Many of the energy-related optimization options available are related to fuel choice and efficiency with respect to the heating/cooling and power requirements for running the various unit operations involved in ethanol production. The goal of all of these options is to decrease the net energy requirement (and consequently the associated cost) of fuel ethanol production relative to the standard process in the United States. All of the options yield significant savings but are not yet in widespread use or considered standard. A more detailed discussion of these and additional options can be found in the full-length version of this three-part series, available at www.ethanolproducer.com/articles/EmergingOptimizationOpportunities.
Pervaporation, Capillary Distillation
The act of purifying ethanol through traditional distillation techniques is the single-largest consumer of energy in ethanol production. Roughly between 50 percent and 60 percent of the energy used in the fermentation process can be accounted for during distillation. Consequently, it's appropriate to examine the distillation process first when identifying energy-saving opportunities. Two techniques for reducing energy consumption during distillation are pervaporation and capillary distillation.
Article Continues After Advertisement
Pervaporation involves the separation of two or more components across a membrane by differing rates of diffusion through a thin polymer and an evaporative phase change comparable to a simple flash step. A concentrate and vapor pressure gradient is used to allow one component to preferentially permeate across the membrane. Using such membranes to replace both the rectification column and the molecular sieve unit has been proposed in a conventional process. This process could save up to 50 percent of the overall energy consumption in distillation while at the same time yielding a 99 percent pure ethanol stream. This technology has been installed on a demonstration scale in an ethanol plant in Chatham, Ontario.
Capillary distillation utilizes fractionating plates with capillary-type passages to alter the vapor-liquid equilibrium of two compounds and yield high tray efficiencies. The overall result is a shorter column requirement and less energy input to affect the same separation as conventional columns.
Cogeneration
Also referred to as combined heat and power, cogeneration is the combined generation of steam and electricity for use in a plant. In a cogeneration facility, steam is generated in a boiler, passed through a turbine to generate electricity and then sent on to a process that requires heating. Alternatively, the hot flue gases from the fuel combustion can be sent directly to a gas turbine for electricity generation and then to a waste heat boiler or other device for further heat recovery. Cogeneration thus allows a plant to eliminate or reduce the need to purchase electricity and allows for a more efficient use of fuel in generating the plant’s energy needs.
While cogeneration technology isn't new, its application in the ethanol industry hasn't been widely implemented. Most of the ethanol plants that utilize cogeneration are wet mills, likely because of their larger size relative to a typical dry-grind plant. While the capital cost of installing or retrofitting cogeneration isn't insignificant, the benefits of reduced utility costs over time makes this a worthwhile option to consider, particularly as fuel costs continue to rise.
Biomass for Fuel
Biomass holds potential not only as an ethanol feedstock but also as a direct fuel source or a feedstock for other fuels that can be used for power generation. Most ethanol plants generate energy for the plant by burning fossil fuels. This is expensive, especially with today's rising fuel prices. The use of low-cost and/or readily available biomass as an alternative fuel can therefore be very attractive.
The most obvious source of biomass for an ethanol plant to consider is that associated with the dead yeast cells and other non-fermentable solids (e.g., protein, fiber) in the stillage. Whether stillage is more valuable as fuel or livestock feed depends on the price of alternative fuel sources (e.g., natural gas) versus the selling price of distillers grains. Other sources of available biomass include agricultural waste (e.g., corn stover), manure and lignin. The most popular ways to convert biomass to fuel are by direct burning, gasification or anaerobic digestion.
Direct burning is the simplest way to extract the energy value of biomass. Distillers grains and condensed distillers solubles (i.e., syrup) have a sufficiently high heating value, so they can be burned as fuel to generate steam or operate the drum dryers used to generate distillers dried grains (without solubles). A Minnesota plant that has burned syrup since 2005 has reduced its natural gas use by up to 54 percent. In addition to the energy savings, burning distillers grains or syrup avoids the logistical difficulties in having to store, transport and sell these coproducts, although it also eliminates the income from their sales.
| 1 2 3 4 | Next Page --> | |
| View Entire Article | ||
