Energy from gasification of solid wastes
V. Belgiorno, G. De Feo*, C. Della Rocca, R.M.A. Napoli
Department of Civil Engineering, University of Salerno, via Ponte Don Melillo, 84084, Fisciano (SA), Italy
Accepted 11 September 2002
Gasification technology is by no means new: in the 1850s, most of the city of London was illuminated by ‘‘town gas’’ produced
from the gasification of coal. Nowadays, gasification is the main technology for biomass conversion to energy and an attractive
alternative for the thermal treatment of solid waste. The number of different uses of gas shows the flexibility of gasification and
therefore allows it to be integrated with several industrial processes, as well as power generation systems. The use of a waste–bio-
mass energy production system in a rural community is very interesting too. This paper describes the current state of gasification
technology, energy recovery systems, pre-treatments and prospective in syngas use with particular attention to the different process
cycles and environmental impacts of solid wastes gasification.
# 2002 Elsevier Science Ltd. All rights reserved.
Today, the world demand for renewable energy sour-
ces is the key factor in the revival of the use of gasifica-
tion systems, which was in strong decline after the
advent of petroleum (Cuzzola et al., 2000). Gasification
systems are successfully applied to the production of
energy from biomass. They also represent an attractive
alternative to the well-established thermal treatment
systems for the recovery of energy from solid wastes.
Gasification is particularly suitable to treat industrial
wastes but there are some problems with municipal
solid wastes related to their heterogeneity.
In this paper, the relative complexity of technology
needed for feasible gasification process cycles is dis-
cussed with particular reference to the different reactors,
energy recovery systems and gas clean up systems.
The aim of this paper is not to determine or to
demonstrate whether gasification is the best process for
the thermal treatment of solid wastes or not. The con-
cept of ‘‘best’’ is valid solely in the context of local
values, limits and problems, such as characteristics of
waste,environmental regulation orcommunities
dimension. Nevertheless modern incineration is de-facto
the standard for comparison of the gasification perfor-
mance (Juniper, 2000).
Combustion, gasification and pyrolysis are the ther-
mal conversion processes available for the thermal
treatment of solid wastes. As shown in Fig. 1, different
products are gained from the application of these pro-
cesses and different energy and matter recovery systems
can be used to treat these.
Gasification can be broadly defined as the thermo-
chemical conversion of a solid or liquid carbon-based
material (feedstock) into a combustible gaseous product
(combustible gas) by the supply of a gasification agent
(another gaseous compound).
The thermochemical conversion changes the chemical
structure of the biomass by means of high temperature.
The gasification agent allows the feedstock to be quickly
converted into gas by means of different heterogeneous
reactions (Di Blasi, 2000; Hauserman et al., 1997; Bar-
ducci, 1992; Baykara and Bilgen, 1981). The combus-
tible gas contains CO2, CO, H2, CH4, H2O, trace
amounts of higher hydrocarbons, inert gases present in
the gasification agent, various contaminants such as
small char particles, ash and tars (Bridgwater, 1994a).
0956-053X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
Waste Management 23 (2003) 1–15
* Corresponding author. Tel.: +39-0-89-964100; fax: +39-0-89-
E-mail address: firstname.lastname@example.org (G. De Feo).
Direct gasification occurs when an oxidant gasifica-
tion agent is used to partially oxidise the feedstock. The
oxidation reactions supply the energy to keep the tem-
perature of the process up. If the process does not occur
with an oxidising agent, it is called indirect gasification
and needs an external energy source (Figs. 2 and 3)
(Hauserman et al., 1997; Staniewski, 1995). Steam is
the most commonly used indirect gasification agent,
because it is easily produced and increases the hydrogen
content of the combustible gas (Hauserman et al., 1997).
Pyrolysis is an indirect gasification process with inert
gases as the gasification agent. As shown in Fig. 2,
resulting from the gasification process and varying with
the temperature at which the process is carried out, the
three major output fractions are (De Feo et al., 2000):
1. a combustible gas;
2. a liquid fraction (tars and oils); and
3. a char, consisting of almost pure carbon plus
inert material originally present in the feedstock.
As shown in Table 1, the heating value of the gas is
significantly affected by the presence of nitrogen. Due to
the absence of nitrogen in the gasification agent, the
indirect gasification process increases the volumetric
efficiency and produces a gas with a higher heating
value (De Feo et al., 2000; Paisley, 1998). The lowering
of gas production rate, typical of indirect gasification,
reduces the cost of energy recovery and gas cleanup
systems but is still complex and increases investment
costs (Hauserman et al., 1997).
Fig. 1. Thermal conversion process and products (Bridgwater, 1994a).
Fig. 2. Gasification and pyrolysis processes.
2 V. Belgiorno et al./Waste Management 23 (2003) 1–15
Direct gasification with pure oxygen has the same
advantages as the indirect gasification process. How-
ever, the cost of oxygen production is estimated to be
more than 20% of the overall electricity production
(Della Rocca, 2001).
Typically, a gasification system is made up of three
fundamental elements: (1) the gasifier, useful to produce
the combustible gas; (2) the gas cleanup system, neces-
sary to remove harmful compounds from the combus-
tible gas; (3) the energy recovery system. The system is
completed with suitable sub-systems useful to control
environmental impacts (air pollution, solid wastes pro-
2. Solid waste and biomass
For a correct and efficient gasification process, a suf-
required. Therefore many kinds of waste cannot be
treated in the gasification process and for certain types
an extensive pre-treatment is required (refuse derived
fuel) (Fig. 4). Instead there are several types of waste
that are directly suitable for the process; they are: paper
mills waste, mixed plastic waste, forest industry waste
and agricultural residues (Juniper, 2000).
The gasifier is the reactor in which the conversion of a
feedstock into fuel gas takes place. There are three fun-
damental types of gasifier: (1) fixed bed, (2) fluidised bed
and (3) indirect gasifier. In Table 2, the main advantages
of the different type of gasifiers are summarised. Pres-
surised reactors, not discussed in this paper, are only
suitable for coal and oil gasification.
Fig. 3. Direct and indirect gasification processes.
Pure oxygen gasification
Fig. 4. Wastes suitable for gasification.
V. Belgiorno et al./Waste Management 23 (2003) 1–153
A key factor of the reactor is the capacity to produce
a gas with low tar content (condensable bituminous
compounds). A high tar concentration causes a lot of
problems to energy recovery systems because of its cor-
3.1. Fixed bed
Vertical fixed bed reactors (VFB) are the most com-
petitive fixed bed gasifiers. As shown in Fig. 5, they are
Updraft is a counter-current gasifier, where the feed-
stock is loaded from the top while air is introduced from
the bottom of the reactor. In the reactor the solid
material is converted into combustible gas during its
downward path (Quaak et al., 1999; Bridgwater, 1994a).
Feedstock is treated in the following sequence starting
from the top: drying, pyrolysis, reduction and combus-
tion (Juniper, 2000; Quaak et al., 1999; Hauserman et
al., 1997; Bridgwater, 1994a). In the combustion zone,
the highest temperature of the reactor is greater than
1200?C. As a consequence of the updraft configuration,
the tar coming from the pyrolysis zone is carried
upward by the flowing hot gas: the result is the produc-
tion of a gas with a high tar content. Typically, the
sensible heat of gas is recovered by means of a direct
heat exchange with feedstock (Bridgwater, 1994a).
In a downdraft reactor, co-current, the carbonaceous
material is fed in from the top, the air is introduced at
the sides above the grate while the combustible gas is
withdrawn under the grate (Juniper, 2000; Quaak et al.,
1999; Hauserman et al., 1997; Bridgwater, 1994a). As a
consequence of the downdraft configuration, pyrolysis
and downdraft gasifiers.
vapours allow an effective tar thermal cracking. How-
ever, the internal heat exchange is not as efficient as in
the updraft gasifier (Quaak et al., 1999; Bridgwater,
3.2. Fluidised bed
Fluidisation is the term applied to the process
whereby a fixed bed of fine solids, typically silica sand,
is transformed into a liquid-like state by contact with an
upward flowing gas (gasification agent) (Juniper, 2000).
Fluidised bed gasification was originally developed to
solve the operational problems of fixed bed gasification
related to feedstocks with a high ash content and, prin-
cipally, to increase the efficiency (Quaak et al., 1999).
The efficiency of a fluidised bed gasifier is about five
times that of a fixed bed, with a value around 2000
kg/(m2h) (Quaak et al., 1999; Bingyan et al., 1994).
Fluidised bed reactors are gasifier types without dif-
ferent reaction zones. They have an isothermal bed
operating at temperatures usually around 700–900?C,
lower than maximum fixed bed gasifiers temperatures.
The bubbling fluidised bed (BFB) and circulating flui-
dised bed (CFB) gasifiers are schematically presented in
In a BFB reactor, the velocity of the upward flowing
gasification agent is around 1–3 m/s and the expansion
of the inert bed regards only the lower part of the gasi-
fier. Bed sand and char do not come out of the reactor
because of the low velocity (CITEC, 2000; Ghezzi, 2000;
Quaak et al., 1999).
The velocity of the upward flowing gasification agent
in a CFB reactor is around 5–10 m/s (CITEC, 2000;
Comparison of different gasifier [modified by (Juniper, 2000; Bridgwater, 1994 a)]
Fixed bed Fluidised bed Indirect gasifier
Sized feed elasticity
Moisture feed elasticity
Ash feed elasticity
Fluffy feed elasticity
a* poor, ** fair, *** good, **** very good, ***** excellent.
4 V. Belgiorno et al./Waste Management 23 (2003) 1–15
Ghezzi, 2000). Consequently, the expanded bed occu-
pies the entire reactor and a fraction of sand and char is
carried out of the reactor together with the gas stream
(De Feo et al., 2000). This fraction is captured and
recycled in the reactor using an air cyclone that inter-
cepts the gas stream (Niessen et al., 1996).
3.3. Indirect gasifier
Indirect gasifiers are the reactors used for the steam
indirect gasification and are grouped as char indirect
gasifiers and gas indirect gasifiers depending on the type
of internal energy source (Fig. 7).
Fig. 6. Fluidised bed gasifiers.
Fig. 5. Fixed bed gasifiers (Quaak et al., 1999).
V. Belgiorno et al./Waste Management 23 (2003) 1–155
A char indirect gasifier consists of two separate reac-
tors: a CFB steam gasifier that converts feedstock into
produced gas and a CFB combustor that burns residual
char to provide the necessary heat to gasify the feed-
stock. Sand is circulated between the two reactors to
transfer heat. Energy is provided by combustion of resi-
dual char, reserving all gaseous and condensable pro-
ducts for gas production (Hauserman et al., 1997; Craig
et al., 1995; Staniewski, 1995). This process is also called
‘‘fast fluidised process’’ because it has the highest
throughputs and yields of gas (Farris et al., 1998; Hau-
sermanetal.,1997; Niessen etal.,1996; Staniewski,1995).
Gas indirect gasifiers use a steam fluidised bed gasifier
within bed heat exchange tubes (Hauserman et al., 1997;
Niessen et al., 1996). A fraction of combustible gas is
burned with air in a pulse combustor and the hot com-
bustion products provide heat to gasify the feed (Hau-
serman et al., 1997; Niessen et al., 1996; Staniewski,
1995). Gas indirect gasification is extremely versatile
with a wide range of feeds (Hauserman et al., 1997).
The main advantage of indirect gasification is the high
quality of the combustible gas produced in contrast with
greater investment and maintenance cost of the reactor.
Therefore it is necessary to improve the quality of gas
with the adoption of a highly efficient energy recovery
4. Energy recovery systems (ERS)
4.1. Steam cycle
The steam cycle is the simplest option for energy
recovery. It does not need gas pre-treatment, because
tar is burned in the combustor and cannot damage the
boiler (Quaak et al., 1999). The maximum net electrical
efficiency of a gasification–steam cycle plant is about
23%, which is comparable with the efficiency of a typi-
cal solid waste incinerator (Consonni, 2000).
A limitation in the traditional waste incineration and
the gasification–steam cycle boiler is the maximum
metal temperature of the superheater tubes, normally
limited to less than 450?C to prevent excessive corro-
sion of the tubes by the HCl that may be present in the
flue gas. This limitation results in a lower steam tem-
perature to the steam turbine and thus a low overall
plant electrical efficiency (Rensfelt and Everard, 1998).
In a gasification–steam cycle plant, this limitation
could be overcome by gas pre-treatment or by integration
with a thermoelectric power plant (Della Rocca, 2001).
Pre-treatment of the gas can remove the HCl before it
goes into the burner, thus the firing of the clean gas in a
modern boiler combination would allow a steam tem-
perature of 520?C, with a 6% improvement in electrical
efficiency (Rensfelt and Everard, 1998).
The integration with conventional power plants is
called ‘‘co-firing’’: it allows to increase the performance
taking advantage of the high efficiency steam cycle of
the thermoelectric power plant. Usually a co-firing sys-
tem is performed in two possible configurations (Con-
sonni, 2000; Nieminen et al., 1999): adopting a gas
burner in a separate boiler only for the water evapora-
tion phase, as shown in Fig. 8, or adopting a gas burner
in the same boiler as the primary fuel, as shown in Fig. 9.
Spark ignition engines, normally used with petrol or
kerosene, can be run on gas alone. Diesel engines can be
converted to full gas operation by lowering the com-
Fig. 7. Indirect gasifiers.
6 V. Belgiorno et al./Waste Management 23 (2003) 1–15
pression ratio and by installing a spark ignition system
(Quaak et al., 1999; FAO, 1993).
Because of the low lower heating value (LHV), engines
converted to gas are less efficient thanthose not converted;
neverthelessamodernenginecorrectlymodified can reach
over 25% of net electricity output (FAO, 1993).
The engines have the advantage of being robust and
having a higher tolerance to contaminants than gas
turbines (Bridgwater, 1994a). Nevertheless if the gas is
compressed into a turbocharger the same condition as
in the gas turbine will result (Bridgwater, 1994a; FAO,
The main disadvantages of gas engines are the low
increase in efficiency obtained using the combined-cycle
mode and the poor economy of scale (Bridgwater,
4.3. Gas turbine
The power plants based on advanced combined cycle
gas turbine could allow an efficiency-rate of around 60%
(Najjar, 1999). The effective net electrical output is lower
than 40% because of the consumption for gas pre-treat-
ment (De Lange and Barducci, 2000; Van Ree et al.,
Fig. 8. A possible configuration of cofiring system with two different boilers.
Fig. 9. A possible configuration of cofiring system with one boiler.
V. Belgiorno et al./Waste Management 23 (2003) 1–157
1997). In fact gas turbines are very sensitive to the quality
of gas, only extremely low levels of contaminants, prin-
cipally tar, alkali metals, sulphur and chlorine com-
pounds, can be tolerated (Bridgwater, 1994a).
The chemical recovery cycle is a new and very inter-
esting option. In this case, the energy content in the tur-
bine exhaust gas is used to feed the pre-treatment process
of gas, such as catalytic cracking of tar or steam reform-
ing process (Della Rocca, 2001; Happenstall, 1998).
Typical gas turbines must be adapted to the low LHV:
for an easier start-up phase, the burners must allow dual
fuel operation and longer combustion chambers are
necessary to improve the control of CO emissions
(Zanforlin, 1995; Becker and Schetter, 1992).
5. Future alternative use of syngas
Future alternative uses of syngas include molten car-
bonate fuel cells and methanol production. Molten car-
bonate fuel cells are very interesting because of the high
efficiency-rate (more than 50%) and the easy integration
with different energy recovery systems (Iacobazzi, 1995).
Methanol, well known as a clean fuel, can be synthe-
sised by a gas containing H2, CO and CO2using a cop-
per catalyser (Jung, 1999; Nowell et al., 1999).
Methanol production by conversion of a homogeneous
waste could be an interesting alternative.
6. Pre-treatment of gas
Pre-treatments of gas can be used to avoid environ-
mental pollution and dangerous components, such as
tar and particulate, for the energy recovery system or to
increase heating value and hydrogen contents. The
design of pre-treatment systems principally depends on
the energy recovery technologies in use (Quaak et al.,
1999; Bridgwater, 1994a).
In Table 3, gas properties related to pre-treatments
are briefly described. While the required gas properties
for different energy recovery systems are given in
Table 4. Finally, Table 5 shows issues and cleanup pro-
cesses related to flue gas contaminants.
6.1. Thermal cracking
Biomass and waste-derived tars are very stable and
refractory to cracking by thermal treatment (Depner
and Jess, 1999; Bridgwater, 1994b). Temperatures
required are around 1000–1300?C (Depner and Jess,
1999; Quaak et al., 1999; Bridgwater, 1994a).
Two competitive different approaches are used in
fixed bed gasifiers to obtain thermal cracking: use of
temperatures of hearth zone and/or increase of gas resi-
Some advanced applications of modified downdraft
gasifiers with internal recycle of gas, proposed for the
automotive gasifier application, can obtain a tar level
lower than 50 mg/Nm3(Susanto and Beerackers, 1996).
Contaminant presence in the gas and relative problems
ParticulatesDerive from ash, char, condensing compounds and bed material
for the fluidised bed reactor
Cause erosion of metallic components
and environmental pollution
Alkali metalsAlkali metals compounds, specially sodium and potassium,
exist in vapour phase
Alkali metals cause high-temperature
corrosion of metal, because of the stripping
off of their protective oxide layer
Fuel-bound nitrogenCause potential emissions problems by forming NOxduring
Sulphur and chlorineUsual sulphur and chlorine content of biomass and waste is
not considered to be a problem
Could cause dangerous pollutants and acid
corrosion of metals
TarIt is bituminous oil constituted by a complex mixture of
oxygenated hydrocarbons existing in vapour phase in the
producer gas, it is difficult to remove by simple condensation
Clog filters and valves and produce metallic
Gas quality requirements/energy recovery system
Boiler EngineGas turbine
Alkali metals (ppm)
8 V. Belgiorno et al./Waste Management 23 (2003) 1–15
6.2. Catalytic cracking
Catalytic processes for the conversion of tars need
reaction temperatures of around 800–900
removal efficiency is 90–95% and dolomite is an effec-
tive and inexpensive tar cracking catalyst (Rensfelt and
Everard, 1998; Delgado et al., 1996; Orio et al., 1996;
Bridgwater, 1994b; Mudge et al., 1987). Dolomite
demand is around 0.03 Kg/Nm3of raw gas (De Lange
and Barducci, 2000; Rensfelt and Everard, 1998).
The process can be carried out both in a fluidised bed
gasifier with catalysts added to the bed or in a special
reactor below the gasifier (Bridgwater, 1994b; Mudge et
al., 1987). The first solution uses the temperature of the
reactor but the catalyst life is not very long. With a
secondary reactor, the catalyst is protected by deactiva-
tors but requires added oxygen to oxidise gas and
increase the temperature.
Fig. 10 shows catalytic processes in gasification
6.3. Steam reforming and CO-shift
Shift and reverse methanation reactions allow an
increase of up to 10% of the gas volume of hydrogen
content by conversion of methane and steam (Aznar et
al., 1998; Caballero et al., 1997). A commercial catalyst
is used for steam reforming and for CO shift; catalysts
are activated at a low temperature.
Steam reforming and CO-shift need a preliminary
abatement of tar because catalysts are easily deactivated
when the tar content is greater than 2 g/Nm3(Aznar et
Fig. 10. Catalytic processes in gasification systems (Bridgwater, 1994b).
Advantages and disadvantages of tar removal systems
System Advantages Disadvantages
Thermal crackingSimple control
Catalytic crackingLHV unchanged
No gas cooled
Scrubber Easy control
Air pollution control
Fuel gas contaminants: problems and cleanup processes
ContaminantRangeExamples ProblemsCleanup method
Alkali metals (g/Nm3)
Fuel nitrogen (g/Nm3)
Sulphur, chlorine (g/Nm3)
3–70 Ash, char, fluid bed material
Sodium and potassium compounds
Mainly NH3and HCN
Clog filters, deposit internally
Condensation and filtration
Tar cracking, scrubbing
V. Belgiorno et al./Waste Management 23 (2003) 1–159
6.4. Scrubber or saturator
Modified scrubbers called saturators are adopted as a
tar control system (Larson, 2000).
A saturator has two separate towers. In the first tower
the gas is saturated by water droplets at a temperature of
40–80?C (Quaak et al., 1999; Bridgwater, 1994a). Tem-
perature and water saturation allow tar condensation on
the droplet (Larson, 2000; Cernuschi, 2000). In the sec-
ond tower a scrubbing process eliminates suspended
droplets and condensed tar. A humidified packed bed is
usually applied to increase the contact surface between
gasandwashwater (Quaak etal., 1999; Cernuschi, 2000).
In Table 6, the main advantages and disadvantages of
tar removal systems are described.
A baghouse is a very effective and proven technology
that allows the removal of particulate matter larger than
0.1 mm with an efficiency-rate of around 99% (Conti
and Lombardi, 2000; Urbini, 2000).
6.6. Alkali condenser
Alkali metals condense at 550?C on the particulate.
Consequently if the gas reaches 550?C and is treated by
a baghouse, alkali metals are removed with the particu-
late (Quaak et al., 1999; Craig et al., 1995).
7. Process cycles
Several elements of a gasification system can be com-
bined to follow both the wastes characteristics and
energy recovery requirements. In the following para-
graphs four different process cycles adopted in different
situations are presented.
7.1. Gasification/steam cycle, stand-alone configuration
This effective and reliable configuration can be easily
integrated with industrial processes for onsite use of
heating and electricity and for the recycling of gasifier
inert residues (SAFI, 1995).
Gas is burned directly to give heat at a stand-alone
steam cycle. Absence of a cleanup system simplifies the
process and reduces the plant cost, nevertheless an air
pollution control system may be necessary to meet
emission limits (SAFI, 1995; Barducci, 1992). The
overall electricity output is around 20% and it can be
increased by 6% if an acid gas control systems is
adopted (De Feo et al., 2000; Rensfelt and Everard,
Fig. 11 shows a synthetic scheme of TPS installation
in Gre ` ve in Chianti (Italy).
7.2. Gasification/steam cycle, co-firing configuration
A co-firing plant uses gas to give energy to a steam
cycle power generation plant. Gas can be used in the
same fuel boiler or in a secondary one which produces
steam, later superheated in the first boiler (Figs. 8 and
9). Efficiency of this process cycle can allow an electrical
output of over 30%.
In the first configuration acid gases are diluted,
whereas in the secondary boiler the temperature of the
steam tubes remains below 180?C and corrosive action
is not effective (Della Rocca et al., 2001; Consonni,
2000). Moreover, a co-firing plant allows the use of high
moisture content feedstock because of the superheated
temperatures, which do not depend on the heating value
of the combustible gas (Consonni, 2000; Nieminen et
Fig. 12 shows a scheme of an installation in Lahti
The gasification of coal and carbon containing fuels
and the use of gas as fuel in internal combustion
engines is a technology that has been utilised for more
than a century (FAO, 1993). The main innovation for
waste and biomass gasification/engine process are clean-
up systems used for removing dust, tar, and alkali
metals from the raw gas (Quaak et al., 1999). Experi-
ence leads to a use of a modified downdraft gasifier for
tar thermal cracking and a hot gas filter for dust
removal. Electrical efficiency is around 25%, because of
the low heating value of gas (Bridgwater, 1994a; FAO,
Fig. 11. TPS gasification plant in Gre ` ve in Chianti (Italy) (De Feo et
10 V. Belgiorno et al./Waste Management 23 (2003) 1–15
7.4. Integrate gasification combined cycle (IGCC)
IGCC could be competitive in a few years (Larson,
2000). An IGCC plant uses a clean gas in a gas turbine
combined cycle to produce energy. Net electrical output
is over 30% and can increase up to 40% if total thermal
power is over 50 MW (Morris and Waldheim, 1998).
The main disadvantage of this plant is the need for a
cleanup system for the control of corrosive gas phase
compounds such as tar, acid gas and alkali metals
(Quaak et al., 1999; Bridgwater, 1994a; Larson, 1992).
Fig. 12. Foster Wheeler gasification plant in Lahti (Finland) (Nieminen et al., 1999).
Fig. 13. TPS/ARBRE gasification plant in Eggborough (UK) (Van Ree et al., 1997).
V. Belgiorno et al./Waste Management 23 (2003) 1–1511
The most interesting element is the system of tar control
through catalytic tar cracking or wet scrubbing. The
first solution is implemented in the Termiska Processer
ARable Biomass Renewable Energy (TPS/ARBRE)
installation in Eggborough (UK), as shown in Fig. 13.
Fig. 14 shows the second solution as adopted in the
Thermie Energy Farm (TEF) installation in Cascina
8. Environmental impacts
8.1. Air pollution
Atmospheric emissions of gasification systems depend
on the installed air pollution control equipment and
energy recovery systems in use. Therefore it is difficult
to compare the air pollution impact of gasification sys-
tems with a conventional combustion process.
However, the use of gas allows for a successful com-
bustion control, better than solid combustion resulting
in an effective reduction in the emission of CO, NOx,
dioxins and unburned compounds (Giugliano, 2000;
Tchobanoglous et al., 1993). Moreover gas pre-treat-
ment can be performed to remove pollution precursors
such as nitrogen and chlorine compounds and improve
emissions (SAFI, 1995; Staniewski, 1995; Tchoba-
noglous et al., 1993).
8.2. Solid waste production
Solid waste production is related to char and ash
extracted from the gasifier, the particulate control
equipment and the possible boiler, with a production of
2–9, 5–10 and 3–6% of treated feedstock, respectively
Gasifier residues could be used to fertilise the
ground (rarely if the feedstock is not agricultural
waste) or disposed in a sanitary landfill. Instead, solid
residues of gas pre-treatment and air pollution con-
trol systems are typically disposed in landfills, because
of their high heavy metal concentration level. Some-
times, solid residues can be used in industrial pro-
cesses,such as cement
integration between gasification and industrial pro-
cesses (SAFI, 1995).
In the gasification process, wastewater may be pro-
duced by the gas cooler and the wet scrubber containing
many soluble and insoluble pollutants, such as acetic
Fig. 14. TEF gasification plant in Cascina (Italy) (De Lange and Barducci, 2000).
12 V. Belgiorno et al./Waste Management 23 (2003) 1–15
acid, sulphur, phenols and other oxygenated organic
compounds (Bridgwater, 1994a). The insoluble fraction
of the wastewater consists mainly of tars.
Wet scrubber effluent production is around 0.5 kg/
Nm3of treated gas (Barducci et al., 1997). If a scrubber
is used for tar removal the effluent needs expensive
treatment, otherwise the usual problems are a low pH
and a high salt content, which can be easily controlled
by neutralisation and chemical precipitation, respec-
tively (Cernuschi, 2000).
In the gasification plant Thermie Energy Farm, one of
the three IGCC projects selected for funding by the
European Union, the sequence of treatment for tar-rich
wastewater is (Barducci et al., 1997; Barducci and Neri,
? precipitation of sulphur by iron sulphate addition;
? recovery of sulphur and dust by filtering;
? disposal of filter cake;
? stripping off gases dissolved in the water and the
major part of the hydrocarbons;
? partial evaporation of water and usage of con-
densate as scrubber make-up;
? discharge of evaporator blow down to conven-
The salt recovered has a very low polluting potential,
and is conveyed to a sanitary landfill. Hydrocarbons
and other stripper gases are recycled in the combustor
for destruction, so that the tar energy content is recov-
ered (Barducci et al., 1997; Barducci and Neri, 1997).
Because of the complexity of treatment and disposal,
the present trend is to develop a system that does not
produce a liquid effluent (Bridgwater, 1994a), never-
theless this is possible only for wastes that do not con-
tain many contaminant precursors.
9. Gasification and waste management
Gasification represents a future alternative to the
waste incinerator for the thermal treatment of homo-
geneous carbon-based waste and for pre-treated hetero-
geneous waste. As shown in Fig. 15, gasification should
be considered as an option for the thermal treatment of
wastes in an integrated waste management system. For
example, co-firing and co-gasification (gasification of
solid waste with coal or biomass in the same gasifyer) are
interesting solutions for bothdecentralised energysystems
and waste management systems in rural communities.
It is difficult to compare the costs of gasification pro-
cesses with conventional combustion on a direct basis.
This is due to the fact that the costs available refer to
different specifications of plant, for example to meet
different emission standards or have a varying ash con-
tent or water treatment requirements (Altmann and
Fig. 15. Integrated waste management system.
V. Belgiorno et al./Waste Management 23 (2003) 1–1513
Kellett, 1999). Moreover it is incorrect to compare the
cost of new technology with the cost of old technology,
because the former also includes the R&D cost.
The gasification process offers considerable energy
recovery and reduces the emission of potential pollu-
tants. It is considered an interesting alternative to the
conventional technology for the thermal treatment of
The principal difficulties of solid waste gasification,
especially for municipal solid waste (MSW), are related
to the heterogeneity of wastes. A possible solution is the
production of a refuse derived fuel (RDF) with homo-
geneous and controlled characteristics. In any case, gasi-
fication is particularly suitable for many homogeneous
agricultural and industrial wastes (waste tyres, paper and
cardboard wastes, wood wastes, food wastes, etc.).
Gasification plants could be integrated with pre-
existing industrial and thermoelectric plants, because of
their flexibility and compactness. The most significant
choices of design are the reactor type and process cycle,
which can be conveniently adopted according to waste
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