86
Water for Energy and Fuel Production
In this process, the preheated oxygen and steam transport coal and unreacted r ecycled
coal (after ash removal) in the molten salt gasifier. A significant portion of oxygen
and steam is also admitted into the bottom of the reactor to provide the necessary
gases for the complete gasification reactions. Sulfur in the coal is accumulated as
sodium sulfide at equilibrium level and it reacts with molten salt as
Na 2 CO 3 + H 2 S → Na 2 S + CO 2 + H 2 O
(4.75)
Ash accumulates in the melt and leaves with a bleed stream of salt where it is separated and the clean salt is recycled back into the reactor. The bleed salt is quenched in
the water to dissolve sodium carbonate and the ash is separated by filtration. Sodium
carbonate is further carbonated to make sodium bicarbonate (NaHCO 3 ), which is
then separated and heated to regenerate sodium carbonate for reuse in the reactor.
The entrained salt and heat in the product gas are recovered and the purified gas
stream is further processed to make synthesis gas, pipeline gas, or synthetic natural
gas (SNG).
4.6.1.4.2 Atgas Molten Iron Coal Gasification
In this process, coal is injected with steam in the molten iron bath [1,3]. Thermal
cracking of coal along with steam dissociation generates a mixture of carbon monoxide and hydrogen. The sulfur in coal is captured by iron and transferred to lime
slag from which elemental sulfur is recovered. The Atgas process produces gases
with a heating value of about 900 Btu/scf. The Atgas molten iron process has the
following advantages over conventional fixed- and fluidized bed steam gasification
processes:
1. Sulfur in coal is recovered as elemental sulfur, which can be sold, and this
helps process economics. The product gas is essentially free of sulfur.
2. Gasification is carried at low pressure; hence, the coal feeding problem in
pressurized operation is eliminated. Coking properties, ash fusion temperature and generation of coal fines, are not problematic.
3. Tar formation is minimal due to high-temperature operation.
4. The system is very flexible and does not cause any environmental problems.
Relatively large coal particles can be handled without any pretreatment.
5. Reactor start-up and shutdown procedures are much simpler compared to
those for fixed and fluidized bed reactors.
The coal and limestone are injected into the molten iron through tubes using steam
as a carrier gas. Coal gasifies and produces carbon monoxide, and sulfur (both inorganic and organic) migrates to slag and reacts with lime to produce CaS. The product
gas at 1425°C is cooled and compressed, and passes through a shift converter to convert CO into water gas with a H 2 -to-CO ratio of 3–1. The carbon dioxide is removed
from the final product, and the gas is again cooled and passed through a methanator to produce methane by the reaction: CO + 3H 2 → CH 4 + H 2 O. Excess water is
removed from the methane-rich product.
Water for Energy and Fuel Production
In this process, the preheated oxygen and steam transport coal and unreacted r ecycled
coal (after ash removal) in the molten salt gasifier. A significant portion of oxygen
and steam is also admitted into the bottom of the reactor to provide the necessary
gases for the complete gasification reactions. Sulfur in the coal is accumulated as
sodium sulfide at equilibrium level and it reacts with molten salt as
Na 2 CO 3 + H 2 S → Na 2 S + CO 2 + H 2 O
(4.75)
Ash accumulates in the melt and leaves with a bleed stream of salt where it is separated and the clean salt is recycled back into the reactor. The bleed salt is quenched in
the water to dissolve sodium carbonate and the ash is separated by filtration. Sodium
carbonate is further carbonated to make sodium bicarbonate (NaHCO 3 ), which is
then separated and heated to regenerate sodium carbonate for reuse in the reactor.
The entrained salt and heat in the product gas are recovered and the purified gas
stream is further processed to make synthesis gas, pipeline gas, or synthetic natural
gas (SNG).
4.6.1.4.2 Atgas Molten Iron Coal Gasification
In this process, coal is injected with steam in the molten iron bath [1,3]. Thermal
cracking of coal along with steam dissociation generates a mixture of carbon monoxide and hydrogen. The sulfur in coal is captured by iron and transferred to lime
slag from which elemental sulfur is recovered. The Atgas process produces gases
with a heating value of about 900 Btu/scf. The Atgas molten iron process has the
following advantages over conventional fixed- and fluidized bed steam gasification
processes:
1. Sulfur in coal is recovered as elemental sulfur, which can be sold, and this
helps process economics. The product gas is essentially free of sulfur.
2. Gasification is carried at low pressure; hence, the coal feeding problem in
pressurized operation is eliminated. Coking properties, ash fusion temperature and generation of coal fines, are not problematic.
3. Tar formation is minimal due to high-temperature operation.
4. The system is very flexible and does not cause any environmental problems.
Relatively large coal particles can be handled without any pretreatment.
5. Reactor start-up and shutdown procedures are much simpler compared to
those for fixed and fluidized bed reactors.
The coal and limestone are injected into the molten iron through tubes using steam
as a carrier gas. Coal gasifies and produces carbon monoxide, and sulfur (both inorganic and organic) migrates to slag and reacts with lime to produce CaS. The product
gas at 1425°C is cooled and compressed, and passes through a shift converter to convert CO into water gas with a H 2 -to-CO ratio of 3–1. The carbon dioxide is removed
from the final product, and the gas is again cooled and passed through a methanator to produce methane by the reaction: CO + 3H 2 → CH 4 + H 2 O. Excess water is
removed from the methane-rich product.