Gasification of Coal, Petrocoke And Biomass | Chemical Technology - Chemical Engineering PDF Download

Under Ground Coal Gasification (UCG) 

Under the process of UCG, gasification of coal happens in situ by controlled burning. About 350m3 of gas can be produced per tone of coal. Bye products of significant importance are hydrocarbon, phenols, anhydrous ammonia, and clean water. In UCG overcomes hazard of underground and open cast mining and ash/slag removal is not required as they remain in the cavities. Some of the advantages of UCG are tapping of energy from vast coal reserves which are not economically viable to mine, increased worker safety, low environmental impact, increased worker safety

Surface Gasification

Surface gasification involves reaction of coal with hot steam and controlled amount of air or oxygen under high temperature and pressure. Produces synthetic gas which can be used be the substitute of Natural gas, for power generation using low Btu gas as fuel, manufacture of chemicals and fertilizer and conversions to liquid fuel by GTL liquid.

 Process Steps in Gasification
Various process steps involved in gasification are :

Feed Preparation: Wet feed system or dry feed system may be used in gasification. Wet feed system employ grinding and slurring with water and pumping to gasifier while dry system consist of grinding of the feed and employ lock hopers to pressurize the feed

Gasification: Gasification is carried out at temperature between 900-1100oC. carbon react with oxygen and steam to raw synthesis gas( CO and H2) and some minor by products which is removed to produce clean synthesis gas which can be used as fuel , for generation of steam and electricity or may be used as chemical feed stock. 

Slag Handling System: The gasifier may be either slagging or non slagging.

Gasification in Petroleum refinery  With increasing use of heavier crude and processing of heavy residue has resulted in increased production of tar, petroccoke and asphalt by various residues upgrading technologies processes like visbreaking, coking and deasphalting. These residues can be gasified for production of hydrogen, syn. gas, electricity ammonia and chemicals.. During recent years petrocoke gasification is getting interest by petroleum refiners for production of syn. gas which can be further process to separate hydrogen or can be used for production of useful chemicals.

Various types of gasifier used are mention in Table M-II 3.3 and shown in Figure M-II 3.2 below: 

Table M-II 3.3: Various Type of Gasifier  

Gasification Reactions:

Where for gas as pure methane m=4 and n=1 hence m/n=4

For oil m/n≈2 hence m=2, n=1

For coal m/n≈1 hence m=1,n=1 

It is exothermic and produces a gas containing mainly CO and H₂. The raw synthesis gas contains small quantities of CO₂, H₂O and H₂S and impurities, such as CH₄, NH₃, COS, HCN, N₂, Argon and ash, the quantities being determined by the composition of the feedstock, the oxidant and actual gasification temperature (1300-1400oC). A small amount of unconverted carbon is also present and ranges from 0.5 to 1.0 by percent wt in liquid feedstock or 50-200 ppm wt in gaseous feedstock. The various reactions can be summarized as follows

Figure M-II 3.2: Various Type of Gasifier Commonly Used 

Figure M-II 3.3: Fluidised bed gasifier  \

NEW GASIFIER DESIGNS 

GE Global/ Unmixed Fuel Processor (UOP):

• Elimination of Air Separation Unit (ASU)
• High Temperature Syngas Clean up
• Higher efficiency
• Lower cost

KBR Transport Gasifier (TRIG^TM) 

• Low rank, high-ash, high-moisture coal compatible
• For power generation, air can be employed as the oxidant
• Lower cost predicted
• Higher availability predicted
• Non-slagging, and refractory issues should therefore be minimal
• Higher predicted efficiency
• Lower emissions (due to higher efficiency)
• Large scale up of the technology still required, by a factor of ~30
• By 2010, this technology will be operating at the scale of E-Gas which has >20 years   experience already in 2007.
• Lower temperatures and short gas residence time may lead to some methane formation, which is detrimental in chemical applications.
• Ash disposal problem if carbon conversion predictions are not met in commercial apparatus.

Catalytic Coal Gasification - Bluegas^TM

• Elimination of oxygen plant
• For SNG objective, little or no catalytic methanation required
• High thermodynamic efficiency potential
• Catalyst cost and recoverability
• Carbon conversion and methane production yields in the gasifier
• Cost of applying the catalyst effectively to the coal
• Inherently must be done in a fluidized bed which have not been scaled up to larger capacities of entrained gasifier (yet)
• Interactions of catalyst with coal ash
• Separation costs of syngas and methane - cryogenic process
• Excess steam requirements
• Unsuitable for chemical synthesis processes due to CH₄ reforming requirement.  

 Coal Gasification in fertilizer Industry: Coal gasification fro production of synthesis gas and ammonia has being used many part of the world where large quantity of coal is present. However, higher energy consumption in coal based ammonia plant has been major constrain.  Relative energy consumption per tonne of ammonia is given below.

Gas  

There has been extensive research for improvement of coal gasification technology to improve the efficiency and reduce the cost.

 Rapid Thermal Processing (RTP ^TM) Process for Conversion of Biomass to Liquid Fuels 

RTP ^TM is fully heat integrated technology that yields over 70percent liquid products from typical biomass feed stocks. It is a fast thermal process where biomass is rapidly of heated in the absence of oxygen. The biomass is vaporized and the vapour cooled to generate high yields of pyrolysis oil. Typical yield of various biomass is given in Table M-II 3.4. Chemical and biochemical conversion of biomass to biofuel is shown in Figure M-II 3.4 and Figure M-II 3.5 respectively.

Table M-II 3.4: Typical Yield of Various Biomass 

Figure M-II 3.4: Chemical Conversion Route of Biomass to Biofuels 

Figure M-II 3.5: Biochemical Conversion Route of Biomass to Biofuels  

 Co-gasification of Coal and biomass for methanol synthesis: Cogasification of coal and biomass combined with power and methanol production can be of considered as a potential fuel-base for gasification and further production of synthesis gas and methanol. Some of the advantage of integrated biomass-coal cogasification system is possible continuous operation of the coal gasification reactor, lower costs of electric-energy production