Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

Introduction 1
1 Introduction
The depletion of global fossil fuels resources has been a major concern for the world recently. In
order not to run out of energy supplies it is essential to focus on the research of alternative systems
for combined heat and power production. Biomass, as in all organic materials that are originated
from plants, has high potentials for both industrialized and developing countries. Thus, biomass
gasification has been proved as an appealing alternative option towards the fuels that have been
used for the past decades. In addition, the concern for the climate change is another reason why
the interest in this technology has been increased recently. Currently, the greenhouse gas emissions
(60% CO2, 20% CH4, 20% N2O) are estimated around 26.6 CO2 Gt /year and it is possible
that they reach the value of 40.4 Gt CO2/ year by 2030.
The use of biomass deriving fuel gas is a CO2 neutral source of renewable fuel as it consumes the
same amount of CO2 from the atmosphere during growth as it is released during combustion.
Hence, if it can be widely applied the greenhouse gas emissions will be considerably decreased.
Specifically, it has been reported that if this technology gets mature enough for wide application the
CO2 emissions will decrease 50-80% by 2050. At the moment, 8.5% (5.6 EJ/year) of energy consumption
derives from biomass (including MSW) in Europe and also contributes 10-15%
(45EJ/year) in the total world energy use. The estimated potential for the use of biomass can reach
almost 50% of the demanded energy by 2050 in the best case scenario (Demirbas,2009).
However, the commercial breakthrough of this potential source of energy is still not achieved.
There are still some drawbacks that render its application difficult. The syngas that is produced
during the gasification of biomass contains apart from the desired combustible components (CO,
H2 and CH4) impurities such as particulates, soot, ash, sulfur species, ammonia and trace quantities
that need to be removed in order to obtain high quality fuel gas. Particulates removal has been
already achieved significantly, as the available technologies are quite efficient. As for the sulfur
species and the trace quantities with the use of appropriate sorbents, very high quality product gas
can be obtained. Hence, the bottleneck of this technology is the removal of the higher hydrocarbons
(usually characterized as tar) that condense on the colder parts of the plant and cause plugging
and corrosion. Therefore, gas cleaning is a crucial step that needs to be applied in order to
have the quality of syngas required for the downstream applications that also affects the optimized
design and operation of the biomass gasifier.
Considerable efforts have been made the past twenty years in order to come up with an efficient
technology to completely get rid of tar compounds. Cold cleanup methods seem to be quite efficient
but the heating value of the gas fuel is substantially diminished and a waste stream created is
difficult to dispose. As a result, hot gas cleaning seems to be more appropriate in order to maintain
the quality of the product gas. Also, high temperature clean-up can give both tar and ammonia-free
syngas. The use of catalysts for tar destruction and decomposition is a very promising technology.
Many types have been tested so far such as dolomite, limestone, olivine etc. that are inexpensive
but not that efficient. It has been proved, though that one type of catalysts, nickel based, can give
completely tar-free gas. However, these ones have other disadvantages such as the fact that they
can’t be used in situ, due to their rapid deactivation mostly from carbon deposition. So the use of a
second catalytic bed increases a lot the cost of the facility. Iron based catalysts have dragged the
attention recently as they are not toxic; they are naturally abundant and cheap. They are not that
efficient compared to nickel catalysts but they can be used in situ as they are not easily deactivated
by carbon. The fact that they can be used inside the gasifier makes them quite attractive as an
alternative technology, but their effectiveness needs to be increased in order to use them in large
scale facilities. As a third option, precious metal catalysts seem quite appealing in terms of effectiveness,
whereas their main disadvantage is their high cost.