ENERGY REVIEW: Reducing Water Usage in Biopower Production
The power industry is second only to agriculture as the largest domestic user of water, accounting for 39 percent of all freshwater withdrawals in the nation, of which 71 percent is used in fossil fuel-based electrical generation. The same technologies used to produce electricity from fossil-based fuels are, and will continue to be, used for a significant amount of biomass-based power production.
Much of the water used for power generation, including biomass power generation, is consumed in the condensing step of the system where the steam exiting the turbine is cooled and condensed back to water. This water is sent back to the boiler for reconversion to steam to spin the generator to produce electricity. This steam is in a closed loop and has small losses from venting of steam and blowdown of water in the steam cycle to reduce impurities in the steam system. It is in the cooling loop, separate from the steam system, where water is used to condense the steam which has the most significant losses. The water in this loop is cooled by evaporation in large cooling towers to cool the water that is then used to condense the steam. The availability of water for use in biomass electric power generation is limited in many parts of the United States, and biomass power plants must compete with other industrial customers, agricultural interests and households for this limited commodity.
Therefore, water is an important factor in obtaining site permits for new biomass power plant construction. Difficulty in obtaining necessary water permits can lead to delayed or abandoned projects. Also, it is often the case that additional infrastructure is required to provide a suitable supply, adding cost and environmental impact. In areas that do not have an adequate water source, biomass power plant construction is often not even considered, even though these locations are ideal in other respects. In addition, potential regulations curtailing CO2 emissions will impact water use. Because of the corrosive nature of carbonic acid, water will need to be removed to low levels prior to pipelining the CO2 to its final destination.
The EERC in conjunction with a commercial partner investigated a unique technology aimed at reducing the water use in power production systems that has many potential applications. This technology is a liquid desiccant dehumidification system (LDDS). The LDDS is an absorption-based system designed to recover moisture from process gas streams. This gas may be the effluent from an ethanol process, the exhaust from a biomass power generation system or any other moisture-rich gas stream.
The LDDS concept has several features that make it potentially attractive for integration with a biomass utilization process:
• The desiccant solution separates the process gas and moisture condensation space. In this respect, the LDDS performs a similar function to a water-selective membrane, but it is more robust and can resist fouling in harsh gas environments.
• The LDDS produces high-quality product water without additional treatment.
• The LDDS does not completely cool the process gas stream, which is an important consideration if the gas is to be exhausted to a stack.
This is just one example where the EERC can play a part in cooperation with the biomass industry to provide value-added services and processes. The EERC is committed to reducing the water footprint of biomass utilization systems, whether it is in power production or in the production of bioproducts or biofuels, and is working with industry to do so.
Author: Bruce Folkedahl
Senior Research Manager,
Energy & Environmental Research Center