Page 198 - Advances in bioenergy (2016)

tendency to remove tar content from the gas and thus helps in improving the quality of the
product gas. At this stage, addition of catalysts, usually dolomites and Ni, at high temperatures
with steam and oxygen could be helpful to deliver higher product gases. In this context,
Ramage and Scurlock have discussed about the high temperature pyrolysis (between 800 and
900°C) that delivers as low as 10% of biochar while converting more than 60% of biomass
28
into gases rich in H and CO. Such high temperature pyrolysis can be a competitor to various
2
conventional gasification methods such as (1) oxygen gasification, (2) air gasification, (3)
steam gasification. But during all three kinds of gasification, the heating value varies from 3 to
3
3
15 MJ/Nm , while in air gasification the heating value ranges from 4 to 6 MJ/Nm with
different by-products such as water, CO , hydrocarbons, and tar etc. Steam gasification (SG)
2
converts the ligno(hemi)cellulosic materials to H , CO, CO , CH , light hydrocarbons, char,
2
2
4
and tar. This SG process can be compared with that of pyrolysis at elevated temperatures
(600–1000°C). With an overlapping of some aspects of both gasification and pyrolysis
process, the material decomposition, hydrogen yield, and energy yield were better with
gasification in comparison with pyrolysis process. Florin and Harris have analyzed SG with
the utilization of calcium oxide (CaO) as a sorbent to capture CO , a novel approach to
2
manifold the H concentrations ranging between 40–50 vol% in a conventional manner, to 40–
2
80 vol% on dry basis and by utilization of CaO. 31

Hydrogen production through SCW gasification is an emerging and promising technology for

the utilization of high moisture content biomass. Critical point of H O (T > 374°C, P > 22
2
MPa) plays a significant role in chemical reactions because of their dielectric constant
variation, both in the gasification method and hydropyrolysis process. When arrived at a
critical point the properties of both liquid and gas become identical, making the supercritical
water (SCW) completely miscible with organic substance and other gases. This SCW
gasification process has the advantage of processing any kind of biomass with any moisture
availability (>50%), by avoiding the unit operation such as drying and pelletization. First one
is at temperatures (HTSWG) higher than 500°C and second one is at temperatures (LTSWG)
lower than 500°C. In the HTSWG case, decomposition of biomass proceeds without the
presence of catalyst, whereas in the LTSWG, the use of catalyst is necessary. Shimura and
Yoshida reported that the Ru catalyst has shown higher activity; where the Ni catalyst has also

been explored for HTSWG processes extensively. 32