20th European Symposium on Computer Aided Process Engineering – ESCAPE20 S. Pierucci and G. Buzzi Ferraris (Editors) © 2010 Elsevier B.V. All rights reserved.
Mathematical modeling and simulation of gasification processes with Carbon Capture and Storage (CCS) for energy vectors poly-generation Victoria Maxim,a Calin-Cristian Cormos,a Ana-Maria Cormos,a Serban Agachi a a
Babes – Bolyai University, Faculty of Chemistry and Chemical Engineering 11 Arany Janos Street, RO-400028, Cluj – Napoca, Romania Tel: +40264593833, Fax: +40264590818, E-mails:
[email protected];
[email protected];
[email protected];
[email protected]
Abstract Gasification of solid fuels is a partial oxidation process which converse the solid feedstock into syngas which can be used in a large number of applications e.g. power generation, manufacture of various chemicals and fuels (hydrogen, methanol, ammonia, fertilizers etc.). Not all of the gasification systems are suitable for energy vectors polygeneration with carbon capture and storage (CCS). This paper is proposing to evaluate various gasification technologies by mathematical modeling and simulation methods (especially for entrained flow types as these gasifiers are more suitable for implementing carbon capture technologies). In this paper a particular accent will be put on the selection of the most promising gasifier, as not all are appropriate for a carbon capture Integrated Gasification Combined Cycle (IGCC) applied for energy vectors poly-generation (with a particular focus on hydrogen and electricity co-production case) with Carbon Capture and Storage (CCS). For the selection of the most appropriate gasifier technologies the process were mathematical modeled and simulated with process flow modeling software (e.g. ChemCAD, Aspen). In the evaluation of various gasification technologies (e.g. Shell, Siemens, GE-Texaco, Conoco-Phillips etc.) a multi-criteria analysis was performed. Keywords: Gasification, Energy Vectors Poly-generation, Mathematical Modeling and Simulation, Multi-criteria Analysis, Carbon Capture and Storage (CCS)
1. Introduction Coal gasification is one of the options for implementation of clean coal technologies. Gasification is the conversion of solid fuels (coal, coke, oil, tar, pitch) with air, oxygen steam or a mixture of this gases at a high temperature (above 800°C) into a gaseous product which can be used either to produce electricity either as a row material for the synthesis of chemicals or liquid fuels. In the early part of the last century the first application of fuel gas was illumination and domestic heating. Gasification of coal generates a wide range of products: power, chemicals, substitute natural gas (SNG) and transport fuels. The chemical composition of syngas varies based on many factors as: coal composition, size and rank, feeding system (dry or slurry), gasification agent used for oxidation (air or oxygen), temperature, pressure, residence time in gasifier, heating rate, gasification island configuration etc. The concept of gasification applied to electricity generation, the integrated gasification combined cycle IGCC, is very attractive for energy vectors poly-generation: electricity, hydrogen, heat and chemicals [1, 2].
V. Maxim et al The gasification reactors may be designed to gasify a wide variety of solid feedstock, either fossil fuels (e.g. coal, lignite peat etc.) or various biomass types (sawdust, agricultural wastes etc.) and solid waste (animal residue, municipal solid wastes, waste paper etc.). All coal types can be gasified, low ash content coals are preferred, but coal utilisation is regarded with concern because of bigger greenhouse gas emissions associated with it. IGCC is one of the power generation technologies having the highest potential to capture CO2 with the lowest penalties in efficiency and cost. In an IGCC, modified for this purpose, the raw syngas (which contains mostly hydrogen and carbon monoxide) is subsequently reacted with steam in a shift converter, to maximize the hydrogen level in the syngas and to concentrate the carbon species in the form of CO2 that can be later capture in a pre-combustion arrangement [3, 4]. The main focus of the article is to evaluate various gasification technologies by mathematical modeling and simulation methods and the selection of the most promising gasifier, as not all are appropriate for a carbon capture IGCC. The selection of the most promising gasifier investigated in the paper will be modelled and simulated using commercial process flow modelling package (ChemCAD) to produce data for the evaluation of gasification reactors. Technologies as Shell, Siemens, GE-Texaco, Conoco-Phillips will be analyzed in this paper considering coal as feedstock.
2. Gasifier options For the gasification processes a wide range of reactors are available. For the commercial use, currently are available three types of gasifiers: moving-bed gasifiers, fluidised-bed gasifiers, and entrained-flow gasifiers [2,5]: • Moving-bed gasifiers (also called fixed bed) are characterized by operation in a bed in which the coal moves slowly downward under gravity as it is gasified by a blast. The main drawback is that synthesis gas contains high levels of phenols, methane and tars. • Fluidised-bed gasifiers in which the fuel and the oxidant coal particles are suspended in the gas flow. These gasifiers are suitable for reactive feedstocks such as lignite. Some of the disadvantages of these reactors are: high level of tar of the product gas, the incomplete carbon burn-out, and poor response to load changes. • Entrained-flow gasifiers operate with feed and blast in co-current flow. They are suitable also for non-reactive feedstocks as coal. The synthesis gas contains low levels of phenols, methane and tars. From the large range of gasifiers not all are suitable for Integrated Gasification Combined Cycle (IGCC) applied for energy vectors poly-generation with Carbon Capture and Storage. For the selection of the most appropriate gasifier several criteria must be used [6,7]: Oxygen purity: in conventional IGCC concept a 95% O2 (vol.) is acceptable. The increase of the oxygen purity above 95% (e.g. 99 %) determines higher power consumption for the Air Separation Unit (ASU) with an increase of 5-10%; Gasifier throughputs, reliability and experience: depends of the plant size taken into evaluation (for instance in this paper in the range of 400 - 500 MW power net); Cold gas efficiency (CGE) and carbon conversion efficiency (CCE): is desirable that this indicators to be as high as possible on condition that hydrocarbons (mainly methane) present in syngas must be as low as possible (hydrocarbons negative influence the carbon capture plant capabilities). In case of entrained-flow gasifiers both CGE and CCE are optimum.
Mathematical modeling and simulation of gasification processes with Carbon Capture and Storage (CCS) for energy vectors poly-generation Syngas cooling options: because of the steam requirement of the carbon monoxide shift conversion (WGS), the water quench type gasifiers are desirable. Unlike the gas quench option, the steam rising potential of the hot syngas leaving the gasifier reaction zone is severely neutralized. Influence of oxygen purity and gasifier feed system for hydrogen purification step: the influence of oxygen purity on hydrogen purification stage (done in a PSA unit) is a compromise between the need not to dilute the syngas with much nitrogen coming from the oxygen stream and decreasing the power consumption of the ASU. Although dry-feed design implies a certain syngas dilution with nitrogen, this is a preferable option against slurry-feed which imply a significant energy penalty by the water introduced with the coal slurry. Hydrogen production potential: similar to CGE is defined as the sum of carbon monoxide and hydrogen content in the syngas and it must be as high as possible, which is in the case of entrained-flow gasifiers. Downstream gas clean up issues: because of clean gas produced the entrainedflow gasifiers are the most desirable. Removing ash, hydrochloric acid, ammonia is possible using a quench system of the hot syngas Implication of gasifier reactor selection on Acid Gas Removal system, having in mind the fact that CO2 capture process based on gas – liquid absorption is positively influenced by an increased pressure is desirable to have a high pressure. Capital cost: is a very important factor in gasifier selection and for the assessment of techno-economical indicators of the plant. However, this paper is not investigated the financial aspect of gasifier selection. Analyzing all the above criteria the most promising reactors for energy vectors polygeneration (mainly hydrogen and electricity) with Carbon Capture and Storage are the entrained-flow gasifiers. The main characteristics of the four gasification technologies which are evaluated in this paper are presented above. • Shell gasifier is a carbon steel vessel that contains a gasification chamber enclosed by a non-refractory membrane wall, which operates at 30-40 bar pressure, temperature range of 1500-1600°C, dry feed and one stage. Pulverised coal is stored under nitrogen,where it is pressurized and then pneumatically transported into the gasifier. The syngas is quenched with cooled recycled product gas and further cooled in a syngas cooler. Raw gas is cleaned in ceramic filters. • Siemens gasifier is a top-fired reactor, where the reactants are introduced through a single centrally mounted burner. It is operating at similar conditions as the Shell gasifier. Unlike the Shell gasifier, which is using a gas quench, Siemens gasifier is using a water quench for cooling the syngas. • GE-Texaco gasifier is a pressure vessel with a refractory lining, which operates at at 70-80 bar pressure, temperature range of 1300-1500°C, slurry feed and one stage. Oxygen and steam are introduced through burners at the top of the gasifier, coal is preprocessed into a slurry by fine grinding and water addition. The syngas is cooled into a water quench. • Conoco Phillips (E-Gas) gasifier is two-stage coal-water slurry feed gasifier in a pressure shell lined with un-cooled refractory. Toward the bottom of the gasifier about 80% of the feedstock, as a coal water slurry is injected through burners. The temperature reaches 1350 - 1400°C at about 30 bar pressure. The syngas formed in the first stage flows upwards into the second stage area, where the remaining 20% of coal water slurry is injected. Hear the temperature is reduced to about 1000°C. The syngas exiting the gasifier in a fire-tube syngas cooler.
V. Maxim et al
3. Modeling and simulation of gasifiers for hydrogen and electricity coproduction scheme For the evaluation of various commercial gasification reactors a multi-criteria analysis will be performed considering all the criteria mentioned above. The gasification technologies presented before are analyzed in Table 1 [6,8]. Table 1. Multi-criteria analysis of coal gasifiers
Conoco Parameters Shell Siemens GE Texaco Phillips Maximum pressure (bar) 40 40 100 40 Temperature (°C) 1400-1600 1400-1600 1200-1450 950-1150 Carbon conversion (%) >99 >99 >98 >98 Steam/oxygen necessity High High High High Syngas clean up issues Low Low Low Medium H2 production potential High High High Medium CGE (%) 75-77 75-77 65-70 68-71 CO2 capture capacity High High High Medium Good OK Bad Bad Overall ranking Considering all the above criteria when choosing a gasifier for hydrogen and electricity co-production scheme with carbon capture and storage, it appears that the most appropriate gasifiers are Shell and Siemens. As mentioned before, the chosen gasifier is an entrained-flow type, operating at high temperatures with a high fuel conversion. From different commercial gasification technologies available on the market four are modeled and simulated using process flow modeling software (ChemCAD). As main design assumption, all gasifier concepts evaluated in the paper were considered Gibbs Free Energy Reactors (GIBS). By this model the calculations are made by the minimization of Gibbs free energy and approaching equilibrium state between reactants and products. Other gasifier design assumptions are: pressure drop 1.5 bar, pressure 40 bar (except GE Texaco 75 bar), vapour or mixed reaction phase, heat duty thermal mode (except GE Texaco which is adiabatic). The diagram for Shell gasification block is presented in Figure 1.
Figure 1. Shell gasification diagram
The coal is advised to have low content of ash and sulphur to reduce corrosion and SOx emissions, but most important is that the ash has to have a relatively high melting point to prevent ash build up on the boiler heat transfer area [8]. The fuel characteristics (coal) are presented in Table 2.
Mathematical modeling and simulation of gasification processes with Carbon Capture and Storage (CCS) for energy vectors poly-generation Table 2. Coal characteristics
Parameter Proximate analysis (% wt)
Coal
Moisture Volatile matter Ash
8.10 28.51 14.19
Ultimate analysis (% wt dry) Carbon 72.04 Hydrogen 4.08 Nitrogen 1.67 Oxygen 14.17 Sulphur 0.65 Chlorine 0.02 Ash 14.19 Lower heating value - LHV (MJ/kg a.r.) 27803.29 Table 3 summarizes the syngas composition, CGE and efficiency of conversion in CO and H2 (hydrogen production potential) for all the gasifiers that have been considered. Cold gas efficiency (CGE) and hydrogen production potential must be as high as possible on condition that hydrocarbons (mainly methane) present in syngas must be as low as possible [8]. Cold gas efficiency (CGE) shows the energy efficiency of gasification process and it is defined as follow:
CGE =
Syngas thermal energy [MW ] Feedstock thermal energy [ MW ]
(1)
* 100
Hydrogen production potential of the gasifier gives a better idea of how much of the thermal energy of coal can be converted into hydrogen and it is calculated with the formula:
Hydrogen production potential = =
CO and H 2 thermal energy [MW ] * 100 Feedstock thermal energy [ MW ]
(2)
Table 3. Overall gasification performance indicators
Properties Fuel flow Oxygen flow Syngas flow H2 CO CO2 CH4 H2S CGE CO+H2 efficiency
Unit
Shell
Siemens
t/h t/h t/h % vol. % vol. % vol % vol. % vol. % %
165.4 145.27 314.52 64.92 25.94 1.2 0.02 0.19 79.07 78.71
165.4 145.27 314.52 64.92 25.94 1.2 0.02 0.19 79.07 78.71
GE Texaco Conoco Phillips 165.4 160.27 382.98 49.29 18.8 4.8 0.4 0.1 73.04 71.60
165.4 132.27 354.98 44.28 30.28 10.04 1.7 0.17 80.27 74.79
V. Maxim et al As can be noticed from the Table 3 on the basis of the efficiency of conversion in H2 + CO, meaning the hydrogen production potential, Shell and Siemens technologies are superior to the other ones. Regarding the mechanism of raw gas production these two processes are identical. The Siemens gasifier has water quench which ensures the optimal condition for shift conversion, precondition for CO2 capture. As Table 3 shows the GE Texaco is the least appropriate for our process, because of the low CGE and hydrogen production potential. Because of the relatively high methane content in the syngas, the Conoco Phillips (E-Gas) technology has high cold gas efficiency. This will be good in a power application, but may not be the optimum choice for a synthesis gas application, in which case the (H2 + CO) yield will provide a better guide to process selection. For gasifiers which produce a syngas with significant concentrations of methane, is difficult to capture 90% of the carbon from the coal.
4. Conclusions The purpose of this paper is to evaluate by modeling and simulation various coal gasification technologies for hydrogen and electricity coproduction, with carbon capture and storage (CCS). The most promising gasification concepts for hydrogen and electricity co-production with carbon capture are all based on entrained-flow gasifiers The aim was to perform a multi-criteria analysis for different gasification concepts by eliminating gasifiers which are unapropriate for this purpose (IGCC plant concept with CCS). Modelling and simulation techniques were used to evaluate four gasification technologies and the main overall performance indicators.
5. Acknowledgements The authors wish to thank for the financial support provided from programs co-financed by The Sectoral Operational Programme Human Resources Development, Contract POS DRU 6/1.5/S/3 – „Doctoral studies: through science towards society” and by Romanian National University Research Council through grant no. 2455: “Innovative systems for poly-generation of energy vectors with carbon dioxide capture and storage based on co-gasification processes of coal and renewable energy sources (biomass) or solid waste”.
References [1] A.G. Collot, 2006, Matching gasification technologies to coal properties, International Journal of Coal Geology, 65, 191– 212 [2] C. Higman, M. Van Der Burgt, 2008, Gasification, Elsevier Science, Second edition [3] E. Tzimas, A. Mercier, C. Cormos, S. Peteves, 2007, Trade-off in emissions of acid gas pollutants and of carbon dioxide in fossil fuels power plants with carbon capture, Energy Policy, 35, 3991 – 3998. [4] International Energy Agency – Greenhouse Gas Programe (GHG), Potential for improvement in gasification combined cycle power generation with CO2 capture, Report PH4/19, 2003 [5]Food and Agriculture Organization of the United Nations www.fao.org [6] C. Cormos, F. Starr, E. Tzimas, S. Peteves, Brown A., Gasifier concepts for hydrogen and electricity co-production with CO2 capture, Third International Conference on Clean Coal Technologies, Cagliari, Sardinia, Italy, 2007 [7]C. Cormos, 2008, Decarbonizarea combustibililor fosili solizi prin gazeificare, Cluj University Press [8] C. Cormos, 2009, Assessment of hydrogen and electricity co-production schemes based on gasification process with carbon capture and storage, International Journal of Hydrogen Energy,34, 6065-6077 [9] Statistical Review of World Energy BP 2008, www.bp.com
