Gasification Technologies: Making Second-Generation Biofuels a Reality
Enerkem's gasification technology provides a solution for some of the challenges facing cellulosic ethanol producers, including high manufacturing cots and the volume of feedstock required.
While there are several operational first-generation biofuel plants that use food feedstocks such as corn and sugarcane, very few second-generation ethanol plants have arrived at the commercial scale. In their quest to develop commercially viable technology, next-generation fuel producers have taken one of two approaches: an enzymatic platform for homogeneous biomass or a thermochemical platform for a variety of feedstock, including nonhomogeneous residues.
The first technological approach targets forest biomass and agricultural residues. Enzymatic technologies seek to recover and ferment sugars that are found in lignocellulosic (tree and plant) materials. However, these technologies face several challenges that are addressed by thermochemical technology, which can use a more diverse range of feedstocks.
The enzymatic approach works to recover sugars in lignocellulosic materials. The sugars are "imprisoned" in complex structures making it hard to break down since nature engineered them to last. In order to recover the sugars, engineered enzymes are used to break down the tree and plant material, after which it is possible to hydrolyze the cellulose into glucose, from which ethanol is easily made. Engineering of these new enzymes is, in many cases, still at the research and pilot stage.
The challenge with this approach is that it only applies to homogenous materials (feedstock that is composed entirely of one type of trees, for instance). This is because the enzymes and the microorganisms that ferment sugars do not adapt to materials that may fluctuate in chemical composition.
The thermochemical technologies are able to use heterogeneous material as feedstock, using heat to convert these carbon-rich materials into gas. This gas is then purified so it can be transformed into alcohols such as methanol and ethanol. It is also possible to produce other fuels, such as synthetic diesel, synthetic gasoline and dimethyl ether as well as green chemical products. Therefore, gasification is a crucial component of a thermochemical technology platform and one that is a key differentiator among second-generation ethanol producers. The Sierra Club in its January report called "Smart Choices for Biofuels" recognizes the advantage of thermochemical technologies: "These technologies can be used to convert almost any kind of biomass into fuel . . . giving them a potential advantage over biochemical technologies that rely on developing specific enzymes to break down specific plant matter."
The gasification process converts carbon-rich residues into a synthetic gas. Enerkem, a waste-to-fuel and green chemical company based in Montreal, is one of few companies that has developed advanced gas purification technology. This allows Enerkem to use heterogeneous (mixed) raw materials that may contain impurities, such as end-wastes that would otherwise be land-filled. Esteban Chornet, Enerkem's co-founder and chief technology officer, is a world-leading scientist in the area of gasification and biomass conversion. He had feedstock dynamics in mind when he designed Enerkem's technology platform.
Enerkem's gasification technology is based on a bubbling fluidized bed reactor with a front-end feeding system that is capable of handling fluffy material with no need to pelletize it. A continuous feed of biomass is carried into a reactor where an inert heat carrier (i.e., sand) is "fluidized" in a zone known as "the bed" under relatively low temperatures of 700 to 750 degrees Celsius (1,292 to 1,382 degrees Fahrenheit) and moderate pressures of ~2 atm. Slurries or liquids can also be fed into the gasifier through appropriately designed injectors. The low severities at which the gasifier operates allows for the use of known, available and inexpensive construction materials and refractories. In this environment, biomass is converted to gas and small amounts of residual inert materials. This bed is a significant departure from other gasification technologies that use plasma and extreme temperatures of 1,400 to 1,550 C (2,552 to 2,822 F) to break down biomass.
Oxygen and steam are used as fluidizing gases and gasification agents. Gasification takes place throughout the bed. The injected oxygen and added steam prevent the residual inert materials from melting and maintain the injection nozzles at appropriate temperatures, so available steels can be used, thus avoiding exotic and expensive ceramics.
The synthetic gas, or syngas, is drawn from the top of the gasifier. In its movement towards the top of the bed, the syngas disengages from the sand and undergoes extensive cracking and reforming in the freeboard stage-key chemical processes, which convert organic intermediates to simple molecules-having the desired molecular characteristics (hydrogen, carbon monoxide, methane and carbon dioxide-the main constitutive molecules in synthetic gas).
The superior mixing, which occurs in the bubbling fluidized bed used by Enerkem, generates high rates of heat and mass transfer which subsequently yield stable temperatures, resulting in well-controlled reaction kinetics. In addition, the feed material does not need to be completely uniform, since heat and mass transfer rates will, de facto, achieve fuel uniformity within the bed itself. Thus, Enerkem's reactor is capable of handling feedstock that is not uniform, is geometrically dissimilar and is heterogeneous. Enerkem's reactor is also efficiently sized due to high throughput rates facilitated by the design-a feature that contributes to the commercial viability of the Enerkem platform.
"In order to be cost competitive, the alternative to gasoline will need to be feedstock flexible," says Enerkem CEO Vincent Chornet. "We believe that's one of the main advantages of our technology: we're able to use a wide range of feedstocks, allowing us to adapt to changing feedstock conditions should input prices or feedstock availability change significantly."
Feedstock Flexibility at Work
Enerkem's recently announced plants in Edmonton, Alberta, and Mississippi demonstrate the range of projects facilitated by the diverse feedstocks its technology uses. Because Enerkem's technology platform is designed to use nonhomogeneous waste as feedstock, the company is able to use negative-cost feedstock such as municipal solid waste (MSW) and used electricity poles. Municipalities actually pay Enerkem a fee, sometimes called a "tipping fee," to remove their waste and free up scarce landfill space, relieving municipalities of some costs related to waste disposal.
"Our plants in Edmonton and Mississippi represent innovative waste management solutions for local governments," Chornet says. "Converting their waste into a new transportation fuel option allows Enerkem to contribute to a greener economy and a more sustainable future for these municipalities. We believe that fostering fuel independence by producing fuels locally is important in addressing today's environmental and economic challenges in both large, urban centers and smaller, more rural communities."
In June 2008, Enerkem GreenField Alberta Biofuels signed a 25-year agreement with the city of Edmonton to build and operate a plant that will produce and sell next-generation biofuels, including methanol and cellulosic ethanol, from sorted MSW. This is the world's first agreement between a large urban center and a biofuel producer to turn municipal waste into ethanol. As part of the agreement, Edmonton will supply a minimum of 100,000 tons of sorted MSW per year. The sorted MSW to be used is the ultimate residue after recycling and composting. These residues would otherwise be landfilled.
Enerkem GreenField Alberta Biofuels will be responsible for constructing, owning and operating the plant, which will be at the Edmonton Waste Management Centre in Edmonton.
The plant will initially produce 10 MMgy of biofuels. In May 2009, Enerkem GreenField Alberta Biofuels completed the necessary environmental regulatory process and was awarded a permit to commence construction of the facility.
In March 2009, Enerkem announced plans to build and operate a $250 million second-generation biofuels production facility in Pontotoc, Mississippi, its first in the U.S. The facility will produce 20 MMgy of ethanol from 370,000 green tons of feedstock (200,000 tons of urban biomass and 170,000 tons of forest/agricultural biomass).
Enerkem's state-of-the-art technology platform is also currently in operation at its pilot plant in Sherbrooke, Quebec, and its first commercial plant in Westbury, Quebec. The company has tested more than 20 types of feedstock at its pilot plant, since 2003, and has produced syngas, methanol and ethanol. The Westbury plant is considered to be the world's first ethanol plant to use negative-cost and unconventional materials-treated wood from used electricity poles.
Enerkem's approach provides a solution to some of the challenges that the production and commercialization of cellulosic ethanol has faced, including high manufacturing costs and the volume of feedstock required. And, with the ability to use MSW and other urban wastes as feedstock, Enerkem finds itself at an advantage. This is a feedstock which will not be depleted any time soon. As revealed by the U.S. EPA in its RFS proposed rulemaking, cellulosic ethanol produced from urban waste will represent more than 2 billion gallons of the projected ethanol volumes in 2022. Chornet is confident that Enerkem won't have any difficulties maintaining a constant supply of feedstock. "The waste streams we're looking into, fortunately for us, have been up in the past 10 years. That's a more subtle answer to the question: ‘Are you going to miss waste someday?'"
Marie-Helene Labrie is vice president of government affairs and communications for Enerkem. Reach her at firstname.lastname@example.org.