Gasification is a thermo-chemical process in which carbonaceous (carbon-rich) feedstocks such as coal, petro-coke or biomass are converted into a gas consisting of hydrogen and carbon monoxide (and lesser amounts of carbon dioxide and other trace gases) under oxygen depleted, high pressure, high-heat and/or steam conditions. The resulting gaseous compound is called Syngas. Carried out under proper conditions, gasification is an efficient energy extracting process that can return double benefits as a waste stream disposal system. Interestingly, this technology is nothing new; it was used extensively during WWII when ration-weary, gasoline-starved farmers used simple homemade gasifiers to turn wood into gas/liquid fuel for their tractors and equipment.
A Little History on Gasification
Gasification is an old technology with a long history of development. The process was mainly used from the mid-1800’s through the early-1900’s to produce “town gas” from coal for heating and lighting purposes. The subsequent development of natural gas fields soon replaced “town gas." World War II brought a resurgence of gasification when petroleum starved Europeans used wood gas generators to power vehicles. But the need for liquid fuels remained and German engineers devised a way to make synthetic liquid fuel from gasified coal.
The 1970’s brought The Arab Oil Embargo and the “energy crisis” which prompted the U.S. government to support industrial scale gasification projects. From this development came the first Integrated Gasification Combined Cycle (IGCC) electric generating plant. Presently, several IGCC power plants are operating throughout the world. And crude oil price spikes and geopolitical instabilities in major oil-producing countries have generated serious interest in using gasification for GTL (Gas To Liquid) synthetic fuel processes.
The Process: Gasification, not Combustion
Combustion is an exothermic (heat releasing) reaction between a high carbon fuel and an oxidizer (a substance that supports combustion, usually oxygen) in which the fuel is burned to produce heat as an energy source.
Carbon fuel + Oxygen → Heat + Water + Carbon DioxideGasification is an exothermic reaction between a high carbon fuel and a carefully controlled and limited supply of oxidizer, in which the fuel yields useful elemental and compound gases that can be made into other products.
Carbon fuel + Oxygen → Hydrogen + Carbon Monoxide (plus trace Water and Carbon Dioxide)
Types of Gasifiers
Many design variations of Gasification Reactors (Gasifiers) exist, however, depending upon the type of flow conditions present in the equipment, they all fall into one of three general categories:
- Moving Bed - Dry carbon fuel is fed through the top of the gasifier. As it slowly drops through the vessel, it reacts with steam and oxygen as they flow in opposite in directions over the bed. The fuel goes through the process until it is completely spent leaving behind low temperature syngas and molten ash. Trace contaminants are later scrubbed from the syngas.
- Entrained Flow - Fuel can be fed dry or wet (mixed with water) into the gasifier. The reactants (steam and oxygen) flow uni-directionally upwards through the gasifier, as the stages of gasification take place, until high temperature completed syngas exits the top of the reactor. Molten slag drops out at the bottom.
- Fluidized Bed - Steam and oxygen flow upwards through the reactor tower while fuel is injected into, and remains suspended in, this stream while gasification takes place. Moderate temperature syngas exits the while dry (unmelted) ash is evacuated at the bottom.
Why Bother with Gasification?
So why go through all of this? Yes, it is a complicated and challenging process, but the reasons for developing gasification are compelling. Hydrocarbon resources throughout the world are both finite and unevenly distributed. For the United States, dwindling reserves of crude oil and natural gas contrasted against abundant coal and biomass resources illustrate the economic viability of gasification. Syngas can be used to not only manufacture all manner of industrial and commercial chemicals, but it can also play an important role in the development and manufacturing of synthetic liquid fuels for transportation.Advantages of Gasification
- Feedstock flexibility
- Product flexibility
- Near-zero emissions
- High efficiency
- Energy security
- Complex multi-stage process
- Up-front processing of feedstock
- Syngas must be cleaned/purified
- Initial setup is expensive
The gasification story is not all glory. It has its drawbacks and trade-offs just like any other complex endeavor, and whether it is ultimately fruitful and sustainable remains to be seen. Looking at the big picture though, gasification, and its resulting Syngas, with its potential to create all types of useful chemicals and synthetic fuels, certainly deserves a spot-at-the-table with other alternatives.