Emerging Energy Optimization Opportunities for Ethanol Facilities

By Philip A. Marrone, Kenneth R. Liberty and David J. | October 06, 2008
In the October issue of EPM, several options for improving ethanol plant operation were presented from a process-related perspective. This article, the second in a three-part series, focuses on energy-related changes that could improve the ethanol production process or reduce energy costs.

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.

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.

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.

If conveniently available, agricultural waste is an ideal fuel or fossil fuel supplement because it provides a use for waste material. Dry manure (less than 20 percent moisture) can also be burned directly. Use of these materials is most economical for ethanol plants located near farms or ranches since transportation costs can outweigh the energy value.

Conventional gasification or reforming uses high temperature and steam with limited or no oxygen to break down organic substances into synthesis gas (primarily carbon monoxide and hydrogen). These gaseous products are valuable for use as a fuel by itself or for building larger organic molecules (including ethanol) via a catalytic Fischer-Tropsch synthesis reaction. Gasification isn't a new technology, but its consideration for biomass feeds such as corn stover has only occurred over the past 30 years. Although there has been considerable recent interest in gasification for generating alternative liquid fuels from biomass, use of this technology in an existing ethanol plant at the present time is unknown.

Anaerobic digestion is a process where organic matter is decomposed by bacteria in the absence of oxygen to generate a gaseous mixture (referred to as biogas) consisting primarily of methane. The process is commonly used for wastewater treatment and has been utilized by several ethanol plants for processing relatively small quantities of off-spec fermentation batches. Digestion of the much larger quantities of stillage that are produced in a typical plant would generate a volume of gas that could justify its recovery and use as a fuel within the plant. Several designs where an anaerobic digestion system replaces the centrifuge, evaporator and dryer within an ethanol plant have recently been proposed.

Anaerobic digestion can easily accommodate other biomass sources such as agricultural waste and manure. The main hurdle that has prevented widespread use of anaerobic digestion by the ethanol industry has been cost (up to 30 percent greater in capital cost). However, elimination of two critical economic factors in plant operation—fuel and distillers dried grains with solubles (DDGS) prices—provides a powerful incentive that may make the initial high cost worthwhile.

Improved Efficiency, Energy Extraction
This is a key reason why energy use in ethanol plants has dropped considerably over the past few decades. The most efficient plants are getting at least two uses out of most of
their energy inputs. One way to increase energy production efficiency is through cogeneration. Plants have also been reusing waste heat and utilizing regenerative heating.

Examples include using hot gaseous effluent from the DDGS drum dryers to heat the thin stillage evaporators and using evaporator waste heat to augment what's needed for distillation. The improved energy costs must be weighed against the higher capital expense and more complex operation.

Another strategy to consider is cooperative energy. Excess heat or power can be captured and transferred from a nearby industrial facility to the ethanol plant or vice versa. This synergistic relationship fosters good neighbors and reduces overall energy costs. The downside to this strategy is the dependence on this synergy. For example, if there is a plant shutdown for maintenance, there needs to be a backup plan so that there is no effect on the collaborating plant. This backup system could result in additional capital costs and maintenance to ensure no disruption to the dependent processes.

In addition, pinch technology is a methodology that seeks to maximize heat recovery in a process facility by systematically identifying all heat gradients and then determining the optimum heat recovery scenarios. Many facilities that have applied this methodology have seen energy savings of 20 percent.

Elimination, Minimization of Drying
Drying distillers grains and solubles is one of the most energy-intensive unit operations in a dry-grind ethanol plant, consuming as much as one-third of a plant's entire energy requirements. Thus, any method of incrementally reducing the energy demand of the drying step can yield substantial savings. Of course, elimination of the dryers and evaporators altogether is the most straightforward solution if there is either a nearby market for distillers wet grains or if the stillage is used for its heating value within the plant.

One way to minimize dryer energy costs is to reduce the load of material or water content that is sent to the dryer. This can be accomplished by eliminating or reducing the flow of condensed distillers solubles, or syrup, to the dryer. Elimination of this liquid from the dryer feed can reduce the dryer energy requirement. Another option is to increase the concentration of the syrup before sending to the dryer, decreasing the water load. While the increase in viscosity can make more concentrated syrup difficult to handle without a properly designed evaporator, the downstream dryer energy savings may be considerable. The syrup could also be concentrated by a less energy-intensive method than evaporation by utilizing one of various types of low-temperature filtration such as reverse osmosis or nanofiltration. Such a system can provide energy savings by both eliminating the evaporator and reducing water load to the dryer.

Raw Starch Hydrolysis
Also referred to as cold saccharification, cold cooking or cold hydrolysis, raw starch hydrolysis is a process that converts starch to glucose directly, without heating. It thus avoids the traditional energy-intensive and high-temperature (more than 158 degrees Fahrenheit) liquefaction and saccharification steps.

In the raw starch process, ground corn is slurried with water and enzymes at a temperature between 86 and 104 degrees Fahrenheit. This is comparable to the temperature range of fermentation, which follows once the starch has been fully hydrolyzed to glucose. Although work on this process began as far back as the 1940s, it wasn't until 2004 that it became a mature commercial-scale process. A key breakthrough for this process was the development of alpha-amylase and glucoamylase enzymes capable of converting solid starch to sugar without heat. Several companies, most notably Poet LLC and the team of Genencor (Danisco Inc.), ICM Inc., and Fagen Inc., have developed versions of this process. As of early 2006, there were at least 10 ethanol plants employing this technology. However, not all companies are fully convinced that the benefits of raw starch hydrolysis outweigh its drawbacks.

The main advantage of the raw starch process is the significant reduction in energy consumption caused by not heating the feed above the temperature of fermentation (i.e., approximately 86 degrees Fahrenheit). The elimination of the liquefaction and saccharification unit operations also provides considerable time and cost savings. Other benefits are reduced water and waste costs, improved conversion efficiency, and increased protein content and quality of feed coproducts. Disadvantages of utilizing a lower temperature include the increased cost and amount of enzymes needed for raw starch hydrolysis, slow conversion, an increased number of undesirable microorganisms and toxins that would normally be destroyed/inactivated by heat, and the incomplete liberation of endosperm starch. While these disadvantages aren't insurmountable, addressing them will minimize the cost savings derived from the process benefits. Each plant needs to weigh the pros and cons subject to its own unique operating conditions to determine whether raw starch technology is worthwhile to implement.

Summary of Energy Optimization Options
Most of the options presented here address energy savings by operation either 1) at lower temperatures, 2) with a more efficient and effective use of energy, or 3) with alternatives to traditional fossil fuels. Many of these options have the initial hurdle of high capital costs but ultimately achieve savings over the long term through decreased energy costs. The key to whether an option is worth implementing for a particular facility will likely depend on how quickly the return on investment is achieved. As energy prices continue to rise, implementation of these energy-saving options may become more favorable. Unlike the process-related optimization options (discussed in the October issue), many of the energy options (e.g., cogeneration, anaerobic digestion, regenerative heat exchange) aren't unique to an ethanol production plant. Many of these options are more fully developed and mature because of their more extensive industrial uses, even if they have yet to be fully embraced by the fuel ethanol production industry. Regardless, the cost of energy plays such a critical role in plant viability that even small improvements can result in significant savings.

The next and final installment of this three-part series will focus on waste and coproduct-related options to improve the ethanol production process.

The authors would like to thank Bryan Yeh of Science Applications International Corp. for his contributions to this article.

Philip A. Marrone is a chemical engineer, Kenneth R. Liberty is a biochemical engineer and David J. Turton is a civil engineer with Science Applications International Corp. Reach Marrone at marronep@saic.com or (617) 618-4686. Reach Turton at david.j.turton@saic.com or (443) 402-9209.