent were conducted in a systematic approach for system characterization with respect to overall conversion, sulfur balance, and residence time. Analytics were based on GC measurement of the synthesis gas covering all relevant volatile gaseous sulfur components such as COS, H2S, CS2, and CH3SH. The achieved desulfurization equilibria in the process application of in situ desulfurization proved to be in good accordance with theoretical values derived from a Factsage 6.4 simulation.
1. INTRODUCTION The future prospect of a broader implementation of biomass gasification to produce substitute natural gas (SNG) or other biofuels is critically dependent on the achievable specific costs for such a renewable complement to natural gas and fossil fuels. As for gasification of wood pellets and chips, the production of SNG is technically feasible and has been demonstrated on scale of several megawatts thermal,1−3 yet is too expensive to compete with natural gas.4 One of the most important cost factors is fuel, and therefore, a cheaper alternative to using wood as fuel is crucial. Cheaper fuels such as herbaceous biomass, felling residue, or sewage sludge, although having the benefit of lower expenses for fuel input, give rise to technical challenges during the gasification process itself, because of a relevantly higher content of impurities in the gas.5−10 In addition to having detrimental properties such as higher ash and chlorine contents, the higher content of sulfur in alternative fuels leads to the bulk formation of gaseous H2S and, therefore, to a substantial increase in the effort required for gas cleaning. The sulfur content in lower-grade fuels ranges from 0.049 wt % for park and public garden wood to 0.22 wt % for Cacao shells or even 0.65 wt % and up to 2.61 wt % for sludge from the paper industry and sewage, respectively.10 Depending on the targeted process of conversion of the synthesis gas (syngas) into energy or chemical substances, different levels of sulfur content are tolerable in the gas.11 In © 2015 American Chemical Society
most cases, especially if the gas is intended for the production of SNG or other chemical substances, a catalytic conversion is necessary.12 To counteract the poisoning effect of sulfur on catalytic sites, a higher expense for sulfur capture upstream of the catalytic treatment is inevitable.13 If the capture of high loadings of sulfur is done by desulfurization over zinc oxide, the higher cost due to sorbent consumption might overcompensate the benefit of a cheaper gasification fuel. Thus, a low-cost process for coarse primary in situ desulfurization under biomass gasification conditions is essential for the implementation of the use of a broad range of alternative fuels. The concept of in situ desulfurization was evaluated in a previous study14 combining the achievable residual sulfur contents of this process with desulfurization over a zinc oxide sorbent in a fixed-bed reactor at lower temperatures and aiming at reduced overall costs through a cheap coarse desulfurization. The pursued concept of gas cleaning is depicted in Figure 1. The benefits of in situ desulfurization would be capturing sulfur during the gasification process inside the fluidized bed without the need for an additional apparatus. Negative Received: Revised: Accepted: Published: 5759
February 10, 2015 May 13, 2015 May 14, 2015 May 22, 2015 DOI: 10.1021/acs.iecr.5b00593 Ind. Eng. Chem. Res. 2015, 54, 5759−5768
Article
Industrial & Engineering Chemistry Research
requirements for suitable sorbents for in situ sulfur capture demand both thermal stability and an equilibrium constant that is high enough to achieve low residual sulfur contents even at elevated temperatures.16 The aim of an in situ desulfurization process would be to lower the content of H2S in the gas to a level that is acceptable for further fine desulfurization and catalytic treatment (Figure 1). With regard to economic suitability, such a coarse desulfurization would have to be much less expensive than the sole application of a mediumtemperature desulfurization over a zinc oxide sorbent in a downstream fixed-bed reactor. Previous investigations17−21 on high-temperature desulfurization were performed mostly in the context of coal-derived syngas, which usually contains a significantly lower content of steam, in the range of 2−20 vol %,17 than syngas from allothermal biomass gasification.14,22,23 According to thermodynamic equilibrium calculations, the water content has a strong influence on the desulfurization equilibrium for most sorbents other than CuO.14 For application to biomass-derived syngas with a steam content of 40 vol %, a mixed melting phase with BaO as the active sorbent24 has a thermodynamically predicted desulfurization equilibrium that is sufficient for further catalytic gas treatment. These theoretical values consider the system to be in thermodynamic equilibrium, which is not the case for real process applications. This is clearly shown by the content of the methane fraction in the permanent gas components, as well as by the formation of tars, which would not occur under thermodynamic equilibrium conditions. Therefore, the predicted values can provide only a qualitative picture concerning the suitability of a sorbent, without confirmation under real process conditions. Other sorbent combinations have shown good sulfur capture results in experimental applications under fixed-bed conditions,25−28 whereby the positive material interactions and
Figure 1. Pursued concept of gas cleaning for high-sulfur-content fuels with optional steps such as hydrodesulfurization (HDS), which converts organic sulfur components to H2S.13 The displayed values for sulfur content represent the target value for each process step.14
restrictions are the fixed conditions of the gasifier, which are unfavorable for the desulfurization reaction MeOx + x H 2S ↔ MeSx + x H 2O
(1)
The desulfurization takes place through sulfidation of metal oxides (MeOx). Its equilibrium depends on the steam content and the temperature. High steam contents and temperatures shift the reaction to the reactant side. Assuming a surplus of sorbent, the sorbent-specific equilibrium constant of desulfurization, Kbeq, can be expressed as a ratio between the gas-phase reactions15
⎛ [H O] ⎞ b Keq =⎜ 2 ⎟ ⎝ [H 2S] ⎠eq
(2)
The formation of a sulfide phase in the sorbent takes place until the ratio of partial pressures between H2O and H2S equals the temperature-dependent equilibrium constant. Under high steam conditions in allothermal biomass gasification, the
Figure 2. Flowchart of the experimental setup. 5760
DOI: 10.1021/acs.iecr.5b00593 Ind. Eng. Chem. Res. 2015, 54, 5759−5768
Article
Industrial & Engineering Chemistry Research
Figure 3. Flowchart of the test gas manifold (temperature-controlled parts indicated with red lines).
minimum amount of sorbent per mole of initial sulfur necessary to reduce the sulfur content to the determined equilibrium value was determined to both estimate the conversion of sorbent and evaluate the process economics.
surface induced desulfurization effects, as proposed by Cheah et al.,23 but are beyond the scope of thermodynamic simulation. The results published for desulfurization sorbents are commonly obtained in experiments in fixed-bed test rigs under conditions of mixed syngas from bottled gases. Especially for in situ desulfurization, where the desulfurization reaction takes place in the fluidized bed and is thus subjected to fluctuating gas compositions and temperatures depending on the cyclic addition of fuel, an experimental investigation under real process conditions is necessary to elucidate further influencing factors. For this purpose, a systematic approach for sorbent characterization has been developed that is suitable to investigate the factors influencing sulfur capture and, therefore, the extent of agreement between experiments and simulations. In a first experimental study, a test rig was adapted to compare simulated results with the application of sorbents in a real process of wood pellet gasification. To visualize the actual desulfurization equilibrium, the H2S content of the gas had to be increased. Only when the initial H2S content was above the desulfurization equilibrium of the investigated sorbent would this sorbent cause a reduction of H2S content in the gas by sulfidation. For a first characterization of the adapted system, lime (CaO) powder was applied as a reference sorbent material because it is easily available and toxically harmless. Furthermore, it has previously been described as a desulfurization sorbent.20,21,29−32 To investigate the chemical equilibrium without sorbent limitations, a first series of experiments was performed with excess addition of sorbent. In this context, a stoichiometric ratio of 10:1 for the number of moles of sorbent to the number of moles of total sulfur was applied to ensure a surplus of sorbent. As the sorbent was continuously added in excess, the ratio of available sorbent in the system increased during these experiments. With these experiments, the practically achievable sorbent-specific residual sulfur content was determined. Further investigations were then performed to investigate the factors influencing the achieved sulfur species concentration values. This included experiments to exclude the influence of kinetics by variation of the residence time. Finally, the necessary
2. EXPERIMENTAL SETUP Experiments were carried out in a bubbling fluidized-bed gasifier (BFBG) unit at TU Graz adapted for high-sulfur equilibrium investigations. Biomass was gasified in a bubbling fluidized bed at a temperature of approximately 800 °C under allothermal conditions with steam used as the fluidization medium. A schematic of the gasification system is depicted in Figure 2. The fluidized bed as the central gasification unit is connected to different parts of an operational periphery, indicated by different colors for the gasification product gas stream; fuel; and fluidization medium, which is superheated steam. A steam generator injects superheated steam at 400 °C through a nozzle at the bottom of the reactor, which diverts the steam to six downward-oriented jets, ensuring good back mixing and stable fluidization conditions. Fluidization is monitored by the spread of temperatures in the fluidized bed, which are assumed to be roughly equal under fluidized conditions. The amount of steam entering the reactor is controlled by the temperature in a steam generator connected to the reactor by a fixed orifice. The inlet of water to the steam generator is measured by a flow meter. Steam and fuel supply had to be calibrated prior to the experimental work. For the experiments described in this work, wood pellets were used as the fuel. The ultimate and proximate data from fuel analysis are summarized in Table S1 (Supporting Information). In the experimental system used, a charging screw in combination with a vibrating conveyor implements the dosage of fuel. Fuel is supplied from a storage tank to the inlet at the top of the reactor and inserted by a system of two combined pneumatic ball valves, which act as an air lock. Because of the cyclical opening of the air lock, some air and thus nitrogen enters the reactor. This can be seen from the nitrogen fraction in the composition of the resulting gas. Every 90 s, a medium amount of 12.5 g of pellets is fed into the 5761
DOI: 10.1021/acs.iecr.5b00593 Ind. Eng. Chem. Res. 2015, 54, 5759−5768
Article
Industrial & Engineering Chemistry Research reactor, resulting in a total fuel mass flow of 500 g/h. With a heating value of 17.6 MJ/kg, this equals a medium fuel input of 2.45 kW. Temperatures in the reactor are controlled by means of two 3.5 kW electrical heating elements. The temperatures in the reactor are kept at an average of 750 °C in the fluidized bed and 830 °C in the freeboard of the reactor. Temperature control is enabled by a series of type-k thermocouples located as indicated in Figure 2. The reactor itself consists of austenitic steel (1.4571) with dimensions as shown in Figure 2. Olivine with a particle size of 200−300 μm and consisting of 50 wt % MnO, 42 wt % SiO2, 8 wt % Fe2O3, and traces of NiO and CaO was used as the bed material. Depending on the addition of sorbent, either 1450 or 1500 g of olivine was added to the reactor prior to each experiment. The sorbent added in the course of the conducted experiments was lime from Peggau in Styria, Austria, with the composition listed in Table S2 (Supporting Information). The particle size fraction used was within 80−500 μm, as the flow velocities were above the minimum fluidization velocity and below pneumatic transport for CaO particles within this range. Depending on the purpose of an experiment (Table S3, Supporting Information), a defined amount of powder was added in batches with a standard interval of 10 min. The added powder was wrapped in negligible amounts of paper to prevent agglomeration upon contact with the humid syngas atmosphere in the air lock of the fuel supply. Wood pellets were chosen as the fuel because the homogeneity and low ash content of this fuel yield long-term stable gasification conditions. However, for the investigation of sorption equilibrium of most sulfur sorbents, the content of sulfur in the gas would be too low, as it is already below the thermodynamic equilibrium of most single metal oxide sorbents.14 Therefore, the reactor was connected to a gas manifold that allowed the injection of different gas mixtures into the BFBG. For the conducted experiments, the sulfur content of the gas was increased by adding CS2 to the bottom of the reactor. The applied gas manifold is depicted in Figure 3. The test-gas supply rig at TU Graz is suitable for various applications such as catalyst deactivation and desulfurization testing in a catalyst test rig. As shown in Figure 3, the gas manifold consists of several bottles of pressurized gas connected through mass flow controllers (MFCs) to a humidifier with subsequent addition of CO2. For adjustment of the sulfur content, a N2 stream is bubbled through a metal container filled with liquid CS2 at a defined temperature. The CS2-rich stream is then diluted by being mixed with additional N2 to prevent condensation. The flow of CS2 into the system can be controlled by means of the temperature of the container and the volume flow of the strip gas stream. The resulting overall inlet of sulfur is known through gravimetric measurement of the container filled with CS2 and then determined from the mass difference in relation to the operating time. The resulting stream with a total flow of 0.16 L/min under standard conditions (0 °C, 1 atm) is then directed to the reactor and injected through the steam nozzle. Upon contact with the steam and permanent gas components of the produced syngas, the CS2 is converted to H2S.33 At a temperature of 750 °C, the formation of H2S from CS2 is shifted to the product side according to equilibrium calculations.14 To ensure that the H2S content of the produced syngas was approximately 1500 ppmv so as to remain in accordance with prior simulations,14 the amount of additional sulfur necessary in the form of gaseous CS2 was estimated. Based on the Antoine equation coefficients
for CS2 determined by Waddington et al.,34 the temperature of the container filled with liquid CS2 was kept at 8 °C. For the purpose of analysis, a part of the product gas stream is separated from the main gas flow and continuously burned in a flare. By default, the separation of streams takes place downstream of the particulate filter consisting of a single metallic filter candle. The filter and all piping are heated to a temperature of 350 °C to prevent the condensation of tars, which could subsequently block the pipes. The gas for analysis is then cooled using a series of impinger bottles (Figure 2). In the first two bottles, water in the gas is condensed at a temperature of 20 °C and subsequently weighed for gravimetric determination of the water content. The composition of permanent gas components is monitored by a permanent gas analyzer (ABB Advance Optima Series),35 which quantifies O2, CO, CO2, CH4, and H2. The difference between 100% and the sum of content of the gas components mentioned is considered to be the N2 fraction, which includes about 1 vol % of olefins, which are present in the synthesis gas. For the measurement of gaseous sulfur components, a Varian CP-3800 gas chromatograph36 equipped with a pulsed flame photometric detector (PFPD) is utilized. The components are separated in a Gaspro Q column, enabling the distinguished analysis of H2S and COS as well as CS2 and CH3SH at moderate temperatures. The duration of chromatographic measurement determines the intermediate time between two measured gas compositions. The interval is about 12 min including the time of measurement and reinitialization of the instrument. As the gas for measurement of the sulfur content is always separated after passing the impinger bottles for water condensation, all values given in ppmv refer to the dry fraction of the gas. With the experimental setup described above, a series of experiments were performed to confirm the functionality of the system and establish a systematic approach of sorbent characterization. An overview of the experiments that contributed to the results presented herein is provided in Table S3 (Supporting Information).
3. RESULTS The gas obtained from the experimental setup described above was assumed to contain about 1500 ppmv H2S, which was achieved by the addition of CS2. The average sulfur content at different operating hours is depicted in Figure 4. The sulfur content measured in the gas was not stable within the first hours of the experiments. This is attributed to the deposition of char in the hot gas filter and subsequent sorption of sulfur on its surface. As a result of this process, the measured H2S content was in the range of 1500 ppmv within the first
Figure 4. Average sulfur content of the gas at different operating hours and sampling points (experiment NS_FI#1). 5762
DOI: 10.1021/acs.iecr.5b00593 Ind. Eng. Chem. Res. 2015, 54, 5759−5768
Article
Industrial & Engineering Chemistry Research
Table 1. Experimental Results for the Composition of Permanent Gases with Values Referring to Dry Basis as Indicateda
wood pellets wood pellets + CS2 wood pellets + CS2 + CaO a
H2 (%, dry)
CO2 (%, dry)
CO (%, dry)
N2 (%, dry)
CH4 (%, dry)
H2S (ppmv, dry)
COS (ppmv, dry)
CH3SH (ppmv, dry)
H2O (%)
39.7 41.0 41.8
19.0 18.4 18.8
25.1 24.2 23.2
7.3 8.5 8.5
9.0 7.9 7.5
18 1195 494
