GASIFICACIÓN

1. Biomass gasification technology: The state of the art
overview
Marta Trninić
University of Belgrade, Faculty of Mechanical Engineering,
Department for Processing Engineering and Environmental
Protection
Belgrade, Serbia
mtrninic@mas.bg.ac.rs
Dragoslava Stojiljković
University of Belgrade, Faculty of Mechanical Engineering,
Department for Material Technology
Belgrade, Serbia
dstojiljkovic@mas.bg.ac.rs
Aleksandar Jovović
University of Belgrade, Faculty of Mechanical Engineering,
Department for Processing Engineering and Environmental
Protection
Belgrade, Serbia
ajovovic@mas.bg.ac.rs
Goran Jankes,
University of Belgrade, Faculty of Mechanical Engineering,
Innovation Center
Belgrade, Serbia
gjankes@mas.bg.ac.rs
Abstract— The reduction of imported forms of energy, and
the conservation of the limited supply of fossil fuels, depends up
on the utilization of all other available fuel energy sources.
Biomass is a renewable energy source and represents a valid
alternative to fossil fuels. The abundance of biomass ranks it as
the third energy resource after oil and coal. Moreover, when
compared to fossil fuels, biomass fuels possess negligible sulphur
concentrations, produce less ash, and generate far less emissions
in to the air. In other words, biomass can deliver significant
greenhouse gas reductions in electricity, heat and transport fuel
supply. The energy in biomass may be realized by different
thermochemical technologies of which gasification is most
promising alternative routes to convert biomass to power/heat
generation and production of transportation fuels and chemical
feedstock. This paper deals with the state of the art biomass
gasification technologies, evaluating advantages and dis-
advantages, the potential use of the syngas and the application of
the biomass gasification. Also, this paper provides short overview
of the current status of the biomass gasification in Serbia.
Keywords— biomass; gasification technology
I. INTRODUCTION
Ever increasing energy demand and the climate change
problem caused by anthropogenic greenhouse gas emissions
have resulted in the worldwide effort to find a sustainable and
environmentally friendly alternative to today`s fossil fuels
dominated energy supply. The potential offered by biomass for
solving some of the world's energy and environmental
problems is widely recognized as environmentally friendly and
renewable energy source. Biomass can meet different kinds of
energy needs, including fuelling vehicles, providing process
heat for industrial facilities, generating electricity and
providing heat for household heating. Through energy
thermochemical conversion, biomass can be used in three
different processes: gasification, pyrolysis, and direct
combustion. Gasification of biomass has proved itself a highly
efficient way to utilize biomass for a series of different
purposes in processes with one or several useful outputs that is
applicable on small or medium scale. The combination of
flexibility and efficiency in biomass conversion has attracted
increasing attention to the gasification technology platform in
recent years [2]. This article has the objective of surveying the
current applications of gasification technology.
II. THEPRINCIPLE OF BIOMASS GASIFICATION
Gasification is the conversion of biomass, or any solid fuel,
into gas as a prime product. Due to the process, it also results
in variable quantities of charcoal, pyroligneous acids, tars and
ash [3]. The gasification process is performed in the presence
of a gasifying agent (for example air, pure oxygen, or steam,
or mixtures of these components) at elevated temperatures
between 500 and 1400 °C and at atmospheric or elevated
pressures up to 33 bar [1, 2]. The produced gas is a fuel gas
mixture consisting primarily of carbon monoxide (CO),
hydrogen (H2), methane (CH4), small quantities of other light
hydrocarbons (CnHm), carbon dioxide (CO2), steam (H2O),
besides the nitrogen (N2) present in the air and supplied for the
reaction [1,3,4]. The gas produced may be classified as
product gas (producer gas) or biosyngas (syngas), depends on
the further gas application. In general, producer gas is formed
at temperatures less than 1000 o
C, and syngas is formed at
temperatures higher than 1200 o
C [5]. Syngas has a higher
content of CO and H2 than producer gas as it is used for the
synthesis of hydrocarbons. The producer gas can be converted
into syngas by thermal cracking or reforming (CO and H2
content is increased) [5]. The lowest heating value (LHV) of
the gas produced by biomass gasification ranges from 4 to 13
MJ/Nm3
depending on the feedstock, the gasification
technology and the operational conditions [6].
The principal reactions of the gasification are endothermic
and the necessary energy for their occurrences, generally,
generated by the oxidation of part of the biomass, through an
allothermal or an auto-thermal phase [6]. In the auto-thermal
process, the gasifier is internally heated through partial
combustion, while in the allothermal process the energy
required for the gasification is supplied externally [6].
Considering the autothermal system, gasification can be
generally seen as a sequence of several stages: drying

2. (endothermic stage), pyrolysis (endothermic stage), oxidation
(exothermic stage) and reduction (endothermic stage). An
additional step, consisting in ash cooling, can be also included
[7].
A. Gasification products
The gasification end products may be distinct in a solid
phase and gas phase (condensable and noncondensable). The
solid phase, ash, consists of the inert material formed of the
mineral matter present in the feedstock and the unreacted
carbon. The carbon in the ashes is in a very low percentage of
the total ash amount as the transformation of the carbon matrix
in gas being the objective of the overall process. The gas phase
is a gas mixture that contains the gases that are incondensable
at ambient temperature. If air is used in the oxidization step as
a gasifying carrier, then inert N2 is present in the gas phase.
Also, minor components such as NH3 and inorganic acid gases
(H2S and HCl) can be found in gas depending on the biomass
composition. According to Molino [6], depending on
gasification technology chosen and the operating variables, the
amount of syngas which can be produced from biomass may
range in 1–3 Nm3
/kg on a dry basis, with a LHV spanning over
4–15 MJ/Nm3
. The condensable phase, tar, is a complex
mixture of condensable hydrocarbons, which includes single
ring to 5 - ring aromatic compounds along with other oxygen-
containing hydrocarbons and complex high molecular weight
polynuclear aromatic hydrocarbons (PAH). The composition
depends on the biomass feedstock, the gasification technology
used and on the chosen operating parameters chosen. The
formation of tar is one of the biggest problems faced during the
gasification of biomass. Tar is undesirable because of various
problems associated with condensation, formation of tar
aerosols and polymerization to form more complex structures.
That can cause problems in the process equipment, primarily in
operation of engines and turbines used in application of the
producer gas.
B. Types of gasifiers
All the gasifiers fall into basically four primary gasifier
types: moving bed (downdraft and updraft), circulating and
fluidized bed, as shown in Figure 1. Each of these types is
defined according to on the contact of the biomass and the gas
phase in the gasifier. A summary of the gasifier types is
presented in Table I.
Fig. 1 The typical gasifier types [12]
TABLE I. SUMMARY OF SELECTED BIOMASS GASIFIER TYPES [12]
Gasifier
Type
Scale
Typical
temp.
(°C)
Fuel
moisture
content (%)
Gas
Characteristics
Downdraft
Fixed Bed
5 kWth -
2 MWth
800 -
1000
<20%
Very low tar,
Moderate
particulates,
charcoal at low
temperatures
Updraft <10 800 - up to 50%-
Very high tar
(10% to 20%),
Fixed Bed MWth 1000 55% Low particulates,
High methane
Bubbling
Fluidized
Bed
<25
MWth
850 <5 to 10%
Moderate tar,
Very high in
particulates
Reduced
charcoal,
Ash does not
melt,
Circulating
Fluidized
Bed
A few
MWth
up to
100
MWth
850 <5 to 10%
Low tar,
Very high in
particulates
Reduced
charcoal,
Ash does not melt
C. Gasification process parameters
The products of gasification process depend on the design
of the gasifier, the characteristics of the biomass and important
operating parameters (temperature, pressure, gasifying agent,
air to fuel ratio) and type of bed materials (presence or absence
of catalytically active substances).
1)Effect of the temperature - The temperature profile is the
most important aspect of operational control for gasification
processes. Temperature has an important effect on the
conversion, the product distribution, and the energy efficiency
of a gasifier. Higher temperature results in increased gas yield
because of higher conversion efficiency. Influence of
gasification temperature on product distribution is shown in
Fig 2.
Fig. 2 Typical gasification temperature for various feedstock and influence of
temperature change on some critical factors [20]
2)Effect of gasifying agent - As gasifying agents, air, steam,
carbon dioxide and pure oxygen are commonly being used the
selection of gasifying agent entirely depends on the
requirement of the product gas quality for different
downstream applications. Under steam gasification conditions,
a H2-rich gas (30–60 vol %) with a heating value of 10–16
MJ/m3
is produced [13]. Addition of external steam with air
increases the H2 concentration, because of the water gas shift

3. reaction. It contributes to balance CO and H2 ratio for Fischer
– Tropsch synthesis . Biomass gasification with pure oxygen
produces, produces a higher quality, nitrogen - free gas with a
LHV of 10 – 18 MJ/m3
[14]. Pure oxygen is suitable for
producing gases with high concentration of CO and H2 and
low concentration of tar; however, pure oxygen itself is an
expensive gasifying agent. When CO2 is set under non-
catalytic conditions, it acts mainly as a diluent of the producer
gas, but also slightly higher carbon conversion and lower tar
release. With increasing CO2 to biomass mass ratio, more
oxygen would be available for the exothermic oxidation
reactions, resulting in higher producer gas temperature, and
higher mole fractions of CO in the producer gas, but lower
mole fractions of H2 and CO2. The influence of gasification
medium on characteristics of gas and tar production is shown
in Table II.
TABLE II. EFFECT OF GASIFICATION MEDIUM ON CHARACTERISTICS OF
GAS AND TAR PRODUCTION [8]
Medium Operating condition
Tar Yield
(g/Nm3
)
LHV
(MJ/Nm3
dry)
Steam
Steam to biomass ratio–
0.9
30-80 12.7 – 13.3
steam/oxygen Steam to oxygen ratio-3 4-30 15.5 – 13
Air ER=0.3 2-20 4.5 – 6.5
3)Effect of equivalence ratio - According to Güell et al.
[14] the optimal equivalence ratio (or excess air ratio) applied
in biomass air gasification is in the range 0.2–0.45. Lower
values than 0.2 led to a decrease of the gasification
temperature and therefore higher amounts of tar, whereas
higher values than 0.45 resulted in a decrease in the heating
value and the energy content of the produced gas [14]. Also, at
temperatures between 750 o
C and 850 o
C, an increase in
equivalence ratio from 0.2 to 0.45 resulted in higher gas
yields, lower amounts of tar and lower heating value of the gas
[14]. An equivalence ratio of 0.25 - 0.3 was proposed as the
optimal value to obtain a good gas quality [7].
4)Effect of the pressure - Depending on the downstream
application of the product gas, gasification of biomass is often
conducted under atmospheric and high pressures. Performing
biomass gasification in pressurized reactors has two main
advantages. First, the size of the gasifier can be reduced to a
large extent [14]. Second, gas compression steps can be
avoided in the production of biofuels, as most of the processes
namely Fischer – Tropsch synthesis depending on the pressure
applied methanol and dimethylether synthesis are carried out
at high pressures [14]. In addition, an increase in the gasifier
pressure reduces the tar yield in the product gas. As pressure
increases, H2, CO2, and CH4 yield increased (enhancing the
water-gas shift reaction), whereas the CO yield decreased
[14]. The evolution of H2, CO, and CO2 with pressure was
attributed to an increase of the apparent experimental water-
gas shift constant. It is due to an acceleration of the reaction
kinetics with pressure and an enhanced catalytic effect of the
charcoal, which increased its hold-up rate in the bed with
increasing pressure, on the reaction [14]. However, some
investigations conducted in the fluidized bed gasifier have
shown that the concentration of tar, mainly naphthalene,
increased with increasing gasifier pressure from 0.1 to 0.5
MPa, and thus the concentration of CO decreased, while CH4
and CO2 increased]. The disadvantage of high pressure
gasification is the expensive technology.
5)Effect of residence time - According to Kinoshita et al.
[15] residence time has little influence on the tar yield, but it
significantly influences the tar composition. Amounts of O2 -
containing compounds tend to decrease with increasing
residence time. Also, yields of 1- and 2-ring compounds
(except benzene and naphthalene) decrease whereas that of 3-
and 4-ring compounds increases in the total tar fraction [8,
11].
6)Effect of the catalysts - Catalysts used in gasification
processes can be categorized into natural catalysts and
synthetic catalysts. Among natural catalysts, dolomite and
olivine are the most widely investigated catalysts [7].
Dolomite has attracted much attention as it is inexpensive and
it reduces the tar content effectively. However, it undergoes
attrition relatively easy, resulting in a loss of catalyst activity.
Ekstrom et al. [16] have achieved almost 100% conversion of
tar at 700–800 °C using Malaga dolomite under steam
reforming conditions. However, they also observed a marked
increase in CH4 and C2H4 at lower temperatures and showed
that calcinad dolomite was 10 times more active than the
uncalcined material. Olivine, in contrast, is reported to be less
active in tar removal but more attrition-resistant than dolomite
thus, being more suitable for fluidized bed biomass
gasification. Devi et al. [17] reported that pre-treatment of
olivine at high temperatures could improve the catalytic
performance as bed material. This increased activity was
explained by the increased iron concentration at the surface of
the catalyst. The presence of dolomite and olivine increased
the production of gas by more than 50 %, resulting in a 20 -
fold reduction in tar content and over 30% reduction in
charcoal. In terms of gas composition, H2, CO, and CO2
increased, related to the decrease in tar and charcoal [23]. On
the other hand, the concentration of CH4 in the gas mixture
remained rather constant, indicating that none of the catalyst is
active for CH4 conversion. Alkali metal catalysts are premixed
with biomass before they are fed into the gasifier [8]. Unlike
dolomite, alkali catalysts can reduce methane in the product
gas, but it is difficult to recover them after use. Many
commercial nickel catalysts are available in the market for
reduction of tar as well as CH4 and NH3 in the product gas [8].
Catalyst activity is influenced by temperature, space time,
particle size, and composition of the gas atmosphere. Wang
and his coworkers [18] reported 95% conversion of NH3 along
with 89% conversion of light hydrocarbons, which they
defined as C2H6, C6H6; C7H8 and C9H8. Use of dolomite or
alkali as the primary catalyst and nickel as the secondary
catalyst has been successfully demonstrated for tar and

4. methane reduction [8]. Steam-reforming nickel catalysts for
heavy hydrocarbons are effective for reduction of tar while
nickel catalysts for light hydrocarbons are effective for
methane reduction [8]. Charcoal, a carbonaceous product of
pyrolysis, also catalyzes tar reforming when used in the
secondary reactor [8]. Chembukulam et al [19] obtained a
nearly total reduction in tar with this.
7) Effect of biomass particle size and shape - The size and
shape of the biomass particles are important to determining the
diffuculty of handeling and transport the biomass, as well as
the behavior of the biomass once it is in the gasifier. Gasifiers
friquently have problems with bridging and channeling of the
biomass. The size and size distribution of the biomass
determine the thickness of the gasification zone, the pressure
drop through the bed, and the minimum and maximum heart
load for satisfactory operation. Smaller particle sizes
contributed to higher total gas yields, higher H2
concentrations, and lower charcoal and tar yields , but also
cause problems of bridging and channeling and bigger
pressure drop. Larger feedstock particle size increase the
temperature gradient inside the particle, so that at a given time
the core of the particle has lower temperature compared to the
particle surface, which resulted to the increase of the charcoal
and liquids yields and decrease in gase. Smaller particles can
obtain faster heating rate due to larger surface area. High
heating rates produce more light gases and less charcoal and
condensate [22].
III. BIOMASS GASIFICATION PROCESS CHALLENGES
Gasification is considered as one of the most attractive
options to convert biomass into mechanical energy and
electricity or to produce high quality synthetic liquid and
gaseous fuels. However, the biomass gasification process
requires optimization to minimize the energy efficiency loss
stemming from a few main problems: biomass must be dried
before conversion; expensive equipment is required to clean
the produced gas from contaminants, then further prevent
pollution during combustion; despite special equipment and
treatments, tar remains a part of the synthesis gas [5].
A. Ash agglomeration mechanism and its reduction
Ash related problems including sintering, agglomeration,
deposition, erosion and corrosion are the main obstacles to
economical and viable applications of biomass gasification
technologies. In general, potassium plays an important role in
forming of the ash melts during the thermochemical processes
of biomass. Potassium will react with Cl, S, Si and P through
different reaction paths. Partial products from these reactions
have low melting temperatures (even lower than 700 o
C); these
are potassium salts, silicates, phosphates and mixtures of them.
These potassium containing compounds or eutectics will be
present as molten phases, leading to adhesion/aggregating of
charcoal and ash grains and further ash sintering to large and
heavy blocks. The alkali silicates and sulfates tend to deposit
on the reactor walls and leave a sticky deposit on the surface of
the bed particles, causing bed sintering and defluidization].
Furthermore, the presence of ash such as alkali in syngas can
cause problems of deposition, corrosion and erosion for
equipment that utilizes syngas (gas engines and turbines).
Methods which can be used for reduction of the ash-related
problems are: leaching, fractionation and use of additives (e.g.,
kaolin and calcite).
B. Tar
The presence of tars in the produced gas may be considered
as the weakest point of biomass gasification affecting the final
use of the produced gas itself: energy production and/or
chemical utilization. Tars removal or their conversion is the
great technical challenge to overcome and develop a successful
application of biomass-derived gas. Several approaches are
available for tar reduction and they can be categorized in two
types depending on the location where tar is removed; either in
the gasifier itself (known as primary method) or outside the
gasifier (known as secondary method) [8]. Primary methods
include: the proper selection of the specific operational
parameters, the use of a proper bed additives or a catalyst
during gasification, and a proper gasifer design/modification of
the gasifier [3]. Another way to realize increased tar conversion
is to create areas of high temperatures in the product gas
usually by adding a post gasification section or in a secondary
gasification reactor put in series with the primary gasifier.
Secondary methods can be chemical or physical treatment as:
tar cracking downstream the gasifier (either thermally or
catalytically), mechanical methods (use of cyclone, baffle,
ceramic filter, fabric filter, rotating particle separator,
electrostatic filter, scrubber and alkali remover).
C. Biomass moisture
The water content in biomass absorbs a considerable
amount of sensible heat for evaporation and heating up inside
the gasifier and this way it adversely affects the gasification
efficiency, more biomass has to be fully combusted to provide
the required heat. Moreover, the maximum temperature inside
the gasifier is reduced, which affects tar and charcoal
conversion negatively. On the other hand, a small amount of
moisture in the biomass is favorable for satisfactory gasifier
performance as water is used in the gasification and (steam)
cracking reactions. The moisture content limits for gasifier
feedstock depend on the type of gasifier used. The highest
moisture content for a downdraft gasifier is generally
considered to be 25 % wet basis and not higher than 50 % for
an updraft gasifier [3]. Predrying of the biomass to <25 wt %
moisture (w.b.) is therefore required.
D. Secondary equipment
The greatest challenge for biomass gasification for energy
production may be the costliness of secondary, or auxiliary,
equipment needed make gas clean and relatively contaminants
free [3]. Auxiliary systems are operations supplemental to the
basic process of gasification and generally are placed into one
of two categories: preparation of the fuel and its introduction in
the gasifier, or cleaning of the produced gas [3]. The need for
cleaning syngas depends on the intended use of the gas, being
particularly important in cases when gas will be used to
synthesize liquid fuel. Typical equipment for dry gas cleaning
are cyclones, baffle filter, bag filter, ceramic filter, candle filter,

5. and separators. Wet gas cleaning use scrubbers, spray towers
and wet electrostatic precipitators [3]. This greatly drives up
the cost of the entire process, accounting for more than half of
the final price of produced biofuel [3].
IV. APPLICATIONS OF BIOMASS GASIFICATION PRODUCTS
The gaseous products can be combusted to generate heat or
electricity, or they can be used in the synthesis of liquid
transportation fuels, H2, or chemicals [4]. On the other hand,
the tar can be used as fuel in boilers, gas turbines or diesel
engines, both for heat or electric power generation [4].
Furthermore, the composition of the gasification gas is very
dependent on the type of gasification process, gasifying agent
and the gasification temperature. Based on the general
composition and the typical applications, two main types of
gasification gas can be distinguished, i.e. syngas and product
gas.
A. Utilisation of product gas
The main application of product gas from gasification is
found in direct or indirect combustion to generate power with
co-production of heat.
1)Co-combustion – The primary products obtained by the
thermochemical conversion of biomass may be added to
conventional fuels (coal, heavy oil or biomass) in power plants
for co-combustion processes. According to Boerrigter and
Rauch [20] the most straightforward application of product
gas is co-firing in existing coal power plants by injecting the
product gas in the combustion zone of the coal boiler. Co-
firing percentages up to 10% (on energy basis) are feasible
without the need for substantial modifications of the coal
boiler [20]. The overall electrical efficiency of these plants is
about 35% [47]. Critical issue in co-firing is the impact of the
biomass ash on the quality of the boiler fly and bottom ash .
Examples of biomass co-firing plants are the AMER 85 MWth
circulating fluidised bed (CFB) gasifier in the Essent power
plant in Geertruidenberg (the Netherlands) and the Foster
Wheeler CFB gasifiers in Lahti (Finland) and Ruien
(Electrabel power plant, Belgium) [45].
2)Combined heat and power (CHP) - Biomass gasification
is one of the most suitable processes for combined heat and
power (CHP) production, being a direct route to utilizeenergy
from renewable resources efficiently. The product gas in CHP
plants the product gas is fired on a gas engine [4, 20].
Modified gas engines can run without problems on most
product gases even those from air-blown gasification that have
calorific values of approximately 5 - 6 MJ/Nm3
[20].
Typically, the energetic output is one-third electricity and two-
third heat [20]. The main technical challenge in the
implementation of integrated biomass gasification CHP plants
has been, and still is, the removal of tar from the product gas.
The use of biomass for district heating and CHP has been
expanding rapidly in countries such as Austria and Germany,
e.g. the plants in Güssing (Austria) and Harboøre (Denmark).
They have in common that they result from long development
trajectories and that the technologies are neither simple nor
cheap [20]. In Finland, biomass-based fuels are used nearly
completely in heat and CHP production. The number of large
scale CHP plants in Finland is nearly 100MW and the total
capacity is over 1500 MW [4]. The Alholmens Kraft CHP
plant in Pietarsaari, Finland, is the largest biofuelled power
plant in the world [4].
3)Biomass Integrated gasification combined cycle (BIGCC)
– this technology holds the promise of efficient, clean and
cost-effective power generation from biomass. A typical
biomass integrated gasification combined cycle (BIGCC)
involves combustion of the hot gas from a gasifier in a gas
turbine to generate electricity in a topping cycle. As a gas
turbine requires a pressurised feed gas the biomass
gasification should be carried out at the pressure of the turbine
(typically 5-20 bar) or the atmospherically generated gas must
be pressurised [20]. The first route is preferred as in that case
only dedusting of the gas and cooling to the turbine inlet
temperature (400-500 °C) is required, whereas in the
alternative route the gas must be completely cleaned and
cooled down to allow compression [20]. Miccio [21] reported
that the overall efficiency of the BIGCC system was 83 % and
the electrical efficiency was 33 %. However, this technology
is not yet commercially available, there are experiments with
gasification for use in high efficiency combined-cycle power
plants, which are in the demonstration phase [4]. Several
projects have been initiated for IGCC applications over the
last decade, however, only two have been implemented, the
SYDKRAFT plant at Värnamo (Finland) and the ARBRE
plant at South of Selby, (North Yorkshre, UK). In general the
operational costs related to the inert gas consumption and
electricity consumption for pressurisation of the inert gas for
solids feeding and the gasification air are a major drawback
for pressurised gasification [20].
4)Biomass gasification fuel cell (BGFC) systems -
electrochemical systems for electricity production via
chemical energy conversion [6]. FC have the advantage of
providing clean, near zero emission conversion of reformed
fuels to electricity, with high conversion efficiency in
relatively small units. In theory, fuel cells have the potential to
achieve higher electrical efficiencies (greater than 40% [6])
compared to simple combustion systems and gas engines. A
fuel cell (such as solid oxide fuel cells (SOFC) and molten
carbonate fuel cells (MCFC)) uses H2 and O2 to produce
electricity and the by-product of heat in the presence of an
electrically conductive electrolyte material. However, the
application of gas in fuel cells for the production of electricity
is still in its early development [20].
5)Synthetic Natural Gas (SNG) - Synthetic Natural Gas
(SNG) is a gas with similar properties as natural gas but
produced by synthesis of methan from H2 and CO from
gasification product gas. Methanation is the catalytic (nickel-
based catalyst) reaction of carbon monoxide and/or carbon
dioxide with hydrogen, forming methane and water. The
methanation reactions of both carbon monoxide and carbon
dioxide are highly exothermic. Such high heat releases

6. strongly affect the process design of the methanation plant
since it is necessary to prevent excessively high temperatures
in order to avoid catalyst deactivation and carbon deposition
[20]. The highly exothermic reaction generally creates a
problem for the design of methane synthesis plants: either the
temperature increase must be limited by recycling of reacted
gas or steam dilution, or special techniques such as isothermal
reactors or fluidised beds, each with indirect cooling by
evaporating water, must be used [20].
B. Utilisation of syngas
1)Transportation fuels production via biomass gasification -
Use of biomass based syngas for liquid fuel generation –
biomass to liquid (BTL), can help lower the economic and
environmental problems caused by the natural gas or coal.
Biomass to liquid (BTL) is suggested to be a positive route to
reducing the inclination towards fossil transportation fuels and
is also a key to keeping the environment clean. For 20% of the
total liquid fuels produced from carbon neutral sources, like
biomass, 15% CO2 emissions reduction could be achieved –
just by fuel replacement. As of now, there are no commercial
scale BTL plants, like those installed for coal to liquid (CTL)
or gas to liquid (GTL) plants. Most of the documented BTL
plants are either on demonstration scale or experimental scale.
2)Synthesis of Fischer–Tropsch fuels - The production of
liquid fuels from syngas has a long history, which goes back
to the pioneering work of Fisher and Tropsch to synthesize
hydrocarbon fuels in Germany in the 1920s. The Fischer-
Tropsch synthesis (FTS) is a process by which gasoline, diesel
oil, wax, and alcohols are produced from CO and H2 gas
mixture [4]. Typical operation conditions for FTS are
temperatures of 200 - 350 °C and pressures between 25 and 60
bar [20]. In the exothermic FTS reaction about 20% of the
chemical energy is released as heat [20]. Several types of
catalysts can be used for the FTS, the most important are
based on iron (Fe) or cobalt (Co). Co catalysts have the
advantage of a higher conversion rate and a longer life (over
five years). The Co catalysts are in general more reactive for
hydrogenation and produce therefore less unsaturated
hydrocarbons (olefins) and alcohols compared to iron catalysts
. Iron catalysts have a higher tolerance for sulphur and
produce more olefin products and alcohols. One of FTS
technical issues is the reduction of inert gases, such as CO2
and contaminants, such as H2S, because the inert gases and
contaminants can lower catalyst activity due to catalyst
poisoning [36].
3)Synthesis of methanol and dimethyl ether from syngas -
Methanol and dimethyl ether (DME) are promising clean
liquid fuels because they are storable and would be
alternatives to gasoline and diesel fuels. Methanol can be
produced by means of the catalytic reaction of CO and some
CO2 with H2 [20]. These reactions are exothermic and proceed
with volume contraction; and a low temperature and high
pressure consequently favours them [20]. Side reactions, also
strongly exothermic, can lead to formation of by-products
such as methane, higher alcohols, or dimethyl ether (DME)
[20]. Methanol is currently produced on an industrial scale
exclusively by catalytic conversion of synthesis gas [20].
Processes are classified according to the pressure used: high-
pressure process (250-300 bar) with uses of zinc-chromium
oxide catalysts, medium-pressure process (100-250 bar) with
uses of copper-zinc-chromium catalyst and low-pressure
process (50–100 bar) [20]. The low-pressure processes are
dominant currently and their main advantages are lower
investment and production costs, improved operational
reliability, and greater flexibility in the choice of plant size
20]. DME can be obtained from methanol via catalytic
dehydration using catalysts based on silica-alumina or using
bifunctional catalysts such as copper-ZSM-5 zeolite and
hybrid copper-allumina based catalysts [6].
4)Ethanol production - According to Xu et al. [522] three
major research areas using lignocellulosic biomass for biofuels
are: enzymatic hydrolysis of cellulose followed by sugar
fermentation, gasification followed by raw syngas
fermentation and gasification followed by Fisher Tropsch
catalysis. Biosyngas fermentation is a microbial process where
syngas is used as carbon and energy source by certain
anaerobic microorganisms (act as biocatalysts) and then
converted into fuels or chemicals (acetic acid, ethanol, 2,3
butanediol, butyric acid, and butanol) [6]. A fermentation
process using a biocatalyst has several advantages in
converting syngas into chemicals and fuels compared with a
thermochemical route (FTS). Griffin and Schultz [23] pointed
out that a biosyngas fermentation route profits from low
temperature, low pressure, the tolerance of a biocatalyst to
several impurities in syngas, and the ability to use flexible
syngas compositions. Thus, the need for an extensive gas
clean up was eliminated. However, relatively low rates of
growth and production by anaerobes, difficulties in
maintaining anaerobic conditions and mass transfer between
gas phase (especially CO and H2) and liquid phase, and
product inhibition have been identified as the main barriers to
commercializing syngas fermentation technology.
5)Synthetic Natural Gas (SNG) - can also be produced from
syngas, however, in that case the biomass to SNG yield is
significantly lower, as no advantage is taken from already
present amounts of methane [20].
6)Chemical synthesis - Syngas is one of the main sources
for hydrogen used in refineries. Biomass gasification followed
by water reforming of CH4 to H2 and CO, water–gas shift
reaction of CO to H2 and CO2 with catalysts such as copper–
zinc, and CO2 adsorption using an adsorbent such as CaO can
produce pure H2. In refineries, hydrogen is used for the hydro-
treating and hydro-processing operations.
V. STATUS OF THE EUROPEAN BIOMASS GASIFICATION
PLANTS
In the last decades, the presence of the gasification process
in the European market has increased. According to IEA
Bioenergy, in Europe there are 77 gasification plants [24] from

7. which 49 are power generation or combined heat and power
generation plants, 15 are co-combustion plants and 13 plants
are dedicated to the production of chemicals.
VI. STATUS OF THE SERBIAN BIOMASS GASIFICATION
PLANTS
In Serbia, several experimental small scale laboratory
gasifiers were designed and tested with different kind of
gasification processes using biomass and waste material at the
Department for Process Engineering of the Faculty of
Mechanical Engineering, University of Belgrade in the
seventies and eighties. Low energy prices and the lack of the
support for R&D work were the reasons for no commercial
application and interest. The importance of the use of biomass
as an energy source was recognized by the study supported by
NPEE and completed in 2007 (NPEE ev.no. 273 020) has
determined that it was economically feasible to build
approximately 400-500 MWe of CHP pants with solid biomass
as a fuel in units of 500 kW to several MW in Serbia. Also,
according to the National Renewable Energy Action Plan
(NREAP) established by Ministry of Energy, Development and
Environmental Protection of the Republic of Serbia in 2013,
until 2020, it is planned to build biomass CHP plants with total
power of 100 MW (640 GWh of electricity and 49 ktoe of
heat). The future investments for these capacities were
estimated to be in total 1 billion EUR. If the development of
domestic equipment would be supported, nearly 70% of this
amount could be covered by equipment and engineering from
domestic companies (it was assumed that it is not economically
reasonable to finance domestic development of gas engines, or
turbines which takes about 30% of plant investments). This
made a good basis for R&D projects in this field. The Faculty
of Mechanical Engineering, University of Belgrade proposed in
2011. Project “Development of CHP demonstration plant with
Biomass Gasification” started with support of the Ministry of
Science and Technological Development of the Republic of
Serbia in 2011. This Project is a continuation of the previous
project “Technologies for using biomass for combined heat and
power generation” (ЕЕ 18026). The aims of project are: a) to
build a demonstration CHP plant with biomass gasification, b)
optimize the gasification processes in terms of maximum
utilization of energy from biomass, c) to determine conditions
for gas engine to gasification coupling. The gas cleaning and
heat recuperation will be tasted, and necessarily testing will be
done in order to determine pollutants emissions of the plant,
and if it is necessarily, segments of equipment will be
improved. The new project will determine possible ways of
development of the future commercial plant. The CHP Facility
with Biomass Gasification is based on down-draft fixed bed
gasifier with use of corn cob (HHV app. 18.6 MJ/kg d. b.) and
with thermal output of 0.5 MWth, PBH regenerative heat
exchanger (for gas cleaning), and gas engine. The location of
the plant is planned to be nearby Belgrade. The available
amount of biomass of the Company (corn cobs) is app. 1000
t/year. Produced gas in demonstration phase will be used as
additional fuel for the existing hot water boilers of the
company, or alternatively, after cooling and dust separation, for
electricity production. The expected electrical power is 150-
180 kW. After introducing Feed-in tariff in Serbia in 2009,
production of electricity with biomass as a fuel became
commercially interesting. The analysis of the efficiency of
investments in development and operation of demonstration
CHP plant was based on “Feed-in” tariff of 13.26 EURc/ kWh
(for electricity produced in biomass CHP plant less than 1
MW) and price of LNG which can be replaced by heat from the
CHP. The Simple pay-back period of 5-6 years is expected.
VII. CONCLUSION
Biomass gasification can be considered as one of the
competitive ways of converting biomass to fuel gas for
combined heat and power generation, fuel cell and synthetic
fuel production. This review leads to the following
conclusions: 1) The parameters with the greatest impact on
the gasification process are the gasification reaction
temperature and the equivalent ratio. The control of these
parameters ensures that a syngas with an acceptable content
of tars and particles is produced and that ash sintering effects
caused by high temperatures in the reactor not appears. 2)
Biomass moisture content is an operating parameter that
reduces gasification efficiency, as part of the energy is used
for drying the biomass. Moisture contents above 15% can
lead to unstable process and reduction of the produced gas
calorific value. 3) The presence of tars in the produced gas is
one of the main technology barriers to the development of
gasification. Several approaches available for tar reduction
can be categorized in two types depending on the location
where tar is removed; either in the gasifier itself (known as
primary method and include gasifier design, optimal settings
of the operating parameters and use of catalysts) or outside
the gasifier (known as secondary method). Once the gas has
been obtained, it is difficult and costly to ensure that it meets
the optimal conditions required for the energy of fuel
production.
At the Department for Process Engineering of the
Faculty of Mechanical Engineering, University of Belgrade,
several experimental laboratory gasifiers have been designed
and tested with different kinds of gasification processes
using downdraft gasifier design biomass and waste material.
According to results of biomass and waste gasification
experiments carried out several years ago at laboratory scale
reactors at the Department, but also according to results of
many projects recently presented in literature, the concept of
downdraft demonstration unit has been developed and the
downdraft gasification unit of thermal power 0.5-07 MW is
designed. After demonstration phase, it is expected that the
plant will be commercialized and used for heat production or
combined heat and electricity production in small-scale
plants. Biomass gasification is important because of the
amount of available biomass in Serbia and because it offers
the possibility of increasing the share of renewable energy
sources in energy balance of Serbia.
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