Page 248 - Synthetic Fuels Handbook

234 CHAPTER EIGHT

Fuel gas + ash
Gas Fuel gas +
Gas
hot inert +
Gas + inert + char residual char
Recycle cyclone
Feedstock Gas phase gas
reaction
Feedstock
Air Combustor
Burner Gasifier Inert +
char
Flus gas
Hot inert +
Bed residual char
Grats
Ash
Steam Steam Steam Steam Air Air
FIGURE 8.5 Gas (left hand side) and char (right hand side) indirect gasifiers. (Source: Kavalov, B., and S. D.
Peteves: “Status and Perspectives of Biomass-to-Liquid Fuels in the European Union,” European Commission.
Directorate General Joint Research Centre (DG JRC). Institute for Energy, Petten, Netherlands, 2005.)

circulating fluidized bed gasifiers and the steam-blown gas or char indirect gasifiers
(Fig. 8.5) are better solutions for biomass-to-liquids production. Both gasifying concepts
reduce significantly the amount of nitrogen in the product gas. In the first case it is
achieved via substituting air with oxygen. In the second case nitrogen ends up in the flue
gas, but not in the product gas, because gasification and combustion are separated—the
energy for the gasification is obtained by burning the chars from the first gasifier in a
second reactor.
The gasification of biomass (e.g., wood scrap) is a well-known process, taking place
in pyrolysis (oxygen supply far below what is required for complete combustion, the frac-
tion called equivalence ratio) or fluidized-bed type of reactors. Conditions such as oper-
ating temperature determine whether hydrogen is consumed or produced in the process.
Hydrogen evolution is largest for near-zero equivalence ratios, but the energy conversion
efficiency is highest at an equivalence ratio around 0.25. The hydrogen fraction (in this
case typically some 30 percent) must be separated for most fuel-cell applications, as well
as for long-distance pipeline-transmission. In the pyrolysis-type application, gas produc-
tion is low and most energy is in the oily substances that must be subsequently reformed
in order to produce significant amounts of hydrogen. Typical operating temperatures are
around 850°C. An overall energy conversion efficiency of around 50 percent is attainable,
with considerable variations. Alternative concepts use membranes to separate the gases
produced, and many reactor types uses catalysts to help the processes to proceed in the
desired direction, notably at a lower temperature (down to some 500°C).
Each type of biomass has its own specific properties which determine performance as a
feedstock in gasification plants. The most important properties relating to gasification are:
(a) moisture content, (b) mineral content leading to ash production, (c) elemental composi-
tion, (d) bulk density and morphology, and (e) volatile matter content.
The moisture content of biomass is defined as the quantity of water in the material
expressed as a percentage of the material’s weight. For gasification processes like gasifica-
tion, preference is given to relative dry biomass feedstocks because a higher quality gas
is produced, that is, higher heating value, higher efficiency, and lower tar levels. Natural
drying (i.e., on field) is cheap but requires long drying times. Artificial drying is more
expensive but also more effective. In practice, artificial drying is often integrated with the
gasification plant to ensure a feedstock of constant moisture content. Waste heat from the
engine or exhaust can be used to dry the feedstock.