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Gasification Technology - Page 5

 Introduction | History | Challenges | Benefits | Current Tech | Future Tech | Conclusions 

By Scott Miller — February 2011


Currently, there are four basic types of gasifiers in commercial use today. The up draft gasifier uses a fixed bed of feedstock with steam and/or oxygen flowing in a counter-current direction. This design typically results in a high thermal efficiency but its throughput limits its usefulness for the application being pursued here (high volume fuel production). The down draft gasifier is similar to the up draft, with the difference being the direction of the flow of steam and/or oxygen.

Fluidized bed reactors are arguably the best models out of today’s options for the large-scale production of fuels. The best description of the fluidized bed reactor I have seen is that it is much like a hot air popcorn popper. The feedstock is generally pulverized and is fed into a chamber where it is floated in a mix of steam and hot air. The gasification reaction occurs at which point gravity and aerodynamics take over and sort out the remaining particles. The process has been in commercial use in petroleum refining since the 1940s and is widely used today for manufacturing chemicals. The gasifiers used in the GPSP are fluidized bed reactors.

The entrained flow gasifier takes a somewhat different approach. Here, the feedstock is either pulverized into a dry powder, atomized as a liquid or converted into fuel slurry and injected along with oxygen into the gasifier. This process works well with most coals and should be convertible to certain dedicated feedstocks (especially wood biomass) however this is a very high temperature process. This method offers the highest current throughput of material but it is very hard on the equipment and the gases must be cooled before being processed with existing technology.

IGCC Co-Production:

A common use for gasification technology today is in Integrated Gasification Combined Cycle (IGCC) power generation systems. These are often referred to as “Clean Coal” systems and involve gasifying the coal feedstock and powering a turbine with the produced syngas. Newer systems are beginning to employ a process of dividing the produced syngas with some going to power generation and the remainder going to fuel and/or chemical production.

Consider that there are currently about 500 conventional coal-fired production plants in the US today. Many of these are older facilities that are only allowed to operate by receiving pollution exemptions due to their age. By converting these plants to IGCC with co-production capabilities, we can eliminate the pollution problems from these facilities while working towards our goal of 10 million barrels per day of domestic production. This would also allow for the opportunity to bring renewable feedstocks into the mix that cannot be used with existing coal power production facilities.
Co-Production of Electricity and Fuels Using IGCC Technology
size: 138 kb - 18 pages

IGCC Co-Production also has the advantage of siting at locations that have generally already been environmentally approved. Through a process called repowering, some existing coal plants can be redesigned as IGCC facilities using much of the existing facility. In other cases, the existing plant would need to be replaced however it is already zoned and located in a position where gasification should be legally permitted.
IGCC Technology Status, Economics and Needs size: 257 kb - 29 pages

Plasma Gasification:

Plasma is an ionized gas that forms when an electrical discharge passes through a gas. The flash from lightning is one example of plasma as is the arc of an arc welder. When applied in a controlled fashion, it can apply an extremely intense amount of heat to break down materials that cannot normally be processed with other methods. The two most common plasma gasifier concepts are Plasma Arc and Plasma Torch designs. Plasma Arc gasifiers use carbon electrodes that are continuously fed into the gasifier. These electrodes strike an extremely high temperature arc that gasifies the feedstock. The Plasma Torch design typically uses a fixed copper electrode that arcs either to a molten slag bath or an electrode of opposite polarity to consume the material. With both designs, the feedstock requires less pre-processing compared to other gasifier designs.

Used commercially in Japan, Canada and India, plasma gasification is usually found in waste management applications where incineration and land filling are not viable options. Most current plasma gasifiers today are used for electricity generation (burning the syngas directly) or for manufacturing chemicals. While the capital costs for plasma are typically higher than with conventional gasification, the technology is also capable of processing a wider variety of feedstocks including mixed feedstocks because of the higher operating temperatures. Plasma also offers a more complete conversion of the feedstock than conventional gasification.

One key environmental advantage of plasma gasification is the elimination of the ash by-product of the process. Conventional gasifiers produce ash that is the inorganic remains of the molecules extracted from the feedstock (metals exit as slag). With plasma gasifiers, because of the extremely high temperatures, this remaining material exits the process as a vitrified, leach-resistant glass that can be used in a variety of construction applications.

 Introduction | History | Challenges | Benefits | Current Tech | Future Tech | Conclusions 

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