Page 21 - Impiantistica Italiana Maggio Giugno 2014

iomass is a renewable energy source, level. In fact, direct biomass gasifcation in an entrai-
considered neutral to the greenhouse ned fow gasifer might require purposely-designed
effect as the carbon dioxide release to feeding systems, which could result in operational
the atmosphere is only the fnal step diffculties and increased investment cost.
along the atmospheric–biospheric Therefore, to avoid changing the standard design
Bcarbon cycle, which starts with the of a commercial gasifer, pre-treatment of the fuel
absorption of carbon dioxide through the process is possible, in order to have a feedstock with the
of photosynthesis. Then, through exploitation of same characteristics of a conventional solid or li-
biomass gasifcation, it is possible to produce a ga- quid fuel. In this respect, two different methods are
seous carbon-based fuel starting from a renewable suitable for treating the feedstock, namely torrefac-
source. Moreover, the low sulfur, ash and other pol- tion and pyrolysis, whose main difference is their
lutants content also results in a limited production operating temperature. The biomass torrefaction is
of solid and liquid waste streams. a thermo-chemical treatment of biomass, carried
Among the different biomass types suitable for out in the range of 200 - 232 °C under atmospheric
being processed by gasifcation, the most signif- conditions and in the absence of oxygen, while the
cant are forest and wood industry residue, wood biomass pyrolysis process consists of a thermo-
chips, agricultural byproducts and waste food. chemical decomposition of organic material at
elevated temperatures in the absence of oxygen,
1. Process scheme typically occurring at a temperature over 43 °C.
However, both these processes have not reached
and technology review a level of maturity suffcient to justify their applica-
tion in large scale commercial plants.
SNG (Substitute Natural Gas) can be produced Another well-proven technology is the fuidized bed
from biomass by using different plant confgura- gasifer, which has the major advantage that bio-
tions, mainly depending on the gasifcation techno- mass can be directly fed to the gasifer, right after
logy. In general, all the major stages of the produc- a simple and conventional drying process. Indeed,
tion process are shown in fgure 1. when biomass or waste fuels are considered, fuidi-
zed bed boilers are often the technology of choice
2. Gasifcation [3]: BFB (Bubbling Fluidized Bed Boilers) have often
been favored in small-scale industrial applications,
Gasifcation is the initial stage of the SNG produc- while CFB (Circulating Fluidized Bed Boilers) have
tion process from biomass, generating synthesis been proven to be more advantageous in larger
gas that is mainly composed of carbon monoxide applications.
and hydrogen. The syngas produced in the ga- Among other factors, the operating temperature is
sifcation step is then appropriately treated in the of major importance. In fact, while the entrained fow
downstream process units, before being used for gasifer operates around 1400 °C, the gasifcation
the production of SNG. in a fuidized bed reactor is normally carried out at
Amongst the different gasifcation technology types, about 900 °C, which guarantees signifcantly higher
the entrained fow gasifer is the most well-known methane content in the syngas. On the other hand,
technology. Nevertheless, its specifc experience this technology leads to the formation of organic
with biomass gasifcation at large scale is confned to compounds (tar) in the syngas (up to 10 mg/Nm ),
4
3
co-gasifcation processes with other fuels, while the which implies the need to use treatment units in the
knowledge on pure biomass gasifcation is generally downstream stages of gasifcation. In addition, the
limited at a pilot plant or small-scale demonstration biggest challenges in biomass-fred CFBs are the
increased risks for agglomeration, fouling, and cor-
rosion for which Foster Wheeler, has developed own
characterization procedures and methods [5].
During the past 30 years, Foster Wheeler has boo-
ked over 360 CFB boilers ranging from 7 to nearly
1000 MW [4]; of these, over 50 are designed for
th
biomass (or bio-mix), resulting in a considerable ex-
perience in this feld. The following sections provide
a deeper description of the Foster Wheeler expe-
rience on gasifcation.

2.1. Foster Wheeler air-blown CFB
gasifcation development

The development of atmospheric air blown gasif-
Fig. 1 - Stages involved in the production of SNG from biomass cation technology and gasifcation modeling tools

Impiantistica Italiana - Maggio-Giugno 2014 17