While engineers have developed better systems for gasification, the initial processing of various types of sludge to obtain a suitable feedstock remains more of a challenge than when converting biomass to biofuel.
While engineers have developed better systems for gasification, the initial processing of various types of sludge to obtain a suitable feedstock remains more of a challenge than when converting biomass to biofuel.
Biosolids result from recycling sewage sludge. They must meet the land application Part 503 requirements before they can be used as fertilizer. Over millenniums farmers have used biosolids; one alternative since the mid-1990s has been to convert organic material to fuel. While the burning of dried dung also has been a fuel source over millenniums, gasification of sewage sludge produced by large scale, modern waste water treatment facilities is a relatively new concept.
U.S. Department of Energy at the National Energy Technology Laboratory affirms that gasification technologies can provide a stable, affordable energy supply for the nation; a further advantage, they can provide high efficiency with near-zero pollutants. Gasification-based systems can provide a wide range of products, e.g., electricity, fuels, chemicals, hydrogen, and steam. And, perhaps most important in a time of higher energy costs, flexible gasification systems provide for operation on low-cost, widely-available feedstock.
While it remains unresolved whether a thermal-chemical or sugar platform is better for biomass, the lumber industry is shifting to IGCC (Integrated Gasification Combined Cycle) in treatment of waste from kraft mills. So, one might wonder whether a similar approach would be profitable for large scale waste water treatment facilities? In other words, what efficiency is achievable with a large scale sewage sludge system when pyrolysis, combustion, then gasification are the processes used to convert the waste to energy.
Gasification refers to transformation of soluble and suspended organic materials into gas. Whereas DoE strongly advocates for the gasification of coal and this post focuses on biosolids, questions remain as to the emissions profile for any, recommended production processes.
In 2002 Doug Williams of Fluidyne Gasification asserted on GAS-L, a discussion list on gasification:
During the years that we have been playing with producer gas, both in engines for power generation and for process heat in timber kilns, at no time has NOx been an emission problem.
Although gasifier temperatures are high, it appears that it just doesn’t happen in this process although I suppose anything is possible given certain conditions existing.
The adiabatic flame temperature of clean producer gas ie tar free has never exceeded 1100 degrees Celcius, in our experience, and in fact is one of the measures we use to check to see if tar is present in the gas stream.
However, Dr. Tom Miles then challenged such assurances:
One should not conclude that gasifiers do not make NOx. We measured NOx from the gasification of fuels with different nitrogen contents and found that, for a given system, NOx generation is proportional to N content.
Comparisons of NOx reduction from gasification with staged combustion and flue gas recirculation with selective non catalytic reduction using ammonia or urea are welcome.
Unfortunately, while there is a growing body of knowledge as to the practical value of gas gasification with pulp mill waste, there is little yet published as to efficiencies of staged combustion and FGR (Flue-Gas Recirculation) techniques for high nitrogen fuels.
Dr. Tom Miles knows of only one one large scale sewage sludge system that has been tested in the Unites States:
Three gasifiers were installed in 1984-1986 by (the former) Combustion Power at the Hyperion sewage treatment plant in Los Angeles County. The sewage sludge was dried using light oil to displace the water in the “Carver Greenfield” process. The resulting dried powder was pneumatically conveyed into a bubbling fluidized bed. Gas was staged using partial oxidation in three stages before final combustion. Input N was about 8% if I recall. NOx was reduced from several thousand ppm to 25 ppm. The plant was dismantled after they couldnt get the drying process to work properly.
Thus, if sufficiently dried, wastewater residuals could comprise feedstock for efficient biomass gasification. According to an April 2002 White Paper from the Residuals and Biosolids Committee — Bioenergy Technology Subcommittee of the Water Environment Federation1 (PDF):
The control of moisture in the feedstock is critical to efficient gasification. As the moisture level increases the heat available from the char is no longer sufficient to maintain gasification and also vaporize the water in the feed. About 1400 to 1700 Btu’s / lb. water evaporated are required for biosolids drying, combustion, and gasification systems.
Reduction in the feedstock moisture concentration will increase overall efficiency of the gasification process. The maximum moisture concentration in the feedstock should not exceed 40% moisture for gasification performance viability. Moisture concentrations of less than 25% are desirable for efficient gasification results.
Capital, operation and maintenance costs currently remain less economical than disposal-only alternatives. However, disposal costs are tied to fuel costs so that rising costs of current disposal-only alternatives, coupled with efficiency improvements and inherent environmental advantages in gasification technologies soon may mean much more development. Still, the initial investment is far from trivial and there are few companies globally to implement systems that with cost-effective modification should remain adequate over the life of the plant.
Currently, sufficient lead time is necessary for plant construction. It is unlikely that many private investors are willing to anticipate higher oil prices by building such capability now, which means that such front-end expenses must be met by bond issues, grants, and dwindling budget reserves achieved through other improvements.
Meanwhile, the feedstock unquestionably is available. Take Iowa as an example.
At least 19 Iowa municipal wastewater treatment plants capture methane for on-site use and electricity generation, collecting more than 1.2 million cubic feet of biogas per day, enough energy to heat 2,555 homes annually. More than 250,000 tons of municipal treated sludge could be available each year for anaerobic digestion in Iowa with the potential to produce 3 trillion BTU, the equivalent to the power and heat consumed by 23,700 homes annually.
Dealing with sewage sludge as a feedstock raises special concerns in areas of health, safety, and the environment. The National Association of Clean Water Agencies, the Water Environment Federation, and U.S. Environmental Protection Agency, provide input into standards established by the National Biosolids Partnership
In March 2006 the Kent County Regional Wastewater Treatment Plant in Milford, DE was recognized as the first public agency in the United States to be certified according to 1) NBP EMS guidelines, 2) the ISO 14001 standard and 3) the OHSAS 18001 standard. It is one of nearly 90 agencies to adopt the environmental management system requirements of the National Biosolids Partnership and the 11th agency in the country to receive NBP certification during 2005.
According to the WEF, meeting such standards requires wet and dry scrubbing technologies. Gas scrubbing must remove 96% to 99% of all particulate matter and tar aerosols. The entire process also must ensure:
- Destruction of all pathogens, viruses, and organochlorinated compounds.
- Immobilization of heavy metals in wastewater residuals.
- Significant reduction in odor problems.
- No threat of groundwater contamination.
Beside gaining energy from waste, thus reducing the amount to organic solids needing disposal, there is additional environmental benefit from minimization of greenhouse gas emissions. Such payoffs have been driving research in the Alternative Fuels and Renewable Energy Program at UC-Riverside. The Bourns College of Engineering - Center for Environmental Research and Technology has developed a “fueled-enhanced” steam flame heated gasification system. Alkalization plus higher temperatures, i.e., molten-salt loops, may be on way to solve input challenges.
University of California-Riverside illustration
The process starts with a slurry, which is pumped to a container, basically a huge autoclave. While under high heat and pressure gas is mixed with the superheated material, then is blown into a reactor for steam reformation. The gaseous products become the feedstock for further processing.
When waste materials are ground into small particles (less than 1 mm in diameter) and mixed with water to form a slurry, which is then pumped into a steam generator that heats the mixture to about 700 C and 30 atmospheres pressure. The superheated steam and hot waste particles are then mixed with hydrogen gas inside a long tubular reactor, known as a hydro-gasifier.
Inside the hydro-gasifier, the hydrogen reacts with any carbon present in the waste particles and forms methane gas. The methane gas and the superheated steam are then fed into a second stage reactor, known as a steam reformer.
In this reactor, the steam reacts with the methane to form hydrogen, carbon monoxide and carbon dioxide gases. Approximately half the hydrogen produced in the steam reformer is recycled back into the first stage hydro-gasifier, making it self-sustaining.
The remaining synthesis gases (hydrogen, carbon monoxide and methane) can either be used in a CE-CERT-developed variable gaseous fueled engine to produce electricity and process-heat, or sent on to a liquid fuel synthesizer designed to produce sulfur-free synthetic diesel fuel and recycled clean water. Molten salt heat transfer loops take heat away from the hydro-gasifier and fuel synthesis reactors and transfer it to the water steam generator and steam reformer reactor to make the system almost thermally self-sufficient. Thus carbonaceous waste and water feeds can be converted into fuels, process-heat, and recovered water in what is expected to be a series of self-sustaining processes.”Converting Wet Bio-Waste Into Energy”
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