Article pubs.acs.org/EF
Syngas Production by Combined Biomass Gasification and in Situ Biogas Reforming Katarina Åberg,* Linda Pommer, and Anders Nordin Energy Technology - Thermal Process Chemistry, Department of Applied Physics and Electronics, Chemical Biological Centre, Umeå University, 901 87 Umeå, Sweden ABSTRACT: For small- to medium-sized streams of biogas (methane) produced at a biorefinery site where cost-efficient reforming by traditional methods are unavailable, combined biomass gasification and methane reforming could facilitate coconversion and increase the H2/CO ratio in the syngas from the gasification plant. In the present work, co-gasification of biomass with CH4 was evaluated by means of a parametric chemical equilibrium study for both wood/CH4 and black liquor/CH4 feedstocks and bench-scale fluidized-bed gasification experiments for a wood/peat/CH4 fuel mixture. The parametric study indicated that high-temperature, and steam and oxygen addition all facilitate a high conversion rate, i.e., methane reforming. Evaluating the influence of the gasification temperature on CH4 reforming and increasing the H2/CO ratio experimentally demonstrated that high temperatures are required for efficient co-conversion.
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INTRODUCTION The concept of a biorefinery is to upgrade and refine biomass feedstock to a variety of value-added end products, such as chemicals, fuels, and energy, using different conversion processes and integration of energy and resources. Pulp mills have several prerequisites to become successful biorefineries, such as established biomass feedstock routes, economy of scale, and a large number of byproduct streams,1,2 several already used for energy purposes or refined to commercial products. The main byproduct stream black liquor, a partly processed biomass that is presently converted to steam and electricity in a recovery boiler, could possibly be refined to more valuable products by means of gasification for syngas production. Furthermore, in addition to using black liquor feedstock, gasification of wood residues and other biomass byproduct streams has significant potential to increase the overall product output from the biorefinery. Syngas, a mixture of mainly hydrogen and carbon monoxide, is a major intermediate in chemical synthesis for production of several valuable petrochemicals and fuels, such as synthetic natural gas (SNG), methanol, dimethyl ether (DME), Fischer− Tropsch diesel, and ammonia. The required composition of the syngas differs for the various synthesis routes and is, thus, dependent upon the end product, particularly with regard to the H2/CO ratio. In general, a ratio in the range of about 1.0− 2.0 is required, often contingent upon whether the water-gas shift (WGS) reaction (CO + H2O ⇌ CO2 + H2) is an active additional reaction in the synthesis. During direct air/oxygen blown gasification of solid feedstocks, the H2/CO ratio in the syngas is usually in the range of 0.7−1.13 and requires subsequent upgrading in a separate WGS process, increasing both capital and operating costs for the plant. Thus, there are incentives to achieve direct production of a syngas with suitable and flexible H2/CO, resulting in increased efficiency and cost reduction by avoiding additional upgrading units.4 One way could potentially be to increase the hydrogen input in the feedstock mix by adding fuels with high relative hydrogen content, such as biogas. © 2015 American Chemical Society
Biogas, containing mainly methane, is frequently produced from pulp and paper industries via wastewater treatment plants as a high-energy but low-bulk byproduct. In Sweden, the biogas is often insufficiently exploited as either combustion fuel at high heat demand periods (winter) or merely flared.5,6 The conventional reforming of methane (from natural gas) to syngas is a well-developed industrial process and can be summarized in three processes: steam reforming (CH4 + H2O → CO + 3H2), partial oxidation (CH4 + 1/2O2 → CO + 2H2), and CO2 reforming (CH4 + CO2 → 2CO + 2H2). Steam reforming7 is the most commonly used method with a high conversion rate (>95%), but requires elevated conditions with regard to the temperature and pressure. Partial oxidation may operate both with8,9 or without a catalyst.10 However, the catalyst used is prone to carbon deposit and deactivation, and for reforming without the use of a catalyst, severe conditions with high pressure and temperature are required and the reported conversion rate is lacking. CO 2 reforming 11 unfortunately suffers from a greater risk for carbon deposition, followed by swift catalyst deactivation. While all routes produce a syngas with sufficient (or too high in the case of steam reforming) H2/CO ratios, the reforming systems are capital-intensive with high energy consumption and, therefore, not economical for small- to medium-sized feedstock streams. In recent years, the concept of co-gasification of coal and methane has been evaluated as either a way to reform unused low-quantity or -quality methane resources, an attempt to increase the H2/CO ratio in the syngas from coal gasification, or both.4,12−15 For biomass, Palumbo et al.16 evaluated hightemperature steam-only co-conversion of biomass and methane in an indirect entrained flow reactor, determining the temperature as the most influencing factor with regard to the Received: February 23, 2015 Revised: April 25, 2015 Published: May 12, 2015 3725
DOI: 10.1021/acs.energyfuels.5b00405 Energy Fuels 2015, 29, 3725−3731
Energy & Fuels
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syngas composition. In air-blown gasification, the necessary reactants (O2, CO2, and H2O) are present in a thermal reactive environment, making gasification a potentially suitable process for methane reforming. Previous work for coal demonstrated that char particles from the solid feedstock provide a catalytic effect on the CH4 reforming; however, the effect of coal ash is harder to determine but may potentially catalyze the CH4 reforming.13,14,17 Because biomass has a more catalytic ash than coal as well as carbon present, the co-gasification of biomass and methane may facilitate cost-efficient reforming at lower temperatures. Therefore, fluidized-bed gasification may provide sufficient conditions for successful co-gasification, making the determination and evaluation of the methane conversion during real, non-equilibrium conditions possible. Fluidized-bed gasification requires operation below the softening and melting temperatures for the biomass ash to avoid bed agglomeration, extensively studied in previous work.18−20 The upper temperature limit of the technology is therefore determined by the choice of fuel and its ash composition, but the common temperature range is 800−900 °C. As a result, the formation of tars and their subsequent removal from the syngas are one of the major issues with the fluidized-bed gasification technology.21 In an integrated biorefinery, co-gasification with black liquor or biomass and small stream(s) of locally generated biogas from waste treatment or other sources appear promising (Figure 1).
Article
MATERIALS AND METHODS
An initial parametric chemical equilibrium study was performed to identify feasible and non-feasible operation settings for co-gasifying wood and black liquor with methane to achieve simultaneous gasification and methane reforming. For each considered parameter, a low and high value were selected (Table 1) and an equilibrium calculation was performed using the software and database FactSage, version 5.2, for every possible combination of settings and fuel types.
Table 1. Design Matrix for the Chemical Equilibrium Parametric Study for Both Wood and Black Liquor temperature (°C) pressure (bar) steam addition (%w) ERa CH4 addition (%en) fuel type a
low
high
800 1 0 0.3 2 wood
1300 30 67 0.5 20 black liquor
ER = equivalence ratio.
The fuel compositions for wood and black liquor presented in Table 2 were used as reactants in the chemical equilibrium calculations. In
Table 2. Input Fuel Analysis Data for Chemical Equilibrium Calculations (Wood and Black Liquor) and Experimental Runs (Wood/Peat) chemical equilibrium fuels wood moisture (%w) ash (%w) C (%DS) H (%DS) O (%DS) N (%DS) S (%DS) Cl (%DS) Si (%DS) Al (%DS) Fe (%DS) Mg (%DS) Ca (%DS) K (%DS) Na (%DS) P (%DS) sum HHV (MJ/kg of DS)
Figure 1. Simple schematic overview of a potential process chain in an integrated biorefinery.
Such a system has several potential advantages: (1) refinement to valuable commodity chemicals or fuels for the biogas byproduct, (2) reforming of methane at less severe conditions than commercial technologies because of the catalytic environment in the gasifier, and (3) increase in the H2/CO ratio of the syngas produced from solely black liquor or biomass gasification. The objective of the present work was, therefore, to evaluate the co-processing of methane and biomass/black liquor in direct gasification. This was achieved by means of (1) a chemical equilibrium study for co-gasification of black liquor or woody biomass feedstock with methane, (2) identifying suitable process settings for co-gasification based on the chemical equilibrium study, and (3) bench-scale screening experiments in a fluidized-bed gasifier. The process was evaluated with respect to the total methane reforming and syngas quality and composition. Although biogas contains other compounds than methane (particularly CO2), the scope of the present work was focused on the methane reforming and, therefore, pure CH4 rather than a biogas mixture was evaluated. Fluidized-bed gasification was selected as the experimental bench-scale technology to enable the study of an extended temperature range from low to medium gasification process temperature (facilitated by use of intelligent fuel mixing to increase the agglomeration temperature).
3.23 48.9 5.9 39.5 1.0 0.21 0.11 0.956 0.207 0.618 0.075 0.744 0.365 0.043 0.112 98.74 19.87
experimental fuel
black liquor
wood/peat
25.2 20.86 36.4 3.5 34.3 0.1 4.8 0.24
10.2 1.53 51.2 6.0 40.7 0.5 0.05 0.043 0.381 0.132 0.381 0.0372 0.426 0.0639 0.0213 0.044 99.98 20.81
2.02 18.6 99.96 13.92
addition to varying the process parameters temperature, pressure, and oxygen equivalence ratio (ER), steam and methane addition were included. Steam was either not added (low level) or added corresponding to 50% fuel moisture content and an additional equal amount of excess steam (high level). Methane was added at 2 and 20% on an energy basis. The gasification and methane co-processing experiments were performed in an atmospheric bench-scale bubbling fluidized-bed gasifier (30 kW) for a wood/peat pellet mixture (Table 2) to study the influence of the temperature on the methane conversion rate. The bench-scale gasifier has been described in detail in previous work,22 and therefore, only a brief description is given in the present paper. The addition of peat to the wood feedstock was made to avoid potential bed agglomeration at the highest temperature settings, because peat has previously been demonstrated to increase the agglomeration temperature in a fuel mixture.23 The fluidized-bed 3726
DOI: 10.1021/acs.energyfuels.5b00405 Energy Fuels 2015, 29, 3725−3731
Article
Energy & Fuels Table 3. Degree of Methane Reforming (CH4 ref.) Based on the Parametric Chemical Equilibrium Calculations wood
black liquor
pressure (bar)
temperature (°C)
steam (%w)
ERa
CH4 ref. (%), + 2%en
CH4 ref. (%), + 20%en
CH4 ref. (%), + 2%en
CH4 ref. (%), + 20%en
1 30 1 30 1 30 1 30 1 30 1 30 1 30 1 30
800 800 1300 1300 800 800 1300 1300 800 800 1300 1300 800 800 1300 1300
0 0 0 0 0 0 0 0 67 67 67 67 67 67 67 67
0.3 0.3 0.3 0.3 0.5 0.5 0.5 0.5 0.3 0.3 0.3 0.3 0.5 0.5 0.5 0.5
98.3 72.5 100.0 99.7 99.9 63.5 100.0 100.0 100.0 79.5 100.0 100.0 100.0 93.1 100.0 100.0
96.5 70.5 100.0 99.4 99.8 64.6 100.0 100.0 99.9 74.8 100.0 100.0 100.0 89.5 100.0 100.0
99.2 77.5 100.0 99.9 99.9 74.1 100.0 100.0 100.0 90.9 100.0 100.0 100.0 97.7 100.0 100.0
98.4 75.1 100.0 99.8 99.9 69.2 100.0 100.0 100.0 87.5 100.0 100.0 100.0 95.9 100.0 100.0
a
ER = equivalence ratio.
Figure 2. Results from the parametric study for wood for 20%en CH4 addition: (a) influence of the temperature at 1 bar pressure, (b) influence of the steam addition at 1 bar pressure, (c) influence of the ER at 1 bar pressure, and (d) influence of the pressure at 800 °C temperature. diameter of >10 μm before entering the sampling section of the gas channel. After the gas sampling section, the syngas was passed through a burner, where an excess of air ensured complete oxidation of the combustible gases. The flue gas was then cooled and passed through a wet scrubber before entering the stack. For each experimental setting, a reference experiment was performed excluding the addition of methane. This was carried out
reactor is built of stainless steel (253MA) with a height of 2.4 m and an inner diameter of 100 mm in the fluidized-bed section and 220 mm in the freeboard section. The distribution plate at the bottom of the fluidizing bed is made of stainless steel with 90 holes (1% open area). A total of 1100 g of quartz sand (>98% SiO2) of the size fraction of 200−250 μm was used as bed material. After the freeboard section, the syngas is led through a cyclone to remove particles with a cut-size 3727
DOI: 10.1021/acs.energyfuels.5b00405 Energy Fuels 2015, 29, 3725−3731
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Table 4. Results from Bench-Scale Gasification of Wood/Peat Pellets (Ref) and Co-gasification Introducing Methane (+CH4)a 800 °C
experiment run actual temperature (°C) actual ERb fuel feed (kg/h) CH4 flow (N L min−1) CH4 addition (%en) primary air flow (N L min−1) dry gas composition (%v) CO H2 CH4 CO2 C2H2 C2H4 C2H6 N2 Ar syngas flow, dry (N L min−1)c a
850 °C
900 °C
950 °C
1000 °C
(Ref)
+CH4
(Ref)
+CH4
(Ref)
+CH4
(Ref)
+CH4
(Ref)
+CH4
805 0.330
803 0.330
849 0.335
852 0.348
901 0.335
900 0.342
950 0.330
957 0.330
1003 0.334
1000 0.330
1.656
1.757
39.92
3.29 19.9 48.86
15.45 14.25 3.00 13.08 0.04 0.92 0.08 53.16 0.64 69.3
13.31 12.0 5.59 12.93 0.03 0.81 0.07 54.67 0.65 79.6
1.781
42.71
3.61 19.8 55.9
17.27 13.99 3.26 12.18 0.06 0.77 0.04 51.98 0.62 73.0
12.14 10.39 5.35 12.97 0.06 0.69 0.02 57.66 0.69 86.7
1.751
39.95
3.65 20.2 56.9
18.59 14.99 3.54 11.28 0.09 0.44 0.01 50.8 0.61 72.5
14.07 11.96 4.70 11.98 0.10 0.42 0.01 56.32 0.67 89.7
1.850
41.87
3.54 19.4 54.91
44.83
3.78 19.9 55.89
18.97 15.29 3.38 11.06 0.09 0.22 0.01 50.56 0.60 73.8
16.58 14.09 3.61 10.3 0.10 0.20 0 54.41 0.65 89.8
19.24 15.63 2.82 10.3 0.04 0.06 0 51.33 0.61 79.4
18.53 17.15 2.93 9.0 0.06 0.07 0 51.44 0.62 96.3
The syngas data were based on the average of three samples. bER = equivalence ratio. cCalculated by mass balance.
CH4 reforming = 1 − nCH4,syngas /nCH4,input
to be able to estimate the amount of methane in the synthesis gas formed solely through gasification of the wood/peat pellets. Dry air was used as the gasification agent and fluidizing medium at a flow rate of ∼0.042 N m3 min−1 for the reference cases and ∼0.055 N m3 min−1 for the co-gasification runs. The fuel was gasified at a bed and freeboard temperature varied from 800 to 1000 °C by increment steps of 50 °C for each experiment. The temperature was maintained by a combination of preheated air (varied between 365 and 512 °C, with an average of 495 °C), electrical wall heaters in the freeboard and upper bed section, and the heat generated through partial combustion of the fuel. The fuel was added at the top of the fluidized bed together with a small nitrogen flow (0.005 N m3 min−1) to reduce back-flush problems. The oxygen ER was set to 0.33 for all experiments. The varied factor in addition to the temperature was the introduction of methane at the bottom of the fluidized bed at either 20% on an energy basis or 0% for the reference cases. All experiments were performed in a consecutive sequence, initiated with the 800 °C reference run and ending with the 1000 °C, +CH4 run. The syngas was sampled immediately after the cyclone using a small sampling probe (2 cm diameter) and with the sampling line heated to 250 °C to reduce condensation of tars. Before each sampling period, the sampling probe was carefully flushed with nitrogen to remove soot and other particles accumulated on the probe surface during continuous gasification. The syngas samples were collected in gastight Teflon bags (1 L) after passing through two parallel quartz filters and a water-cooled condenser to remove retained particles and tars, respectively. The sampling time was 100 s with an average flow rate of 0.23 N L min−1, and three replicates were performed for each experimental setting. The samples were analyzed within 2 days using a gas chromatograph equipped with a thermal conductivity detector [Varian CP-3800 Permanent Gases Analyzer, gas-valve injection, packed column configuration] for H2, O2, N2, CO, CH4, CO2, C2H2, C2H4 and C2H6. The carrier gas for the permanent gas channel (Ar, O2, N2, CO, CO2, CH4, C2H2, C2H4, C2H6, and H2S) was He, and for the hydrogen analyzer (H2), the carrier gas was N2. The column used for separation of Ar/O2, N2, CO, and CH4 was a Molsieve 13X (CP81071). For CO2, C2H4, C2H6, C2H2, and H2S separation, Haysep T (CP81072) and Haysep Q (CP81073) columns were used. The methane reforming was calculated on a molar basis as
CH4 reforming = 1 −
where nCH4,syngas is the amount of CH4 detected in the syngas for the co-gasification runs/calculations, nCH4,biomass is the amount of CH4 produced from solely biomass gasification during the experimental reference runs, and nCH4,input is the added amount of CH4 during cogasification. The molar amount of CH4 in the syngas was calculated from the syngas composition analysis for each experimental setting and a mass balance based on the fuel and gas compositions. Approximations made were that no other hydrocarbons, except for CH4, C2H2, C2H4, and C2H6, were formed, all feedstock hydrogen not accounted for in the gas samples formed H2O, and no losses of carbon because of char formation were included.
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RESULTS AND DISCUSSION The results from the parametric chemical equilibrium study for co-gasification of wood or black liquor with methane are summarized in Table 3. Complete CH4 reforming (100%) was predicted for more than half of the parametric runs. The difference in the CH4 conversion rate with 2 and 20% added CH4 on an energy basis was relatively small at equilibrium and follows the same trend for both feedstocks. For all calculations, the CH4 conversion rate was slightly higher for black liquor compared to wood because of higher amounts of catalytic alkali components present. The influence of the temperature, ER, pressure, and addition of steam are illustrated in Figure 2 for wood feedstock and 20%en CH4 addition. It is clearly demonstrated that, because of the thermal instability of CH4, a higher temperature results in a higher degree of CH4 reforming. At 1300 °C, the CH4 reforming is complete at 1 bar pressure, regardless of other settings (Figure 2a), and below 100% for only one calculation (30 bar pressure and ER of 0.3; see Table 3). The addition of steam and more oxygen available (ER) follow the same trend as the temperature with a positive effect on the CH4 reforming (panels b and c of Figure 2, respectively). Figure 2d illustrates the negative influence of increased pressure, by far the most influential parameter, on the CH4 conversion rate at 800 °C. The equilibrium partial pressure of CH4 increases with increasing total pressure, favoring the equilibrium toward the
nCH4,syngas − nCH4,biomass nCH4,input
(2)
(1)
for the experimental runs and for the chemical equilibrium study as 3728
DOI: 10.1021/acs.energyfuels.5b00405 Energy Fuels 2015, 29, 3725−3731
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Energy & Fuels
indicate that the fluidized-bed gasification operation temperature range is insufficient for effective methane reforming, because only the highest evaluated gasification temperatures (950 and 1000 °C) demonstrate a degree of reforming higher than 75%. The H2/CO ratio was significantly improved by the addition of CH4 to the gasifier at all evaluated temperatures, except at 800 °C, when only a small improvement can be observed. The relative increase in the H2/CO ratio by methane addition was moderately constant at 850−950 °C but increased at the highest experimental temperature of 1000 °C, corresponding to the highest CH4 conversion rate of 84.3%. To determine the contribution of methane reforming into the main syngas components, the molar yields for the major non-inert gas components for the entire experimental temperature range with and without introducing CH4 were evaluated (Figure 4). For the desired constituents of syngas, H2, and CO, the yields predictably increase with the temperature, while a slight decrease is observed for CO2 and CH4 for both reference and CH4 addition runs. The introduction of CH4 appears to have no or a negative effect on the yields of both H2 and CO at temperatures up to 850 and 900 °C, respectively, but a positive influence at the higher temperatures. Correspondingly, the CO2 and CH4 yields increase with the addition of CH4 for the entire temperature range but most significantly for temperatures up to 900 °C. At higher temperatures, the disparity steadily decreases for each temperature setting. However, the reduced difference with the temperature between the reference runs and +CH4 runs together with the increased yield for H2 and CO indicate that more and more CH4 has been reformed to H2 and CO with increased temperature. Also, a significant amount of CH4 passes through the reactor without reforming, although the amount decreases with the temperature. There is a significant difference between the experimental CH 4 reforming results at 800 °C compared to the corresponding chemical equilibrium calculations (Table 3), indicating that, at 800 °C, the process did not attain chemical equilibrium. To evaluate the deviation from equilibrium at the higher temperatures, the hydrocarbons, except CH4, measured in the syngas were plotted as a function of the gasification temperature (Figure 5). At the corresponding equilibrium calculations (1 atm pressure and ER of 0.33), the yields of C2H2, C2H4, and C2H6 all approach zero (
