managing e-waste since 1995

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. 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.

Julian Powell
chemical engineer