Digester Technology Developments for Cheaper Renewable Fuel

A very popular idea currently gaining publicity is a very old concept: the methane digester. The methane given off during the decomposition of the manure is captured and burned, providing either heat or power, for electrical generation. These promise a minor revolution in small and medium scale energy generation from methane, with a scale smaller than wind turbines, but still significant in terms of national adjustments to high oil prices.

However, digesters have been criticized for being inefficient and unstable in operation. But, the technology of anaerobic digestion has been largely ignored until the last run on oil prices about 5 years ago (about 2003), when for the first time for as long as anyone can remember the oil price exceeded the production cost for fuel produced as methane by digestion.

Five years has been scarcely long enough for some half a dozen to one dozen AD plants to be designed, constructed and commissioned. These should be considered to be a first generation of a new breed of reactors using this technology. This is a bit like the people who criticized the motor car for being slow while the law (in the UK certainly) required all automobiles to be preceded by a man holding a flag to warn pedestrians.

Many did criticize the automobile at that time, but do you want to do so for digesters, as I think that you will be looking as silly as those flag wavers were just ten years later, when the motor car became an established mode of transport.

There are many ways in which the efficiency of Anaerobic Digester bio-reactors are being improved, and the first is by using sophisticated ultrasonic technology to break up the particles and so allow breakdown of a bigger proportion of the organic content. 

In some of the other processes being developed the excess liquor from the process is used to re-wet incoming biowaste as it contains useful bacterial populations. This method can produce a faster reaction then the original start-up. 

It is important because on-farm Digester (Anaerobic Digestion) projects can provide needed services to farmers; develop local, renewable electrical generation; enhance environmental quality; and generate income for the community.

Other researchers have identified the fact that if you have fluctuating temperatures, then you will not be able to establish an optimum microbial population. The digester stirring system must be efficient and operational at all times to ensure that the cold, newly introduced sludge, is mixed with the warm older solids and the bacteria. This sounds easy but in a large tank with a fairly viscous sludge mass it can be surprisingly onerous on the mixing technology.

Anaerobic digestion consists of a series of reactions which are catalyzed by a mixed group of bacteria and through which organic matter is converted in a stepwise fashion to methane and carbon dioxide. Polymers such as cellulose, hemicellulose, pectin, and starch are hydrolyzed to oligomers or monomers, which are then metabolized by fermentative bacteria with the production of hydrogen (H2), carbon dioxide (CO2), and volatile organic acids such as acetate, propionate, and butyrate. Clearly, this is a complex reaction which e can be greatly improved by better knowledge gained by further academic study which can now take place given the raised awareness and importance of this technique. This will most likely yet result in big advances in how man designs and runs its new digesters.

In the developing world another angle for them is selling carbon credits from the renewable energy created by anaerobic digestion on the worldwide market. Those credits should be a source of income for as well as providing a way to readily obtain seed capital for these projects from the banks.

However, the process also produces a solid and a liquid digestate in the slurry. The use of the process would not be sustainable without an environmentally safe method of disposal, and better still preferably a 'beneficial use' of the output from digesters. 

The market for the digestion processing outputs is still undeveloped just about everywhere. However, there are some positive signs reported that the outputs will be genuinely useful, and indeed a source for additional revenue for the operators of these plants.

The adoption of manure digesters at animal operations is much more advanced in Europe than in the U.S. But, there are many successful AD plants in operation throughout the U.S. 

Northern Concrete has one such installation and has reported on its digestion process. They have said that the feedstock (animal byproduct) goes into a holding area until it is ready to enter the digester. It sits in the digester for 22 days and is released as useful by-products like methane and a grassy sawdust-like product that can be used as fertilizer, animal bedding or after further processing for floor boards.

There is certainly other evidence of progress in selling AD outputs. Another operator (Pro-Gro Mixes of Tualatin, Ore.) is thought to have contracted to market the solids material or digested fiber to the wholesale nursery and landscape industries, reportedly. It is understood to be selling between 1,000 to 3,000 yards of digested fiber, under the FiberLife brand, per month in the Willamette Valley. 

There is also potential for the methane to be burnt in efficient turbines, rather than today's ubiquitous reciprocating engines. Here the heat from turbine exhaust is used to maintain the optimum digester temperature and sustain bio-gas production. The resultant bio-gas is collected from one such system and cleaned, then used to fire the turbines. The results have reportedly been way above expectations, with a significant increase in production, higher yield and fewer rejects being recorded. The digester in question is thought to qualify as a small-power production facility, which means it follows a funding schedule, enabling projects to gain rapid approval.

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About the Author: Digesters can be considered for a wide variety of agricultural and
industrial and commercial sites. From agricultural
community scale Anaerobic Digesters to supermarkets with
waste food, to municipal authorities with organic waste in their
collected waste streams. All should now be considering the installation
of a digester of one type of another. For more information visit the Steve Evans Renewable Energy Hub Page
web site, compiled by Steve Evans.