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Two-stage dual fluidized bed gasification: Its conception and application to biomass
F U E L P R O CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9 ) 1 3 7–1 4 4
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Two-stage dual fluidized bed gasification: Its conception and application to biomass Guangwen Xu a,b,⁎, Takahiro Murakami a,c , Toshiyuki Suda a , Yoshiaki Matsuzaw a , Hidehisa Tani a a
Research Laboratory, IHI Corporation, Ltd., Isogo-Ku, Yokohama 235-8501, Japan Institute of Process Engineering, Chinese Academy of Sciences, Haidian, Beijing 100080, China c Clean Gas Group, National Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8569, Japan b
AR TIC LE D ATA
ABSTR ACT
Article history:
The quoted two-stage dual fluidized bed gasification (T-DFBG) devises the use of a two-stage
Received 14 November 2007
fluidized bed (TFB) to replace the single-stage bubbling fluidized bed gasifier involved in the
Received in revised form
normally encountered dual fluidized bed gasification (N-DFBG) systems. By feeding fuel into
5 August 2008
the lower stage of the TFB, this lower stage functions as a fuel gasifier similar to that in the
Accepted 13 August 2008
N-DFBG so that the upper stage of the TFB works to upgrade the produced gas in the lower stage and meanwhile to suppress the possible elutriation of fuel particles fed into the
Keywords:
freeboard of the lower-stage bed. The heat carrier particles (HCPs) circulated from the char
Dual fluidized bed gasification
combustor enter first the upper stage of the TFB to facilitate the gas upgrading reactions
Multi-stage fluidized bed
occurring therein, and the particles are in turn forwarded into the lower stage to provide
Biomass
endothermic heat for fuel pyrolysis and gasification reactions. Consequently, with T-DFBG it
Tar elimination
is hopeful to increase gasification efficiency and decrease tar content in the produced gas.
In-bed gas upgrading
This anticipation was corroborated through gasifying dry coffee grounds in two 5.0kg/h
Calcium
experimental setups configured according to the principles of T-DFBG and N-DFBG, respectively. In comparison with the N-DFBG case, the test according to T-DFBG increased, the fuel C conversion and cold gas efficiency by about 7% and decreased tar content in the produced gas by up to 25% under similar reaction conditions. Test results demonstrated also that all these upgrading effects via adopting T-DFBG were more pronounced when a Ca-based additive was blended into the fuel. © 2008 Elsevier B.V. All rights reserved.
1.
The conception
Because it isolates combustion reaction for endothermic heat required by pyrolysis/gasification reactions, the dual fluidized bed gasification (DFBG) has the advantage of producing middle-caloric gas free of serious dilution by N2 of combustion air [1]. A distinctive feature of DFBG is its involvement of two fluidized beds (FBs), causing it to have different technical options resulting from combining two different-type FBs [2–5]. Literature study demonstrated that the technically superior choice for DFBG is a combination of a bubbling/turbulent
fluidized bed gasifier and a pneumatic riser combustor in terms of facilitating gasification reactions and suppressing tar evolution [5]. Fig. 1(a) conceptualizes a normal DFBG (N-DFBG) system, showing that fuel gasification (including pyrolysis) takes place in a dense bubbling/turbulent fluidized bed (BFB). The produced gas passes through the freeboard of the BFB gasifier to evolve, causing the gas to have generally high content of tars. Meanwhile, there has to have some fuel loss via elutriation. Incorporating gas upgrading function into the gasifier is a viable way to avoid or alleviate these problems.
⁎ Corresponding author. Institute of Process Engineering, Chinese Academy of Sciences, P. O. Box 353#, Beijing 100080, China. Tel./fax: +86 10 62550075. E-mail address: [email protected] (G. Xu). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.08.007
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Fig. 1 – Conception illustration for (a) the existing normal dual fluidized bed gasification (N-DFBG) and (b) the newly proposed two-stage dual fluidized bed gasification (T-DFBG).
This resulted in our proposal of the two-stage dual fluidized bed gasification (T-DFBG) system conceptualized in Fig. 1(b) [6]. The BFB gasifier for the N-DFBG [Fig. 1(a)] is replaced with a two-stage fluidized bed (TFB). Fuel feed is into the freeboard of the lower (or first) stage of the TFB. The circulated heat carrier particles (HCPs) enter first the TFB's upper (or second) stage and moved into its lower stage via overflow. Consequently, the downcomer of the overall system is immersed into the particle bed of the TFB's upper stage. Between the upper and lower stages of the TFB there is an overflow pipe of particles, which is immersed into the particle bed of the lower stage. The pneumatic riser of the T-DFBG is connected to the lower stage of the TFB so that this stage is essentially equivalent to the BFB gasifier of N-DFBG. Fuel pyrolysis and gasification occur in the lower stage of the TFB by contacting the HCPs from the upper stage and interacting with the gasification reagent (air + steam) fed into this stage. The generated gas, mixed with overfed gasification reagent, then flows up and passes through the TFB's upper stage. There, via interacting with hot particles gas upgrading reactions, including tar/hydrocarbon reforming and water gas shift (WGS), are expected to occur. Because there are much fewer endothermic reactions in the upper stage, the particle temperature Ts of this stage should be very close to that of the riser combustor (Tg) but higher than Tb in the lower stage. Consequently, the tar and hydrocarbon reforming and WGS must be easier to proceed in the upper stage. These gas upgrading reactions, on the other hand, would not considerably reduce the temperature of the HCPs so that the particles can still work on the highly endothermic fuel pyrolysis and
gasification reactions in the lower stage of the TFB when they are conveyed to there. Different from N-DFBG where high-temperature HCPs are circulated directly into fuel gasifier, the T-DFBG depicted above first takes advantage of the high temperature of HCPs to upgrade produced gas and in turn sends the particles to the endothermic reaction zone, the lower stage of the TFB. The gas upgrading would increase the conversion of tars into gas and enhance WGS, thus hopefully increasing gasification efficiency and causing the produced gas to have less tar and hydrocarbon but more H2. All these effects of T-DFBG should be rather pronounced when a catalytic material, such as Cabased ore, is used. In the upper stage of the TFB, not only the catalytic material has its highest activity on catalyzing reforming and WGS reactions (because the material being just activated in the combustor) but the high reaction temperature in the upper stage also facilitates the catalytic reactions. Meanwhile, the upper stage of the TFB suppresses elutriation of fuel particles fed into the freeboard of the TFB's lower stage. In comparison with the other usual BFB gasifiers, this effect is very like that allowed by feeding fuel into the inside of fluidized particle bed, say, enhancing gasification reactions and lowering tar evolution with the produced gas [7]. Nonetheless, the fuel feed into the T-DFBG of Fig. 1 is obviously easier because the feed is essentially into the freeboard of the TFB's lower-stage bed. In summary, we may hope that T-DFBG is an advanced DFBG technology that increases gasification efficiency, suppresses tar generation and upgrades producer gas quality. The
F U E L P R O CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9 ) 1 3 7–1 4 4
present study is designated to demonstrate all of these superiorities of T-DFBG and thereby to show its technical feasibility.
2.
Experimental section
Gasification tests according to N-DFBG and T-DFBG were conducted. The experimental apparatus (5.0 kg/h) and detection method involved in the tests following N-DFBG are available in our previous publications [5,8]. Modifying the gasification setup for the N-DFBG tests according to the conception highlighted in Fig. 1(b) resulted in the pilot TDFBG setup sketched in Fig. 2 (the slightly slanted downcomer being a causative case resulting from the modification). The setup consisted of a pneumatic riser combustor and a two-stage fluidized bed (TFB) reactor with its lower (or first) stage being the fuel gasifier. The upper or second stage of the TFB had an expanded cross section (see the side view of TFB in the double dash-line box) in order to reduce gas velocity and increase gas residence time in this stage. The riser was 52.7 mm in i.d. and 6400 mm high, while the TFB was a rectangular bed with a total height of 1980 mm. The cross section area and height of the TFB were 80 × 370 mm2 and 980 mm for its lower stage and 180 × 370 mm2 and 700 mm for its upper stage, respectively. All ducts connecting the riser and
139
TFB and the particle overflow pipe inside the TFB had the same i.d. of 52.7 mm. The overflow pipe was extended to 300 mm above the distributor of the upper stage. Following Fig. 1(b) the overflow pipe and downcomer immersed into the particle beds of the lower and upper stages of the TFB, respectively. The adopted distributors for the lower and upper stages were nozzle type and the nozzle diameter was 1.4 mm. A cut was made for the windbox of the gasifier (see Fig. 2) at a position just below the seal annotated in Fig. 2. Hence, independent gas feeds to the seal and the lower stage of the TFB were possible. The riser and TFB had their independent cyclones, heat exchangers, bagfilters and induction fans, enabling thus independent control of pressures in both the reactor vessels. These reactors were electrically heated and temperatures of up to 1173K were allowed. The produced gas evolving from the upper stage of the TFB was first burnt off in a tube furnace (GCB) and in turn sent to the condenser mounted in the TFB's exhaust line. The involved experimental procedure and detection method were similar to those taken in the tests with N-DFBG [5,8] except for the points mentioned here. The fuel fed with a table feeder was carried into the freeboard of the lower stage of the TFB by a well-metered argon stream. Being the gasification reagent, steam at about 673K was supplied into the lower stage of the TFB. To the seal of the TFB was a N2 stream of 3.0 Ln/min during fuel feeding. In this work, only the gas evolving from
Fig. 2 – A schematic diagram of the employed 5.0 kg/h experimental setup following the principle of T-DFBG.
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Table 1 – Properties of fuel and other materials referred to in this article Fuel (dried coffee grounds) Proximate analysis [wt.%] Moisture Volatile matter Fixed carbon Ash Ultimate analysis (db-wt.%) C H N O S HHV [kcal/kg-db] Bulk density [kg/m3] Size
10.5 71.8 16.7 1.0 52.97 6.51 2.80 36.62 0.05 5260 350 b 2 mm
Silica sand (HCPs) Sauter mean size [µm] Bulk density [kg/m3]
190 1600
CaO for physical mixing Lightly calcined CaCO3 ore Bulk density [kg/m3] Sauter mean size [µm] Surface area [m2/kg] Pore volume [L/kg]
1050 70 (b1.2 mm) 2.0–5.0 0.01
the upper stage was sampled to measure its tar intake and molar composition. Using a fabric filter to capture the tars escaping from our tar trapping system confirmed that the tar trapping efficiency was over 90% in the tests. The major parameters referred to herein were defined as: Xi ¼
Mig TðGas production rateÞ 100k ði ¼ c; hÞ; Mif TðFuel f eed rateÞ
ð1Þ
ge ¼
ðProduct gas HHVÞTðGas production rateÞ 100k; ðFuel HHVÞTðFuel f eed rateÞ
ð2Þ
dry tar weight ; ðSampled gas volumeÞTð1 CAr Þ
ð3Þ
teb ¼
and Tar content ¼
Ar molar concentration in the produced gas. In Eq. (1), Mig and Mif shows the mole of element i (= c, h) in every normal cubic meter of produced gas and every kilogram of fuel treated, respectively. The tested fuel was coffee grounds dried with the method of slurry dewarting in kerosene [9]. Summarized in Table 1 are the major property parameters of the fuel, showing that the fuel was rich in volatile and oxygen. The sizes of the tested fuel were below 2.0mm. In addition to bare coffee grounds, the fuel blended with CaO according to a CaO-to-fuel mass ratio of 1/20 was also tested in the article to investigate the effect of Ca additive. The principle of T-DFBG expects that in this case the Ca additive should have more pronounced effects than in NDFBG. The mass ratio 1/20 adopted here is arbitrary but it is indicative to a certain degree of our intention to keep the ratio as low as possible (much higher ratios being suggested in the literature [10]). The employed CaO was calcined CaCO3 ore (sieved to below 1.2mm), whose characteristics were listed also in Table 1. Silica sand of 190 µm in Sauter mean diameter was used as the heat carrier particles (HCPs). Table 2 summarizes the experimental conditions of all tests referred to in this article, where the ones numbered as T1 and T2 refer to the tests according to T-DFBG and those indexed by N1 and N2 are the tests with N-DFBG. The tests T2 and N2 were for the fuel without Ca addition. All the tests had the similar fuel feed rate F of about 4.0kg/h and were under similar reaction temperatures of 1103 and 1093K in the riser combustor (Tg) and fuel gasifier (Tb), respectively. The superficial gas velocity Ub in the gasifier was around 0.2m/s at the operating bed temperature Tb. However, the two tests following T-DFBG (i.e. T1 and T2) had much higher gas velocity Ug in the riser combustor (5.8m/s against 2.6m/s for N1 and N2). This led to a much higher particle circulation rate for the T-DFBG tests and in turn to a much shorter explicit residence time teb of particles (including fuel) inside the fuel gasifier estimated by
where Gas production rate ¼ ðVolume rate of ArÞ=ð1 CAr Þ: In Eq. (3) the quoted tar content is with respect to the product gas volume free of argon tracer, and CAr refers to the
Particle amount in the gasif ier : Particle amount circulated in unit time
ð4Þ
The quick particle circulation at higher Ug for T-DFBG not only allows the particles in the upper stage of the TFB to be quickly refreshed to realize a better gas upgrading effect but leads as well to a lower teb for gasification reactions in the TFB's lower stage so that the superiority demonstration for TDFBG can be more definite. The particle mount in the gasifier was about 20kg (determined from the pressure drop over the
Table 2 – Condition parameters adopted in the tests reported in the article No.
With a Ca?
F b [kg/h]
S/F [kg/kg]
Tb [K]
Ts [K]
T1 1:20 3.65 1.10 1093 1093 T2 – 4.27 0.93 1090 1093 N1 1:20 3.67 1.31 1095 – N2 – 3.70 0.92 1103 – Argon to gasifer: 10.6 Ln/min N2 to windbox below the seal in face to riser: 3.0 Ln/min Air ratio to 30% C of fuel in riser: ~1.2 for N1 and N2, but 3.0 for T1 and T2. Total load of silica sand: ~ 40 kg for T1 and T2, but ~ 24 kg for N1 and N2. a b
The mentioned ratio refers to the mass ratio of Ca to fuel with moisture. The fuel feed rate refers to the value without Ca.
Tg [K]
Ub [m/s]
Us [m/s]
Ug [m/s]
1093 1100 1103 1113
0.21 0.25 0.25 0.18
0.10 0.10 – –
5.80 5.90 2.65 2.55
teb [s] 110 110 1000 1000
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consequently in their respective constants, which were about 3650kcal/m3n, 85.0%, 78.0% and 110.0%, respectively. The H conversion higher than 100% indicates a fact that a considerable large amount of H is transferred from steam to product gas via steam-involved reactions like steam gasification/ reforming and water gas shift. Fig. 4 compares the attained fuel C (Xc) and H (Xh) conversions, cold gas efficiency (ηe) and tar content in the produced gas free of Ar tracer for the tests performed according to N-DFBG (open bars) and T-DFBG (solid bars). There, Fig. 4a is for the tests T2 and N2 regarding bare coffee grounds and Fig. 4b for T1 and N2 with respect to the CaOblended fuel. Meanwhile, Table 3 presents a comparison for the corresponding product gas compositions and the H2/CO ratios and HHVs estimated from the compositions. The figured and tabulated values for an individual test all refer to the averages over the period in which the test became quasisteady (for instance, between 20 and 70min in Fig. 3). Furthermore, the molar concentrations in Table 3 were the values normalized to the mentioned gas species so that the sum of the concentrations was constantly 100% for all tests. As for a given fuel, either bare coffee grounds (T2, N2) or with 1/20 CaO (T1, N1), Table 2 shows that the tests according to N-DFBG (i.e. N1 and N2) had much longer residence time teb (1000s vs. 110s) and the same or slightly higher S/F ratio and reaction temperature Tb (~ 1093K). Even so, in Fig. 4 the tests with T-DFBG exhibited apparently higher C and H conversions and cold gas efficiency, while their tar contents in the produced gases (free of Ar tracer) were obviously lower. Actual Fig. 3 – Typical time series of (a) molar composition of rude product gas and (b) the corresponding fuel conversion, energy recovery efficiency and HHV for the tests with T-DFBG. The plotted test is the T1 specified in Table 2.
particle bed of the TFB's lower stage), and the particle amount circulated in unit time was measured experimentally at actual running temperature using the method of Xu et al. [11]. In the tests T1 and T2 the temperature of the TFB's upper stage Ts was controlled at a value very close to Tb of the gasifier.
3.
Results and discussion
With the test T1 as an example, Fig. 3 typifies the time series of measured product gas composition (Fig. 3a) and its corresponding performance (Fig. 3b). The similar time series was reported also for the tests according to N-DFBG in Ref. [8]. The displayed test in Fig. 3 lasted for about 70min but in the first 15min there was no measurement for the product gas composition. The data demonstrate a very good stability after 30min of fuel feed, showing that the test's quasi-steady sate should be surely achievable in about one hour. Fig. 3a clarified an unvaried order of molar concentration among all the gas species presenting in the product gas. While H2 content took the top, the other gas species next to it were CO, CO2, CH4, C2H4, C2H6 and C3H6 in succession. The corresponding HHV of the produced gas (right Y), cold gas efficiency ηe, and fuel C and H conversions Xc and Xh (left Y) remained
Fig. 4 – Comparison of fuel conversion and energy recovery efficiency realized with N-DFBG and T-DFBG for (a) pure coffee grounds and (b) coffee grounds mixed with CaO according to a CaO-to-fuel mass ratio of 1/20. Detailed test conditions are in Table 2.
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Table 3 – Molar composition (normalized), H2/CO ratio and HHV of product gas corresponding to the performance data plotted in Fig. 4 No. a
T1 T2 N1 N2
Normalized b molar concentration [mol%]
H2/CO
HHV c
H2
CO
CO2
CH4
C2H4
C2H6
C3H6
[–]
[kcal/m3n]
31.2 23.7 25.4 22.1
28.2 36.6 33.7 37.4
17.3 12.4 14.2 12.3
13.6 15.8 15.7 17.8
6.32 7.56 6.68 6.82
3.23 3.84 3.40 3.55
0.06 0.11 0.13 0.09
1.106 0.648 0.754 0.596
3640 3750 3890 3920
a
The test Nos. correspond to those in Table 2. Normalization of the molar concentration was with respect to all the listed gas species so that their concentration values were summed to 100%. c The listed HHV was calculated on the basis of the molar composition of rude product gas with Ar tracer and trace amounts of N2 and O2. b
increases in Xc, Xh and ηe via T-DFBG were about 3.5, 5.0 and 4.0 percentage points for bare coffee grounds (Fig. 4a) and approximately 4.5, 14.0 and 6.0 percentage points for the fuel blended with CaO. Tar content in the product gas decreased by about 7.0g/m3n in both cases. According to Xu et al. [8], more increase in the gasification efficiency and decrease in the tar content of the product gas can be expected, if the residence time of fuel inside the TFB gasifier (now 110s, see Table 2) is similar to that for the N-DFBG case (1000s). Increasing fuel conversion and energy recovery efficiency (into product gas) via T-DFBG may be relative to its suppression of fuel loss via elutriation. Lowering the tar content in the produced gas, however, should be surely due to the enhanced tar reforming or destruction reactions. What is dominant can be clarified by comparing the product gas composition for the two cases. Table 3 shows that for a given fuel the produced gas via T-DFBG had higher H2 but lower CO contents, leading to its higher H2/CO ratio than that via N-DFBG (see T1 against N1, or T2 against N2). As a result, the HHV of the produced gas was more or less lower for T-DFBG. All of these effects were distinctive in the tests with CaO-blended fuel (i.e. T1 and N1). While the test T1 according to T-DFBG had a H2 content (31.2mol%) higher than that of CO (28.2mol%), the test N1 with N-DFBG led to a CO content (33.7mol%) much higher than H2 concentration (25.4mol%). Meantime, all hydrocarbons were more in the gas produced with N-DFBG, whereas the CO2 concentration was higher for T-DFBG. Consequently, both reforming of tars/hydrocarbons and WGS truly occurred to a greater degree in the test T1, even though its steam-to-fuel mass ratio (i.e. S/F) was slightly lower than that for the test N1 (1.10 against 1.31). Responding to this the highest H2/CO ratio (1.106) and lowest HHV (3640kcal/m3n) are shown for test T1 in Table 3. The situation for the tests T2 and N2 with regard to bare coffee grounds was different. Although T-DFBG (test T2) elevated slightly H2 content and lowered CO content of the produced gas, CO2 concentration in the gas remained to have little change and the contents of hydrocarbons (except for CH4) were somehow higher in comparison with those from the test N2. Thus, in this test there should not be greatly enhanced catalytic reforming of tars and hydrocarbons. Correspondingly, enhanced thermal decomposition of tars into higher-C hydrocarbons (tars may be hardly broken into CH4) via T-DFBG can be considered to be responsible for the slightly lower tar and CH4 contents but slightly higher hydrocarbon concentrations in the product gas from T2.
Therefore, the addition of CaO facilitated the upgrading effects of T-DFBG on gasification efficiency and tar elimination. These effects were realized through enhancing the reactions of tar/hydrocarbon reforming and WGS. Indeed, the calcined Ca-base ores have some activity to catalyze such reactions [4,10,12 and Refs. therein]. In T-DFBG the Ca-base additive can maintain its most active state in the TFB's upper stage where the reaction temperature Ts (≈ Tg) is higher than Tb in the lower stage. In N-DFBG, the activated additive in the riser combustor is circulated into the fuel gasifier directly. There, not only the temperature of the active additive is quickly lowered to Tb from Tg (Tb b Tg), but the active sites of the additive may be also quickly blocked by ash and deposited carbon in the contact and interaction of the additive with fuel and tarry pyrolysis gas. Without addition of any catalytic material, the T-DFBG enables gas upgrading merely by providing the produced gas an additional reaction space (i.e. the TFB's upper stage) at a higher temperature Ts. Hence, the available effects should be confined to thermal decompositions of tars and higher-C hydrocarbons. Nonetheless, the tar content in the produced gas from the test T1 was still high. Literature studies shown that physically mixed Ca-base additive may reduce the tar content of product gas to a few g/m3n [4,10]. The additive amount, however, was usually higher than 30wt.% of the bed material, while the additive itself may be other highly active materials (than CaO), such as dolomite, olivine, even well-designed catalysts [4]. Therefore, the low blending ratio (only 1/20) and low catalytic activity of CaO, in addition to the use of coffee grounds fuel that is highly tar-productive, should be responsible for the tar content high as 25 to 40g/m3n shown in our tests. In Fig. 3a both H2 and CO2 concentrations tended to increase slightly, while CO concentration became gradually lower with reaction time. These shown possibly a gradually increased effect of CaO additive that became more with test (because CaO being blended into the tested fuel). Notwithstanding, the adopted CaO ratio of 1/20 enabled a good demonstration of the effect of T-DFBG, as is delineated above. In spite of the preceding merits of T-DFBG, the adoption of a TFB has to increase the system's pressure drop. Depending on the desired particle bed height (usually 300–400mm) for the upper stage, the pressure drop increase may be 3000 to 5000Pa. In actual fluidized bed systems it has no problem to tackle this pressure drop increase. For the T-DFBG a critical issue might be whether its TFB runs stably because the upper-stage distributor may be blocked up via entrained fine particles
F U E L P R O CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9 ) 1 3 7–1 4 4
Fig. 5 – Pressure drop across the upper-stage distributor of the TFB involved in the T-DFBG experimental setup during temperature rise (before time 0) and fuel feed. The data are from the test T1.
during running. In our test this problem did not occur, and Fig. 5 verifies the point through a time series of pressure drop between the freeboard of the TFB's lower stage and a location 50 mm above the upper-stage distributor. The data are from the test T1, and the plotted time zero refers to the onset of fuel feed. Before fuel feed the system was run to raise the bed temperature. Within 2 h the pressure drop remained to have an average that was little changed, indicating the fact that the upper-stage distributor was never blocked up. We had run this T-DFBG facility for several other fuels (including brown coal), and in all cases the TFB's upper-stage distributor remained in its dynamic steady state. During fuel feed there were some instantly high or low pressures, which appeared as sharp pulses in Fig. 5. This shows that some fine fuel particles would be carried with gas to pass through the nozzles (1.4 mm in i.d.) of the upper-stage distributor, but the worried blockage of the nozzles by such particles did never happen.
4.
Conclusions
The study devises the use of a two-stage fluidized (TFB) bed to replace the single-stage bubbling fluidized bed reactor involved in a generally-encountered normal dual fluidized bed gasification (N-DFBG) system. This forms the newly proposed two-stage dual fluidized bed gasification (T-DFBG) process that is anticipated to decrease tar evolution and increase gasification efficiency and H 2 production by enhanced in-bed gas upgrading effect. The anticipations were demonstrated by gasifying coffee grounds (dried in advance to about 10 wt.% water) in two pilot gasification setups constructed according to the principles of T-DFBG and N-DFBG, respectively. At gasifier temperatures of about 1093 K and steam-to-fuel mass ratio of about 1.0, the T-DFBG increased absolutely the fuel C conversion and cold gas efficiency by about 5 percentage points and decreased tar content in the produced gas by 7.0 g/m3n in comparison with N-DFBG, although the explicit fuel particle residence time for the T-DFBG test (110 s) was much lower than that for N-DFBG (1000 s). In relative scale these upgrades are equivalent to about 7% increase in fuel C conversion and 20–25% decrease in
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tar evolution, and these upgrading effects would be surely further large by running the gasifier of T-DFBG at longer fuel particle residence time. The upgrading effect was more pronounced when CaO additive was blended into the fuel (only 1/20 of the fuel mass in this article). In this case the increase in H conversion was distinctively high and reached 14 percentage points, clarifying that reforming and water gas shift reactions were greatly enhanced via CaO additive. Without Ca additive, the T-DFBG exhibited a similar tendency to upgrade the produced gas but to a lower degree. Analysis revealed that in this case the enhanced thermal decomposition of tars in the upper stage of the TFB is the excluding cause for the realized upgrading effects.
Nomenclature F Mcg Mcf Mhg Mhf S teb Tb Tg Ts Ub Ug Us Xc Xh
fuel feed rate, kg/h moles of C in one normal m3 of product gas, mol/m3n moles of C in 1 kg of treated fuel, mol/kg moles of H in one normal m3 of product gas, mol/m3n moles of H in 1 kg of treated fuel, mol/kg steam feed rate into fuel gasifier, kg/h explicit residence time of fuel in gasifier, s characteristic temperature of fuel gasifier, K characteristic temperature of riser combustor, K characteristic temperature of the upper stage of TFB in T-DFBG, K superficial gas velocity in fuel gasifier at its operating temperature, m/s superficial gas velocity in riser combustor at its operating temperature, m/s superficial gas velocity in the upper-stage bed of TFB at its operating temperature, m/s conversion of fuel C into product gas, % conversion of fuel H into product gas, %
Greek letters cold gas efficiency, % ηe
Acknowledgements The work was conducted during a technical program on upgrading and gasification of high water content biomass financed by The New Energy and Industrial Technology Development Organization, Japan (NEDO). The document work was done by the first author under financial support of National Natural Science Foundation of China (NSFC) under project Nos. of 20606034 and 20776144. The authors are also grateful to Mr. Minoru Asai and Ms. Kumiko Uchida of the same company for their help in the experiment.
REFERENCES [1] A.V. Bridgwater, The technical and economic feasibility of biomass gasification for power generation, Fuel 74 (1995) 631–653. [2] M.A. Paisley, R.D. Litt, K.S. Creamer, Gasification of refuse-derived fuel in a high throughput gasification system,
144
[3]
[4]
[5]
[6]
[7]
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In Energy from Biomass and Waste XIV (P012990), Florida, January, 1990. A. Zschetzsche, H. Hofbauer, A. Schmidt, Biomass gasification in an internally circulating fluidized bed, Proceedings of the Eighth European Conference on Biomass for Agriculture and Industry, vol. 3, 1998, pp. 1771–1777. C. Pfeifer, R. Rauch, H. Hofbauer, In-bed catalytic tar reduction in a dual fluidized bed biomass steam gasifier, Ing. Eng. Chem. Res. 43 (2004) 1634–1640. G. Xu, T. Murakami, T. Suda, Y. Matsuzawa, H. Tani, The superior technical choice for dual fluidized bed gasification, Ind. Eng. Chem. Res. 45 (2006) 2281–2286. G. Xu, T. Murakami, T. Suda, S. Kusama, T. Fujimori, Gasification method and apparatus incorporating gas upgrade for solid fuels, Patent, 2006, Appl. No. PCT/JP2006/ 305785. J. Corella, J. Herguido, F.L. Alday, Pyrolysis and steam gasification of biomass in fluidized bed. Influence of the type and location of the biomass feeding point, in: A.V. Bridgwater, J.L. Kuester (Eds.), Research in Thermochemical
[8]
[9]
[10]
[11]
[12]
Biomass Conversion, Elsevier Applied Science, London, 1988, pp. 384–397. G. Xu, T. Murakami, T. Suda, Y. Matsuzawa, H. Tani, Gasification of coffee grounds in dual fluidized bed: performance evaluation and parametric investigation Energy Fuels 20 (2006) 2695–2704. Y. Mito, N. Komatsu, I. Hasegawa, K. Mae, Slurry dewatering process for biomass, In 2005 International Conference On Coal Science and Technology (ICCS&T), IEA-Clean Coal Center: London, 2005, Paper 2E01. J. Corella, M.P. Aznar, J. Gil, M.A. Caballero, Biomass gasification in fluidized bed: where to locate the dolomite to improve gasification? Energy Fuels 13 (1999) 1122–1127. G. Xu, T. Murakami, T. Suda, Y. Matsuzawa, H. Tani, Particle circulation rate in high-temperature CFB: measurement and temperature influence, AIChE J. 52 (2006) 3626–3630. G. Xu, T. Murakami, T. Suda, S. Kusama, T. Fujimori, Distinctive effects of CaO additive on atmospheric gasification of biomass at different temperatures, Ing. Eng. Chem. Res. 44 (2005) 5864–5868.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Two-stage dual fluidized bed gasification: Its conception and application to biomass Guangwen Xu a,b,⁎, Takahiro Murakami a,c , Toshiyuki Suda a , Yoshiaki Matsuzaw a , Hidehisa Tani a a
Research Laboratory, IHI Corporation, Ltd., Isogo-Ku, Yokohama 235-8501, Japan Institute of Process Engineering, Chinese Academy of Sciences, Haidian, Beijing 100080, China c Clean Gas Group, National Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8569, Japan b
AR TIC LE D ATA
ABSTR ACT
Article history:
The quoted two-stage dual fluidized bed gasification (T-DFBG) devises the use of a two-stage
Received 14 November 2007
fluidized bed (TFB) to replace the single-stage bubbling fluidized bed gasifier involved in the
Received in revised form
normally encountered dual fluidized bed gasification (N-DFBG) systems. By feeding fuel into
5 August 2008
the lower stage of the TFB, this lower stage functions as a fuel gasifier similar to that in the
Accepted 13 August 2008
N-DFBG so that the upper stage of the TFB works to upgrade the produced gas in the lower stage and meanwhile to suppress the possible elutriation of fuel particles fed into the
Keywords:
freeboard of the lower-stage bed. The heat carrier particles (HCPs) circulated from the char
Dual fluidized bed gasification
combustor enter first the upper stage of the TFB to facilitate the gas upgrading reactions
Multi-stage fluidized bed
occurring therein, and the particles are in turn forwarded into the lower stage to provide
Biomass
endothermic heat for fuel pyrolysis and gasification reactions. Consequently, with T-DFBG it
Tar elimination
is hopeful to increase gasification efficiency and decrease tar content in the produced gas.
In-bed gas upgrading
This anticipation was corroborated through gasifying dry coffee grounds in two 5.0kg/h
Calcium
experimental setups configured according to the principles of T-DFBG and N-DFBG, respectively. In comparison with the N-DFBG case, the test according to T-DFBG increased, the fuel C conversion and cold gas efficiency by about 7% and decreased tar content in the produced gas by up to 25% under similar reaction conditions. Test results demonstrated also that all these upgrading effects via adopting T-DFBG were more pronounced when a Ca-based additive was blended into the fuel. © 2008 Elsevier B.V. All rights reserved.
1.
The conception
Because it isolates combustion reaction for endothermic heat required by pyrolysis/gasification reactions, the dual fluidized bed gasification (DFBG) has the advantage of producing middle-caloric gas free of serious dilution by N2 of combustion air [1]. A distinctive feature of DFBG is its involvement of two fluidized beds (FBs), causing it to have different technical options resulting from combining two different-type FBs [2–5]. Literature study demonstrated that the technically superior choice for DFBG is a combination of a bubbling/turbulent
fluidized bed gasifier and a pneumatic riser combustor in terms of facilitating gasification reactions and suppressing tar evolution [5]. Fig. 1(a) conceptualizes a normal DFBG (N-DFBG) system, showing that fuel gasification (including pyrolysis) takes place in a dense bubbling/turbulent fluidized bed (BFB). The produced gas passes through the freeboard of the BFB gasifier to evolve, causing the gas to have generally high content of tars. Meanwhile, there has to have some fuel loss via elutriation. Incorporating gas upgrading function into the gasifier is a viable way to avoid or alleviate these problems.
⁎ Corresponding author. Institute of Process Engineering, Chinese Academy of Sciences, P. O. Box 353#, Beijing 100080, China. Tel./fax: +86 10 62550075. E-mail address: [email protected] (G. Xu). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.08.007
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F U E L P RO CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9) 1 37 –1 4 4
Fig. 1 – Conception illustration for (a) the existing normal dual fluidized bed gasification (N-DFBG) and (b) the newly proposed two-stage dual fluidized bed gasification (T-DFBG).
This resulted in our proposal of the two-stage dual fluidized bed gasification (T-DFBG) system conceptualized in Fig. 1(b) [6]. The BFB gasifier for the N-DFBG [Fig. 1(a)] is replaced with a two-stage fluidized bed (TFB). Fuel feed is into the freeboard of the lower (or first) stage of the TFB. The circulated heat carrier particles (HCPs) enter first the TFB's upper (or second) stage and moved into its lower stage via overflow. Consequently, the downcomer of the overall system is immersed into the particle bed of the TFB's upper stage. Between the upper and lower stages of the TFB there is an overflow pipe of particles, which is immersed into the particle bed of the lower stage. The pneumatic riser of the T-DFBG is connected to the lower stage of the TFB so that this stage is essentially equivalent to the BFB gasifier of N-DFBG. Fuel pyrolysis and gasification occur in the lower stage of the TFB by contacting the HCPs from the upper stage and interacting with the gasification reagent (air + steam) fed into this stage. The generated gas, mixed with overfed gasification reagent, then flows up and passes through the TFB's upper stage. There, via interacting with hot particles gas upgrading reactions, including tar/hydrocarbon reforming and water gas shift (WGS), are expected to occur. Because there are much fewer endothermic reactions in the upper stage, the particle temperature Ts of this stage should be very close to that of the riser combustor (Tg) but higher than Tb in the lower stage. Consequently, the tar and hydrocarbon reforming and WGS must be easier to proceed in the upper stage. These gas upgrading reactions, on the other hand, would not considerably reduce the temperature of the HCPs so that the particles can still work on the highly endothermic fuel pyrolysis and
gasification reactions in the lower stage of the TFB when they are conveyed to there. Different from N-DFBG where high-temperature HCPs are circulated directly into fuel gasifier, the T-DFBG depicted above first takes advantage of the high temperature of HCPs to upgrade produced gas and in turn sends the particles to the endothermic reaction zone, the lower stage of the TFB. The gas upgrading would increase the conversion of tars into gas and enhance WGS, thus hopefully increasing gasification efficiency and causing the produced gas to have less tar and hydrocarbon but more H2. All these effects of T-DFBG should be rather pronounced when a catalytic material, such as Cabased ore, is used. In the upper stage of the TFB, not only the catalytic material has its highest activity on catalyzing reforming and WGS reactions (because the material being just activated in the combustor) but the high reaction temperature in the upper stage also facilitates the catalytic reactions. Meanwhile, the upper stage of the TFB suppresses elutriation of fuel particles fed into the freeboard of the TFB's lower stage. In comparison with the other usual BFB gasifiers, this effect is very like that allowed by feeding fuel into the inside of fluidized particle bed, say, enhancing gasification reactions and lowering tar evolution with the produced gas [7]. Nonetheless, the fuel feed into the T-DFBG of Fig. 1 is obviously easier because the feed is essentially into the freeboard of the TFB's lower-stage bed. In summary, we may hope that T-DFBG is an advanced DFBG technology that increases gasification efficiency, suppresses tar generation and upgrades producer gas quality. The
F U E L P R O CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9 ) 1 3 7–1 4 4
present study is designated to demonstrate all of these superiorities of T-DFBG and thereby to show its technical feasibility.
2.
Experimental section
Gasification tests according to N-DFBG and T-DFBG were conducted. The experimental apparatus (5.0 kg/h) and detection method involved in the tests following N-DFBG are available in our previous publications [5,8]. Modifying the gasification setup for the N-DFBG tests according to the conception highlighted in Fig. 1(b) resulted in the pilot TDFBG setup sketched in Fig. 2 (the slightly slanted downcomer being a causative case resulting from the modification). The setup consisted of a pneumatic riser combustor and a two-stage fluidized bed (TFB) reactor with its lower (or first) stage being the fuel gasifier. The upper or second stage of the TFB had an expanded cross section (see the side view of TFB in the double dash-line box) in order to reduce gas velocity and increase gas residence time in this stage. The riser was 52.7 mm in i.d. and 6400 mm high, while the TFB was a rectangular bed with a total height of 1980 mm. The cross section area and height of the TFB were 80 × 370 mm2 and 980 mm for its lower stage and 180 × 370 mm2 and 700 mm for its upper stage, respectively. All ducts connecting the riser and
139
TFB and the particle overflow pipe inside the TFB had the same i.d. of 52.7 mm. The overflow pipe was extended to 300 mm above the distributor of the upper stage. Following Fig. 1(b) the overflow pipe and downcomer immersed into the particle beds of the lower and upper stages of the TFB, respectively. The adopted distributors for the lower and upper stages were nozzle type and the nozzle diameter was 1.4 mm. A cut was made for the windbox of the gasifier (see Fig. 2) at a position just below the seal annotated in Fig. 2. Hence, independent gas feeds to the seal and the lower stage of the TFB were possible. The riser and TFB had their independent cyclones, heat exchangers, bagfilters and induction fans, enabling thus independent control of pressures in both the reactor vessels. These reactors were electrically heated and temperatures of up to 1173K were allowed. The produced gas evolving from the upper stage of the TFB was first burnt off in a tube furnace (GCB) and in turn sent to the condenser mounted in the TFB's exhaust line. The involved experimental procedure and detection method were similar to those taken in the tests with N-DFBG [5,8] except for the points mentioned here. The fuel fed with a table feeder was carried into the freeboard of the lower stage of the TFB by a well-metered argon stream. Being the gasification reagent, steam at about 673K was supplied into the lower stage of the TFB. To the seal of the TFB was a N2 stream of 3.0 Ln/min during fuel feeding. In this work, only the gas evolving from
Fig. 2 – A schematic diagram of the employed 5.0 kg/h experimental setup following the principle of T-DFBG.
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F U E L P RO CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9) 1 37 –1 4 4
Table 1 – Properties of fuel and other materials referred to in this article Fuel (dried coffee grounds) Proximate analysis [wt.%] Moisture Volatile matter Fixed carbon Ash Ultimate analysis (db-wt.%) C H N O S HHV [kcal/kg-db] Bulk density [kg/m3] Size
10.5 71.8 16.7 1.0 52.97 6.51 2.80 36.62 0.05 5260 350 b 2 mm
Silica sand (HCPs) Sauter mean size [µm] Bulk density [kg/m3]
190 1600
CaO for physical mixing Lightly calcined CaCO3 ore Bulk density [kg/m3] Sauter mean size [µm] Surface area [m2/kg] Pore volume [L/kg]
1050 70 (b1.2 mm) 2.0–5.0 0.01
the upper stage was sampled to measure its tar intake and molar composition. Using a fabric filter to capture the tars escaping from our tar trapping system confirmed that the tar trapping efficiency was over 90% in the tests. The major parameters referred to herein were defined as: Xi ¼
Mig TðGas production rateÞ 100k ði ¼ c; hÞ; Mif TðFuel f eed rateÞ
ð1Þ
ge ¼
ðProduct gas HHVÞTðGas production rateÞ 100k; ðFuel HHVÞTðFuel f eed rateÞ
ð2Þ
dry tar weight ; ðSampled gas volumeÞTð1 CAr Þ
ð3Þ
teb ¼
and Tar content ¼
Ar molar concentration in the produced gas. In Eq. (1), Mig and Mif shows the mole of element i (= c, h) in every normal cubic meter of produced gas and every kilogram of fuel treated, respectively. The tested fuel was coffee grounds dried with the method of slurry dewarting in kerosene [9]. Summarized in Table 1 are the major property parameters of the fuel, showing that the fuel was rich in volatile and oxygen. The sizes of the tested fuel were below 2.0mm. In addition to bare coffee grounds, the fuel blended with CaO according to a CaO-to-fuel mass ratio of 1/20 was also tested in the article to investigate the effect of Ca additive. The principle of T-DFBG expects that in this case the Ca additive should have more pronounced effects than in NDFBG. The mass ratio 1/20 adopted here is arbitrary but it is indicative to a certain degree of our intention to keep the ratio as low as possible (much higher ratios being suggested in the literature [10]). The employed CaO was calcined CaCO3 ore (sieved to below 1.2mm), whose characteristics were listed also in Table 1. Silica sand of 190 µm in Sauter mean diameter was used as the heat carrier particles (HCPs). Table 2 summarizes the experimental conditions of all tests referred to in this article, where the ones numbered as T1 and T2 refer to the tests according to T-DFBG and those indexed by N1 and N2 are the tests with N-DFBG. The tests T2 and N2 were for the fuel without Ca addition. All the tests had the similar fuel feed rate F of about 4.0kg/h and were under similar reaction temperatures of 1103 and 1093K in the riser combustor (Tg) and fuel gasifier (Tb), respectively. The superficial gas velocity Ub in the gasifier was around 0.2m/s at the operating bed temperature Tb. However, the two tests following T-DFBG (i.e. T1 and T2) had much higher gas velocity Ug in the riser combustor (5.8m/s against 2.6m/s for N1 and N2). This led to a much higher particle circulation rate for the T-DFBG tests and in turn to a much shorter explicit residence time teb of particles (including fuel) inside the fuel gasifier estimated by
where Gas production rate ¼ ðVolume rate of ArÞ=ð1 CAr Þ: In Eq. (3) the quoted tar content is with respect to the product gas volume free of argon tracer, and CAr refers to the
Particle amount in the gasif ier : Particle amount circulated in unit time
ð4Þ
The quick particle circulation at higher Ug for T-DFBG not only allows the particles in the upper stage of the TFB to be quickly refreshed to realize a better gas upgrading effect but leads as well to a lower teb for gasification reactions in the TFB's lower stage so that the superiority demonstration for TDFBG can be more definite. The particle mount in the gasifier was about 20kg (determined from the pressure drop over the
Table 2 – Condition parameters adopted in the tests reported in the article No.
With a Ca?
F b [kg/h]
S/F [kg/kg]
Tb [K]
Ts [K]
T1 1:20 3.65 1.10 1093 1093 T2 – 4.27 0.93 1090 1093 N1 1:20 3.67 1.31 1095 – N2 – 3.70 0.92 1103 – Argon to gasifer: 10.6 Ln/min N2 to windbox below the seal in face to riser: 3.0 Ln/min Air ratio to 30% C of fuel in riser: ~1.2 for N1 and N2, but 3.0 for T1 and T2. Total load of silica sand: ~ 40 kg for T1 and T2, but ~ 24 kg for N1 and N2. a b
The mentioned ratio refers to the mass ratio of Ca to fuel with moisture. The fuel feed rate refers to the value without Ca.
Tg [K]
Ub [m/s]
Us [m/s]
Ug [m/s]
1093 1100 1103 1113
0.21 0.25 0.25 0.18
0.10 0.10 – –
5.80 5.90 2.65 2.55
teb [s] 110 110 1000 1000
F U E L P R O CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9 ) 1 3 7–1 4 4
141
consequently in their respective constants, which were about 3650kcal/m3n, 85.0%, 78.0% and 110.0%, respectively. The H conversion higher than 100% indicates a fact that a considerable large amount of H is transferred from steam to product gas via steam-involved reactions like steam gasification/ reforming and water gas shift. Fig. 4 compares the attained fuel C (Xc) and H (Xh) conversions, cold gas efficiency (ηe) and tar content in the produced gas free of Ar tracer for the tests performed according to N-DFBG (open bars) and T-DFBG (solid bars). There, Fig. 4a is for the tests T2 and N2 regarding bare coffee grounds and Fig. 4b for T1 and N2 with respect to the CaOblended fuel. Meanwhile, Table 3 presents a comparison for the corresponding product gas compositions and the H2/CO ratios and HHVs estimated from the compositions. The figured and tabulated values for an individual test all refer to the averages over the period in which the test became quasisteady (for instance, between 20 and 70min in Fig. 3). Furthermore, the molar concentrations in Table 3 were the values normalized to the mentioned gas species so that the sum of the concentrations was constantly 100% for all tests. As for a given fuel, either bare coffee grounds (T2, N2) or with 1/20 CaO (T1, N1), Table 2 shows that the tests according to N-DFBG (i.e. N1 and N2) had much longer residence time teb (1000s vs. 110s) and the same or slightly higher S/F ratio and reaction temperature Tb (~ 1093K). Even so, in Fig. 4 the tests with T-DFBG exhibited apparently higher C and H conversions and cold gas efficiency, while their tar contents in the produced gases (free of Ar tracer) were obviously lower. Actual Fig. 3 – Typical time series of (a) molar composition of rude product gas and (b) the corresponding fuel conversion, energy recovery efficiency and HHV for the tests with T-DFBG. The plotted test is the T1 specified in Table 2.
particle bed of the TFB's lower stage), and the particle amount circulated in unit time was measured experimentally at actual running temperature using the method of Xu et al. [11]. In the tests T1 and T2 the temperature of the TFB's upper stage Ts was controlled at a value very close to Tb of the gasifier.
3.
Results and discussion
With the test T1 as an example, Fig. 3 typifies the time series of measured product gas composition (Fig. 3a) and its corresponding performance (Fig. 3b). The similar time series was reported also for the tests according to N-DFBG in Ref. [8]. The displayed test in Fig. 3 lasted for about 70min but in the first 15min there was no measurement for the product gas composition. The data demonstrate a very good stability after 30min of fuel feed, showing that the test's quasi-steady sate should be surely achievable in about one hour. Fig. 3a clarified an unvaried order of molar concentration among all the gas species presenting in the product gas. While H2 content took the top, the other gas species next to it were CO, CO2, CH4, C2H4, C2H6 and C3H6 in succession. The corresponding HHV of the produced gas (right Y), cold gas efficiency ηe, and fuel C and H conversions Xc and Xh (left Y) remained
Fig. 4 – Comparison of fuel conversion and energy recovery efficiency realized with N-DFBG and T-DFBG for (a) pure coffee grounds and (b) coffee grounds mixed with CaO according to a CaO-to-fuel mass ratio of 1/20. Detailed test conditions are in Table 2.
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F U E L P RO CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9) 1 37 –1 4 4
Table 3 – Molar composition (normalized), H2/CO ratio and HHV of product gas corresponding to the performance data plotted in Fig. 4 No. a
T1 T2 N1 N2
Normalized b molar concentration [mol%]
H2/CO
HHV c
H2
CO
CO2
CH4
C2H4
C2H6
C3H6
[–]
[kcal/m3n]
31.2 23.7 25.4 22.1
28.2 36.6 33.7 37.4
17.3 12.4 14.2 12.3
13.6 15.8 15.7 17.8
6.32 7.56 6.68 6.82
3.23 3.84 3.40 3.55
0.06 0.11 0.13 0.09
1.106 0.648 0.754 0.596
3640 3750 3890 3920
a
The test Nos. correspond to those in Table 2. Normalization of the molar concentration was with respect to all the listed gas species so that their concentration values were summed to 100%. c The listed HHV was calculated on the basis of the molar composition of rude product gas with Ar tracer and trace amounts of N2 and O2. b
increases in Xc, Xh and ηe via T-DFBG were about 3.5, 5.0 and 4.0 percentage points for bare coffee grounds (Fig. 4a) and approximately 4.5, 14.0 and 6.0 percentage points for the fuel blended with CaO. Tar content in the product gas decreased by about 7.0g/m3n in both cases. According to Xu et al. [8], more increase in the gasification efficiency and decrease in the tar content of the product gas can be expected, if the residence time of fuel inside the TFB gasifier (now 110s, see Table 2) is similar to that for the N-DFBG case (1000s). Increasing fuel conversion and energy recovery efficiency (into product gas) via T-DFBG may be relative to its suppression of fuel loss via elutriation. Lowering the tar content in the produced gas, however, should be surely due to the enhanced tar reforming or destruction reactions. What is dominant can be clarified by comparing the product gas composition for the two cases. Table 3 shows that for a given fuel the produced gas via T-DFBG had higher H2 but lower CO contents, leading to its higher H2/CO ratio than that via N-DFBG (see T1 against N1, or T2 against N2). As a result, the HHV of the produced gas was more or less lower for T-DFBG. All of these effects were distinctive in the tests with CaO-blended fuel (i.e. T1 and N1). While the test T1 according to T-DFBG had a H2 content (31.2mol%) higher than that of CO (28.2mol%), the test N1 with N-DFBG led to a CO content (33.7mol%) much higher than H2 concentration (25.4mol%). Meantime, all hydrocarbons were more in the gas produced with N-DFBG, whereas the CO2 concentration was higher for T-DFBG. Consequently, both reforming of tars/hydrocarbons and WGS truly occurred to a greater degree in the test T1, even though its steam-to-fuel mass ratio (i.e. S/F) was slightly lower than that for the test N1 (1.10 against 1.31). Responding to this the highest H2/CO ratio (1.106) and lowest HHV (3640kcal/m3n) are shown for test T1 in Table 3. The situation for the tests T2 and N2 with regard to bare coffee grounds was different. Although T-DFBG (test T2) elevated slightly H2 content and lowered CO content of the produced gas, CO2 concentration in the gas remained to have little change and the contents of hydrocarbons (except for CH4) were somehow higher in comparison with those from the test N2. Thus, in this test there should not be greatly enhanced catalytic reforming of tars and hydrocarbons. Correspondingly, enhanced thermal decomposition of tars into higher-C hydrocarbons (tars may be hardly broken into CH4) via T-DFBG can be considered to be responsible for the slightly lower tar and CH4 contents but slightly higher hydrocarbon concentrations in the product gas from T2.
Therefore, the addition of CaO facilitated the upgrading effects of T-DFBG on gasification efficiency and tar elimination. These effects were realized through enhancing the reactions of tar/hydrocarbon reforming and WGS. Indeed, the calcined Ca-base ores have some activity to catalyze such reactions [4,10,12 and Refs. therein]. In T-DFBG the Ca-base additive can maintain its most active state in the TFB's upper stage where the reaction temperature Ts (≈ Tg) is higher than Tb in the lower stage. In N-DFBG, the activated additive in the riser combustor is circulated into the fuel gasifier directly. There, not only the temperature of the active additive is quickly lowered to Tb from Tg (Tb b Tg), but the active sites of the additive may be also quickly blocked by ash and deposited carbon in the contact and interaction of the additive with fuel and tarry pyrolysis gas. Without addition of any catalytic material, the T-DFBG enables gas upgrading merely by providing the produced gas an additional reaction space (i.e. the TFB's upper stage) at a higher temperature Ts. Hence, the available effects should be confined to thermal decompositions of tars and higher-C hydrocarbons. Nonetheless, the tar content in the produced gas from the test T1 was still high. Literature studies shown that physically mixed Ca-base additive may reduce the tar content of product gas to a few g/m3n [4,10]. The additive amount, however, was usually higher than 30wt.% of the bed material, while the additive itself may be other highly active materials (than CaO), such as dolomite, olivine, even well-designed catalysts [4]. Therefore, the low blending ratio (only 1/20) and low catalytic activity of CaO, in addition to the use of coffee grounds fuel that is highly tar-productive, should be responsible for the tar content high as 25 to 40g/m3n shown in our tests. In Fig. 3a both H2 and CO2 concentrations tended to increase slightly, while CO concentration became gradually lower with reaction time. These shown possibly a gradually increased effect of CaO additive that became more with test (because CaO being blended into the tested fuel). Notwithstanding, the adopted CaO ratio of 1/20 enabled a good demonstration of the effect of T-DFBG, as is delineated above. In spite of the preceding merits of T-DFBG, the adoption of a TFB has to increase the system's pressure drop. Depending on the desired particle bed height (usually 300–400mm) for the upper stage, the pressure drop increase may be 3000 to 5000Pa. In actual fluidized bed systems it has no problem to tackle this pressure drop increase. For the T-DFBG a critical issue might be whether its TFB runs stably because the upper-stage distributor may be blocked up via entrained fine particles
F U E L P R O CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9 ) 1 3 7–1 4 4
Fig. 5 – Pressure drop across the upper-stage distributor of the TFB involved in the T-DFBG experimental setup during temperature rise (before time 0) and fuel feed. The data are from the test T1.
during running. In our test this problem did not occur, and Fig. 5 verifies the point through a time series of pressure drop between the freeboard of the TFB's lower stage and a location 50 mm above the upper-stage distributor. The data are from the test T1, and the plotted time zero refers to the onset of fuel feed. Before fuel feed the system was run to raise the bed temperature. Within 2 h the pressure drop remained to have an average that was little changed, indicating the fact that the upper-stage distributor was never blocked up. We had run this T-DFBG facility for several other fuels (including brown coal), and in all cases the TFB's upper-stage distributor remained in its dynamic steady state. During fuel feed there were some instantly high or low pressures, which appeared as sharp pulses in Fig. 5. This shows that some fine fuel particles would be carried with gas to pass through the nozzles (1.4 mm in i.d.) of the upper-stage distributor, but the worried blockage of the nozzles by such particles did never happen.
4.
Conclusions
The study devises the use of a two-stage fluidized (TFB) bed to replace the single-stage bubbling fluidized bed reactor involved in a generally-encountered normal dual fluidized bed gasification (N-DFBG) system. This forms the newly proposed two-stage dual fluidized bed gasification (T-DFBG) process that is anticipated to decrease tar evolution and increase gasification efficiency and H 2 production by enhanced in-bed gas upgrading effect. The anticipations were demonstrated by gasifying coffee grounds (dried in advance to about 10 wt.% water) in two pilot gasification setups constructed according to the principles of T-DFBG and N-DFBG, respectively. At gasifier temperatures of about 1093 K and steam-to-fuel mass ratio of about 1.0, the T-DFBG increased absolutely the fuel C conversion and cold gas efficiency by about 5 percentage points and decreased tar content in the produced gas by 7.0 g/m3n in comparison with N-DFBG, although the explicit fuel particle residence time for the T-DFBG test (110 s) was much lower than that for N-DFBG (1000 s). In relative scale these upgrades are equivalent to about 7% increase in fuel C conversion and 20–25% decrease in
143
tar evolution, and these upgrading effects would be surely further large by running the gasifier of T-DFBG at longer fuel particle residence time. The upgrading effect was more pronounced when CaO additive was blended into the fuel (only 1/20 of the fuel mass in this article). In this case the increase in H conversion was distinctively high and reached 14 percentage points, clarifying that reforming and water gas shift reactions were greatly enhanced via CaO additive. Without Ca additive, the T-DFBG exhibited a similar tendency to upgrade the produced gas but to a lower degree. Analysis revealed that in this case the enhanced thermal decomposition of tars in the upper stage of the TFB is the excluding cause for the realized upgrading effects.
Nomenclature F Mcg Mcf Mhg Mhf S teb Tb Tg Ts Ub Ug Us Xc Xh
fuel feed rate, kg/h moles of C in one normal m3 of product gas, mol/m3n moles of C in 1 kg of treated fuel, mol/kg moles of H in one normal m3 of product gas, mol/m3n moles of H in 1 kg of treated fuel, mol/kg steam feed rate into fuel gasifier, kg/h explicit residence time of fuel in gasifier, s characteristic temperature of fuel gasifier, K characteristic temperature of riser combustor, K characteristic temperature of the upper stage of TFB in T-DFBG, K superficial gas velocity in fuel gasifier at its operating temperature, m/s superficial gas velocity in riser combustor at its operating temperature, m/s superficial gas velocity in the upper-stage bed of TFB at its operating temperature, m/s conversion of fuel C into product gas, % conversion of fuel H into product gas, %
Greek letters cold gas efficiency, % ηe
Acknowledgements The work was conducted during a technical program on upgrading and gasification of high water content biomass financed by The New Energy and Industrial Technology Development Organization, Japan (NEDO). The document work was done by the first author under financial support of National Natural Science Foundation of China (NSFC) under project Nos. of 20606034 and 20776144. The authors are also grateful to Mr. Minoru Asai and Ms. Kumiko Uchida of the same company for their help in the experiment.
REFERENCES [1] A.V. Bridgwater, The technical and economic feasibility of biomass gasification for power generation, Fuel 74 (1995) 631–653. [2] M.A. Paisley, R.D. Litt, K.S. Creamer, Gasification of refuse-derived fuel in a high throughput gasification system,
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[3]
[4]
[5]
[6]
[7]
F U E L P RO CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9) 1 37 –1 4 4
In Energy from Biomass and Waste XIV (P012990), Florida, January, 1990. A. Zschetzsche, H. Hofbauer, A. Schmidt, Biomass gasification in an internally circulating fluidized bed, Proceedings of the Eighth European Conference on Biomass for Agriculture and Industry, vol. 3, 1998, pp. 1771–1777. C. Pfeifer, R. Rauch, H. Hofbauer, In-bed catalytic tar reduction in a dual fluidized bed biomass steam gasifier, Ing. Eng. Chem. Res. 43 (2004) 1634–1640. G. Xu, T. Murakami, T. Suda, Y. Matsuzawa, H. Tani, The superior technical choice for dual fluidized bed gasification, Ind. Eng. Chem. Res. 45 (2006) 2281–2286. G. Xu, T. Murakami, T. Suda, S. Kusama, T. Fujimori, Gasification method and apparatus incorporating gas upgrade for solid fuels, Patent, 2006, Appl. No. PCT/JP2006/ 305785. J. Corella, J. Herguido, F.L. Alday, Pyrolysis and steam gasification of biomass in fluidized bed. Influence of the type and location of the biomass feeding point, in: A.V. Bridgwater, J.L. Kuester (Eds.), Research in Thermochemical
[8]
[9]
[10]
[11]
[12]
Biomass Conversion, Elsevier Applied Science, London, 1988, pp. 384–397. G. Xu, T. Murakami, T. Suda, Y. Matsuzawa, H. Tani, Gasification of coffee grounds in dual fluidized bed: performance evaluation and parametric investigation Energy Fuels 20 (2006) 2695–2704. Y. Mito, N. Komatsu, I. Hasegawa, K. Mae, Slurry dewatering process for biomass, In 2005 International Conference On Coal Science and Technology (ICCS&T), IEA-Clean Coal Center: London, 2005, Paper 2E01. J. Corella, M.P. Aznar, J. Gil, M.A. Caballero, Biomass gasification in fluidized bed: where to locate the dolomite to improve gasification? Energy Fuels 13 (1999) 1122–1127. G. Xu, T. Murakami, T. Suda, Y. Matsuzawa, H. Tani, Particle circulation rate in high-temperature CFB: measurement and temperature influence, AIChE J. 52 (2006) 3626–3630. G. Xu, T. Murakami, T. Suda, S. Kusama, T. Fujimori, Distinctive effects of CaO additive on atmospheric gasification of biomass at different temperatures, Ing. Eng. Chem. Res. 44 (2005) 5864–5868.