The Effect of Bed Particle Inventories with Different Particle Sizes in a

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The Effect of Bed Particle Inventories with Different Particle Sizes in a Dual Fluidized Bed Pilot Plant for Biomass Steam Gasification Stefan Koppatz,* Johannes C. Schmid, Christoph Pfeifer, and Hermann Hofbauer Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, 1060 Vienna, Austria ABSTRACT: The paper reviews recent results obtained at the dual fluidized bed pilot plant for steam gasification of biomass at the Vienna University of Technology. The dual fluidized bed reactor system involves the combination of a steam fluidized bubbling bed and an air blown fast fluidized bed. Bed particles of olivine with different mean particle sizes are applied as solid inventory in the fluidized bed system. The applied mean particle sizes are 520 and 260 μm, and therefore differ in coarse and fine solid particle inventories. Increased conversion of higher hydrocarbons is assumed for the fine particle inventory due to higher turbulence (improved gas−solid contact), higher gas residence times in the bubbling bed, and the higher specific particle surface which is exposed to the gas phase. Experimental test runs with conventional wood pellets were conducted by varying the fuel feed, gasification temperature, and the steam-to-fuel ratio. The results are presented according to hydrodynamic considerations, gas composition, tar content, and tar composition, and further specific data (e.g., gas yields, gas residence times, water conversion, and deviation from equilibrium state). The effect of the fine particle inventory is found to significantly influence the tar content of the GC−MS detectable tar fraction; it is reduced by about 50%. Furthermore, it is found that the main gas composition is broadly independent of the particle inventory.

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INTRODUCTION The share of renewables in the energy supply is rapidly growing worldwide. In particular, the utilization of biomass is of high importance, since this carbon source is considered as a precursor for the substitution of carbon-containing products (e.g., liquid fuels, chemicals, synthetic natural gas) for conventional heat and power supply which are largely based on fossil fuels. High quality product gas and syngas are the precursors for these applications and are produced by gasification followed by gas cleaning and gas upgrading processes. Fluidized bed processing is preferably applied for the gasification of biomass. The thermal conversion of the solid fuel into a gasesous product is intensly promoted by the benefits of fluidized bed processing (e.g., excellent gas−solid contact and heat transfer, fuel flexibility). Allothermal gasification with steam is enabled within dual fluidized bed reactor systems, which separates the heat supply from the basic gasification. Thus, a nitrogen free gas is produced with a heating value of 12−15 MJ/Nm3db. The general performance of the dual fluidized bed (DFB) system which is identified by the qualitiy and quantity of the generated product gas is usually influenced by the applied feedstock, the operating conditions (temperature, steam-to-fuel ratio, pressures) and the character of the solid particle inventory. Most studies in the field of biomass gasification by fludized bed processing have focused on the variation of the feedstock, the general operating parameters of the reactor system (temperature, steam-to-fuel ratio), the bed material (natural or synthetic materials), and their impact on tar conversion. Commonly referred-to publications on these aspects in the field of steam gasification are refs 1−6. Based on the chemical properties, the bed particles might be of noncatalytic or catalytic character with regard to the gaseous conversion, in particluar to the decomposition of higher hydrocarbons (tar). However, the mean particle size and the particle size distribution of the bed inventory may have an influence on the reactor performance in terms of the © 2012 American Chemical Society

hydrodynamics, gas production, and the gaseous output. These issues and the context have been less addressed in the literature.7,8 This study focuses on the impact on the biomass gasification of whether fine or coarse solid particles are applied in the dual fluidized bed. The experiments were carried out at a dual fluidized bed system of pilot plant scale (nominal fuel input of 100 kWth). The particle size influences the fluidized bed characteristics (minimum fluidization velocity, minimum bubbling velocity). Hence, the applied mean particle size might have an effect on the hydrodynamics and the conversion characteristics as the gas−solid contact behavior differs depending on whether there are coarse or fine particles.

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FUNDAMENTALS Thermodynamics of Biomass Steam Gasification, Reactions and Definitions. The process of gasification effects a decomposition of the solid feedstock at high temperatures (800−900 °C) into certain gasesous species. The chemical composition (dry and ash free basis) of wood is in general: 50 wt % carbon, 6 wt % hydrogen, and 44 wt % oxygen. Within the steam gasification, a hydrogen-rich product gas is produced, due to the water−gas shift reaction which contributes mainly to the content of the hydrogen and the specific yield in hydrogen. The bulk reaction of steam gasification is given in eq 1. By means of the stoichiometry of the system CxHyOz and (x−z)H2O, the stoichiometric H2O ratio (λH2O) is defined by eq 8. Furthermore, the stoichiometric steam demand (ΦH2O) related to the system CxHyOx is defined by eq 7. With regard to the wood pellets Received: Revised: Accepted: Published: 10492

October 13, 2011 May 15, 2012 June 20, 2012 June 20, 2012 dx.doi.org/10.1021/ie202353b | Ind. Eng. Chem. Res. 2012, 51, 10492−10502

Industrial & Engineering Chemistry Research

Article

applied within the experiments (see Table 3), the stoichiometric steam demand amounts to: • ΦH2O = 14.76 [molH2O/kgdaf,N,S,Cl free] • ΦH2O = 0.266 [kgH2O/kgdaf,N,S,Cl free] These numbers show the theoretical quantities which are involved in the idealized and overall conversion. However, in practical applications, λH2O fairly exceeds 1 and is determined by fluidization requirements. This calculation considers the full decomposition of CxHyOz toward CO and H2, which is an idealized system without the assessment of side reactions. Thus, the possible splitting of H2O in contact with catalytic solids is not taken into account in this consideration. The progress of gasification involves numerous solid−gas and gas−gas reactions. Relevant reactions for steam gasification are listed below (eqs 2−6, ΔHR,850 is calculated with HSC Chemistry9). The reaction heat of this overall reaction complex is endothermic. Within the dynamic process of gasification, the distribution of the species involved progresses toward a certain equilibrium concentration. This concentration is determined by the thermodynamic conditions, reactions kinetics, residence times, and catalytic support by the solid particles. In terms of the steam gasification, the bulk input mass balance which involves the feedstock and the gasification agent is usually characterized by the steam-to-fuel ratio (φSF,wt), eq 9. The steam-to-fuel ratio describes the ratio of total steam to fuel (dry and ash free basis) fed into the gasifier. Furthermore, the steam-to-carbon ratios φSC,wt and φSC,mol relates the total steam input to the carbon input at weight and molar basis, respectively, equations 10 and 11. The quantity of water consumed in the gasifier due to a gas−gas or gas−solid reaction might be indicated by the relative water conversion, wherein the amount of water (ṁ H2O) which is converted in the gasifier is related to the fuel input, eq 12. Furthermore, the absolute water conversion in the gasifier is defined by eq 13, which relates the amount of water converted (ṁ H2O) to the total input of water into the gasifer. The deviation of the actual product gas composition according to the CO-shift (eq 6) equilibrium might be assessed by means of the logarithmic deviation from the CO-shift equilibrium, see eq 14, which relates the actually measured gas phase partial pressures of species i (CO, H2O, CO2 and H2) to the equilibrium constant Kp calculated from pure substance thermodynamic data. Futher details (and the source of the thermodynamic data) are given elsewhere.10 ⎛y ⎞ Cx H yOz + (x − z)H 2O → xCO + ⎜ + x − z⎟H 2 ⎝2 ⎠

C + H 2O → CO + H 2

(x ≥ z)

ΔHR,850 = 135.7 kJ/mol

λ H 2O =

ṁ H2O,act ṁ H2O,stoich

φSF,wt =

ṁ steam + μH Oṁ fuel 2

(1 − μH O − μash )ṁ fuel

ΔHR,850 = 169.4 kJ/mol

y⎞ ⎛ Cx Hy + x H 2O → xCO + ⎜x + ⎟H 2 ⎝ 2⎠

y Cx Hy + xCO2 → 2xCO + H 2 2

ΔHR,850 > 0 kJ/mol

φSC,wt =

ṁ steam + μH Oṁ fuel 2

μC ṁ fuel

(10)

2 ṁ steam + μH2Oṁ fuel 3 μC ṁ fuel

φSC,mol =

X H2O,rel =

(11)

ṁ H2O (1 − μH O − μash )ṁ fuel

(12)

2

X H2O,abs =

ṁ H2O ṁ steam, fluidization + μH O ·ṁ fuel

=

2

X H2O,rel φSF,wt

⎡ ⎤ ∏i piνi ⎥ pδeq,CO − shift(pi , T ) = log10⎢ ⎢⎣ K p,CO − shift(T ) ⎥⎦

(13)

(14)

Bed Particle Size in Fluidized Beds and Hydrodynamic Considerations. The behavior of fluidized beds is fundamentally influenced by the applied bed particles in terms of their hydrodynamics, operable ranges, and the intensity of chemical conversions in the reactor. The mean particle sizes which are usually used in bubbling fluidized beds differ from those used in circulating fluidized beds. Overall, bubbling fluidized beds (BFB) are operated with rather coarse particles compared to those of circulating fluidized beds (CFB), which rather have smaller particle sizes, due to the required particle entrainment. An overview of typical particle size ranges for BFB, CFB, and DFB referred to in the literature are given in Table 1. Table 1. Typical Particle Diameters (μm) for BFB, CFB, and DFB author/group/ institution Grace et al. Kunii and Levenspiel Plass et al.

(1) (2) (3)

Fernwärme Vienna/Austria

(4)

ΔHR ,850 > 0 kJ/mol (5)

CO + H 2O → CO2 + H 2

(9)

2

Hamada Boiler

C + CO2 → 2CO

(8)

ΔHR,850 = − 33.6 kJ/mol

subject general overview general overview combustion/ gasification in the Lurgi CFB industrial application industrial application, combustion in BFB

author/group/ institution

subject

CHP Güssing, Austria CHP Oberwart, Austria

industrial application industrial application

BFB (μm)

CFB (μm)

ref

30−3000

50−500

11

200−2000

50−400

12

50−300

13

150−250

14

1000−2000 800

15

DFB (μm)

ref

540

16

600

17

(6)

ΦH2O = x − z

The overview in Table 1 is related to particles whose density is sandlike in the range of 2400−2900 kg/m3. According to the

(7) 10493

dx.doi.org/10.1021/ie202353b | Ind. Eng. Chem. Res. 2012, 51, 10492−10502

Industrial & Engineering Chemistry Research

Article

literature, there is no common agreement concerning the particle size, which is also governed by the particle density. Further impact on the fluidized bed behavior is effected by the particle size distribution which can be wide, narrow, or bimodal character. Properties which are the inherent character of the bubbling fluidized bed might be: (1) maximum bubble size or bubble volume fraction; (2) bed expansion; (3) bubble rise velocity. The latter properties are often discussed in the literature. However, there is no common agreement in the literature whether these aspects are to be considered independent of the mean particle size or not. A comprehensive review and study of the effect of particle size and distribution is given by Geldart7 and Grace and Sun.8 The investigation of bubbling bed characteristics (bubble size, rise velocity) is not the aim of this study. However, we anticipate that the application of a fine particle inventory has an influence on the gas conversion, in particular on the conversion of the higher hydrocarbons (tars) since this inventory may provide higher turbulence and higher specific particle surface. Thus, in the case of a fine particle solid inventory in contrast to a coarse particle inventory we assume: • enhanced gas−solid contact behavior/turbulence in the bubbling bed • higher exposure of particle surface to the gas due to higher specific particle surface • higher fraction of particles in diluted phase • higher particle entrainment into the splash zone • different ratio of dense phase to diluted (bubble) phase • lower gas velocities to reach similar fluidization regime

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EXPERIMENTAL SECTION DFB Pilot Plant and Diagnostic Methods. The pilot plant is based on the DUAL FLUID bed (DFB) reactor system for steam gasification of biomass. Details on the DFB concept and process are given by Hofbauer et al.18,19 or Koppatz et al.10 Figure 1 shows schematically the reactor part of the pilot plant. The steam blown bubbling fluidized bed reactor and the air blown fast fluidized bed reactor are connected via an upper and lower steam fluidized loop seal. The configuration of the fuel feeding system enables the fuel feed from diverse positions: (1) direct fuel feeding into the bubbling bed (feedstock hopper 1 and 2) and (2) top-down fuel feeding directly onto the bubbling bed (feedstock hopper 3 and 4). In this study the fuel feeding was carried out with the feedstock hopper 2, directly in the bubbling bed. The nominal fuel power capacity of the pilot plant is 100 kW. The maximum fuel particle size of about 40 mm is limited by the screw feeder system. Optionally, the combustion reactor is fed with additional fuel (light fuel oil) for control and influence of the temperature. The additional fuel is injected into the lower part of the combustor. Nitrogen is used as a seal gas for the fuel hoppers. The geometry of the reactor is given in Table 2. Superheated steam (250−300 °C) for fluidization of the gasifier and the loop seals is provided by an electrically heated steam generator and preheating of the pipes. The combustion air (primary and secondary air injection nozzels) is injected at two different levels into the reactor. The secondary air is preheated to about 300−350 °C. The primary air serves to constantly hold up solid particles. Fast fluidization solid particle transport to the top of the riser is done with the secondary air. A solid separator is

Figure 1. Scheme of the DFB reactor system of the pilot plant.

Table 2. Geometric Data of the Gasification and Combustion Reactor of the Pilot Plant parameter fluidization agent fluidization regime cross section geometry reactor free height reactor inner diameter

gasification reactor

combustion reactor

steam

air

bubbling fluidized bed

fast fluidized bed

conical bottom section with square shaped upper freeboard section 2.35 m

circular

304 mm (equivalent cylindrical diameter at square shaped freeboard section)

3.9 m (central tube section) 98 mm

arranged on the top of the riser to separate the solid particles from the gas−solid stream. The product gas stream exits the gasifier and passes through a thermo-oil heat exchanger. A sampling point for analysis of the product gas composition is installed after the heat exchanger. The product gas stream and flue gas stream are merged together into a postcombustion chamber for complete combustion of burnable species. A cyclone separates the particles from the exhaust gas stream. The pilot plant and experimental results are also reported by Koppatz et al.10 or Schmid et al.6 The general process data (temperatures, pressures, volume-/mass flows) are recorded using LabVIEW. The measuring points for temperature and pressure are evenly positioned over the reactor height and entire pilot plant. The 10494

dx.doi.org/10.1021/ie202353b | Ind. Eng. Chem. Res. 2012, 51, 10492−10502

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different mean particle size. Figure 2 shows the particle size distribution and the particle cut sizes: dp10, dp50, and dp90. The

temperature sensing elements are chromel-alumel (Ni/CrNi) thermocouples of Type K (range from 20 to 1200 °C). Pressure measurement is done with Honeywell piezoresistive pressure sensors and Rosemount (1151) pressure transmitters. The following measuring devices are used to determine the composition of the product and flue gas: (1) product gas, Rosemount NGA 2000 (CO, CO2, H2, CH4, O2), and online gas chromatograph Syntech Spectras GC 955 for cross-checking of carbon species and N2 content and (2) fuel gas, Rosemount NGA 2000 (CO, CO2, O2). The tar content of the product gas is determined by using the wet chemical principle to condense and dissolve the condensable hydrocarbons into impinger bottles, which are filled with toluene as the solvent. Details on the tar sampling set up and the measurement method, which is applied at the DFB pilot plant at Vienna University of Technology, can be found in Koppatz et al.10 and Wolfesberger et al.20 Basically, this method for tar determination yield two data for the tar content, GC−MS detectable tars and GC−MS undetectable very high molecular tars (increasing size), gravimetric tars. However, the values are not considered to be independent of each other, since the areas of detection overlap. Besides, this method does not measure the BTX components (benzene, toluene, and xylene) since toluene is used as a solvent, due to the high steam content of the product gas. Fuel Characterization. The proximate and ultimate analysis of the wood pellets used for the experiments are listed in Table 3.

Figure 2. Size distribution of the coarse and fine particle inventory.

particle inventories are labeled as “coarse” particle inventory (CPI), mean particle diameter dp50 = 520 μm and as “fine” particle inventory (FPI), mean particle diameter dp50 = 260 μm. Thus, the solid particle distribution of the coarse particle inventory is somewhat wider than that of the fine particle inventory. The mechanical properties of olivine are reported in Table 4. Table 4. Mechanical Properties of Olivine for CPI and FPI

Table 3. Proximate and Ultimate Analysis of the Wood Pellets parameter

unit

raw basis

dry basis

water content ash content volatiles fixed carbon heating value carbon hydrogen nitrogen oxygen sulfur chlorine

[wt %] [wt %] [wt %] [wt %] [MJ/kg] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %]

6.11 0.27 81.17 12.72 17.5 47.16 5.67 0.05 40.73 0.005 0.003

0.29 86.45 13.55 18.8 50.23 6.04 0.05 43.48 0.005 0.003

parameter

unit

CPI

FPI

hardness particle density BETa surface area bulk density specific surface areab of dp50 specific surface areab of dp50 in bulk

Mohs scale [kg/m3] [m2/g] [kg/m3] [m2/kg] [m2/L]

6−7 ∼2850