Article pubs.acs.org/EF
Evaluation of Performance of Industrial-Scale Dual Fluidized Bed Gasifiers Using the Chalmers 2−4-MWth Gasifier Anton Larsson,*,† Martin Seemann,† Daniel Neves,‡ and Henrik Thunman† †
Department of Energy and Environment, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Department of Environment and Planning, Centre of Environmental and Marine Studies, University of Aveiro, Campus Universitário de Santiago, PT 3810-193 Aveiro, Portugal
‡
S Supporting Information *
ABSTRACT: A general approach to evaluating the performance of industrial-scale dual fluidized bed (DFB) gasifiers was developed in this work. The approach is intended to simplify comprehensive evaluation of DFB gasifiers and to highlight important parameters, some of which are often missed or omitted in the literature. By applying this procedure, experimental results can be generalized, which is verified in this work using the Chalmers 2−4-MWth DFB gasifier. In a DFB gasifier, some of the fuel is converted to the desired calorific gas, while the remaining portion is combusted to meet the heat demands of the process. As shown here, the total heat demands limit the amount of chemical energy that can be restored from the fuel into the produced gas, whereby the main heat demands are from the drying and heating of the fuel, in addition to heating the combustion air and steam. By establishing a heat balance across the system, the chemical efficiency can be estimated. With lower heat demands, higher chemical efficiency is achievable, whereas with higher heat demands, more of the fuel must be burned and a lower chemical efficiency is achieved. It is experimentally complicated to quantify the level of fuel conversion and heat demands of a DFB gasification system. In this work, an experimental procedure is presented and implemented using the Chalmers gasifier to quantify the fuel conversion and heat demands. Furthermore, it was investigated how a variation in the amount of steam used for fluidization of the gasifier affects fuel conversion and other important parameters. To establish a reference case, silica sand was used as bed material and wood pellets was used as fuel to minimize the effects of ash and the bed material. By increasing the level of fluidization steam, the average residence time of the gas was decreased and the gas temperature, gas velocity, and steam-to-fuel ratio were increased, which resulted in increased conversion (up to 36%) of organic compounds (OC). However, limited char conversion was achieved (0%−4%), and the chemical efficiency remained unaffected by the amount of steam added to the process. The chemical efficiency of the Chalmers gasifier was determined to be 74% when using wood pellets as fuel. This is comparable to results from thermo-economic modeling of second-generation biofuels production processes, which, based on the heat demand, report the chemical efficiency of the DFB gasifier as being in the range of 74%−77% to maximize the overall efficiency. This shows that the required chemical efficiency is achieved, even with low char conversion, when using a fuel with a high content of volatiles, such as wood pellets.
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INTRODUCTION Gasification is a process that is based on the thermochemical conversion of a solid fuel into a calorific gas, hereinafter referred to as “raw gas”. The raw gas from gasifiers can be used for heat and power production, as, for example, is commercially achieved by the Güssing unit,1 or it can be synthesized into various liquid and gaseous fuels, such as substitute natural gas (SNG). A demonstration unit for SNG is being constructed in Göteborg, Sweden in the GoBiGas project,2 using a dual fluidized bed (DFB) gasification system, and the long-term goal is to produce ∼800 GWh of SNG from biomass annually. The demonstration unit is planned to be fully operational at the end of 2013 with an SNG production level of ∼160 GWh/yr. The configurations applied to gasification processes are diverse, and the optimal setup differs based on the fuel used and the desired end-products. To understand the differences between the various setups, it is important to understand the nature of the thermochemical conversion of the fuel. By using biomass as feedstock for a gasifier, the raw gas can be considered to be CO2-neutral;3 for this purpose, biomass is the main fuel source considered in the present work. Since biomass © 2013 American Chemical Society
feedstocks vary significantly in their characteristics (i.e., volatiles, ash, and moisture contents, as well as density and particle size, all of which can affect fuel conversion), a flexible system that can still deliver stable gas quality is required. Biomass conversion includes three general steps: drying, devolatilization, and char conversion. The dry part of the fuel consists of volatiles and char. The volatile part of the dry fuel is defined as the fraction of the fuel that can be converted to gas with heat as the only driving force. The remaining part of the dry fuel consists of ash and char. The char, which mainly consists of carbon, can be converted in the presence of a reactant (e.g., H2O, CO2, or O2) and a sufficiently high temperature. The char can be gasified by providing H2O or CO2 as the reactant, which yields a calorific gas through endothermic reactions. However, to achieve a thermally stable process, part of the fuel should be combusted to meet the heat demands of the process. This is achieved either by introducing Received: May 24, 2013 Revised: August 30, 2013 Published: September 2, 2013 6665
dx.doi.org/10.1021/ef400981j | Energy Fuels 2013, 27, 6665−6680
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THEORY To enable comparison of different gasifiers, there is a need to define the general parameters that describe the reactor layout. For a DFB gasifier, this is not a straightforward procedure, because there are two interconnected reactors, each of which affects the other. However, by defining three control volumes, the description of the process can be simplified, as further illustrated by the dotted lines in Figure 1. In this case, the volume of the gasifier, CVG1, is defined to include the main gasification reactor and the parts of the particle seals wherefrom the gas ends up in the raw gas. CVG2 is the volume in the raw gas pipe from the gasifier the sampling point. CVC is the volume of the combustion reactor, including part of the loop seals from where the gas enters the combustor. The gasification reactor is described by the power-to-volume ratio: (Q̇ /V)G1 [MW/m3], in combination with a shape factor, βV. The shape factor describes the difference in volume of the reactor relative to a cube with the same internal area and characteristic length lcar3 = Vcube. From the characteristic length, a shape factor for the bed span (βl,bed = lbed/lcar) and the cross-sectional area of the bed (βA,bed = Abed/lcar2) is formulated. Furthermore, part of the gasifier is occupied by solids that are described by the volume fraction, εG1=Vs,G1/VG1, where the subscript “s” stands for solids. The description of the layout should also involve the type of fuel feed and the positioning of the solids inflow and outflow, since these factors can influence how well the fuel is converted. For instance, Kern et al.7 showed how the positioning of the fuel feed affects the performance of a 90 kW DFB gasifier in terms of higher chemical efficiency, more tar, and a gas composition that is further from the equilibrium when feeding on top of the bed, as compared with in-bed feeding. However, it remains to be determined whether this effect holds true for larger-scale gasifiers. When investigating the functionality of a gasifier in terms of performance, it is important to describe the means by which the fuel is converted and how different process parameters affect the conversion. A simplified scheme for the conversion of a solid dry fuel particle subjected to steam gasification is illustrated in Figure 2. The fuel is primarily converted through
O2 to the gasifier for combustion of part of the gas yielding direct heating or by burning part of the fuel in a separate reactor for indirect heating.4 Thus, in the direct gasification concept, the flue gases are mixed with the raw gas and, consequently, the heating value of the gas is reduced. By locating the combustion to a dedicated chamber, as is the case of indirectly heated gasifiers, the flue gas is separated from the raw gas. This creates additional options for controlling the airto-fuel ratio (ARF) for the heat-generating reactor without affecting the quality of the raw gas. In this manner, stable gas quality and high heating value of the gas are ensured by an indirectly heated gasifier. The present work focuses on DFB gasifiers, in which the heat from the combustion chamber is transported with fluidized bed material to heat the gasifier indirectly. Figure 1 shows the general concept behind the DFB gasification system. The main idea is to use the bed material
Figure 1. Schematic of a dual fluidization bed (DFB) gasification system.
for transporting heat from the combustor to the gasifier, thereby supplying the heat needed for the endothermic gasification reactions and heating of the fuel. To prevent gas leakage between the reactors, the solids passes through a loop seal that is fluidized with steam. The fuel is fed to the gasifier, where it is partly converted to gas by devolatilization and steam gasification. The unconverted fraction of the fuel is transported with the solids through a second loop seal to the combustor, where it is combusted in the presence of air. This process assures the production of a raw gas that contains very little nitrogen. The only nitrogen source in these systems is a minor purge gas flow, which is commonly used in fuel feeding systems for suppressing the back-mixing of raw gas from the gasifier to the fuel storage. Relevant large-scale DFB biomass gasifier plants have recently been reviewed by Kaushal and Tyagi5 and Göransson et al.6 However, a detailed comparison of the performances of these gasifiers is difficult, as energy and mass balances together with important process parameters are not usually reported. As such, it becomes problematic to formulate general correlations or identify important fundamentals. To address this problematic, the aim of this work was to establish a general procedure for evaluating the performance, and important process parameters, of large-scale DFB gasifiers. A generalized description of the effect of the level of fluidizing steam is presented based on experimental results from the Chalmers gasifier, which was quantified through the proposed procedure.
Figure 2. Simplified description of solid fuel conversion. Y is the yield from the devolatilization, and X is the degree of conversion of the devolatilization products; the subscript “cg” stands for cold gas, and the subscript “rg” stands for raw gas.
devolatilization, where the mass yields of char (Ych), organic compounds (YOC), cold gas (or permanent gas), and steam depend on the fuel properties and process parameters. During the devolatilization, the temperature and the heating rate are the dominant parameters.8 Here, the OC is defined as the sum of the organic compounds in the raw gas, with the exception of the hydrocarbons measured in the cold gas (i.e., CH4, C2Hx, and C3Hx). In the secondary conversion step, the converted mass fractions of char and OC are described by Xch and XOC, 6666
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SNG production, revealing a heat demand of 23% of the fuel input when using a gasifier temperature of 850 °C, preheating of steam to 300 °C, a SFR of 0.5, 50% fuel moisture, and airdrying to 20% moisture. Tock et al.11 made a similar evaluation for Fischer−Tropsch (FT) crude, methanol (MeOH) and dimethyl ether (DME), revealing heat demands of 23%, 26%, and 26% of the fuel input, respectively, using a gasifier temperature of 850 °C, preheating of steam to 400 °C, a SFR of 0.5, 50% fuel moisture, and air-drying to 25% moisture. These heat demands need to be covered by the fuel input and can give an estimate of the raw gas efficiency. For example, the raw gas efficiencies of the gasifier for the suggested biofuel production are 77% for SNG and FT-crude production and 74% for DME and MeOH production. It should be stressed that it is the raw gas efficiency of the gasifier that is considered here, rather than the overall plant efficiency. Furthermore, it is clear that the heat demands and, thereby, the raw gas efficiency can change with the level of integration10,11 or in the case of combination with heat and power production.12 Figure 3 shows that, when wood pellets are used for SNG and FT production, a small fraction of the char is available for
respectively. Based on the devolatilization and secondary conversion, a simplified description of the fuel conversion is given by eqs 1−3: Yi = f (fuel properties, Tbed ̅ , heff , bed material, layout) i = ch, cg, steam, &OC
(1)
Xch = f (fuel properties, Tbed ̅ , τf̅ , SFR, μp , μO , bed material, u − umf , layout)
(2)
XOC = f (fuel properties, Tfree ̅ , τrg̅ , SFR, μp , μO , bed material, u − umf , layout)
(3)
where T̅ is the average temperature, heff is the effective heattransfer coefficient to a fuel particle, τ ̅ is the average residence time, SFR is the steam-to-fuel ratio, u − umf represents the superficial velocity minus the minimum fluidization velocity, and μ is the mass per kg of dry-ash-free (daf) fuel (kg/kgdaf fuel). The subscript “bed” indicates the bed section of the gasifier, the subscript “free” represents the freeboard, the subscript “p” stands for purge gas, the subscript “O” denotes oxygen, the subscript “ch” denotes char, the subscript “cg” denotes cold gas, and the subscript “rg” is for the raw gas. The performance of a gasifier can be described by the chemical efficiency (η), which is defined as the chemically stored energy of the gas in relation to the energy stored in the fuel. The chemical energy in the raw gas and cold gas are described as the raw gas efficiency and cold gas efficiency, respectively. Assuming that the fuel is completely devolatilized in the gasifier, the raw gas efficiency of a DFB gasifier is directly coupled to the char yield (Ych) and the degree of conversion (Xch). Figure 3 (presented later in this work) illustrates the raw gas efficiency as a function of char conversion from devolatilization to the maximum theoretical char conversion (solid lines) and estimated raw gas efficiencies possible for the different end-products (dash-dotted lines). The maximum theoretical raw gas efficiency of a DFB gasification system is, by definition, equal to unity, assuming no heat losses and thermally neutral conversion of the volatile fraction. For the theoretical case, for the sake of simplicity, the char is considered to consist of pure carbon, which means that the maximum theoretical char conversion in the gasifier (Xch,max) is given by (1 − Xch,max )Δhcomb = Xch,max Δhgasif
Figure 3. Theoretical raw gas efficiency of a DFB gasifier as a function of char conversion for fuels with char contents typical for biomass (Ych = 0.16), peat (Ych = 0.28), and bituminous coal (Ych = 0.78), and, the raw gas efficiencies of a gasifier integrated for production of substitute natural gas (SNG),10 Fischer-Tropsch (FT) crude,11 dimethyl ether (DME),11 and methanol (MeOH).11
(4)
where Δhcomb is the enthalpy of reaction for the combustion of pure carbon with oxygen (−393.5 kJ/mol)9 and Δhgasif is the enthalpy of reaction for the gasification reaction of pure carbon with steam (+131.3 kJ/mol).9 This calculation gives the maximum theoretical char conversion of Xch,max ≈ 0.75 for any fuel. Indeed, the higher the char yield from a fuel, the more important the char conversion becomes for the raw gas efficiency. Considering a plant for the production of second-generation biofuels, multiple process steps must be integrated to maximize the total efficiency. Through process modeling, the heat demands to be covered by the fuel burned in the combustion section of an integrated DFB gasifier can be estimated. Gassner et al.10 performed thermo-economic modeling of a plant for
gasification, whereas for DME and MeOH production, all the char is required for heat production. These examples show how the external heat demand can determine the amount of char to be gasified or combusted, respectively. Considering overall plant efficiency, the heat and power production and/or consumption levels should be included, as described by Heyne and Harvey:12 μprod LHVprod + ηsys =
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LHVf +
(Pel− − αref Q−) ηref, el
(αref Q− − Pel−) ηref,el
+
+ Q+ ηref, q
Q− ηref, q
(5)
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where LHV is the lower heating value, P is the electrical power, Q is the heat, αref is the electricity-to-heat ratio of a reference plant, and ηref is the efficiency of a reference plant. The subscript “prod” denotes the chemical product of the plan, the subscript “el” indicates the electricity, and the subscript “q” denotes the district heating product. The following reference values are used here: ηref,el = 0.32, ηref,q = 0.76, and αref = 0.41. These values are defined for a reference biomass combined heat and power (CHP) plant, as described by Heyne and Harvey.12 Using the values from Gassner et al.10 and Tock et al.,11 eq 5 gives a plant efficiency of ηsys,SNG = 0.75 and, for liquid fuels, ηsys = 0.33−0.68. It is clear that a high consumption of electricity related to pressurized synthesis siginificantly decreases the efficiency. These scenarios do not consider any production of district heating, which could further increase the plant efficency. For example, Heyne and Harvey12 included district heating and showed a system efficiency for SNG where ηsys,SNG = 0.82−0.84. Process Parameters. This work considers two types of parameters for the evaluation of a large-scale DFB gasifier: the process parameters and the performance parameters. The process parameters, which affect fuel conversion (as stated in eqs 1−3) are SFR, heff, T̅ bed, T̅ free, τr̅ g, τs̅ , u − umf, μp, and μO, and these parameters are defined below. The SFR of a DFB biomass gasifier affects the gas composition and the amount of tar, as described, for instance, by Hofbauer and Rauch,13 who showed that the tar level decreased with higher SFR. The SFR is defined as a mass of total water added to the gasifier per mass of dry ash-free (daf) fuel: SFR = μst ζ + μm = (μst,bed + μst,S1 + μst,S2 )ζ + μm
the char yield can conveniently be investigated by proximate fuel analyses, it should be investigated with a heating rate analogous to that of a fluidized bed (FB); otherwise, an adjustment for the char yield should be applied so as to be relevant to a fluidized-bed situation.4 As described by Palchonok,17 the maximum heat-transfer coefficient to a single fuel particle in a fluidized bed can be estimated by heff = 0.85
(8)
Nu = 0.85Arin 0.19 + 0.006Arin 0.5Pr 0.33
Arin =
(9)
gd in 3(ρin − ρg ) νg 2ρg
(10)
where Nu is the Nusselt number, k is the thermal conductivity, d is the particle diameter, Ar is the Archimedes number, Pr is the Prandtl number, g is the gravitational constant, ρ is the density, and νg is the viscosity of the bulk gas. The subscript “in” denotes an inert particle and the subscript “g” indicates the bulk gas. The factor of 0.85 in eq 8 was proposed to compensate for the observed effect of fuel particles floating on the surface of the bed,17 which reduces the heat transfer to the fuel particle. The average temperature of the bed (T̅ G) affects the heating rate and reaction rate, and an increase in bed temperature leads to a higher gas yield and lower level of tars.6,15 There is no clear lower limit for the temperature at which to operate the gasifier. Nevertheless, at temperatures of ≥600 °C, most of the volatiles are released if the residence time is sufficient.8 Therefore, operating the gasifier at
