IGCC Power Generation

Integrated coal Gasification Combined Cycle (IGCC) power plant is the most environmentally friendly coal-fired power generation technology. Most importantly, coal gasification offers the immediate opportunity to generate power with near zero greenhouse gas emissions and the pathway to a future hydrogen economy.

Description
Coal gasification is the process of converting coal to a gaseous fuel through partial oxidation. The coal is fed into a high-temperature pressurized container along with steam and a limited amount of oxygen to produce a gas. The gas is known as synthesis gas or syngas and mainly consists of carbon monoxide and hydrogen. The gas is cooled and undesirable components, such as carbon dioxide and sulphur are removed. The gas can be used as a fuel or further processed and concentrated into a chemical or liquid fuel.

Integrated gasification combined-cycle (IGCC) systems combine a coal gasification unit with a gas fired combined cycle power generation unit. The first stage is the coal gasification process as mentioned above. The second stage takes the cleaned gas and burns it in a conventional gas turbine to produce electrical energy, and the hot exhaust gas is recovered and used to boil water, creating steam for a steam turbine which also produces electrical energy. In typical plants, about 65% of the electrical energy is produced by the gas turbine and 35% by the steam turbine.

In general the advantages of IGCC are:

• It can achieve up to 50% thermal efficiency. This is a higher efficiency compared to conventional coal power plants meaning there is less coal consumed to produce the same amount of energy, resulting in lower rates of carbon dioxide (CO2) emissions
• It produces about half the volume of solid wastes as a conventional coal power plant.
• It uses 20-50% less water compared to a conventional coal power station.
• It can utilise a variety of fuels, like heavy oils, petroleum cokes, and coals.
• Up to 100% of the carbon dioxide can be captured from IGCC, making the technology suitable for carbon dioxide storage.
• carbon capture is easier and costs less than capture from a pulverised coal plant
• A minimum of 95% of the sulphur is removed and this exceeds the performance of most advanced coal-fired generating units currently installed.
• Nitrogen oxides (NOx) emissions are below 50ppm. This is lower than many of today’s most advanced coal-fired generating units.
• The syngas produce from a gasifier unit can be burned in a gas turbine for electricity generation, or used as a fuel in other applications, such as hydrogen-powered fuel cell vehicles

Coal Gasification
There are three basic gasifier designs used for coal gasification in IGCC plants.

• Entrained flow gasifiers,
• Fixed bed (sometimes referred to as moving bed) gasifiers and
• Fluidised bed gasifiers.

Entrained flow gasifier

Entrained flow gasifier
Entrained flow is the most aggressive form of gasification, with the pulverised coal and oxidising gas flowing cocurrently. High reaction intensity is provided by a high pressure (20-30 atm), high temperature (>1400°C) environment. Extremely turbulent flow sees the coal particles experience significant backmixing, and residence times are measured in seconds.

Entrained Flow Gasification (EFG) is specifically designed for low reactivity coals and high coal throughput. Single pass carbon conversions are in the range of 95-99%.

To experience smooth operation, the gasifier temperature must lie above the coal AFT or fluxes, which lower the melting temperature of the coal mineral matter, must be used. A number of system constraints impose an economic limit on gasification temperature at 1400–1500°C.

• Extremely high temperatures shorten refractory life.
• There is a limit to the heat that can be recovered from the raw fuel gas, without water quench becoming necessary.
• A cost is incurred in providing oxygen and coal to sustain the gasifier temperature.

Consequently, it may be necessary to add fluxes or blend high AFT coals to achieve good slagging characteristics at economic gasifier temperatures.

BGL Fixed Bed Gasifier

Fixed Bed Gasifiers
Fixed bed gasifier operate at 26 bar and closely resemble a blast furnace. Coal and fluxes are placed on the top of a descending bed in a refractory lined vessel. On moving downwards, the coal is gradually heated and contacted with an oxygen enriched gas flowing upwards counter currently. Pyrolysis, char gasification, combustion and ash melting occur sequentially.

The temperature at the top of the bed is typically 450°C, and at the bottom approximately 2000°C. All coal mineral matter melts and is tapped as an inert slag. Ash melt characteristics influence bed permeability, and fluxes may need to be added to modify slag flow characteristics.

Fixed bed gasifier offgas contains tars – which must be condensed and recycled. The production of tars makes downstream gas cleaning more complicated than with other IGCC processes.

The residence time of fixed bed gasifiers is between 30 minutes and one hour, which places stringent restrictions on the coal’s physical and chemical properties. Coal size distribution must be controlled to ensure good bed permeability. Excess fines can be injected with oxygen at the tuyeres or briquetted to maintain stable operation.

Long residence times mean that fixed bed gasifiers have a low throughput and hence have limited application in large scale IGCC plants.

To ensure stable fluid bed operation, gasification temperatures are kept below the AFT of the coal. Above this temperature, particles become sticky and excessive levels of agglomeration will occur, resulting in bed defluidisation.

Fluidised Bed Gasifier

Fluidised bed gasifier
A fluidised bed reactor is a vessel in which fine solids are kept in suspension by a gas such that the whole bed exhibits a fluid like behaviour. This type of reacting system is characterised by high heat and mass transfer rates (increased reaction rates) between the solid and gas.

In fluid bed gasification, rising oxygen enriched gas reacts with suspended coal at a temperature of 950–1100°C and pressure of 20-30 bar. High levels of back mixing result in fluid bed gasifiers having a uniform temperature distribution.

The unusual characteristic of fluid bed gasifiers is that the majority of the bed material is not coal but accumulated mineral matter and sorbent (for in-situ desulphurisation). Operating with a high inventory of inert bed material has a number of advantages:

• The coal experiences high heat transfer rates on entry.
• The gasifier can operate at variable load (high turndown flexibility).

The low temperature operation limits the use of fluidised bed gasifiers to reactive and predominantly low rank coals. Most fluid bed gasifiers have a high level of entrained fines recycle to achieve 95–98% carbon conversion.

To reduce the size of the fines recycle stream, it has been proposed that the gasifier is linked with a fluid bed combustor (Air Blown Gasification Cycle). In this process, the coal is first gasified to 70-80% carbon conversion. The unreacted char is then fed to the combustor where generated heat is used for steam production. The gasifier-combustor combination enables the use of low reactivity, high Ash Fusion Temperature (AFT) coals in an IGCC system.

For more information contact:

Dr Noel Simento
Manager Technology
CCSD
[ www ]: www.ccsd.biz