Interest in the thermochemical conversion of biomass into a synthesis gas that can be run through a turbine for the production of electricity, used to replace natural gas or converted into biofuel, is gaining ground. Biomass Magazine probes several experts for explanations to demystify the processes used to make syngas.
"Syngas production is sort of an orphan," says Alexander Koukoulas, senior technology consultant for ANL Consultants LLC, which services the pulp and paper, packaging, chemicals and bioenergy industries. "It hasn't really seen much in the way of publicity even though it's a much more mature technology that has been used at commercial scale for quite sometime."
To harness the energy stored in the chemical bonds of agricultural waste, forest residues or any other of the profusion of carbohydrate-containing leftovers that can serve as renewable energy feedstocks, engineers tinker with the deforming powers of heat and pressure with the aim of breaking the linkages that hold these molecules together and capturing the chemical energy released in the process. This chemical energy is contained in a mixture of molecules collectively called synthesis gas because it's suitable for the synthesis of various fuels and chemicals. The principal components of syngas are carbon monoxide and hydrogen but the concentrations of these and the presence of other minor molecules can be tailored by using different thermochemical reaction conditions.
This star diagram shows the multitude of biobased products that can be produced from syngas.
The main method of producing syngas from biomass feedstocks is called gasification. Although gasification reactions can take many forms, these processes are defined by cranking up the temperature to between 650 and 1,400 degrees Celsius (1,202-2,552 Fahrenheit). There are two approaches to achieving these elevated temperatures: direct heating and indirect heating. In direct heating, a relatively small amount of oxygen is added to the reactor. If this gas is made up of more than 90 percent oxygen, the resulting syngas will be rich in carbon monoxide and hydrogen, explains Jerod Smeenk, engineering manager for Frontline BioEnergy LLC, a biomass gasifier developer and process engineering firm. A contrasting approach uses various means of indirect heat transfer to achieve high operating temperatures, including hot sand circulation and exotic alloy heat exchangers, Smeenk says. "It comes down to an economic consideration," he says. "One must consider many factors including simplicity of design, upfront costs, operating costs, scale-up potential and the potential replacement costs for exotic alloy heat exchange components."
The least expensive approach to biomass gasification is the direct approach, which adds air-not pure oxygen-to the system with simple blower technology. The gas released from this approach is called producer gas because although this method is a money saver, nitrogen from the air becomes a major component of the gas. Although producer gas doesn't have as high a concentration of carbon monoxide and hydrogen as syngas, it can be made very clean with appropriate gas conditioning and as such it can be used as a replacement for natural gas and burned to fuel equipment like fired boilers and direct-fired dryers, Smeenk explains.
This is the type of system that Frontline is in the process of installing at Chippewa Valley Ethanol Co. LLC in Benson, Minn. The gasification system will be constructed as an island so as not to disrupt the primary workings of the facility. The burners at the boilers and dryers will be replaced with special multi-fuel burners that can run on producer gas, natural gas or a combination of both. "Whereas some other biomass systems require a complete overhaul, our technology lends itself to doing a retrofit of an existing facility," Smeenk says. The first phase of the CVEC project, which is expected to be complete in February, will process 75 tons of locally available wood waste thereby displacing 25 percent of the natural gas consumed by the plant. "Ultimately our objective is to displace more than 90 percent of the plant's natural gas requirement."
But what if the goal of the ethanol producer or pulp and paper mill owner is to produce a rich syngas for the production of electricity? The method of choice in this case is a thermochemical reaction called steam reforming. It's a type of indirect gasification that is also referred to as high-temperature pyrolysis.
In pyrolysis, biomass is heated to temperatures ranging from 400 to 800 C (752 to 1,472 F) in an oxygen-starved reactor. In fast pyrolysis, the reaction is run in the middle of this temperature range and the amount of time that the biomass is exposed to heat is limited. These conditions maximize the production of a liquid product called pyrolysis oil or py-oil, "which can be used in much the same way as a crude oil can be used," Koukoulas says.
In high-temperature pyrolysis, steam is used to heat the biomass. "We put energy in but convert all the biomass to syngas," explains Dan Burciaga, president of ThermoChem Recovery International Inc., a biomass-to-energy company that commercializes gasification technologies. "This makes it a very thermally efficient process." TRI's steam reformer technology is currently being used at the Norampac Inc. containerboard mill in Trenton, Ontario. The feedstock for the steam reformer is black liquor, which is essentially the lignin left over from the pulping process. The syngas that's made is combusted in a boiler to produce steam, which offsets some natural gas requirements. The system started up in September 2003 and transitioned from commissioning to full commercial operation in October 2006.
The use of syngas as a natural gas replacement is just the start. "Once you have syngas you have optionality," says Chris Doherty, vice president of contracts for TRI. "Syngas has the building blocks to create all the products and chemicals currently generated in the petrochemical industry." Perhaps the ultimate and most demanding application of syngas is as a precursor for liquid fuel.
Producing this liquid fuel is the goal of a new partnership between TRI, Flambeau River Papers LLC of Park Falls, Wis., and Syntroleum Corp. of Tulsa, Okla. TRI and Syntroleum will serve as technology partners for Flambeau's planned 37 MMgy wood-to-syngas-to-liquid fuel plant colocated with the company's paper mill. TRI will provide the gasification technology for the project while Syntroleum will provide the gas-to-liquids technology. The latter converts syngas into a low-sulfur replacement to crude oil through a cobalt catalyzed process called Fischer-Tropsch. "We're very excited about this biofuels plant," says Bob Byrne, president of Flambeau River Biofuels LLC. In mid-August, the company submitted a grant application to the U.S. DOE to help fund the construction of the plant. Byrne says they expect to hear from the agency this month. "If we get a government grant it will be a significant tail wind for us to put this project together and move quickly to funding, permitting, construction and startup," he says. However, the development of the biofuels plant doesn't hinge on this grant. "If we don't get the grant, I still expect that we will put the project together but it may take us longer to get it funded." Either way, Byrne expects to break ground this spring. "And that's not the end all," he says. "We're hopefully establishing the model that helps make the North American pulp and paper industry that much more competitive with the global economy that we face," says Bill Johnson, director of government affairs and public relations for Flambeau.
Jessica Ebert is a Biomass Magazine staff writer. Reach her at firstname.lastname@example.org or (701) 738-4962.