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The Production of Hydrogen by Steam Reforming of Trap GreasesProgress in Catalyst Performance Stefan Czernik,* Richard J. French, Kimberly A. Magrini-Bair, and Esteban Chornet National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401 Received March 26, 2004. Revised Manuscript Received July 12, 2004
Most hydrogen is currently produced via the steam reforming of natural gas. An environmentally preferable option is to produce hydrogen from renewable materials. Waste vegetable oil from food processing (“trap grease”) is a low-cost, widely available renewable material that currently has no commercial use. In this work, we produced hydrogen via the fluidized-bed catalytic steam reforming of trap grease using commercial and experimental Ni/Al2O3 catalysts under conditions similar to those of commercial natural gas reforming operationsstemperatures in excess of 800 °C, molar steam-to-carbon ratio of five, and a methane-equivalent volumetric space velocity of ∼1000 h-1. During operation for 150 h, yields of 25 g of hydrogen per 100 g of trap grease were obtained. The process performance decreased with time, because of catalyst attrition and deactivation. We succeeded in extending the catalyst time-on-stream through the use of a laboratory-prepared attrition-resistant catalyst. Another strategy used was a two-step process where the thermal decomposition of grease preceded the catalytic fluidized-bed reforming and pyrolysis vapors instead of the liquid trap grease were fed to the reformer.
Introduction Presently, hydrogen is produced commercially, mostly from natural gas, liquified petroleum gas (LPG), and naphtha, by catalytic steam reforming and from heavy oil fractions by partial oxidation. Consequently, hydrogen production, because it is based on fossil fuels, is a net contributor to carbon dioxide (CO2) emissions and the greenhouse effect. For example, life-cycle analysis of the production of hydrogen in a steam reforming plant based on natural gas with a capacity of 1.5 × 106 Nm3/ day shows that the amount of fossil CO2-equivalent released into the atmosphere to produce 100 kg of hydrogen gas is 1374.5 kg.11 An environmentally preferable alternative is to produce hydrogen from renewable resources (for example, via the thermal conversion of lignocellulosic biomass2). The rationale behind this approach is the fact that the CO2 released into the atmosphere during thermochemical conversion of biomass is offset by the uptake of CO2 during biomass growth. The challenge is to convert those materials into hydrogen at a cost similar to that from existing hydrocarbon-based reforming technologies. Because of the low hydrogen content in biomass, this condition can be met by integrating the production of hydrogen with highervalue biomass-derived byproducts, as discussed elsewhere.3,4 * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Spath, P. L.; Mann, M. K. Life Cycle Assessment of Hydrogen Production via Natural Gas Steam Reforming. Report No. TP-57027637, National Renewable Energy Laboratory (NREL), Golden, CO, 2000. (2) Wang, D.; Czernik, S.; Montane´, D.; Mann, M.; Chornet, E. Ind. Eng. Chem. Res. 1997, 36, 1507.
Another way to improve the economics of the production of hydrogen from biomass is through the use of lowcost feedstocks, especially wastes and byproducts, which also provides additional environmental benefits. We have been studying hydrogen production from a variety of byproducts, such as hemicellulose-rich liquid effluent from wood fractionation or glycerin from biodiesel production.5 This research explores the application of waste grease as a feedstock for producing hydrogen. “Trap grease” that is widely available throughout the country has significant potential for producing hydrogen. It is recovered from two main sources: (i) traps installed in the sewage lines of restaurants and food processing plants (from which it is pumped into trucks) and (ii) wastewater treatment plants, where it flows in through municipal sewage systems. Presently, trap grease is largely treated as a waste stream: for example, grease-trap-pumping companies in the Boston area pay tipping fees of $0.11/gallon for discharging pump trucks at waste processing facilities. The amount of the recovered waste grease is estimated at 13 lbs (∼6 kg) per person per year;6 so far, this material has not found any economically viable application, although the biodiesel industry considers it a potential feedstock for transesterification. Trap grease collected in the United States has the potential to deliver 0.5 million tonnes of (3) Spath, P.; Lane, J.; Mann, M.; Amos, W. Update of Hydrogen from BiomasssDetermination of the Delivered Cost of Hydrogen. NREL Process Analysis Task Milestone Report, April 2000. (4) Chornet, E.; Czernik, S. Nature 2002, 418, 928-929. (5) Czernik, S.; French, R.; Feik, C.; Chornet, E. Ind. Eng. Chem. Res. 2002, 41, 4209-4215. (6) Wiltsee, G. Urban Waste Grease Resource Assessment, 1998 Report prepared by Appel Consultants, Inc., for NREL, Subcontract No. ACG-7-17090-01.
10.1021/ef0499224 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/26/2004
H2 Production via Steam Reforming of Trap Grease
hydrogen annually. Currently, pretreated (dewatered) trap grease is available for $0.04-$0.08/kg. Because >30 kg of hydrogen can be potentially obtained from 100 kg of the grease, the feedstock costs would be only $0.13-$0.26 per kg of hydrogen. With current plant gate prices for hydrogen in the $0.7-$1/kg range, trap grease offers an attractive opportunity for competitive production of hydrogen from renewable resources. In this work, we first attempted to reform trap grease using a commercial steam-reforming catalyst in a benchscale fluidized-bed reactor. This reactor has been successfully used previously to reform several biomassderived liquids.5 Although the commercial catalyst worked well initially, we observed physical attrition of the catalyst and a loss of catalyst from the reactor. Therefore, we developed a custom catalyst prepared on a hardened support and compared its performance to that of the commercial catalyst. We also tested a twostep pyrolysis/reforming configuration that provides an opportunity to better control inorganic contaminants that may poison the catalyst. Results are presented below. Experimental Section Feedstock. Trap grease for our tests was obtained from Pacific Biodiesel, who collected over 40 samples from different sites in the United States. The grease samples contained both saturated and unsaturated C16 and C18 fatty acids, fats, and small amounts of solids.7 Based on the chemical composition, we divided the samples into three groups. Trap grease in the first group (batch 1) had a high content of free fatty acids (>70%) and low ash (35%) and low ash (70% of the stoichiometric potential for 16 h of operation. At 850 °C, S/C ) 3.5, and GC1VHSV ) 1300 h-1, the yield of hydrogen was >80% of the stoichiometric potential and the carbon-to-gas conversion was >99% for 6 h of processing. The longer-duration catalyst performance reforming test was performed on the aforementioned batch 1 washed and filtered trap grease at 850 °C, S/C ) 5, and an initial GC1VHSV value of 970 h-1 (feed rate of 42 g of grease/h). Although the higher temperature promotes cracking of feed to coke, these conditions are more favorable for the removal of carbonaceous material from the catalyst by gasification than lower temperature, less steam, and more feed but are still close to parameters used in commercial reforming. Also, similar to natural gas reforming, a small amount of hydrogen (99%. No carbon deposits on the catalyst were detected by thermoemission electron microscopy. However, out of 280 g of the C11-NK catalyst used at the beginning of the experiment (260 g after reduction), only 65 g remained in the reactor after the test (165 g were recovered from the cyclone and the filter and ∼30 g were not accounted for).9 Such a big loss of the catalystsat the end of the test, the space velocity was almost 4000 h-1swas probably the main reason for the decrease in
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production of hydrogen and the high amounts of hydrocarbons generated by thermal decomposition of grease. NREL Catalyst. Because the large catalyst losses would be too expensive to tolerate in an industrial process, further experiments were conducted under the same processing conditions (850 °C, S/C ) 5, GC1VHSV ) 1000 h-1), using a fluidizable catalyst that was developed at NREL.10 The tests proceeded very smoothly, and the small upsets observed were due to fluctuations of the grease feed rate. Throughout the operation, the conversion of carbon from the grease to gas was 100%, with the overall mass balance closure being 99%. Figure 4 shows the product gas composition obtained by reforming the batch 2 trap grease that was fed as liquid directly to the reformer. During 170 h of operation, the hydrogen concentration in the product gas gradually decreased from 74 vol % at the beginning of the experiment to 64 vol % at the end, which is comparable to tests using the commercial catalyst. However, a significant difference was observed in concentrations of CO and CO2. Using the commercial catalyst, the concentration of CO2 was ∼18% and that of CO was 10% for almost the entire duration of the test. After 3 h on stream of the NREL attrition-resistant catalyst, the concentration of CO2 decreased from 16% to 12%, whereas that of CO increased from 10% to 14% and then remained relatively constant throughout the rest of the test, indicating a significant loss of the water-gas shift activity. The decrease in the concentration of hydrocarbons at 130 h coincides with an interruption of feeding grease to the reactor, resulting in a partial regeneration of the catalyst by steam gasification during that time. However, this effect was short-lasting and catalyst deactivation proceeded rapidly when feeding restarted. The yield of hydrogen, expressed as a percentage of the stoichiometric potential, is presented as a function of time in Figure 5. Overall, 1.29 kg hydrogen was produced from 7.1 kg of waste grease during 170 h of operation, which is 53% of the cumulative theoretical yield. In the first 2 h, the production exceeded 90% of that which was theoretically possible; the production then stabilized at the level of 65% of the stoichiometric potential, before decreasing to 99%, although ∼50 g of carbon, measured as CO2 recovered by burnoff after the test, accumulated in the sand bed, which corresponded to 0.9% of the total feed. Figure 6 shows (10) French, R.; Magrini-Bair, K.; Czernik, S.; Parent, Y., Ritland, M.; Chornet, E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2002, 47 (2), 759.
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Figure 4. Composition of product gas from the reforming trap grease (batch 2) with an NREL catalyst. Conditions are as follows: t ) 850 °C, S/C ) 5, GC1VHSV ) 970 h-1.
Figure 5. Yield of hydrogen produced by the reforming trap grease (batch 2) with an NREL catalyst. Conditions are as follows: t ) 850 °C, S/C ) 5, GC1VHSV ) 970 h-1..
the gas composition, as a function of time, observed during the processing of trap grease (from batch 3), with pyrolysis performed at 450 °C and reforming at 850 °C, using the NREL fluidizable catalyst. In the first 3 h of operation, the hydrogen and CO2 concentrations dramatically decreased, whereas those of CO, CH4, and C2H4 increased. This effect was much stronger than that observed during the previous, single-stage reforming. We believe that this was due to the formation and accumulation of carbon in the pyrolyzer, resulting in a lower feed carbon rate to the reformer. At the same time, the portion reaching the reformer must have been unusually refractory to account for the relatively large percentage of “hydrocarbon slipping” by the reformer. However, after that short period, hydrogen in the product gas started to increase slowly and the concentration of hydrocarbons decreased, indicating steadily improving performance of the catalyst. The rapid growth in the concentration of CH4 and C2H4 at 60 h probably resulted from catalyst oxidation by steam, because of a feed interruption. Also, the hydrogen supply to the bottom of the reactor failed 123 h into the run, causing a shift in the equilibria. Nevertheless, at the end of the test, the concentration of hydrogen was higher (and that of hydrocarbons was lower) than that during the onestep operation, and the downward trend beginning at 120 h was not observed. Overall, 0.98 kg of hydrogen was generated from 6.32 kg of grease during the experiment, which is only 44% of the stoichiometric potential. However, the yield was 54% at the final stage of the process and had a tendency to improve, as shown in Figure 7. This yield would be 72% of that predicted theoretically if CO reacted further in a shift reactor, which is significantly better than that for the reforming of liquid grease using the same catalyst at the same time-on-stream.
Czernik et al.
Figure 6. Composition of product gas from the pyrolysis/ reforming trap grease (batch 3) with an NREL catalyst. Pyrolysis occurred at 450 °C, and reforming occurred under the following conditions: t ) 850 °C, S/C ) 5, GC1VHSV ) 970 h-1.
Figure 7. Yield of hydrogen produced by pyrolysis/reforming trap grease (batch 3) with an NREL catalyst. Pyrolysis occurred at 450 °C, and reforming occurred under the following conditions: t ) 850 °C, S/C ) 5, GC1VHSV ) 970 h-1.
Comparisons. The overall process performances are summarized in Table 1. As the results are considered together, some patterns emerge. We believe that differences between the experiments are not due to the feedstock; despite the differences in the degree of hydrolysis, the feeds were otherwise almost identical. The reforming process operates under sufficiently severe conditions that most bonds can be broken readily, so slight variances in chemical composition should not be significant. One apparent pattern is a decrease in activity during the first several hours of each test, which could be attributed to the catalyst reduction conditions preceding the reforming tests. The rapid reduction in the absence of steam has a tendency to form very fine nickel crystallites, which, although very active, are not physically stable and therefore have a tendency to sinter into larger particles.11 Catalyst deactivation due to steam oxidation is not likely, because of the hydrogen that is fed in the bottom of the reformer.12 The prevailing reason for the unsatisfactory performance in the last phase of the first test is the continuous loss of catalyst from the reactor due to elutriation, leading to an excessive space velocity. At the end of the experiment, the space velocity was four times greater than that observed initially, resulting in an overloading of the catalyst. Experience with the ground catalyst suggests that the losses were spread over the entire (11) Ridler, D.; Twigg, M. V. Steam Reforming. In Catalyst Handbook, Second Edition; Twigg, M. V., Ed.; Wolfe Publishing, Ltd.: London, 1989; pp 244-247. (12) Goodman, D. Handling and Using Catalysts in the Plant. In Catalyst Handbook, Second Edition; Twigg, M. V., Ed.; Wolfe Publishing, Ltd.; London, 1989: p 163.
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Table 1. Results of the Tests catalyst/feedstock
pyrolysis
time (h)
yield of H2 (g H2/100 g feed)
yield of H2 (% stoichiometry)
CH4 (% v/v)
C2H4 (% v/v)
benzene (% v/v)
C11-NK/1 C11-NK/1 NREL/2 NREL/2 NREL/3 NREL/3
no no no no yes yes
0-120 120-212 0-120 120-170 0-120 120-152
25 20 23 19 15 19
74 58 65 54 41 53
1.5 2.3 1.8 2.9 2.4 2.8
0.7 1.4 1.3 2.7 2.6 2.9
0.03 0.08 0.05 0.12 0.04 0.05
duration of the test; however, performance decreased rather rapidly after 135 h on stream. This is probably because the catalyst suddenly reached a critical point where the number of catalytic sites became too few or the residence time of the feed in the bed became too short for effective conversion to occur. There may also have been a flow upset or the catalyst may have been weakened by the regeneration. The custom catalyst performed well, considering that it had much less nickel and much less surface area than the commercial catalyst. However, it was not as active as the commercial catalyst, with respect to the watergas shift reaction. This is of lesser concern for our purpose, because a separate water-gas shift reactor can be added to the process to increase the overall hydrogen yield. More important is the loss of the reforming activity, which can be observed in Figure 3 as a sharp increase in the concentration of hydrocarbons after 120 h on stream, which, again, is very similar to the behavior of the commercial catalyst. At this point, it is not clear how much of the loss in performance is due to attrition, sintering, coking, or inorganic poisons. Preliminary analyses (transmission electron microscopy (TEM), inductively coupled plasma (ICP) spectroscopy) of the catalyst from run 1 did not show any significant changes in the catalyst. Chemical analysis of the catalyst from run 2 showed a significant increase in phosphorus (400 ppm), which is a known component of biomass and (as phosphates) a food additive. ICP analyses of the beds from the third run gave 4.6% iron, 2.9% calcium, 1.2% phosphorus, and 1.3% sulfur in the sand from the pyrolysis reactor on a carbon-free basis, compared to much lower levels in the fresh sand. Also, although phosphorus was not detected in the catalyst (800 °C, steam-tocarbon (S/C) ratio of 3-5, methane-equivalent space velocity (GC1VHSV) of 900-1200 h-1. Under such conditions, 100% of the grease was converted to gas. The hydrogen yield during the first 120 h of operation was very good: the highest value was 25 g per 100 g of grease, which is 74% of that possible for stoichiometric conversion. This yield could increase to 28 g per 100 g of grease if a secondary water-gas shift reactor followed the reformer. (2) Physical loss of catalyst and poisoning by inorganic contaminants such as phosphates have been identified as likely sources of loss in performance after 100 h of operation. The fluidizable catalyst prepared in our laboratory was less active than the commercial catalyst, because of significantly lower surface area and nickel content. However, it was resistant to attrition and was not entrained from the reactor during the tests. (3) The combination of washing and filtering the feedstock, along with a two-step process (pyrolysis, followed by reforming) protects the catalyst from inorganic contaminants, which leads to higher yields at 150 h. The accumulation of carbon (
