1. Biomass and Sludge Gasification for Syngas
Synthesis and CHP
Jad Halawi, Nour Sehnaoui, Ali Al Hady Hatoum, and Marwan Fakhr
Department of Chemical and Petroleum Engineering
American University of Beirut
Jch01@mail.aub.edu, nys04@mail.aub.edu, Anh15@mail.aub.edu, and mkf06@mail.aub.edu
Abstract-The purpose of this project is to produce and
recover the maximum amount of energy, from municipal
solid waste and waste water treatment sludge, by syngas
production via gasification and the combined heat and
power cogeneration.
I. INTRODUCTION
Lebanon is currently facing a serious energy problem in
addition to a solid waste management crisis. The current
demand for electricity in the Lebanon is around 2,300 MW
[1]. Electricité Du Liban, EDL, is struggling to ensure the
continuous delivery of power with its maximum production
in the range of 1,500 MW [2]. The Lebanese energy sector
suffers from crippling inefficiencies in its infrastructures
including power plants, transmission and distribution
networks[2]. In spite of the efforts, the electricity network
does not represent a reliable source for the industrial and
commercial sectors, where most beneficiaries have been
compelled to install and use their own private generators to
overcome the electricity shortages and ensure the continuity
of their operations. To add to that Lebanon has recently been
facing a dire solid waste crisis, due to poor governance of the
sector and to the shutdown of the Nemeh landfill. Based on
the state of the energy and waste management sectors in
Lebanon, this project opts to solve a part of the problem.
Gasification of biomass accompanied combined heat and
power cycle (CHP) would tackle the two problems at hand
simultaneously. Gasification is a technique used for reducing
the volume of the organic portion of solid waste in a cost
efficient manner whilst producing synthesis gas or syngas to
power gas turbines to fuel a CHP cycle.
This paper presents a complete design of gasification plant
developed to be implemented in Lebanon.
II. GASIFICATION
Gasification is the process of partially combusting a
carbonaceous fuel in the presence of a hypo-stoichiometric
amount of oxygen at an elevated temperature. [3]Due to this
partial combustion a gas stream that is rich in H2 and CO is
generated, i.e. syngas or synthesis gas. Thus this makes
gasification an energy-efficient technique for reducing the
volume of MSW and the energy recovery in the form of
hydrogen fuel.
However, the gasification step itself is only the last step in a
gasification system. In a typical gasification system the raw
material i.e. Organic-MSW (O-MSW) and Waste Water
Treatment-sludge (WWT-sludge) undergo the following
steps: dehydration, pyrolysis, and gasification.
1) In Dehydration, whatever amount of moisture left in the
feed or any other volatile liquid would evaporate due to
the high temperature. Commonly this step occurs at a
temperature range between 100o
C and 160o
C [3].
2) Pyrolysis or De-volatilization or thermal cracking is the
thermal break down of bonds to produce char, tars, CO,
CO2, H2O, NOx, SOx, H2S, and NH3 alongside a
multitude of other products. Char is the carbon rich solid
that remains after pyrolysis and is an integral part of the
gasification process. The typical range of temperature
that pyrolysis takes place at is between 550o
C and 700o
C.
The operating temperature generally depends on the Ash
melting temperature [3].
3) Char gasification occurs at high temperatures exceeding
950o
C and could reach up to 1200o
C [3]. Moreover, char
gasification could be summarized in nine chemical
reactions six of which are exothermic and the rest are
endothermic reactions. This mix of endothermic and
exothermic reactions is crucial for the gasification
system, as we would dedicate a portion of the produced
fuels and char to completely combust thus releasing heat
to sustain the high temperature required for pyrolysis,
i.e. designing an auto-thermal reaction system. This
controlled combustion is achieved by varying the O2 or
air flow into the reactor, and maintaining oxygen at
hypo-stoichiometric levels.
The reactions of char gasification are:
 Combustion reaction:
C + O2  CO2 (Exothermic) (1)
 Methanation reaction:
C + 2 H2  CH4 (Exothermic) (2)
 Shift conversion reaction:
CO + H2O  CO2 + H2 (Exothermic) (3)
 Water-gas reaction:
C + H2O  CO + H2 (Endothermic) (4)
 Boudouard reaction:
C + CO2  2CO (Endothermic) (5)

2.  Combustion reaction of CO:
CO +
1
2
O2  CO2 (Exothermic) (6)
 Combustion reaction of H2 :
H2 +
1
2
O2  H2O (Exothermic) (7)
 Combustion reaction of CH4 :
CH4 + 2O2  CO2 + 2H2O (Exothermic) (8)
 Steam methane reforming reaction:
CH4 + H2O CO +3H2 (Endothermic) (9)
III. GASIFICATION SYSTEM SELECTION
There are multiple types of gasifier systems that are currently
employed in the industry. In this project the type of gasifier
that was deemed most suitable and efficient for the
parameters of MSW in Lebanon, i.e. high moisture content
[4], is the bubbling fluidized bed gasifier. In a bubbling
fluidized bed gasifier the oxidant, i.e. air or oxygen, also
plays the role of the fluidizing agent and is fed to the bed at
the bottom of the gasification unit through a distributor plate.
The bottom of the unit contains packing material usually
silica sand particles and the waste is fed into the bottom of
the bed of the unit to be mixed into the inert silica sand. The
fluidizing agent should maintain a specific superficial
velocity greater than the fluidization velocity of the bed to
keep the bed in a fluidized state, which enhances reaction
kinetics by increasing surface area of contact as well as heat
and mass transfer parameters [5]. The optimal gasifying
agents were found to be steam and air simultaneously. The
optimal ratios of air to biomass and steam to biomass are 1:1
and 1.2:1, and the optimal temperature is 800o
C. This high
temperature will not consume energy to achieve as the reactor
will be operating under auto thermal conditions as a fraction
of the fed reactants into the gasifier will undergo complete
combustion to liberate heat [6]. The gasification process was
simulated using the ASPEN PLUS®
simulation environment,
the kinetic reaction rates presented in Table 1 and the feed
conditions presented in Table 2. With the aforementioned
conditions the process was capable of generating 1224.87
kmol/hr of H2 gas alongside an array of gases such as H2S,
SO2, NO2, CH4, CO, and CO2. The ASPEN PLUS®
simulation of the process is presented in Figure1.
Table 1 Reaction Rate Kinetics
Table 2 Reactor Parameters
IV. SYNGAS PURIFICATION
The syngas obtained from the gasification process should be
scrubbed and purified. Acid gas removal is an integral part of
our process to minimize hazardous emissions and prevent
equipment corrosion and erosion. The major corrosive and
acidic constituents of the syngas obtained are mainly
hydrogen sulfide, carbon dioxide, sulfur dioxide, nitrogen
dioxide, and ammonia.
The main reason behind the scrubbing of acidic gas is to
minimize the environmental impact of the process.
Moreover, the syngas is scrubbed to avoid undesired
incidents such as corrosion of the pipelines and units
downstream. In fact, CO2 corrosion is also a major issue in
the oil and gas industry because it is the main cause of
equipment failure. The basic CO2 corrosion reaction starts by
the dissolution and hydration to form carbonic acid as seen in
the reaction equations (10) and (11) [7].
𝐶𝑂2 (𝑔)→𝐶𝑂2(𝑎𝑞) (10)
𝐶𝑂2+𝐻2 𝑂→ 𝐻2 𝐶𝑂3 (11)
Reaction name Reaction rate (s-1
)
Boudouard 𝑟1 =
𝑘21 𝑝 𝐶𝑂2
1 + 𝑘22 𝑝 𝐶𝑂2
+ 𝑘23 𝑝 𝐶𝑂
Water-gas 𝑟2 =
𝑘31 𝑝 𝐻2 𝑂
1 + 𝑘32 𝑝 𝐻2 𝑂 + 𝑘33 𝑝 𝐻2
Methanation 𝑟3 =
𝑘41 𝑝 𝐻2
2
1 + 𝑘42 𝑝 𝐻2
Combustion (CO) 𝑟4 = 𝑘5 𝐶 𝐶𝑂
𝑛1
𝐶 𝑂2
𝑚1
Combustion (H2) 𝑟5 = 𝑘6 𝐶 𝐻2
𝑛2
𝐶 𝑂2
𝑚2
Combustion (CH4) 𝑟6 = 𝑘6 𝐶 𝐶𝐻4
𝑛3
𝐶 𝑂2
𝑚3
Shift conversion 𝑟7 = 𝑘81 𝐶 𝐶𝑂
𝑛4
𝐶 𝐻2 𝑂
𝑚4
− 𝑘82 𝐶 𝐶𝑂2
𝑛5
𝐶 𝐻2
𝑚5
Steam methane
reforming 𝑟8 = 𝑘9(𝑃𝐶𝐻4
−
𝑃 𝐶𝑂 𝑃 𝐻2
3
𝑘 𝑒𝑞2 𝑃 𝐻2 𝑂
)
Parameter
Temperature (o
C) 800 o
C
Pressure (kPa) 101.325 kPa
Mass flow (kg/hr) 19.96 kg/hr
Figure 1 ASPEN PLUS Simulation of Gasification

3. The CO2 corrosion will lead to the production of FeCO3 that
will have different behaviors depending on its environment.
H2S corrosion is also problematic. A proposed mechanism
for H2S corrosion will lead to two different paths and end
with the formation of FeS. Furthermore, the corrosion rates
for H2S and CO2 will vary as a function of the pH and the
temperature of the system.
SO2 and NO2 corrosion are also important issues [8]. SO2
plays a very important role in atmospheric corrosion in urban
and industrial atmospheres; also it is mainly corrosive in the
presence of moisture, as it will lead to the formation of
H2SO4, a very hazardous component that can have severe
effects on the health [9][10] . When H2SO4 comes in contact
with carbon steel, it will cause the formation of ferrous
sulfate [11].
𝐻2 𝑆𝑂4+𝐹𝑒→𝐹𝑒𝑆𝑂4+𝐻2 (12)
However the corrosion rate of SO2 decreases when it comes
in contact with stainless steel due to the anti-corrosive
properties of stainless steel.
After surveying multiple techniques for the removal of acid
gas, adsorption through an activated carbon column was
found to be the most efficient in order to minimize energy
consumption of the process.
The design of the adsorption column depends on many
factors including the densities of the fluidizing agent and the
adsorbent material, the void fractions, adsorption isotherms,
and pressure drop through the bed. Moreover, the particle
size distribution is used to specify the particles uniformity.
For the rate of adsorption will increase as the particle size
decreases.
The activated carbon adsorption process primarily relies on
the diffusion and adsorption kinetics of chemical compounds
through and on the activated carbon; and thus modeling this
process mathematically should be done based on a mass
balance that factors in the rates of mass diffusion, convection,
and adsorption in the column. After performing a mass
balance on the system the resultant equation is presented
hereafter [9],
𝑑𝐶
𝑑𝑡
= 𝐷𝑒 (
𝐶
𝐶0
)
𝑛−1 1
𝑟2
𝜕
𝜕𝑟
(𝑟2 𝜕𝐶
𝜕𝑟
) (13)
The above equation is a second order partial differential
equation of Concentration C with respect to time (t) and
radius (r) of the activated carbon bed where [9],
Where,
𝐷𝑒 =
𝜀 𝑃 𝐷 𝑃
(1−𝜀 𝑃)𝜌 𝑃 𝑛𝑘
𝐶0
1−𝑛
(14)
Where,
 𝜀 𝑃 : Porosity of the adsorbent bed
 𝜌 𝑃: Density of adsorbent (kg/m3
)
 n: Fruendlich isotherm constant
 k: Fruendlich isotherm constant
 DP: Diffusivity (m2
/s)
The Fruendlich isotherm is a mathematical relationship of the
maximum concentration of a particular contaminant that
could be adsorbed; it assumes that the adsorbent has a
heterogeneous surface composed of adsorption sites with
different adsorption potentials and based on empirical data
[7].
𝑥
𝑚
= 𝐾𝐶
1
𝑛 (15)
Where,
 x: Amount of solute adsorbed (mg)
 m: Mass of adsorbent (mg)
The next step in modeling the physical adsorption process of
an activated carbon bed, is to solve the above partial
differential equation (PDE) for each compound based on its
given constants. To solve the aforementioned PDE the
mathematical modeling software MATLAB was employed
and the PDEPE function was used. The results of the
MATLAB simulation are represented in Figure 2. Moreover
the Freundlich isotherm constants employed are presented in
the table below.
Table 3 Freundlich Isotherm Constants
V. COMBINED HEAT AND POWER
COGENERATION
Implementing a combined heat and power cogeneration cycle
(CHP) in this process requires the presence of several
elements. Those elements are a Gas turbine, a Heat Recovery
System and a Steam turbine. After purification, the gas
H2S SO2 NO2 CH4 CO2 CO H2
Dp
cm2
/s
33000 94 106 0.188 0.106 0.22 1.6
K 0.012 0.759 0.025 0.006 0.067 0.189 0.011
n 0.81 0.93 1.46 1.48 2.05 1.34 0.9
Figure 2 Change in Concentration of SO2 through the activated carbon bed

4. produced due to gasification would mainly contain hydrogen.
Hydrogen would be sent to be combusted inside the gas
turbine in order to generate electricity. The gases coming out
of the gas turbine aren’t combustible but are at a high
temperature of around 1000 o
C. Thus to make use of the heat,
a heat recovery system should be implemented. Hence the
high temperature of the flue gases will be used to produce
steam using a heat recovery steam generation unit (HRSG),
and thus generating more electrical power via a steam
turbine. The steam coming out of the turbine, will be
condensed back to water, recycled and used again. Hence a
closed steam cycle would be formed. [16]
Various types of HRSG are present. For this process it was
determined that a once through steam generation unit
(OTSG) would be optimal because it’s easy to operate and
maintain and has a lower cost than traditional HRSG units.
OTSG is a unit with a bundle of tubes. Water passes through
the tubes and gets transformed into steam after being exposed
to flue gases that enter the OTSG unit from below and exit as
stack into open air from above [17].
Using Aspen HYSYS®
, the temperature, pressure and flow
rates of entering and exiting fluids are determined. Hence a
heat and mass balance on the OTSG gives the size of the
proposed unit. The unit consists of 10 entry passes, where the
inlet water is distributed equally. Steam exits from the lower
part of the OTSG unit and is sent to a steam turbine as
mentioned previously [17].
The complete results of the complete design process of the
OSTG unit is presented in Table 4.
This project shows a promising solution to the two problems
Lebanon has been facing for the last 40 years. The project
aims at generating electricity using waste. Although one CHP
power plant can’t cover Lebanon’s need, but constructing
several CHP power plants in several locations would help
close the gap between supply and demand of electricity.
Table 4 Specifications of OSTG unit
The net power generation by the implemented CHP cycle is
presented in Table 5. Assuming that the plant operates for 24
hours a day, then the produced energy over a year (330 days)
is 88,625 MWh/year. According to statistics conducted by
the World Bank, the average consumption of electricity in
Lebanon per capita is 3500 kWh per capita in one year [1].
Hence from this designed plant some 25,321 people would
be provided with a 3500 kWh over a year. Assuming an
average household contains four people, then process could
provide electricity for up to 6300 households.
Table 5 Summary of Power Production
VI. PROPOSED PLANT LOCATION
The plant location is a critical part of the process, there are
many factors that have to be taken into account when
choosing the preferable location. Although the plant should
not be very far from Beirut for transportation of solid waste,
it should be distant from residential areas as well as
agricultural lands, urban areas and cities. Furthermore, the
land should be in the coastal area in order to make sure that
no major topographical limitations exist in that location such
as mountains or sharp slopes and to be able use the sea water
as a cooling fluid in the plant. These specifications were
translated to the following four constraints:
1. Three kilometers away from the sea
2. Two hundred meters from agricultural land
3. Two hundred meters away from main roads(primary
and secondary)
4. Plant area is greater than or equal to ten thousand
square meters
In order to meet the constraint set, a program, geographic
information system (GIS), was used. The constraints were
Specifications Value
Number of tubes 1130
Tubes/row 10
Number rows 113
Pipe inner diameter 5.4 cm
Pipe thickness 0.5 cm
Pipe Length 8 m
Transverse pitch 3 cm
Longitudinal pitch 3 cm
OTSG height 12 m
OTSG length 10m
OTSG width 3 m
Unit Produced (MW) Consumed (MW)
Pumps 0.5
Compressors 14.87
Gas turbine 20
Steam Turbine 6.56
Total 26.56 15.37 11.19 MW
Figure 3 Schematic of an HRSG

5. inputted into the software which allowed it to optimize the
location and give the best fit locations.
The approximate location of one of the areas that was found
by GIS to meet the applied constraints is in the village of
Damour, which lies south of Beirut. Hence, and according to
the calculations mentioned in the previous pertaining to the
amount of power generated, the neighboring towns such as
Saadiyat, Damour and Mechref could be provided with
electrical power from the plant.
VII. ECONOMIC FEASIBILITY
A rough economic analysis was performed on the entering
and leaving materials of the process and yields the following.
For every 1 ton of biomass processed and gasified this would
provide 100 $ which is the price paid by municipalities for 1
ton after checking what other companies in Lebanon are
charging for it [15]. 27.944 (wet) tons/hr of biomass and
sludge are taken by the process. Hence a profit of 22,131,648
$ per year is obtained.
In addition a profit is gained from electricity which is being
sold to neighboring areas. 1kWh of electricity costs 5 cents
[15]. The process provides a net of 84,189.6 MWh per year.
Hence a profit of around 4,000,000 $ is made from the
electricity each year. Thus, this amounts to a gross revenue
of $ 26M per year.
VIII. CONCLUSION
The gasification of municipal solid wastes and waste water
treatment sludge is a green process that not only produces
syngas and power however, also aids in minimizing solid
waste volume which is a troubling aspect of waste
management. Furthermore, at the beginning of this project
we set forth to recover as much power as possible from
otherwise useless waste.
Our plant turned out to be producing around 11 MW of net
electric power, which could power around 6000 households
in Lebanon, and is comparable to a small scale electric power
plant.
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Figure 4 Possible Plant Location