The Superior Technical Choice for Dual Fluidized Bed Gasification

By far, most of these quoted FB combinations have been actually employed to design ... 2d earlier appeared in several Chinese patents disclosed from 1...
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Ind. Eng. Chem. Res. 2006, 45, 2281-2286

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The Superior Technical Choice for Dual Fluidized Bed Gasification Guangwen Xu,* Takahiro Murakami, Toshiyuki Suda, Yoshiaki Matsuzawa, and Hidehisa Tani Ishikawajima-Harima HeaVy Industries (IHI) Co., Ltd., Shin-Nakahara-cho 1, Isogo-ku, Yokohama 235-8501, Japan

Dual fluidized bed gasification (DFBG) technology requires a combination of two fluidized beds. Fluidized bed (FB), on the other hand, has different types depending on, for example, the gas velocity inside the bed. The present article intends to clarify what types of FBs should be chosen for the FB combination of the DFBG in terms of improving gasification efficiency and enhancing tar-destruction capability. Gasification tests in a 5.0 kg/h pilot gasification plant demonstrated that under the same conditions the combination of a dense low-velocity FB (LVFB) fuel gasifier and a high-velocity pneumatic riser (HVPR) char combustor allowed higher gasification efficiency and lower tar production than the combination of an HVPR fuel gasifier and an LVFB char combustor did. Consequently, the article concludes that the superior technique choice for the DFBG is to deploy its fuel gasification into an LVFB and its char combustion into an HVPR. An HVPR char combustor is necessary because it maintains the steady circulation of particles between the two involved beds. 1. The Problem Dual fluidized bed gasification (DFBG) technology for solid fuels such as coal and biomass is catching an increasingly greater interest of academic researchers and industrial engineers because the technology can provide N2-free feedstock for chemical synthetic processes1 and produce CO2-captured H2-rich product gases.2 This has resulted in various DFBG systems devised and put into technical development. Figure 1 displays a recently proposed conception based on the DFBG idea, which was disclosed in a NEDO-financed technical program.3 The proposal intended to build a biomass DFBG technology with a compact configuration that integrates the involved two fluidized beds into one unit. The base of the unit, i.e., the first reactor of the system, is a fluidized bed (FB) operated in the dense bubbling/ turbulent fluidization regime. Seating on the base FB (usually at its vertical center), another reactor is a pneumatic riser upward prolonged and having its bottom section immersed in the particle bulk of the base FB. The base FB and the riser have independent cyclones. The solid particle circulation between both beds is via the cyclone of the riser and the interstice between the riser bottom and the distributor of the base FB. As illustrated in Figure 1a, the original design feeds biomass fuel into the pneumatic riser. Corresponding to this, steam is injected into the riser as the fluidizing gas and gasification reagent. Through contact with hot sand, the heat carrier particles (HCPs) from the base FB, the fuel is pyrolyzed and further its resulting char is gasified by reacting with steam reactant inside the riser. As shown in the figure, the produced gas is independently drawn out of the system as the product gas, while the unreacted char, captured in the cyclone of the riser, is returned to the base FB with the HCPs to bring about therein the exothermic char combustion and in turn to reheat the HCPs. Because the base FB has its independent exhaust, the combustion-resulting flue gas has no way to leak into the riser gasifier to intermix with the product gas. This is why we view the design as DFBG-based, even though in appearance the configuration is integrated. Our question is why the design does not adopt the converse deployment indicated in Figure 1b. There, fuel and steam reagent * To whom correspondence should be addressed. Tel.: 0081-45759-2867. Fax: 0081-45-759-2210. E-mail: [email protected].

Figure 1. System conception of (a) a recently proposed biomass gasification plant and (b) its reversion.

are fed into the base FB, while air is injected into the pneumatic riser. The resulting DFBG thus refers to a reversed combination of a dense fluidized bed fuel gasfier and a pneumatic riser char combustor. Of the two technical designs, which (Figure 1a versus Figure 1b) is superior in terms of fuel gasification efficiency and tar elimination capability? The essence of the problem is what types of FBs should be used for the fuel gasifier and char combustor of a DFBG combination. As a matter of fact, a fluidized bed has different fluidization statuses or regimes depending its operating conditions.4 At least, there is the distinction of FBs running at high and low gas velocities. While the former leads to massive particle conveyance and somewhat low particle suspension concentrations inside the bed, the latter causes generally high particle concentrations without intensive particle elutriation. These two types of FBs correspond just to the pneumatic riser and bubbling/turbulent base bed respectively shown in Figure 1. Hence, in a more general sense the DFBG technology should have at least four different options of technique deployment, as is conceptualized in Figure 2, according to its involved FB types.

10.1021/ie051099r CCC: $33.50 © 2006 American Chemical Society Published on Web 02/24/2006

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Ind. Eng. Chem. Res., Vol. 45, No. 7, 2006 Table 1. Principal Hydrodynamic Features of HVPR and LVFB

Figure 2. Four technical options for dual fluidized bed gasification (DFBG) technology under different combinations of low-velocity fluidized bed (LVFB) and high-velocity pneumatic riser (HVPR).

The two FBs for a DFBG system can be a combination of (Figure 2a) two low-velocity dense fluidized beds (LVFBs), (Figure 2b) two high-velocity pneumatic risers (HVPRs), or (Figure 2c,d) an LVFB coupled to an HVPR. As for the last case, there are still two different deployments for the LVFB and HVPR. The LVFB can be used as (Figure 2c) the char combustor or (Figure 2d) the fuel gasifier so that the coupled HVPR is as (Figure 2c) the fuel gasifier or (Figure 2d) the char combustor correspondingly. By far, most of these quoted FB combinations have been actually employed to design various types of DFBG technologies. In the 1970s, the combination Figure 2b was adopted in the USA by the so-called FERCO SilvaGas process for gasifying waste (RDF) and biomass.1,5 Later, Ebara Co., Ltd. in Japan patented the combination of Figure 2c in 2003,6 while the combination of Figure 2d earlier appeared in several Chinese patents disclosed from 1992.7-9 Meanwhile, the combination of Figure 2d was adopted also in the so-called FICFB biomass gasification plant developed in Austria in the 1990s.10,11 The general purpose of the present article is to clarify which deployment illustrated in Figure 2 is superior in terms of gasification efficiency and tar elimination capability. The answer is surely applicable also to the comparison of the two technique options illustrated in Figure 1. Although various types of DFBG technologies have been actually developed, as was highlighted above, the engaged developers tested merely their own technical deployment so that none has so far compared the technical options indicated in Figure 2 to clarify their respective superiorities. 2. Theoretical Concerns From the viewpoint of facilitating particle circulation, the combination of two LVFBs shown in Figure 2a should be first excluded from the queue for concern. In addition to its difficulty in controlling the particle movement between the two beds, the combination would be also make achieving high particle circulation rates difficult. For the DFBG, however, a suitably high particle circulation rate is indispensable because the circulated particles (i.e., HCPs) provide the gasifier with the only heat input to maintain its reaction temperature. Therefore, the options left for consideration are the combinations illustrated in Figure 2b-d. Its solution relies on whether the fuel gasification for a DFBG system should be deployed into an LVFB or an HVPR to facilitate the fuel pyrolysis, char gasification, and tar destruction. As for the DFBG, the conversion of supplied solid fuel into product gas is subject to the time and intensity of the contact between fuel particles and hot HCPs circulated into the gasifier from the char combustor.

feature

HVPR

LVFB

particle conveyance particle residence time particle concentration particle-particle interaction gas-particle contact gas-particle interaction time

high 3-10 s low bad good a few seconds

no flexible high good bad 3-10 s

Consequently, the technology prefers having an intensive and possibly long particle-particle contact for its gasifier. The tar destruction during fuel pyrolysis and char gasification, on the other hand, desires an extensive gas-particle interaction because the involved particles provide efficient sites for chemical cracking/reforming and physical adsorption of tars. Nonetheless, beyond the interaction extensity the gas-particle interaction time, i.e., the gas residence time in the particle bed, and the available particle concentration may dominate the gasifier’s performance for tar destruction. In summary, we believe that the fuel gasifier for the DFBG technology desires a bed that allows intensive as well as longtime particle-particle interactions. Table 1 compares the principal hydrodynamic characteristics of LVFB and HVPR. While the LVFB allows flexibly long particle residence time and efficient particle-particle interaction, the HVPR enables conversely an intensive gas-solid contact. In the HVPR, however, the particle concentration is low. This causes its intensive gas-particle interaction to be favored probably only in the applications that require high conversions of solid reactant through reactions with the gas. The gas, on the other hand, may hardly gain a high conversion because of the gas-rich and brothlike flow conditions. An actual example is the circulating fluidized bed combustion (CFBC), where solid carbon (C) is required to have high conversions whereas the conversion requirement to the gas (air) is much looser. Therefore, for tar destruction during fuel gasification the intensive gas-solid interaction prevailing in the HVPR may hardly lead to high destruction efficiency because of the counteraction stemming from the bed’s low particle concentration. Furthermore, the gas possesses the similarly long time, say, for a few seconds, to contact particles in both the LVFB and HVPR. In light of all of these, we think that the fuel gasifier for the DFBG technology should take an LVFB. When this is true, it means that the combination conceptualized in Figure 2d is superior to the others. Nonetheless, the anticipation entails experimental demonstration because none has so far compared the performances of gasifiers operated as LVFB and HVPR under similar conditions. This comprises thus the subject that the present article will address. If the experimental test shows a result contrary to the above anticipation, this means that the fuel gasifier for the DFBG should adopt an HVPR. In turn, according to what can the combinations in Figure 2b,c be distinguished? The much quicker rate of char combustion than char gasification and also the good gas-particle contact in the HVPR imply that this kind of FB would be of high priority for being an efficient char combustor. In fact, the CFBC technology was born on these points. Applying to the DFBG system, we consequently think that the combination in Figure 2b should be considered prior to the combination in Figure 2c because the former arranges the char combustion into an HVPR. 3. Experimental Section Figure 3 shows the experimental facility employed. It was a dual fluidized bed system consisting of a pneumatic riser (i.e., an HVPR) and a bubbling/turbulent fluidized bed (i.e., an

Ind. Eng. Chem. Res., Vol. 45, No. 7, 2006 2283

Figure 3. Schematic diagram of the employed experimental dual fluidized bed gasification facility.

LVFB). The riser was 52.7 mm i.d. and 6400 mm high, and the LVFB had a rectangular cross section of 80 mm × 370 mm and a height of 980 mm, with a 700-mm-high expanded section of 180 mm × 370 mm as its freeboard. Both the downcomer between the riser’s cyclone and LVFB and the duct bridging the bottoms of the LVFB and riser had the same 52.7 mm i.d. With a particularly designed structure12 the LVFB prevented well the gas leakage from the rise to LVFB and vice versa. The riser and the LVFB were both electrically heated, and bed temperatures of up to 1173 K were possible. As mentioned in Figure 3, a cyclone was available also for the LVFB to capture the fine particles elutriated from this bed. The distinctive feature of the facility is that it allows gasification tests both in the LVFB and in the riser. Thus, in Figure 3, the fuel feed “F” was set both for the LVFB and for the riser. While the fuel feed to the LVFB was above the surface of fluidized particles, the feeding port on the riser was at 500 mm above its gas distributor. Corresponding to the fuel gasification inside the LVFB or in the riser, air was supplied, respectively, to the riser or the LVFB to combust the char particles coming from the fuel gasifier. The product gas sample was taken via a suction pump just behind the cyclone or at the top of the riser for the gasification tests using the LVFB or the riser as the fuel gasifier, respectively. Thus, in the latter case a microparticle trap was particularly employed to separate the sample gas and its entrained particles (mentioned as “solid trap” in Figure 3). For both cases the sampled gas entered the same tar trap system (detailed below) to have its tar intake dropped. Until the tar traps the gas was warmed to above 523 K via external heating with tape heaters to prevent tar deposition. The tar-stripped sample gas was finally led to a micro-GC for measuring its molar composition after the gas was detected for volumetric

flow rate (1-2 nL/min) in a wet gas volumeter and dewatered in a CaCl2 column. The LVFB and riser had their respective exhaust lines. In both lines, the effluent gas was vented through an induction fan (IDF) after it was cooled and water removed in a condenser and dedusted in a bag filter. To the LVFB exhaust line, however, a gas combustion tube (GCB) was additionally available ahead of its condenser. Because the facility was made exclusively for the gasification test employing the LVFB as the fuel gasifier, the combustion of the product gas in the GCB was anticipated to reduce CO emission with venting. The adopted tar trap system was made of, in succession, a water condenser and three water bubblers immersed in an icewater bath. The condenser worked at temperatures below 278 K, so nearly all the water and most of the tars carried with the sample gas could be dropped in this step. On the other hand, by controlling the gas flow rate at reasonable values, it was guaranteed that the gas residence time inside the tar traps was longer than 2 min. Hence, we believed that the tar trapping from the sample gas was efficient, although not thoroughly completed. The trapped tars were extracted by following a procedure of collecting the tarry water, washing the condenser, water bubblers, and silicon rubber tubes (for connection) using acetone, filtering the acquired water/acetone liquid, vacuum evaporating water and acetone from the filtered liquid (at temperatures below 333 K), and drying the resulting tars, i.e., the solid residual of evaporation operation, in an air flow of 323 K. The tar content of product gas reported herein refers to the ratio of the extracted dry tar weight over the sample gas volume measured in a wet gas volumeter during tar trapping. The tested fuel was a kind of dried coffee grounds, whose properties were summarized in Table 2. A table feeder was used to feed the fuel quantitatively and continuously, and the fed

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Table 2. Properties of the Tested Coffee Grounds proximate analysis moisture volatile matter fixed carbon ash ultimate analysis C H N O S size bulk density HHV a

10.5 wt % 73.3 wt % 17.0 wt % 1.0 wt % 52.97 wt %, dba 6.51 wt %, dba 2.80 wt %, dba 36.62 wt %, dba 0.05 wt %, dba below 1.2 mm 350 kg/m3 5260 kcal/kg, dba

Dry basis.

Table 3. Employed Experimental Conditionsa Fuel Gasifier temperature steam rate fuel rate air-flow rateb argon ratec Char Combustor temperature air-flow rated

∼1065 K 4.0 kg/h ∼3.6 kg/h 12 nL/min 12 nL/min ∼1083 K 70 nL/min

HCPs (Silica Sand) Sauter Size density loaded amount

190 µm 2600 kg/m3 ∼24 kg

a The conditions shown for the fuel gasifier and char combustor are applicable to both the riser and LVFB. Superficial gas velocity at the bed temperature was 0.21 and 0.16 m/s for the LVFB gasifier and combustor, respectively, whereas the superficial gas velocity at the bed temperature was 2.82 and 2.12 m/s for the riser gasifier and combustor, respectively. b O /C was about 0.06 mol/mol. c Air ratio to 30 wt % fuel C was 1.2. 2

fuel was carried into the gasifier via an argon stream monitored in a mass flow controller. The argon stream was also a gas tracer for determining the volume (i.e., moles) of the produced gas. Steam was the gasification reagent adopted, while air was the oxidant applied to the char combustor. In response to the purpose of the present work, the feeds of steam and air were available to both the LVFB and the riser (see Figure 3). During heating of the facility (before steam feed), both beds were fluidized with air. Consequently, the air flows to both beds were controlled with their respective rotameters, whereas the flow of steam, either to the LVFB or to the riser, was controlled through measuring the water amount in a water pump. Under the intention of speeding up the temperature rise during bed heating, supplies of propane to both the riser and LVFB were available. 4. Results and Discussion Table 3 summarizes the test conditions employed, which were the same for the tests using the riser and the LVFB as the fuel gasifier. The gasification temperatures were around 1065 K. At these temperatures the superficial gas velocities for the riser and LVFB gasifiers were 2.82 and 0.21 m/s, respectively. Thus, the explicit residence time of fuel particles as well as of gas inside the riser gasifier was 2-3 s. Corresponding to this, in the LVFB gasifier the residence time of fuel particles was about 1000 s, which was calculated according to a plug granular flow assumption and the measured particle circulation rate at the experimental temperatures.13 Nonetheless, the time that the gas passed through the particle bed in the LVFB was about 3 s because in the bed the fluidized particles were about 600 mm high. For the char combustor an air ratio of about 1.2 with respect to 30 wt % fuel C was adopted in both tested cases.

Figure 4. Typical axial profiles of time-averaged temperatures in the two reactors (riser and LVFB) of the experimental facility for the cases using (a) the riser and (b) the LVFB as the fuel gasifier.

Figure 4 exemplifies the axial profiles of temperatures inside the riser (0) and LVFB (b) of the experimental apparatus averaged in the last 15 min of the tested time. There, parts a and b of Figure 4 are for the cases taking the riser and LVFB as the fuel gasifier, respectively. During the tests it was found that the temperatures in both beds tended to become quasi steady after undergoing a dynamic period of about 15 min with obvious variations since the fuel feed. In response to the combustion of chars inside the LVFB, Figure 4a shows that the bottom section of this bed had slightly higher temperatures (b). Corresponding to this, in Figure 4b the highest temperature for the LVFB emerged at the bed top when the bed was arranged as the fuel gasifier. This reflects the fact that inside the particle bulk of the LVFB endothermic fuel pyrolysis and char gasification reactions realistically occurred. For both cases the temperature inside the riser was almost axially uniform (0 in Figure 4). Thus, herein the average temperature of all the measured points for the riser was employed to represent the reaction temperature of the riser gasifier, while that averaged on the four points at the bed bottom was adopted as the representative temperature of the LVFB gasifier. Figure 5 exemplifies the time-series composition of the product gas containing argon tracer for the tests adopting the riser (Figure 5a) and LVFB (Figure 5b) of the experimental facility as the gasifier. Obviously, in both cases the operation approached its quasi-steady state in the tested time of about 35 min, as indicated by the product gas composition itself which shifted to change little after 10 min of fuel feed. Furthermore, Figure 5 shows that all the gas species present in the product gas exhibited a similar relationship in amount for the two displayed tests. That is, when CO manifested the highest content, the other species adhered to a gradually decreasing order according to H2, CH4, C2H4, C2H6, and C3H6 in succession. The result reveals essentially that the use of different types of FBs as the fuel gasifier (here riser versus LVFB) did not vary the concentration distribution of product gas. The HHVs (higher heating values) corresponding to the gas compositions in Figure 5 were around 3000 kcal/nm3. This value is much higher than the usual HHVs of the gases from partial oxidation gasification with air being the oxidant, which are hardly over 1000 kcal/nm3. On the other hand, the HHVs for the tested dual bed process can be rather higher in actual application plants because no tracer gas is required there. As a matter of fact, the HHVs of the gases free of argon tracer corresponding to Figure 5 were as high as 3800 kcal/nm3. This high caloric value verifies the distinctive merit of the dual fluidized bed gasification technology. That is, the technology can produce product gas free of N2 dilution, even if air is

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Figure 5. Typical time-series molar composition of tracer-containing product gas for the tests using (a) the riser and (b) the LVFB as the fuel gasifier.

employed to combust a part of char to generate the required endothermic heat. Figure 6 compares the cold gas efficiencies (Figure 6a) and the corresponding conversions of C (Figure 6b) and H (Figure 6c) estimated on the basis of the gas composition exemplified in Figure 5 and the dry gas volume determined according to the flow rate of argon tracer. The formulations employed in the estimations were

cold gas efficiency )

total HHV of product gas total HHV of supplied fuel

and

element conversion )

Figure 6. Comparison of (a) cold gas efficiency, (b) C conversion, and (c) H conversion between the cases using the riser (O, b) and LVFB (0, 9) as the fuel gasifier.

moles of element in product gas moles of element in supplied fuel

To ensure the reliability of the acquired result, the test for each of the examined cases, e.g., taking the riser or LVFB as the fuel gasifier, was conducted twice. In Figure 6 the data for the riser gasifier (O, b) suffered much higher fluctuations, demonstrating the fact that in this case the fuel feed into the riser experienced more instant variations than the feed into the freeboard of the LVFB. The figure demonstrates that throughout the tested time the gasification in the LVFB (0, 9) led to evidently higher cold gas efficiency and accordingly higher C and H conversions than that in the riser did (O, b). Even if the reaction temperature for the case indicated by “9” was slightly lower (1048 K), its resulting cold gas efficiency (Figure 6a) and fuel C conversion (Figure 6b) were still a few percent higher than those from the riser gasifier at a higher temperature of 1064 K. Under the similar temperature of about 1065 K, one can see that the differences in cold gas efficiency, C conversion, and H conversion reached about 15% (Figure 6a), 13% (Figure 6b), and 24% (Figure 6c), respectively (0 versus O, b). Figure 7 compares further the content of tars in the product gas. The plotted tar content refers to the grams of dry tars in a cubic meter of dry gas free of Ar tracer. Evidently, the gasification in the LVFB resulted in lower tar contents (compar-

Figure 7. Comparison of the tar content in tracer-excluding product gas between the two tested cases.

ing particularly the feed rates 3.65 and 3.62 kg/h), although both cases had somewhat high tar productions under the tested conditions. The result complies with the analysis shown under Theoretical Concerns. That is, although the gas-solid flow in the riser can provide the particles extensive contact with the gas, it does not ensure that all the gas will have efficient contact with the particles due to the low particle concentrations therein. For tar destruction, however, it requires an efficient contact of gas with particles such that the riser may have therefore little superiority over the LVFB. Inside the LVFB gasifier, on the other hand, the much higher particle concentration possibly

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enables the gas to contact more particles. Under a similar explicit gas residence time of 2-3 s, it is then not surprising that the LVFB gasifier caused a relatively lower tar production than the riser gasifier did. In addition, Figure 7 reveals that a lower fuel feed rate yielded a slightly lower tar content in the tracer-free product gas. This is plausible because a lower amount of treated fuel led to less total tar so that tars have more chances to contact particles and in turn to be destructed. Consequently, we can see that all the experimental results shown above verified the theoretical concerns detailed in section 2. Then, for a dual fluidized gasification system its fuel gasification should be better put into a low-velocity fluidized bed (usually a bubbling fluidized bed), making thus the combination illustrated in Figure 2d superior to all the other combinations conceptualized in the same illustration. In turn, of the two technical deployments mentioned in Figure 1 the one in Figure 1b must be superior to that in Figure 1a. However, one may feel that even for the LVFB gasifier the acquired cold gas efficiency (