Page 51 - Biofuels Refining and Performance

34 Chapter One

Human excreta and other animal excreta are equally useful for the
same purpose. In fact, all such domestic excreta and pulped organic
refuge may be mixed together to enrich the feed to the gobargas plant.
Social practices and inhibitions prevent people from combining the feed-
stock materials. The common septic tank system can also be modified
in design and be made to deliver biogas. The quantity of human excreta
per family is relatively small, and hence, the gas evolved will hardly
meet even the partial requirement of the family, if the biogas plant is
fed exclusively with night soil.
The disappearing forests and forage have a cyclic relation in the
ecosystem. Rising cost of animal feed of all kinds adds to the crisis.
Keeping of cattle in small village households may not be an attractive
proposal very soon. A major part of the animal dung is not collected by
the owner of the cattle while animals graze. The space required to keep
cattle and have a biogas plant will be considered a poor investment, due
to soaring price of land, even in remote villages. Considering these and
a few more unforeseen factors, better prospects of gobargas plants in a
distant future may not be a correct speculation.

1.14 Biomass, Gasification, and Pyrolysis
1.14.1 Biomass
Imitating the coal-based process, biomass conversion has also been tried
and looks promising. Main sources of biomass are agricultural, horti-
cultural, and forest wastes. Municipal organic solid wastes (which are
also plenty) are potential resources as well. Considering biomass as a
renewable resource, the bioconversion may be pyrolytic, where biogas
and bio-oil are the main products and yet the residue contains some calo-
rie value which can be further utilized (as adsorbents, filter beds, chars,
etc.). Supercritical conversion and superheated steam reformation of bio-
mass are recent techniques. During 1990–1997, quite a few reports
appeared in the literature showing success and promise of catalytic or
uncatalytic reformation of biomass to hydrogen (almost to 18% v/v)
without any char or residues.
Temperature ranges of 340–650 C, with pressures of 22–35 MPa, are
cited with as low as 30-s residence time, through supercritical flow reac-
tors. The raw materials are widely varying: water hyacinth, algae,
bagasse, whole biomass, sewage sludge, sawdust, and other effluents rich
in organic matters. In some efficient carbon bed–catalyzed reactors, other
products (i.e., carbon mono- and dioxides and methane) were also detected.

1.14.2 Gasification and pyrolysis
Gasification, an exothermic reaction, yields mostly producer gas, a
mixture of carbon monoxide, hydrogen, and methane at temperatures