Coal Gasification:
19th century technology for 21st century energy needs
1. History of coal gasification
Coal gasification, the chemical conversion of coal to a gaseous state, was first used to produce gas for lighting and heat in the US and UK in the early 19th century. Prior to the discovery of 'natural' gas it was the fuel of choice, lighting cities across the US and Europe. At the time natural gas was discovered it was so abundant that coal gas was quickly abandoned in the late 19th century.
The energy crisis of the 70's led to a resurgence of interest in coal gas as an energy alternative. A major effort began to commercialize the technology on a large scale for clean energy and chemical feedstock production. The abundant coal reserves of the United States and other countries posed a challenge to engineers – how to utilize this reserve of fossil fuels in a fashion that did not cause the environmental harm of traditional methods of coal combustion.
Decades
of research and development have resulted in a mature technology that
is currently being 'scaled up' as larger and larger plants are built,
allowing further improvements in the process. Recently announced
projects are on the scale of large conventional PC (pulverized coal)
power plants. The Southern Company is currently building a 400 MW
facility near Orlando, FL, American Electric Power recently announced
plans to build a 600 MW unit in West Virginia and a 900 MW unit is
planned in Poland. Worldwide there are 62 operational coal
gasification units and 24 in the planning stages. Generating unit
construction costs are somewhat higher than PC combustion units and
will likely be on a par in the near future as design costs are
amortized over more units and standardized designs emerge
2. How it works
Coal
gasification units partially burn coal with pure oxygen and steam to
generate synthetic gas or 'syngas', a mixture of carbon dioxide,
hydrogen and carbon monoxide. Syngas is burned in conventional
natural gas turbines to generate electrical energy using high
efficiency gas turbines that are engineered to eliminate soot and
minimize formation of nitrogen oxides (NOx), precursers to ozone smog
and acid rain.
Coal gasification
for electric power generation enables the use of a technology
common in modern natural gas fired power plants, the use of 'combined
cycle' technology to recover more of the energy released by burning
the fuel. In a combined cycle plant the gas is burned in a turbine that
generates electricity, then steam is generated from
the hot exhaust and used to power a second generator. This method
achieves an efficiency of 45-50% compared to traditional power
plants which only use one cycle to create
electricity at only 35% efficiency.
Coal gasification units
offer unique flexibility in that the syngas can also be chemically
converted to liquid fuels for use in transportation (coal
liquefaction). Alternatively the hydrogen can be refined from the
syngas to produce hydrogen fuel. An IGCC power plant could be
used for electric power production during peak demand times then
shifted to liquid fuel production at night.
3. Environmental advantages
The
problems inherent in coal combustion are well known – from the
particulates to the acid rain caused by high-sulfur coal, to mercury
pollution, not to mention the CO2 and mountaintop removal
mining. Burning coal in any fashion
is a dirty process,
and trying to remove the pollutants from flue gases is challenging to
say the least.
One of the major
environmental advantages of coal
gasification is the opportunity to remove impurities such as sulfur
and mercury and soot before burning the fuel, using readily
available chemical engineering processes. In addition the ash
produced is in a vitreous or glasslike state which can be recycled as
concrete aggregate, unlike PC plants which generate ash that must be
landfilled, potentially contaminating groundwater.
The increased
efficiency of the 'combined cycle' for electrical power generation
results in a 50% decrease in CO2 emissions compared to conventional
coal plants. As the technology required to develop economical
methods of carbon sequestration, the removal of CO2 from
combustion by-products to prevent its release to the atmosphere, coal
gasification units could be modified to further reduce their
climate change impact because a large part of the CO2 generated can
be separated from the syngas before combustion.
Chemical Engineer