By Drew Robb, freelance writer
Gas-electric hybrid engines are gaining popularity as a way to boost fuel efficiency. Automotive fuel producers are also adopting a hybrid approach to improve the efficiency of their plants – using combined heat and power (CHP) to cut fuel costs and emissions.
“Ethanol plants are a very good application for CHP, since they need a lot of steam and a lot of electricity,” says John Cuttica, director of the Midwest CHP Application Center, which the U.S. Department of Energy established at the University of Illinois at Chicago in 2001. “There is a good coincidence between the electric and thermal requirements and when that happens, it can be a good environment for CHP.”
While efficient, CHP is not always the most cost-effective choice. It depends on the cost of electricity from the utility, properly sizing the equipment to the needs for steam and electricity and the reliability of the equipment selected. The selection process is generally one between a gas turbine/heat recovery steam generator (HRSG) or a boiler/steam turbine design. This article covers the factors to consider in making the choice and gives an example from a site which selected a modular boiler by Rentech Boiler Systems.
The Need for Higher Efficiency
Brazil had long been the world leader in ethanol production, with an automotive fuel program dating back 30 years. Over the past decade, however, the United States has taken over the top spot. According to the Renewable Fuels Association’s Outlook 2009, the number of ethanol plants in the U.S. had more than tripled since the start of the millennium, from 54 in January 2000 to 170 in January 2009. Plant capacity over that same time frame had grown six-fold—from 1.75 billion gallons a year to 10.57 billion gallons.
And that is just the beginning. The EPA’s Renewable Fuels Standard, in accordance with the Energy Independence and Security Act of 2007, calls for 36 billion gallons of renewable fuel to be blended with gasoline by 2022.
This rush to increase ethanol production, however, has its critics. One bone of contention is that ethanol is derived mainly from a food source (corn) which led to rising food prices. This problem has a solution: a switch from corn starch to cellulose derived from grass or waste as a feedstock for the ethanol plants. The downside, however, is that currently it costs about twice as much to produce cellulosic ethanol as it does to produce corn ethanol.
Another criticism is that ethanol production requires high energy inputs. According to a report published in the January 2006 issue of Science magazine, it takes 0.76 joules of fossil fuels to produce 1 joule of corn ethanol.1
Those figures, though, are improving. May Wu of the Center for Transportation Research at Argonne National Laboratory looked at 2006 data covering 22 corn ethanol facilities with a combined 1.813 gallons of annual fuel ethanol production (“Analysis of the Efficiency of the U.S. Ethanol Industry 2007”). She compared those statistics with data compiled by the U.S. Department of Agriculture in 2001. Wu found that over that five-year period, total energy usage had decreased 21.8 percent at dry mills and 7.2 percent at wet mills. Dry mills also reported a 15.7 percent decrease in grid electricity use.
|A modular boiler prior to being lifted into place. Photo courtesy Rentech Boilers.|
Boiler or HRSG
A key factor in reducing energy usage is to use CHP at the ethanol plants. There are two main approaches to this: using a boiler to create process steam along with a steam turbine generator set or a combustion turbine to generate electricity and a HRSG to convert the exhaust heat into process steam.
The largest corn ethanol plants use the wet mill process which entails soaking the grain in water and sulfurous acid for up to two days to separate the grain into its component parts. The parts are then separately processed into products such as corn oil, corn syrup, corn starch, ethanol and protein. Dry milling plants, on the other hand, grind and process the entire grain without separating its parts.
According to Wu, the dry milling process requires considerably less energy input than wet milling (a mean of 31,070 Btu per gallon of ethanol vs. 47,409 Btu/gal). Figures vary widely, with the most efficient plants using less than half the amount of energy as the least efficient.
That doesn’t mean however, that wet mill ethanol production is more expensive than at dry mills. Large wet mills primarily use coal-fired boilers to produce steam and generate most of their electricity on site. The smaller dry mill plants tend to use more natural gas and make less use of cogeneration.
Archer Daniels Midland Co., one of the largest ethanol producers with a capacity of 1.65 billion gallons a year, runs coal- and biomass-fired cogeneration plants ranging up to several hundred megawatts at several of its ethanol production facilities in Illinois and Iowa.
Smaller dry mill plants tend to use a gas turbine/HRSG set up rather than a boiler/steam turbine system. For example, POET LLC of Sioux Falls, SD, which produces over a billion gallons of ethanol at 26 plants, runs a Solar Turbines GT genset at its 56-milion-gallon-a-year plant in Ashton, Iowa. Operating at 69 percent efficiency, the CHP system produces up to 7.2 MW of electricity and 56,000 pounds of steam per hour.
Ken Ulrich, an engineer for ICM Inc., which provides the technology in use at more than 100 dry mill ethanol plants says cogeneration, says his firm has installed Dresser Rand steam turbines at nine of those plants.
“It is all about economics,” he says. “Typically the ethanol plants are in the corn belt, where there is a lot of reasonably priced, coal-generated electricity, so they don’t pay for themselves easily. But they are cost effective out west in places like Kansas and Colorado where electricity costs are higher.”
The ethanol plant ICM built for East Kansas Agri-Energy LLC uses cogeneration effectively. The 35 million-gallon-a-year dry ethanol plant has a natural gas boiler producing about 7 million pounds of low pressure (150 psi) steam daily. Since most of the production process uses steam at near-atmospheric pressures rather than passing through a pressure-reducing valve, some of the steam goes through a Dresser Rand KD2 backpressure steam turbine genset which converts the pressure to electricity before the steam is used for the ethanol process. Though the generator is capable of generating 1.6 MW, the plant’s steam requirements limit the electrical output to 750 kW. Nevertheless, that is more than enough to make it viable. According to plant manager Doug Sommer, the generator provides close to a third of the plant’s electrical needs, saving the company $15,000 a month.
Sizing the Solution
Whether one goes with a GT/HRSG or boiler/steam turbine design, the economics depend on correctly sizing the equipment to meet the plant’s needs. Bruce Hedman, director of distributed generation market and technology for Energy and Environmental Analysis Inc. advises against getting too large a GT. He says it should be large enough that, with supplemental duct firing, it meets most of the plant’s steam needs. But it shouldn’t go above the plant’s electrical needs, since selling power back into the grid can be difficult.
Boiler/ST units are more flexible, but again the boiler needs to be sized properly. Jim Lopata, owner of Lopata Technical Services in Chicago, tells of a boiler he specified for a large dry mill plant. The job specs called for 290,000 lbs of steam per hour at 250 psig and 555 F superheat. In addition, the boiler needed to be able to rapidly ramp up or down in response to demand. These requirements created certain problems.
“The proper design of a boiler that will cycle rapidly up and down is to have a maximum heat release of less than 80,000 Btu per hour per cubic foot,” says Lopata. “That limits the size of a boiler that can be shipped on a truck or rail to about 200,000 pounds per hour, but they needed 290,000 pounds.”
One option was to buy two smaller package boilers which could be shipped. A second was to buy one package boiler that was undersized and then put a larger burner in it.
“That would be like putting a Dodge hemi engine into a Volkswagon,” says Lopata. “It will fly like the wind until it tears itself apart.”
A third option was to erect a custom, stick-built boiler on site. This would meet the performance requirements, but not the budgetary ones.
The solution finally selected by Lopata was a modular boiler from Rentech Boiler Systems. Rentech built the convection section and the mud and steam drums in the shop and shipped them to the field for installation. The tubes were then attached in the field.
“That process turns out to be about half the price of a stick-built boiler of the same size,” says Lopata.
A low purchase price and high efficiency, however, are only important if the equipment can reliably meet the production needs.
“The most important thing when you are talking about efficiency is that it runs all the time and doesn’t break down,” he continues. “The Rentech boiler can swing loads very quickly, from 20 percent maximum continuous rating (MCR) to 100 percent in about five minutes without damaging the boiler.”
He says that several features in the boiler allow it to achieve these swings without breaking down due to the thermal stress. One is that there is no refractory in the boiler, which eliminates the largest maintenance problem. The boiler also has oversized downcomers that are out of the heat path. This eliminates deviation from nucleate boiling—where steam goes up the downcomers and fights with the water coming down—which is the most common cause of rupture in steam boilers. Finally, the boiler comes with an oversized steam drum.
“This is a safety factor,” says Lopata. “If there is an unscheduled power outage or incident, since you have a lot more steam in the drum, you have more time to do an orderly shutdown.”
As a result of those features, the Rentech boiler proved to be the most reliable boiler in the plant. And because of this, the plant owner ordered a second, identical boiler to replace another boiler in use at the facility.
Reference: 1(Farrell AE, Plevin RJ, Turner BT, Jones AD, O’Hare M, Kammen DM “Ethanol can contribute to energy and environmental goals”. Science 311: 506–508)