Microsoft Word - 476hernandez.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 2015 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-34-1; ISSN 2283-9216 Novel Coal Gasification Process: Improvement of Syngas Yield and Reduction of Emissions Andrea Bassania, Flavio Manentia*, Eliseo Ranzia, Nádson N. M. Limab, Lamia Zuniga Linanb. aPolitecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Piazza Leonardo da Vinci 32, 20133 Milano, Italy bUniversity of Campinas (UNICAMP), Department of Processes and Products Development, 13083-852, Campinas/SP, Brazil flavio.manenti@polimi.it This article is intended to propose and model an innovative process layout for coal gasification that improves the production of syngas and also reduces the sulfur and CO2 emissions. The typical coal gasification process uses Sulfur Recovery Units to convert H2S to sulfur, but these have some disadvantage, e.g low sulfur price, coal charge with low sulfur flow rate, use of Tail Gas Treatment unit. Compared to the Claus process, this solution converts H2S and CO2 into syngas (economically appealing), reduces emission of H2S and CO2 and allows the use of coal charge with high sulfur flow rate, e.g. 9.5% mol/mol. The novel process takes advantage of a double amine wash, a thermal regenerative furnace and considers the recycle of the acid gases coming from the catalytic reactor to further promote the H2S conversion. In particular, the double amine wash is useful to purify the H2S stream to be sent to the thermal furnace from the syngas and CO2, in order to reduce the reactor inlet flow rate. The regenerative furnace is simulated using a large detailed kinetic scheme to appropriately describe the minor species (among them, pollutants like CS2 and COS). As a result, the recycle appears to substantially reduce the pollutant emissions. In addition, the conversion of the Claus process into the novel process doesn’t require any change in the main equipment, just needing for a variation in the layout and the operating conditions. 1. Introduction Hydrogen sulfide (H2S) and carbon dioxide (CO2) are two of the most critical chemical by-products discharged by power, petrochemical and manufacturing plants. Despite their massive production, they do not represent a major feedstock or commodity for subsequent industrial purposes. Then, they are pollutants and the venting to atmosphere, when admissible, leads to environmental issues; otherwise, expensive neutralization processes must be adopted. Moreover, H2S is a poison for industrial catalysts and its combustion products (e.g. SOX) are strong pollutants. Consequently, desulphurization of hydrocarbons and coal and sweetening of natural gas are mandatory operations. Due to chemical acid behavior, gaseous effluents from desulphurization units are called “acid gases”. Since H2S is toxic, it cannot be vented to atmosphere, but must be properly neutralized. Therefore, acid gases represent a dangerous and costly waste. At H2S concentrations above 40%, the most important and spread neutralization method is based on oxidation of H2S by air (i.e. Claus process): the acid gas is fed to a furnace along with a sub-stoichiometric amount of air and then oxidized to sulfur dioxide (SO2) at high temperature: 2 2 2 2 1 2H S O SO H O+ → + (1) The SO2 is next reduced by residual H2S by means of catalytic reactors for obtaining elemental sulfur: 2 2 2 32 2xH S SO S H Ox+ → + (2) DOI: 10.3303/CET1543248 Please cite this article as: Bassani A., Manenti F., Ranzi E., Lima N.N.M., Zuniga Linan L., 2015, Novel coal gasification process: improvement of syngas yield and cut of emissions, Chemical Engineering Transactions, 43, 1483-1488 DOI: 10.3303/CET1543248 1483 The elemental sulfur is partially used in process industry to produce sulfuric acid; nevertheless, since most portion of sulfur is stocked and commercial demand for sulfuric acid can be weak, the neutralization process represents a significant plant cost. CO2 is the major responsible for impacts on ecological and environmental system, involving the greenhouse effect. Due to its thermodynamic stability and low chemical value, CO2 has presently minor industrial uses (e.g. urea synthesis). As a result, nowadays H2S and CO2 are strongly undesired coal gasification by-products, to be accounted as a huge cost in the budget of industrial plant. Nonetheless, despite his bad reputation, H2S is quite an interesting chemical since it contains a highly noble species, the hydrogen. Consequently, H2S should deserve more attention than to be considered just a waste and then oxidized in Claus plants to obtain elemental sulfur and water. Besides, as shown in above reforming synthesis reactions, CO2 as well is a potential interesting molecule: it may work as oxidizing agent (e.g. reforming synthesis reactions). 2 4 22CO CH CO H+ → + (3) All these considerations have pushed research and development for a captive use of H2S and CO2, to improve the accounting of these by-products. In this work, proven and mature technology used in the Claus process has been realized to be potential source of innovation. Taking advantage of extensive scientific works and operating experience on acid gas oxidation by air at high temperatures, H2S and CO2 have been observed under a different light, like a couple of oxidizing and reducing species; so, reduction of CO2 by means of H2S looks to be a promising and valuable new route. Although some aspects are still unresolved, thermodynamics, kinetics (Manenti et al., 2013a) and process control (Signor et al., 2010) of Claus process have been plentifully studied so far; consequently, the H2S and CO2 conversion process starts from a conceptual and technological validated basis. This new technology, developed in this study, presents three principle advantages: the recycle of acid gas (i.e low H2S emissions), the reduction of CO2 that is vent into atmosphere and the production of H2 and CO that is economically appealing. Therefore, this technology can be used not only for coal gasification, as in this case, but also for all chemical process in which there are acid gases, such as hydrodesulfuration. 2. Novel Process Description In this section, the overall layout of the novel coal gasification process is discussed and then each part is analyzed. For each section, some numerical results are presented in order to prove the validity of the process and highlight the possible advantages and disadvantages. The process is simulated with the use of an industrial plant simulation software: Aspen Hysys. A Peng-Robinson equation of state is used for the entire process except for the amine wash section where is used the amine package, included in Aspen Hysys. The Figure 1 shows a simplified BFD of the novel process. Figure 1 Process layout 1484 The case study simulation, developed in this work, is built up starting from a syngas stream coming from the gasification section (Table 1). This is a typical composition of coal gasification products launching from coal charge with a high sulfur flow rate, e.g. 9.5% mol/mol. Table 1 Syngas stream composition coming from coal gasification section xCO xH2 xCO2 xH2O xCH4 xAr xN2 xH2S xCOS xO2 ntot [kmol/h] Syngas from Coal Gasification (3) – vapor 0.19 0.13 0.09 0.55 0.00 0.00 0.00 0.03 0.00 0.00 3375.68 2.1 Main Amine Wash The syngas must be purified form acid gases (H2S and CO2). For this work is decided to use an amine wash, using MDEA (Methyl DiEthanolAmine) that is chosen for its industrial application and its specific selectivity to hydrogen sulfide. Furthermore, MDEA has low vapor pressure, high resistance to degradation and requires low energy absorption for the regeneration (approximately 100 kg of steam for 1 m3 of MDEA) compared to DEA (DiEthanolAmine) and MEA (MonoEthanolAmine). In the process, is used two distinct amine washes (Figure 1): 1) The first amine wash aims to remove almost all of the H2S from the acid gas stream. Simultaneously is limited the absorption of CO2, that is the main source of CO and at the same time a "heat absorber" in the furnace. If there is an excess of CO2, more oxygen should be required to reach the desired temperature, leading to a major consumption of H2S for oxidation instead of consumption of H2S for pyrolysis. 2) The second amine wash aims to remove the CO2 from the syngas stream. However, a small percentage of CO2 remain in the stream but in several processes, such as methanol synthesis, it can be useful. The output syngas is used for sale or for some other downstream processes, while the stream with high concentration of CO2 is sent to burner or to storage. 2.2 Reaction Section ( Furnace and Catalytic Reactor) In industry, there is a different configuration of the Claus furnace, called "regenerative", which allows to produce a greater amount of H2. This configuration (Figure 2) present a preheating for input acid gas stream before the combustion of gases. Then, the gases are sent, as usual, to the WHB (Waste Heat Boiler). Figure 2 Regenerative Furnace Scheme The hydrogen is produced almost all by the H2S pyrolysis (Binoist et al., 2003). Therefore, if the temperature of the acid gases rises up to 750-850°C before the combustion, the oxygen flow rate, required to reach the standard temperatures in the furnace (1200-1300°C), is lower and so the H2S available for pyrolysis is greater. Another advantage of this configuration is that a stream of pure oxygen is used. This lead to limit the effects of "heat absorber" of nitrogen. This reactor is modeled as a series of ideal reactors (Manenti et al., 2012): 1) Plug Flow Reactor with a linear temperature profile. The inlet temperature is the same of acid gas stream (about 200-250°C) while the output temperature (before the combustion chamber) is about 750-850°C. 2) "Gibbs Reactor" (Reactor implemented in Hysys that minimized the Gibbs energy). This reactor simulate the combustion chamber. 3) Plug Flow Reactor with a linear temperature profile. The inlet temperature is given by the output stream from the Gibbs Reactor, while the output temperature is evaluated from heat balance with the reactor (1). 4) Non-isothermal PFR reactor with fixed external temperature and heat transfer coefficient. This reactor simulates the WHB. 1485 It is important to emphasize that the reactors (1), (3), (4) are simulated through the use of DSMOKE (software for ideal reactor simulation with a detailed kinetic scheme), which is integrated within Aspen Hysys with the use of Visual Basic programming. This allows to use a detailed kinetic scheme within a commercial package for the simulation of chemical plants. The detailed scheme is made up of three different subsets reaction that describe the kinetic of Carbon (methane oxidation (Ranzi et al., 1994b), propane and n-butane oxidation (Ranzi et al., 1994a) with later revision of CO-H2 system (Frassoldati et al., 2007)), Sulfur (Glarborg studied the effect of SO2 (Glarborg et al., 1996), Binoist studied the pyrolysis of H2S (Binoist et al., 2003) and Manenti revised the kinetic scheme with particular attention to H2S and COS species (Manenti et al., 2013)) and Nitrogen (Faravelli et al., 2003). The catalytic reactor is modelled as a series of four reactors that are already implemented in Aspen Hysys. These reactors are characterized as follows: 1) Conversion reactor for CS2 hydrolysis: in this reactor, the following reaction occurs at complete conversion: 2 2 2CS H O COS H S+ → + (4) 2) Conversion reactor for COS hydrolysis: in this reactor, the following reaction occurs at complete conversion: 2 2 2COS H O CO H S+ → + (5) 3) Conversion reactor for Claus reaction: in this reactor, the following reaction occurs at complete conversion: 2 2 2 2 32 22H S SO S H O+ → + (6) 4) Equilibrium reactor. This the reactor allows, at different temperature, to evaluate the different composition between the various allotropic species of sulfur. The hydrolysis and the Claus reaction are evaluated at complete conversion because is consider that the system works in the most favorable conditions for the sustainability of the process. In fact, the hydrolysis reactions produce H2S, which can be recycled to the regenerative furnace and can be further source of H2. The Claus reaction allows to complete the conversion of SO2. In this way can be produced the greatest amount of sulfur. 2.3 Secondary Wash Section This wash is located after the catalytic rector. It is similar to the first amine wash of the main section and has the purpose of separating the H2S (with a low percentage of CO2) from the syngas and the remaining CO2. Moreover, this stream (syngas and CO2) is sent to the compressor and then again to section of the main wash (Figure 1). This stream has a reduced flow rate, consequently is reduced the compression costs. Instead, the stream containing H2S is sent directly to the furnace. 3. Results and Discussions In this paragraph are presented and discussed the results of the case study simulations for each section. 3.1 Main Amine Wash Results The numerical results of this section are summarized in Table 2. Is possible to note that: • The configuration of the first amine wash is suitable for stripping of all H2S from the syngas stream by ensuring that only a part (about 25%) of CO2 remains in the acid gas. • The configuration of the second amine wash is suitable for complete stripping of CO2 and H2S present in the syngas stream from the first wash. In the syngas stream is left an amount of CO2 equal to about 4% molar. • In the syngas stream to be sold, there is not H2S. • The other output stream ("CO2 to Stock") is sent to a burner or stored under pressure. This is the only outlet stream in which there is H2S. However, its amount is about 40 ppm and so the aim of reducing emissions is achieved. • The yield of syngas washing is: 2 2 ( ) 100 99.2% ( ) out in H S CO H S CO + = + (7) 1486 The surplus syngas produced deserves a special mention. In fact, the comparison between the input syngas with the output syngas should be made with the amount of syngas "clean" obtained from the same amount of initial syngas (Table 1), making one wash that allows to completely remove acid gases (traditional process). Therefore, is simulated a traditional washing section in Aspen Hysys in order to obtain an output syngas stream "clean" with a percentage of H2S and about 4% of CO2. In the syngas stream "clean”, the hydrogen flow rate is equal to 445.86 [kmoli/h], while carbon monoxide flow rate equal to 639.60 [kmoli/h]. So is calculated the surplus of syngas as follow: 2 2 2 (( ) ( ) ) 100 1.17% ( ) new process traditional traditional H CO H CO H CO + − + = + (8) Table 2 Simulation results of the main amine wash section xH2 xH2O xCO xN2 xO2 xCO2 xH2S xSO2 xCOS xCS2 ntot [kmol/h] Syngas to 1st amine wash (4) - vapor 0.28 0.00 0.40 0.00 0.00 0.25 0.06 0.00 0.00 0.00 1635.40 Acid Gases from 1st amine wash (8) - vapor 0.00 0.05 0.00 0.00 0.00 0.50 0.41 0.00 0.03 0.00 226.48 Syngas to 2nd amine wash (5) - vapor 0.32 0.00 0.46 0.01 0.00 0.21 0.00 0.00 0.00 0.00 1416.37 CO2 to Stock (7) - vapor 0.00 0.05 0.00 0.00 0.00 0.94 0.00 0.00 0.00 0.00 269.58 Syngas Out (6) - vapor 0.39 0.00 0.56 0.01 0.00 0.04 0.00 0.00 0.00 0.00 1156.01 3.2 Reaction Section ( Furnace and Catalytic Reactor) Results In Tables 3, 4 and 5 the results of a case study simulation of the reaction section are reported. It is useful to highlight that the gases are preheated in the “regenerative" furnace up to a temperature of 800°C before coming in contact with oxygen. Looking at the numerical results it can be stated that: • The amount of sulfur is lower (about 40%) than that of a classic Claus process, but it is a possible revenue that can financially support the process. • The recycling stream (Table 5) is the heart of the process. This makes it possible to reuse all the H2S and to have no emissions. In fact, in a traditional Sulfur Recovery Unit, this stream must be sent to a tail gas treatment process. Table 3 Simulation results of reaction section (1): Furnace xH2 xH2O xCO xN2 xO2 xCO2 xH2S xSO2 xCOS xCS2 xS2 ntot [kmol/h] Total Acid Gas to Furnace (8+15) - vapor 0.01 0.05 0.01 0.00 0.00 0.40 0.51 0.00 0.02 0.00 0.00 330.08 Oxygen (9) - vapor 0.00 0.00 0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 45.00 Acid Gases to Catalytic Reactor (10) - vapor 0.01 0.33 0.04 0.00 0.00 0.30 0.15 0.00 0.03 0.00 0.13 379.35 Table 4 Simulation results of reaction section (2): Catalytic Reactor (Xsn (n=2,4,6,8) are the allotropic configuration of Sulfur). xH2O xS2 xS4 xS6 xS8 ntot [kmol/h] Sulfur (11) - liquid 0.14 0.15 0.14 0.29 0.28 16.67 Table 5 Simulation results of reaction section (3): Catalytic Reactor xH2 xH2O xCO xN2 xO2 xCO2 xH2S xSO2 xCOS xCS2 xSx ntot [kmol/h] Recycle (12) - vapor 0.01 0.33 0.04 0.00 0.00 0.39 0.23 0.00 0.00 0.00 0.00 328.11 1487 3.3 Secondary Wash Section Results Looking at the numerical results (Table 6) it can be stated that: • The main purpose of this amine wash (reduction of the stream to be compressed in order to be then recycled), is achieved since the recycle stream is "lightened" by about 40% molar. • In addition, is prevented that large amounts of H2S are recycled to the main section of wash. Thus limits the problems of excessive emission of H2S. Table 6 Simulation results of secondary wash section xH2 xH2O xCO xN2 xO2 xCO2 xH2S xSO2 xCOS xCS2 ntot [kmol/h] Syngas form 3rd amine wash (16) - vapor 0.00 0.05 0.00 0.00 0.00 0.19 0.76 0.00 0.00 0.00 95.76 Acid Gases from 3rd amine wash (15) - vapor 0.02 0.03 0.11 0.00 0.00 0.84 0.01 0.00 0.00 0.00 132.43 4. Conclusions In this work is presented a novel process for conversion and treatment of acid gases (in particular H2S) coming from coal gasification. The idea is to exploit the oxidizing capacity of CO2 with H2S to ease the recovery of hydrogen, which is a chemical species more economically attractive than sulfur. Moreover, the emissions of H2S and CO2 is reduced without using any units for gas treatment. Given the innovative nature of the process, this technology requires more detailed analysis before it can be use on a real industrial plant, but this first simulation highlight that the novel process is very interesting and economically appealing. References Binoist M., Labegorre B., Monnet F., Clark P. D., Dowling N. I., Huang M., Archambault D., Plasari E., Marquaire P. M, 2003, Kinetic study of the pyrolysis of H2S, Industrial & Engineering Chemistry Research, 42(17), 3943-3951. Faravelli T., Frassoldati A., Ranzi E., 2003, Kinetic modeling of the interactions between NO and hydrocarbons in the oxidation of hydrocarbons at low temperatures, Combustion and Flame, 132(1-2), 188-207. Frassoldati A., Faravelli T., Ranzi E., 2007, The ignition, combustion and flame structure of carbon monoxide/hydrogen mixtures, International Journal of Hydrogen Energy, 32(15), 3471-3485. Glarborg P., Kubel D., DamJohansen K., Chiang H. M., Bozzelli J. W., 1996, Impact of SO2 and NO on CO oxidation under post-flame conditions, International Journal of Chemical Kinetics, 28(10), 773-790. Manenti F., Papasidero D., Cuoci A., Frassoldati A., Faravelli T., Pierucci S., Ranzi E., Buzzi-Ferraris G, 2012, Reactor network analysis of Claus furnace with detailed kinetics, Computer Aided Chemical Engineering, 30, 1007-1012. Manenti F., Papasidero D., Frassoldati A., Bozzano G., Pierucci S., Ranzi E., 2013a, Multi-scale modeling of Claus thermal furnace and waste heat boiler using detailed kinetics, Computers & Chemical Engineering, 59, 219-225. Manenti F., Papasidero D., Ranzi E., 2013b, Revised Kinetic Scheme for Thermal Furnace of Sulfur Recovery Units, Chemical Engineering Transactions, 32, 1285-1290, DOI: 10.3303/CET1332215 Ranzi E., Faravelli T., Gaffuri P., Pennati G. C., Sogaro A., 1994a, A Wide-Range Modeling Study of Propane and N-Butane Oxidation, Combustion Science and Technology, 100(1-6), 299-330. Ranzi E., Sogaro A., Gaffuri P., Pennati G., Faravelli T., 1994b, A Wide-Range Modeling Study of Methane Oxidation, Combustion Science and Technology, 96(4-6), 279-325. Signor S., Manenti F., Grottoli M. G., Fabbri P. and Pierucci S., 2010, Sulfur Recovery Units: Adaptive Simulation and Model Validation on an Industrial Plant, Industrial & Engineering Chemistry Research, 49(12), 5714-5724. 1488