The majority of ethanol in the United States is produced during dry grind fermentation of total mash from crops such as corn, wheat, barley, milo/sorghum and potatoes. Distillers dried grain with solubles, a cellulosic coproduct of the dry grind process, is valuable as animal feed or fertilizer.
Figure 1 . Potential energy savings per 1 MMBtu
SOURCE: MERICHEM GAS TECHNOLOGY PRODUCTS
Wet milling is more elaborate than dry grind because grain must be separated into its components. After corn milling, corn is heated in a water solution to loosen germ and fiber.
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In subsequent steps, germ is removed from the kernel, corn oil is extracted, germ meal is added to hulls and fiber to make corn gluten feed, and gluten is separated to create corn gluten meal for use in animal food. Only the starch of the corn is fermented to produce ethanol. The process also produces wastewater that must be treated before disposal.
Although more ethanol is produced by dry versus wet milling, increasing demand for ethanol as a biofuel, has prompted increased use of new wet milling processes to maximize the yield of ethanol from many cellulosic materials such as corn stalks, grain straw, paper pulp, municipal wastes, switchgrass and other sources. New processes for fermenting woody and herbaceous biomass are being developed to yield more ethanol. These processes are projected to yield more sulfurous wastewater and sludge.
Wastewater and sludge from the new wet milling processes can be treated in anaerobic digesters before disposal. During this digestion process, sour gas containing hydrogen sulfide is produced. When the gas stream is treated by LO-CAT, hydrogen sulfide is removed, yielding gas that may be used as an alternative fuel source while meeting environmental standards. Treated digester gas can be used to lower ethanol plant energy costs by lowering natural gas usage.
Potential Energy Savings
Natural gas prices have almost doubled since January 2000, reaching $11.06/MMBtu this January. At press time the New York Mercantile Exchange natural gas futures contract settlement price was $11.60/MMBtu. Typical digester off-gas can yield as much as 600 Btu/standard cubic foot (SCF) versus natural gas at 1,000 Btu/SCF.
Ethanol production energy costs could be reduced by more than half if energy requirements are supplemented through use of LO-CAT cleaned digester off-gas (Figure 1). Every 1,000 SCF of treated digester off-gas could yield 0.6 MMBtu, equivalent to $6.96 in saved natural gas cost. Actual energy savings may vary based on the availability and composition of treated digester gas.
Sour gas is routed to a LO-CAT absorber. The type of absorber used is dependent upon the product gas hydrogen sulfide specification. The conventional liquid full scheme is used when approximately 99.99 percent hydrogen sulfide removal efficiency is required. In fact, as low as 1 parts per million by volume hydrogen sulfide can be achieved in the treated gas. The venturi-mobile bed option is used when moderate hydrogen sulfide removal efficiency is required and low pressure feed gas, less than 15 pounds per square inch, is treated.
In the absorber, sour digester gas is contacted with LO-CAT solution. Hydrogen sulfide is absorbed from the gas into the LO-CAT solution where chelated iron catalyst promotes a reaction to convert hydrogen sulfide to elemental sulfur. Treated gas then exits the top of the absorber and sulfur rich LO-CAT solution flows to the oxidizer.
In the oxidizer, LO-CAT solution is mixed with air as elemental sulfur settles to the bottom of the vessel. Oxygen in the air oxidizes (regenerates) the chelated iron in the LO-CAT solution returning it to its active state. In its active state, iron and LO-CAT solution are recycled to the absorber to remove more hydrogen sulfide.
Elemental sulfur that settles to the bottom of the vessel forms a 15 weight percent slurry which is removed using a filter. Sulfur is then washed and filtered resulting in 60 percent to 65 percent sulfur cake. The balance of the cake is water and trace amounts of LO-CAT solution. Wash water is recycled back to the oxidizer.
LO-CAT Chemistry
The LO-CAT process was developed to provide an isothermal, low operating cost method for carrying out the modified Claus reaction:
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