Is biomass gasification the answer to biomass conversion? Biomass is a rich salad of chemicals that contains many highly valuable materials that are precursors to fine chemicals, nutrients, and pharmaceuticals. But this strength is also a weakness of biomass. The assortment of materials in biomass is bound together in a complex, stable structure in varying amounts. Processing biomass to generate higher value products comprises disentangling and modifying the useful fragments in a series of complex and costly steps. The challenge in biomass upgrading and utilization is to balance the value of the chemicals produced against the cost of the feed and its transformation.
Biomass conversion can be viewed in terms of the extent of the transformation of its chemical structure. Very little processing is required to use biomass as food, as a construction material, or as a combustion fuel. With just a bit of transformation, biomass can be a source of cellulose fibers and paper on a grand scale. In some cases, valuable finished products, like the nutritional supplemental omega-3 fatty acids EPA and DHA, may be extracted directly from biomass. Biochemical transformations under mild conditions, such as fermentation, are capable of producing ethanol, butanols, etc.
The most extreme transformations of biomass produce gases. Bio-gas forms slowly by simply allowing the biomass to sit under anaerobic conditions, as in a landfill. Bio-gas is roughly a 60/40 mix of methane and CO2, with small amounts of impurities, so it can be substituted for natural gas with only modest separation.
A different sort of gas can be produced from biomass by gasification. In the gasification, process biomass is converted to syngas (CO, H2, CO2, and a few impurities) at high temperatures. Bio-syngas can be converted to methanol, DME, or FT products via the same commercial processes practiced at a massive scale in the monetization of coal or natural gas.
Utilizing biomass as gas allows one to sidestep many of the limitations of biomass. Gas mixtures are easier to handle than solids and have fewer corrosion, stability, or fouling problems compared to complex liquid bio-oil mixtures. Established purification methods are available that can be adapted to clean or adjust the composition of bio-derived syngas. A host of companies are developing technologies to convert bio-derived gases to liquids, including Synthesis Energy Systems, Primus Green Energy, Sundrop Fuels, Maverick Synfuels, Siluria, INFRA, Oberon, GTI, and Velocys.
Like coal gasification, biomass gasification operates at high temperatures (> 700 ° C) with steam or air or oxygen, and can be tuned to make “synthetic natural gas” (SNG), i.e. methane, or synthesis gas (CO, H2, CO2).
Several processes occur in a gasification reactor. Solid biomass enters the upper part of the reactor and descends as a slowly moving mass. Air can be introduced with the biomass or further down the reactor. Drying, pyrolysis, combustion, and reduction take place in succession as the material descends. At the bottom of the reactor the ash is allowed to fall to the reactor bottom and the produced gas is removed as a side stream. Other gasification reactor designs include up-flow gasifiers, fluid bed gasifiers, and circulating fluid bed gasifiers.
In the drying zone of the gasifier, steam is evolved from the moisture in the biomass and by dehydration reactions, and in the pyrolysis zone, volatile organic molecules vaporize from the solid char. The volatiles and char move into the oxidation and reduction zones where they react with gasifying agents to produce product gases. Air, steam, carbon dioxide, and/or pure oxygen are used depending on the requirement of the downstream application. Using air in the gasifier generates a producer gas that is diluted about 50% with nitrogen, so its energy content is only about 1/8th that of natural gas. Processes that use steam require a great deal of energy in order to drive the reactions, thus reducing thermal efficiency, although increasing the H2 concentration. Carbon dioxide can act as a gasifying agent but the reaction is slow.
The US DOE 2015 Gasification Database lists 8 ‘active’ biomass gasification projects, although all appear to be in the planning/construction phase or have run into operating or funding difficulties. Sundrop Fuels began commissioning and operation of an integrated bioreforming demonstration facility in Longmont, Colorado in December 2015. Sundrop’s technology includes biomass preparation by ‘powderizing’ the feedstock and a high-temperature biomass reformer system claimed to provide high syngas yields and eliminate organic tars. Syngas upgrading comprises conventional methanol synthesis and methanol-to-gasoline processes.
Fulcrum Bioenergy is building a plant to gasify municipal solid waste (MSW) to make syncrude via FT conversion. The gasification process is a variant of steam reforming that will be installed at their $200 million 700 bpd Sierra BioFuels Plant in Nevada, opening in 2017.
SNG can be produced directly or through a syngas mixture that is catalytically converted to methane in a second step. Göteberg Energi used the indirect process in their 20 MW GoBiGas plant, which opened in 2015. The operation was plagued by tar formation and benzene breakthrough, however, and low oil prices sidelined the plant, written off for €90 million.
GreatPoint Energy is developing its bluegas™ technology, a catalytic gasification process to convert coal, petcoke, and biomass into a methane-rich SNG. In 2012 GreatPoint and China Wanxiang Holdings signed an agreement to build a $1.25 billion coal to SNG plant, but to date, construction has not started.
Synthesis Energy Systems (SES) uses a single-stage, bubbling fluidized-bed gasification system to produce low-to-medium heating value syngas from coal, biomass, or wastes using oxygen or air in the gasifier. The so-called U-GAS™ coal gasification process licensed from the Gas Technology Institute is being marketed as “Growth With Blue Skies.” SES has operated a $250 million coal-to-methanol plant in Henan Province in China since 2012 and has an $85 million hydrogen plant in the works. SES claims success with biomass such as wood waste, straw, pelletized alfalfa stems, bagasse, rice straw, and chicken litter.
Raw biomass gasification product gas includes particulate matter, ammonia, sulfur compounds, hydrochloric acid, and alkali metal species as impurities. Different biomass feedstocks, gasifier designs, gasifying agents, and gasification conditions produce different gas compositions and impurities. Tar and particulate concentrations above the acceptable range are problematic for turbines, fuel cells, and methanol or FT synthesis processes.
A major problem with biomass gasification is the production of tars that clog equipment and complicate separation and recovery processes. Tars include complex polycyclic aromatics and oxygen-containing hydrocarbons. Upflow gasifiers and fluid bed gasifiers operating at lower temperatures (700-900 °C) suffer more from tar formation than downflow gasifiers operating at higher temperatures (> 1000 °C). Higher temperatures also favor the yield of H2 and CO but suffer from ash melting to form a slag that coats or clogs equipment.
Two strategies for eliminating tars include gas cleanup and in-bed tar conversion. Gas cleanup is akin to the cleanup of steam cracker products containing heavies. Cracker gas cleanup normally includes a liquid quench or scrubber where tars are isolated for conversion, recycle, or disposal. ECN and Dahlman have developed the OLGA cleanup system for recovering and recycling tars from their MILENA biomass gasification reactor in an integrated gasification combined cycle (IGCC) process. It features both condensation and scrubbing units that recover and recycle tar to the gasifier. Olefins and aromatics in the products may provide more value as chemical intermediates than by combustion, provided they can be recovered economically. In-bed tar conversion processes have focused on dolomite, Ni, olivine, and FCC-type catalysts, but catalyst cost and stability are concerns.
An even more significant hurdle for the competitive economics of biomass gasification is the high oxygen content of biomass compared to natural gas or coal. For SNG or diesel fuel, all of the oxygen needs to be removed, and for syngas, at least 40% needs to be rejected, requiring high temperatures and energy, and reducing yield. Integrated gasification/methanol synthesis/hydrocarbon synthesis processes are projected to achieve about 19% mass yield of gasoline and diesel range products.
Gasification of coal or petroleum coke has been successful where the products are chemicals rather than fuels. CRV Refining annually produces 335,000 tonnes of ammonia and 574,300 tonnes of urea ammonium nitrate solution from pet coke at its Coffeyville, KS refinery. Dakota Gasification is building a plant in Beulah, ND to produce 1,100 tons of urea and 153 million cubic feet of SNG per day. By contrast, Mississippi Power’s Kemper IGCC plant to convert coal to electricity via gasification was started in 2010, is still not complete, and has ballooned from an original budget of $2.8 to $6.7 billion.
The experiences with coal gasification serve as both guide and warning for biomass gasification. With lower heating value and higher oxygen content, the need to aim for higher value products than fuel is vital.
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