Biomass- and waste-derived gasification processes are advantageous in many ways, providing syngas streams for low-carbon energy, renewable fuels and chemical precursors.
However, the complexity of the biomass and waste feedstock is a concern due to the presence of a variety of byproducts and impurities, including tar, ammonia, and sulfur. Tar is a complicated mixture consisting of condensable hydrocarbons and has a high boiling point of 180-350°C, which causes polymerization and condensation of the material in heat exchangers, exit pipes, or particulate filters. This results in choking and attrition, eventually lowering system efficiency and raises process cost.
An economical and efficient catalytic reforming approach developed by Nexceris engineers can address this problem. The conversion of tar into syngas by the TARGET™ catalysts removes the problematic tar as a condensable species. TARGET™ catalysts provide a more convenient and energy efficient approach in comparison with liquid scrubbing, and increase the fuel value by transforming light hydrocarbons and tart into syngas (CO and H2).
Performance of TARGET™ Catalysts
Various tests were carried out to demonstrate the efficiency of the TARGET™ catalysts in the destruction of tars under different operating conditions.
Testing on the granular catalyst of 35-60 mesh size was carried out in a tubular reactor to reform oak wood gasification products. Toluene was employed as a probe for ethylene represented olefins and tar compounds. The gas hourly space velocities used were 10,000-24,000 ml/g-hr. After loading the catalyst, the next step was to heat it to 750-850°C in the flow of the biomass gasification gas. A mass spectrometer was used to analyze the feed gas and the product.
Washcoating the catalyst on a cordierite monolith with 400 CSPI channel density was carried out to convert tar, light hydrocarbons, and methane into syngas in a simulated biomass gasification product stream. Naphthalene was used as a model compound. The reaction conditions were temperature = 900°C and GHSV = 10,000 h-1. A gas chromatograph (GC) was used to analyze the gas composition.
As shown in Figure 1, the use of the granular TARGET™ catalyst in the reformation of the oak wood gasification product helped to achieve a complete tar conversion at 800-850°C. The catalyst delivered a consistent performance throughout testing. There was no change in the reforming activity on stream in 600 hours in the presence of sulfur (Figure 2).
A pre-reduction process is not required for the Nexceris’ TARGET™ catalyst, compared to other conventional Ni- based reforming catalysts, due to its ability to self-activate in flowing biomass gasification at a temperature of 800-850°C before operation.
Figure 1. Tar reforming activity on granular TARGET™ catalyst under the conditions of 41.5%H2O, 20.0%N2, 13.5%H2, 9.6%CO2, 7.7%CO, 5.8%CH4, 1.9%C2H4, 880 ppm toluene, 20 ppm H2S and GHSV = 24,000 ml/g-h.
Figure 2. Preliminary lifetime on granular TARGET™ catalyst under the conditions of 41.5%H2O, 20.0%N2, 13.5%H2, 9.6%CO2, 7.7%CO, 5.8%CH4, 1.9%C2H4, 880 pm toluene, 20 ppm H2S and GHSV = 10,000 ml/g-h.
Figure 3 shows the reforming activities of hydrocarbons on TARGET™-washcoated monoliths. The conversion of C2+ hydrocarbon and methane was 87% and 20%, respectively, in the presence of H2S. The concentration of hydrogen in the product was 35%, whereas the ratio of H2/CO was roughly 1.8. For the C2+ hydrocarbons, the absence of propane and propylene in the product indicates complete conversions. The conversion of ethane and ethylene was roughly 85%.
Figure 3. Reforming of biomass gasification products on a TARGET™ catalyst-washcoated cordierite monolith in the presence of H2S under the conditions of 900°C, GHSV = 10,000 h-1, 14.4% H2, 10.4% CO, 9.8% CO2, 5.8% CH4, 13.2% N2, 44.6% H2O, 1.6% C2H6 and C2H4, 0.2% C3H8 and C3H6, 30ppm naphthalene (when used) and 22ppm H2S.
From this data, the reforming activities of light hydrocarbons lowered in the following order: C3 hydrocarbons (100%) > C2 hydrocarbons (85%) > methane (20%) on the TARGET™ catalyst- washcoated monolith. The catalytic performance remained unchanged during the addition of 30 ppm naphthalene as a model tar compound to the feed for 165 hours. The mass spectrometer also didn't detect naphthalene in the exhaust. Deactivation was not observed while testing the catalyst in the presence of H2S for 500 hours (Figure 3).
After testing the TARGET™ catalyst-washcoated monolith for 1000 hours, the next step was adding 1000 ppm NH3 to the feed without changing the other conditions. The results showed that the catalyst performance remained unchanged even after the addition of NH3 (Figure 4). The removal of H2S from the feed stream was carried out at 1145 hours, but the concentration of NH3 was maintained at 1000 ppm. The conversion of hydrocarbon and methane increased to 100% and more than 95%, respectively (Figure 4).
Figure 4. Reforming of biomass gasification products on a TARGET™ catalyst-washcoated cordierite monolith in the presence of NH3 and H2S under the conditions of 900°C, GHSV = 10,000 h-1, 14.4% H2, 10.4% CO, 9.8% CO2, 5.8% CH4, 13.2% N2, 44.6%H2O, 1.6% C2H6 and C2H4, 0.2% C3H8 and C3H6, 1000ppm NH3 and 22ppm H2S (when used).
In the product, there was an increase in the H2 concentration to 53% and the ratio of H2/CO was roughly 2.7. The catalytic performance was almost the same as it was in the absence of ammonia and sulfur. The rapid increase in methane and C2+ hydrocarbon conversions indicates the reversible degradation effect of H2S on the catalyst. The catalyst activity was rapidly recovered upon the removal of H2S. This monolith catalyst did not show any deactivation even after testing for more than 2000 hours in the presence of sulfur.
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A unique combination of design advantages are offered by TARGET™ to system developers. The easy thermal integration of the catalyst with gasifier systems is another advantage over other tar collection methods, which need gas stream cooling for effective operation. Sulfur tolerance of this catalyst completes its design advantage, as the sulfur removal step can be carried out after tar removal. The TARGET™ shows high activity in the reformation of light hydrocarbons to syngas, and has the ability to self-activate in flowing biomass gasification gas at 700-850°C before the operation. These design features provide unique value to system designers. The TARGET™ catalysts enable developers to address a major obstacle in the commercialization of biomass gasification process, allowing an efficient use of biomass for the production of power, valuable chemicals, and liquid fuels.
Nexceris’ TARGET™ catalyst can transform hydrocarbons and tar into syngas at elevated temperatures by CO2 reforming and steam reforming. The catalyst remains unchanged in the presence of ammonia and sulfur, enabling complete tar conversion during reforming of oak wood gasification product at 800-850°C. The catalytic performance of washcoated monoliths remains unchanged even after more than 2,000 hours. With a range of design features, the TARGET™ catalysts can be easily integrated into gasification systems.
This information has been sourced, reviewed and adapted from materials provided by Nexceris, LLC.
For more information on this source, please visit Nexceris, LLC.