“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.
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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.
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