Douglas G. Tiffany

A Corn Stover Supply Logistics System

Published in Applied Engineering in Agriculture, Vol. 26(3): 455‐461, 2010. American Society of Agricultural and Biological Engineers

Characterization of Feed Streams and Emissions from Biomass Gasification/Combustion at Fuel Ethanol Plants

Fuel and emission characteristics of co-products of the ethanol dry-grind process [distillers wet grains (DWG), concentrated distillers solubles (referred to as “syrup”), and distillers dried grains with solubles (DDGS)], and corn stover are evaluated. Biomass co-products from the dry-grind ethanol production process, particularly DDGS, are good sources for the electrical and thermal energy needed to operate the plant, and even contain sufficient energy to produce excess power that can be sold. However, the number of subsequent process steps required to reduce emissions to compliance levels, particularly nitrogen oxide, will be important in determining the economic viability of using biomass co-products from dry grind ethanol plants for energy production. The alkali metal content (potassium and sodium) of the ash is high (22 to 34 %) for co-products and corn stover. Such high levels of alkali metals can lead to ash fouling in combustion and steam generation units, and to potential agglomeration of bed material in fluidized bed systems.

Commercial scale tub grinding of corn stover and perennial grasses

Performance of a commercial mobile tub-grinder [463 kW (630 hp)] was studied to produce coarsely ground particles from round-baled, plastic net-wrapped corn stover and native perennial grasses. The effects of grinder screen opening size [25.4 to 127.0 mm (1.0 to 5.0 in.)] and feedstock type on ground particle size, bulk density of ground biomass, throughput, specific energy consumption, and cost of grinding were studied. The bulk density of ground biomass ranged from 69 to 207 kg/m3 with the moisture content of particles ranging from 14% to 39% (w.b.). The throughput of the grinder ranged from 6 to 45 t/h. The specific energy consumption ranged from 93 to 661 MJ/t. The estimated cost of grinding ranged from

Integrating Biomass to Produce Heat and Power at Ethanol Plants

4.39 to

A tub-grinding/roll-press compaction system to increase biomass bulk density: Preliminary study

31.23/t. Increasing the grinder screen opening size increased the ground particle size and throughput, but decreased the bulk density of ground biomass, specific energy consumption, and cost of grinding. High moisture bales (= 25% w.b.) decreased the performance of the grinder. The results from this study would be useful for developing a biomass supply logistics system involving mobile grinding.

BIOMASS FOR ELECTRICITY AND PROCESS HEAT AT ETHANOL PLANTS

Dry-grind ethanol process co-products and corn stover can be used to provide electricity (both for the plant and sale to the grid) and process heat. These biomass fuels can reduce process energy costs and increase the renewable energy balance for fuel ethanol production. An Aspen Plus model of the dry-grind ethanol process was used as a basis for the integration of biomass-fueled combined heat and power systems. Several combinations of combustion and gasification systems, power production cycles, biomass fuel combinations, and air emission control technologies are evaluated. Suitable configurations for incorporating biomass to produce heat and power at typical 190 million liters (50 million gallons) per year dry-grind ethanol facilities are analyzed.

ECONOMICS OF BIOMASS GASIFICATION/COMBUSTION AT FUEL ETHANOL PLANTS

Transforming biomass into a product with bulk density of 240 kg/m3 (15 lb/ft3) would enhance logistics of truck transport to users throughout the year. We evaluated a system involving tub-grinding followed by roll-press compaction to increase the bulk density of biomass such as corn stover, corn cobs and native perennial grasses. Preliminary experiments were conducted to study the effect of particle size (tub-grinder screen sizes of 19.1, 76.2, and 203.2 mm) on the bulk density of roll-press compacted biomass materials. The bulk density of tub-ground biomass ranged from 50-100 kg/m3 (3-6 lb/ft3). Tub-grinding followed by roll-press compaction increased the bulk density by two- to three-fold, and resulted in densified products with bulk density of 190-240 kg/m3 (12-15 lb/ft3). Additional increases in bulk density appear possible with improved design of the feeding system for the roll-press compaction machine.

A Biomass Supply Logistics System

Biomass can provide electricity and process heat at dry-grind ethanol plants to both reduce costs and improve the net energy value of ethanol production. Distillers dried grains with solubles (DDGS), which are coproducts of ethanol production, can potentially be used for energy. Cornstover is another potential biomass energy source for ethanol plants. Biomass (DDGS and cornstover) alternatives to provide process heat and electricity at corn dry mill ethanol plants are evaluated. Corn dry grind ethanol production using biomass (DDGS or cornstover) to meet process energy needs and generate electricity achieves net energy values in the range of 20 to 30 MJ/L (72,000 to 108,000 Btu/gal) of ethanol, which equals or exceeds previous estimates for biomass ethanol production. There are significant annual energy cost savings/returns for a 150 million liter (40 million gallon) per year plant capacity over a range of natural gas and biomass prices to apply to additional capital and operating costs required for a biomass energy system. Electricity generation is potentially an important contributor to the annual energy cost savings/returns because of the ability to effectively use waste heat from electricity generation to meet process energy needs. An important next step is to evaluate capital and operating costs of biomass combustion/gasification, emission control, biomass fuel handling, and electricity generation technologies to determine overall economic feasibility.

Reducing Life-Cycle Greenhouse Gas Emissions of Corn Ethanol

Dry-grind ethanol plants have the potential to reduce their operating costs and improve their net energy balances by using biomass as the source of process heat and electricity. We modeled various technology bundles of equipment, fuels and operating activities that are capable of supplying energy and satisfying emissions requirements for dry-grind ethanol plants of 190 million and 380 million liter (50 and 100 million gallon) per year capacity using corn stover, distillers dried grains and solubles (DDGS), or a mixture of corn stover and “syrup” (the solubles portion of DDGS). Results showed favorable rates of return on investment for biomass alternatives compared to conventional plants using natural gas and purchased electricity over a range of conditions. The mixture of corn stover and syrup provided the highest rates of return in general. Factors favoring biomass included a higher premium for low carbon footprint ethanol, higher natural gas prices, lower DDGS prices, lower ethanol prices, and higher corn prices.

Biomass Integrated Gasification Combined Cycle for Heat and Power at Ethanol Plants

We evaluated a corn stover logistics system that included collection and transport by round bales to local storages within 3.2 km (2 miles) of the field during the fall harvest period. This stage was followed by processing at the local storage sites throughout the year using mobile units which converted the bales to bulk material by tub-grinding and roll-press compacting to 240 kg/m3 (15 lb/ft3) to achieve 22.7 t (25 ton) loads for truck delivery to an end user within a 48 km (30 mile) radius. The total cost, fossil energy consumption, and greenhouse gas (GHG) emissions for delivering the bulk corn stover to end users were