(The Gist of Science Reporter) High Ash Indian Coals Gasification Strategy [FEBRUARY-2019]

(The Gist of Science Reporter) High Ash Indian Coals Gasification Strategy


High Ash Indian Coals Gasification Strategy


  • 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, membrane wall).

Coal Properties

  1. 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 agglomeration/slagging/clinker formation.
  2. 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.
  3. 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.
  4. Caking properties: Indicates difficulties due to plastic zone formation in MBG and FBG.
  5. Specific surface area and porosity: Decides approach of gasifying agents to available active sites and hence gasification reactivity.
  6. 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, biomass, etc.
  • 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, thermal applications.
  • 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 parallel programmes.

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Courtesy: Science Reporter