62
Water for Energy and Fuel Production
simultaneous capture of CO 2 , the CaO/coal ratio is (1) a key parameter to obtain
optimal product distribution, (2) a free parameter to be decided by the process
designer, and (3) required to have relatively high values, clearly higher than 2 and
perhaps as high as 80 or more. The type of coal or the types of ISs in the coal have
some influence in the reaction network existing in the gasifier, but its influence is
less than the effect of the temperature. Both product distribution and the usefulness
of CaO are more controlled by temperature than AAEM, IS, and the nature of CaO.
Recently, Sharma [67,68] outlined a stepwise scheme to improve steam gasification reactivity of coal. In this scheme, coal is first refined using coal-derived solvents
such as anthracene oil and paraffin oil. The refined coal has a higher amount of inorganic materials that can act as catalyst for the steam gasification to produce chemicals and char. The particle size of coal has no effect on the gasification reactivity,
and the catalytic effects of minerals follow the order: Na > K > Ca > Ni [67]. The
char is further subjected to steam gasification to produce syngas that can be further
refined using steam reforming reaction. According to Sharma [67], the simplified set
of reactions occurs during coal/char gasification as follows:
C 90 −120 −240 H 6 −9−20 O x S y N z + O 2 + H 2 O → C n H m +
(4.39)
Other products (CO, CO 2 2 , etc.)
C n H m → nC + mH
(4.40)
C + O 2 → CO 2
∆H = −40.59 kJ/mol
(4.41)
C + O 2 → 2CO
∆H = 159.7 kJ/mol
(4.42)
C + H 2 O → CO + H 2
∆H = 118.9 kJ/mol
(4.43)
CO + H 2 O → CO 2 + H 2
∆H = −40.9 kJ/mol
(4.44)
C + 2H 2 → CH 4
∆H = −87.4 kJ/mol
(4.45)
Shift reaction takes place only at high concentration of steam. The last reaction
is important under pressure. Sharma [67] concluded that the main factors for the
steam reactivity of gasification are (1) refining of coal that increases the surface
area of coal, (2) volatile matters in residual coal and char (the more the volatile matter the more the reactivity), and (3) the concentration of mineral matter in coal and
char. Sharma [68] also studied steam gasification reactions that can be useful for the
reactor design. Exxon examined steam gasification of coal liquefaction residue [7].
Exxon technology utilized steam to sequentially gasify and hydrogenate both raw
coal and carbon residue left in coal gasification. The study also used calcium hydroxide or a similar alkaline earth metal compound as possible catalysts for the process.
4.5.1.2 Biomass
In the recent years, the steam gasification of biomass is gaining more importance
because it produces gaseous fuel with high hydrogen content that can either produce
Water for Energy and Fuel Production
simultaneous capture of CO 2 , the CaO/coal ratio is (1) a key parameter to obtain
optimal product distribution, (2) a free parameter to be decided by the process
designer, and (3) required to have relatively high values, clearly higher than 2 and
perhaps as high as 80 or more. The type of coal or the types of ISs in the coal have
some influence in the reaction network existing in the gasifier, but its influence is
less than the effect of the temperature. Both product distribution and the usefulness
of CaO are more controlled by temperature than AAEM, IS, and the nature of CaO.
Recently, Sharma [67,68] outlined a stepwise scheme to improve steam gasification reactivity of coal. In this scheme, coal is first refined using coal-derived solvents
such as anthracene oil and paraffin oil. The refined coal has a higher amount of inorganic materials that can act as catalyst for the steam gasification to produce chemicals and char. The particle size of coal has no effect on the gasification reactivity,
and the catalytic effects of minerals follow the order: Na > K > Ca > Ni [67]. The
char is further subjected to steam gasification to produce syngas that can be further
refined using steam reforming reaction. According to Sharma [67], the simplified set
of reactions occurs during coal/char gasification as follows:
C 90 −120 −240 H 6 −9−20 O x S y N z + O 2 + H 2 O → C n H m +
(4.39)
Other products (CO, CO 2 2 , etc.)
C n H m → nC + mH
(4.40)
C + O 2 → CO 2
∆H = −40.59 kJ/mol
(4.41)
C + O 2 → 2CO
∆H = 159.7 kJ/mol
(4.42)
C + H 2 O → CO + H 2
∆H = 118.9 kJ/mol
(4.43)
CO + H 2 O → CO 2 + H 2
∆H = −40.9 kJ/mol
(4.44)
C + 2H 2 → CH 4
∆H = −87.4 kJ/mol
(4.45)
Shift reaction takes place only at high concentration of steam. The last reaction
is important under pressure. Sharma [67] concluded that the main factors for the
steam reactivity of gasification are (1) refining of coal that increases the surface
area of coal, (2) volatile matters in residual coal and char (the more the volatile matter the more the reactivity), and (3) the concentration of mineral matter in coal and
char. Sharma [68] also studied steam gasification reactions that can be useful for the
reactor design. Exxon examined steam gasification of coal liquefaction residue [7].
Exxon technology utilized steam to sequentially gasify and hydrogenate both raw
coal and carbon residue left in coal gasification. The study also used calcium hydroxide or a similar alkaline earth metal compound as possible catalysts for the process.
4.5.1.2 Biomass
In the recent years, the steam gasification of biomass is gaining more importance
because it produces gaseous fuel with high hydrogen content that can either produce