(The Gist of Science Reporter) High Ash Indian Coals Gasification
High Ash Indian Coals Gasification
- In India, coal based energy generation meets around 70 percent of our
energy needs. However, bulk coal reserves (about 7.1%) are of Inferior grade
non-cooking coals with ash contents 45-50%, having moderately high
reactivity and high ash fusion temperature. Indian coals are not only high
in ash, hut also the association of mineral matter with carbon matrix is
very close and in dispersed form.
- All three main types of gasifier i.e. Entrained Flow Gasifier (EFG),
Fluidised Bed Gasifier (FBG) and Moving Bed Gasifier (MBG) can be used to
gasify die coal: however, gasifier efficiency and stability are ensured
under a range of values of certain characteristics of the coal. The size of
the project may also have an effect.
Gasifiers & Gasification Technologies
- Gasifiers are usually classified by the flow regime inside the reactor.
There are three main categories viz. MBG, FBG and EFG. Each type of gasifier
depending on the operating characteristics can be further classified through
operating pressure, the way heat is produced and transferred (Auto-thermal
or allothermal), nature of gasification agents employed fair or oxygen), ash
removal (Slagging, dry ash or agglomerate), ang Gasifier lining (refractory,
- Proximate analysis: Coal moisture content decides whether the
coal fed into the gasifier should lie dry or slurry. Volatile matter of the
fuel determines the extent and rate of gasification reactions and also a
fleets the syngas generation capacity. Ash content decreases the system
efficiency, increases the slag production and disposal cost. It can cause
- Ultimate (elemental) analysis: Carbon, hydrogen, nitrogen,
sulfur, and oxygen are important for material and energy balance and
decision of gasifying agent feeding per unit mass of fuel.
- Mechanical strength: Coals with high Hard Grove Index (HGI) are
more suitable for slurry fed EFG. MBG and FBG need sufficient cold and hot
strength of fuel.
- Caking properties: Indicates difficulties due to plastic zone
formation in MBG and FBG.
- Specific surface area and porosity: Decides approach of gasifying
agents to available active sites and hence gasification reactivity.
- Gasification reactivity of coal: Reactivity dictates the
dimensions of reduction zone that are required and the residence time
required for complete gasification. Coals with low char yield and high char
reactivity (as lignites or brown coals) arc more suitable for FBG.
However, gasifiers with slagging ash can operate with higher rank coals. The
reactivity decreases with coal rank and increases with the oxygen functional
groups content of the fuel.
Ash composition analysis:
Analysis of major elements of ash such as Si, Al, Fe. Na, K, Ca, Mg, P, etc.
as well as minor elements (Chlorine, fluorine. phosphorus, Mercury) plays
important role in ash behaviour inside the gasifier. Certain substances
contained in the ash (CaO, SiO2, Fe2O3), Sodium
compounds, alkali matter can attack the refractory material and reduce the life
of the refractory as well as adhere on the pipes walls at downstream.
Agglomeration is another ash-derived problem that can affect the process.
Ash fusion temperature:
The AFT values under Oxidizing & reducing conditions are strongly dependent
on the ash composition and the applied atmosphere. Depending on the ash disposal
characteristics of the gasifier, operating temperature is maintained above or
below the ash fusion temperature. For dry ash removal system gasifier
temperature must be lower than the AFT, while for slagging gasifier it should be
above AFT. Softening temperature (ST) is considered as the temperature where
agglomeration starts. In slagging gasifiers, the slag viscosity should be
maintained above the critical viscosity for slag flow between bed particles. In
agglomerating gasifier operating temperature must be over the softening
temperature and below the fluid temperature.
Coal Properties vis-a-vis Matching Gasifier
Selection of gasification technology is based on technical features of
gasifier and key coal properties. Following are important properties for each
type of gasifier.
Entrained Flow Gasifier: For entrained flow gasifier, ash content, ash
composition and ash/slag behaviour are important properties as slag flowability
is the most important aspect of the EFG. Further, to form flowable slag layer,
temperature of gasifier is maintained above the Ash Fusion Temperature (AFT),
Slag formed from ash in coal is utilized as a protective layer for refractory or
membrane wall lining and quantity of slag, ash composition and viscosity decides
the thickness of slag layer Further, apart from the above mentioned gasifier
selection criteria, techno-economic features of the gasifiers listed below are
also important to take decision about suitable gasifier selection.
- Reliability: Maturity of technology in view of technological
risks, operational and maintenance problems.
- Feedstock flexibility: Wide variety feed stocks, coal, lignite,
- Syngas Applications: Syngas contaminants & impurities, allowed
gas purity (S, CO2 , etc.) and cleanliness (tars, soot, ash) for
applications such as synthesis of liquid fuels & chemicals, IGCC, IGFC,
- Efficiency: Carbon conversion efficiency, Cold Gas Efficiency.
- Economics: CAPEX & OPEX.
- Unit Capacity: Fuel Feed Rate/ area (Throughput) & Scale up
Prospects, Turndown ratio.
Gasification Strategy for Indian Coals
- In view of utilisation of Indian coats through gasification route, the
following strategy may be adopted.
- Development of indigenous high pressure oxy-brown fluidized bed pilot
plants to be established followed by establishing operation Development of
membrane-based entrained flow pilot technology for low ash Indian coals and
blends of high ash coal & Petcoke.
- Performance testing of the above mentioned two types of gasifier pilot
plants with high ash Indian coals and identification of suitable type of
gasification process based on which a demo plant can be set up at different
mine mouths Mapping of Indian coals towards gasification potential and
updating coal properties database towards gasification for newly explored
coal resources/newly started mines.
- Testing of Indian coals in demo scale facilities of respective
technology providers at their end can be preferred.
- Hence, both the routes demo scale unit development and commercial
technology adoption may be conducted in parallel.
- Programmes need to be executed with joint venture between R&D
institutions, engineering houses and industries instead of independent
Courtesy: Science Reporter