IGCC

Gasification technology is the cleanest way to make energy from coal. A typical coal plant emits more sulfur dioxide and nitrogen oxide in a few weeks than a state-of-the-art gasification plant produces in a year.

The process

Gasification of solid fuels to produce chemicals is a mature industry, with hundreds of plants worldwide used to make ammonia, methanol, substitute natural gas, and diesel fuel.  A smaller number-- around 16 plants-- use gasification to produce electricity.
 
In gasification, steam and oxygen convert coal into synthesis gas or “syngas.” Syngas is composed of hydrogen (“H2”) and carbon monoxide (“CO”) with relatively small amounts of other impurities that must be removed.

Environmental benefitsSO2 Emision Rates

  Gasification has many environmental benefits:
  • Vastly lower levels of sulfur dioxide, nitrogen oxides, and mercury emissions compared to conventional coal-fired power plants.
  • Carbon dioxide capture technology for syngas has been commercially available for decades.
  • Less solid waste - Solids from gasifiers are half the volume of conventional coal plants. The waste is also vitrified (enclosed in a glass-like substance) and therefore less likely to leach contaminants into the ground.
  • Less water use. IGCC plants use 20% -50% less water than conventional coal plants.

 

 

 

NOx Emissions ChartElectricity generation from gasification

Two principal options are used to make electricity with gasification:  Integrated Gasification Combined Cycle (IGCC) and Substitute Natural Gas (SNG).

IGCC

In an IGCC plant, syngas is burned in a turbine to produce electricity.  Excess heat from this process is then captured and used to power a second turbine that makes more electricity. Power plants like this are called “combined cycle” plants. The complete power plant including gasification and combined cycled unit is called an “integrated gasification combined cycle”, or “IGCC”, power plant.

SNG

Alternatively, syngas produced from coal gasification can be further processed to make substitute natural gas or SNG.  SNG can then be burned in a natural gas combined cycle plant to produce electricity.  These plants often make excess SNG which can then be sold through interstate natural gas pipelines to other gas plants.


Approximately 90% of the carbon dioxide created in the substitute natural gas process can be captured and stored.  This results in a net 50% or more reduction in CO2 emissions compared to the original coal when the substitute natural gas is burned in a conventional natural gas power plant. Read More (.pdf) >>

Gasification Carbon Capture

Carbon capture from gasification is a mature, available technology which has been used commercially for decades in chemical plants.  In these instances, CO2 is removed to produce certain specifications for syngas - a mixture of hydrogen, carbon monoxide and carbon dioxide - necessary for the manufacture of chemicals - including methanol, diesel, substitute natural gas, ammonia.

CO2 in these instances is typically vented to the atmosphere because the capture step was needed for processing, not environmental protection.  Technically, CO2 captured through this type of process could have been compressed for use in enhanced oil recovery or stored permanently underground.

CO2 capture from gasification starts with a water shift reaction(rollover that describes this step- A chemical reaction whereby carbon monoxide and water react to form CO2 and H2),followed by CO2 removal using any number of commercial processes. These processes remove about 90% of the CO2 found in syngas.

CO2 capture is not required to make electricity from an IGCC power plant, but is instead an added step requiring additional electricity.  This electricity adds to what is known as the parasitic load required to operate the plant.  The higher the parasitic load, the lower the overall efficiency of the plant.  Because of the added capital and fuel costs, few IGCC plants currently operating employ CO2 capture systems.

Several IGCC plants located at European oil refineries use liquid refinery waste (not coal) and capture CO2 (which is vented to the atmosphere) in order to produce hydrogen for refinery use. The refineries are the Pernis refinery in the Netherlands and the Sarlux refinery in Italy.

Typical Capture Levels

Several levels of capture can be used on an IGCC plant.[1][2] They include:
  • 90% Capture - All carbon in the syngas is converted to CO2 using a shift reaction resulting in nearly all of the CO2 removed.  The remaining hydrogen fuel is burned in a special hydrogen turbine to produce electricity.
  • 50%-60% Capture - Most, but not all carbon in the syngas is converted to CO2 using a shift reaction.  After CO2 removal, the resulting fuel burned in the turbine is a mixture of hydrogen and carbon monoxide.  The resulting emissions rival a conventional natural gas fired power plant.  This option costs less than the 90% option, and uses industry-standard syngas turbines rather than special ones designed only for hydrogen.
  • 18%-30% capture-  No shift reaction is used to increase the H2 and CO2 content of the syngas.  Only the native CO2 in the syngas is removed, with the resulting H2 and CO burned in a turbine.  Depending upon the gasification technology and type of coal, the resulting CO2 emissions are reduced by 18% -30% compared to a non-capture IGCC plant.


[1] GE Energy IGCC vs. Carbon (.pdf) >> 

[2] Mesaba Energy Project Partial Carbon Dioxide Capture Case (.pdf)>>