Article pubs.acs.org/IECR
Chemical Processing in High-Pressure Aqueous Environments. 9. Process Development for Catalytic Gasification of Algae Feedstocks Douglas C. Elliott,*,† Todd R. Hart,† Gary G. Neuenschwander,† Leslie J. Rotness,† Mariefel V. Olarte,† and Alan H. Zacher† †
Pacific Northwest National Laboratory, P.O. Box 999, MSIN P8-60, Richland, Washington 99352, United States ABSTRACT: Through the use of a metal catalyst, gasification of wet algae slurries can be accomplished with high levels of carbon conversion to gas at relatively low temperature (350 °C). In a pressurized-water environment (20 MPa), near-total conversion of the organic structure of the algae to gases has been achieved in the presence of a supported ruthenium metal catalyst. The process is essentially steam reforming, as there is no added oxidizer or reagent other than water. In addition, the gas produced is a medium-heating value gas due to the synthesis of high levels of methane, as dictated by thermodynamic equilibrium. As opposed to earlier work, biomass trace components were removed by processing steps so that they did not cause processing difficulties in the fixed catalyst bed tubular reactor system. As a result, the algae feedstocks, even those with high ash contents, were much more reliably processed without plugging the feeding systems or the fixed catalyst bed. High conversions were obtained even with high slurry concentrations. Consistent catalyst operation in these short-term tests suggested good stability and minimal poisoning effects. High methane content in the product gas was noted with significant carbon dioxide captured in the aqueous byproduct in combination with alkali constituents and the ammonia byproduct derived from proteins in the algae. High conversion of algae to gas products was found with low levels of byproduct water contamination and low to moderate loss of carbon in the mineral separation step. Further development is required to demonstrate protection of the catalyst bed from sulfur poisoning.
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INTRODUCTION Catalytic hydrothermal gasification of biomass provides a highly efficient pathway to medium-Btu fuel gas. This gas product can be used directly in heat and power applications or has potential to be cleaned to pipeline quality gas. As compressed or liquefied natural gas, it has the potential to displace imported petroleum used in transportation applications. The hydrothermal processing described here utilizes water-based slurries at medium temperature (350 °C) and sufficient pressure (20 MPa) to maintain the water in the liquid phase. The processing option is particularly applicable to wet biomass feedstocks, such as algae. Earlier papers in this series have addressed the processing environment,1 catalyst systems for this environment,2,3 continuous-flow reactor tests with fixed beds of catalyst in a tubular reactor,4 and process development tests with wet biomass feedstocks.5 It has been reported both as a means of recovering useful energy from wet organic wastes and as a water treatment system for wet organic contaminants. Here we report the preliminary results of continuous-flow reactor studies with wet algae feedstocks. This paper describes tests in a bench-scale reactor system, which does not include the heat recovery that is key to an energy efficient process, as described in our other work.4
certain components, like alkaline earths, to allow extended use with catalysts, have also been documented.8 More recently, we have demonstrated more stable catalyst formulations for hydrothermal gasification as described in patents claims.9,10 This article provides additional results of gasification with these improved catalysts using wet algae slurries. Recently algae biomass has received a very high level of interest as a renewable biomass resource for fuels production because of the relatively high growth rates attained.11 The primary focus has been the recovery from the algae of fatty acid triglycerides as a feedstock for biodiesel production. After the recovery of triglycerides, significant biomass is left (sometimes called Lipid-Extracted Algae or LEA), which still contains significant energy. Moreover, not all algae are high fatty acid producers, and those that are must be grown under controlled conditions, which may be less than optimal for rapid growth, in order to maximize fatty acid production. An alternative algae utilization strategy is to grow algae in a wild and/or mixed culture at optimum growth conditions in order to maximize total biomass without consideration of fatty acid production. For both LEA and algae which are not high fatty-acid producers, an appropriate biomass conversion process to utilize such materials without drying is desired to minimize parasitic energy requirements. Hydrothermal gasification can be used in this application for fuel gas production from algae.
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BACKGROUND The use of hydrothermal processing (high-pressure, hightemperature liquid water) has received relatively limited study.6 Although work has been performed with actual biomass, containing mineral components, discussion of the fate of these materials is limited.7 Attempts to pretreat biomass by removing © XXXX American Chemical Society
Received: April 9, 2012 Revised: June 22, 2012 Accepted: July 26, 2012
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dx.doi.org/10.1021/ie300933w | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 1. Process flow schematic of the bench-scale continuous-flow reactor system (CRS).
Bioproducts (NAABB), whose mission is to lay the technical foundations for a scalable, responsible, and affordable renewable biofuels industry based on algae feedstocks.18
Recent reports on hydrothermal gasification of wet algae biomass slurries have so far been limited to batch reactor testing. Minowa12 first reported hydrothermal gasification of microalgae (Chlorella vulgaris) using a nickel metal catalyst, which became noticeably deactivated readily. Vogel’s group also has reported algae gasification, using ruthenium catalyst at supercritical water conditions. A number of strains have been tested including both green algae and cyanobacteria, but the work focused on Spirulina platensis13 and Phaeodactylum tricornutum.14 For these tests “high gasification efficiency” was claimed but reported to be only 68−74% in these 1-h batch tests. The adverse effect of sulfur release from the algae on the Ru catalyst’s performance was demonstrated. The potential effect of leached nickel from the reactor system was evaluated relative to algae growth by tests with 10 ppm nickel and found to be significant. Vogel13 has also reported a means to separate the mineral content from the supercritical processing environment by recovery of a brine. Others have attempted noncatalytic gasification of algae at supercritical water conditions, but reports also are only for batch reactor tests. Tests with microalgae (Nannochloropsis sp) showed carbon gasification ranging from
