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Welcome to the Energy & Environment (E&E) Exchange, a blog dedicated to science and engineering topics that are (generally) related to energy and the environment. This blog is meant to encourage discussion about the challenges and possibilities surrounding sustainability through science and technology. The blog's owner, cheme_wordsmithy, is a former technical writer and engineering editor at IEEE GlobalSpec, the company that powers CR4.

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A New Development In Waste Gasification

Posted December 02, 2011 10:00 AM by cheme_wordsmithy

Gasification is the process by which carbon-based materials are converted to a combustible syngas at high temperatures in a controlled oxygen environment. The technology has been around since the mid-1800s, developed originally as a means to convert coal to usable gas for fuel, heat, and light. Through its evolution, gasifiers have been used for producing heat, generating power, and making liquid fuels and chemicals.

How Gasification Works

Gasification is the reaction of carbonaceous materials with steam under high temperature (anywhere between 650 and 1,400 degrees Celsius) to produce carbon monoxide and hydrogen. The basic chemistry is: C + H2O à H2 + CO, though other reactions take place based on the composition of the feedstock and atmosphere in the reaction chamber. By-products of the process can include ash and char or slag.

Gasification systems can vary in a number of different ways. Most significantly is whether the heat for the reaction is supplied internally through oxidizers (direct gasification) or from an external heat source (indirect gasification). Pyrolysis is an indirect gasification process in an inert atmosphere.

Image Credit: BVSDE

As It Stands Today

Major developments in gasifier technology today revolve around Integrated Gasification Combined Cycle (IGCC) power plants. Instead of directly combusting the fossil fuels, these facilities use the fuel to make syngas which, after cleaning, can then be used at elevated temperatures and pressures to heat steam to run a turbine. Electricity from syngas is an attractive alternative to direct combustion, as it can be produced from low value feedstock sources and has the potential to be cleaner and more efficient, reducing emissions and creating useful or disposable byproducts from captured pollutants.

But the pipedream of sustainable gasifier technology involves the use of municipal solid waste (MSW) as the feedstock, meaning it acts as both a waste reduction and energy generation process. This is (in concept) a much more desirable disposal option than landfill dumping and produces more energy than waste-incineration plants.

Beneath the obvious benefits of this waste-to-energy process lie a lot of ugly problems and logistical issues, however. For one, MSW generally requires extensive pretreatment before it can be put into a gasifier. This includes mechanical and manual sorting to remove recyclables and undesirable materials, and crushers to grind the material into a usable form. Secondly, the waste generally requires preheating to lower the moisture content to a reasonable level. Effectively cleaning the dirty syngas is also a problem, as MSW gasification will generate NOX, SOX, and various VOCs. Also, some types of gasifiers cannot handle the inconsistent composition of MSW in the reaction chamber, and those that can require energy from another source for startup. Finally, the average carbon content of MSW is only around 20%, meaning a large amount (tens to hundreds of tons) of trash per day would be needed to generate a noticeable amount of electricity for a community.

The Promises of MAGS

Terragon Environmental Technologies Inc. is tackling waste-to-energy on a smaller scale. Their Micro Auto Gasification System (MAGS), designed alongside the Office of Naval Research (ONR), is first and foremost a waste-disposal system. It was developed to provide a simple operating, compact solution for handling waste streams in naval vessels, short facilities, and isolated areas. The capacity of a single MAGS system can handle the waste generation of about 1000 soldiers or a community of 500 people.

Here is a video describing the MAGS system.

The gasification method is controlled pyrolysis, in which up to 40kg of waste is heated in a chamber at 750 degrees Celsius in an inert atmosphere. The result is a low emissions char product along with syngas which is diverted through a cleaning system to a combustion chamber to supply heat for the process. Glass and metal are left intact and ready for recycling, and the wastewater and emissions of the process are said to be well within the standards, according to developers.

"Decades ago, the idea of harvesting energy from trash was just a side show in the environmental movement," said Steve McElvany, the MAGS program officer at ONR. "Now, the technology is mature enough to where the Department of the Navy is seriously evaluating its practical and tactical benefits."

They also suggest that MAGS technology will be beneficial for small communities and businesses to reduce both waste and energy costs. Needless to say, the history and past difficulties regarding waste gasification are enough to draw plenty of skepticism towards the technology. But small-scale developments like these could be the right approach to making the process practical.

Sources:

ONR - MAGS

BVSDE - Energy from gasification of solid wastes

IJESD - Waste to Energy: A Case Study

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