i
STOCKHOLM
March, 2013
High Temperature Air/Steam
Gasification (HTAG) Of Biomass –
Influence of Air/Steam flow rate in a
Continuous Updraft Gasifier
Master thesis
Muhammad Jalil Arif
DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING
DIVISION OF ENERGY AND FURNACE TECHNOLOGY
ROYAL INSTITUTE OF TECHNOLOGY
SE- 100 44 STOCKHOLM
ACKNOWLEDGEMENTS
In the name of Allah, the most gracious, the most merciful
I thank Allah Almighty for His immense blessings, help provided to me throughout my life, for
giving me strength and ability to complete not only my thesis, but also the whole process of
obtaining the Master’s Degree.
First and foremost, my utmost gratitude to my supervisor, Associate Professor Weihong Yang
from the Royal Institute of Technology, his expertise and supervision has guided me throughout
my work. I am very thankful to him for giving me this golden opportunity to work under him.
His continuous guidance, support, inspiration helped me during my thesis work. I thank him for
her kindness and for the incredible encouragement throughout my studies.
This thesis would not be in this shape if not because of my co-supervisor, Post Doctor Jan
Chmielewski. I am most grateful for the time he spent providing insightful critique and
guidance when it was most necessary. I Thank Jan for his helpful comments and suggestions for
improvements to this thesis. He always provided insightful feedback whenever it was required
and was a great support throughout.
Deepest gratitude is also due to all members of the Energy and Furnace Technology Division,
especially Pelle, Efthymios, Chunguang for the generous support and all the help during
experiments.
Finally, I would like to thank all people who have helped and inspired me during my Master
study. Special thanks to all my awesome friends in Stockholm for their continuous support.
Last but not least, my deepest heartfelt gratitude to my parents, my family and my friends back
home in Pakistan, for their continuous love, support and for all the good wishes.
TABLE OF CONTENT
1. ABSTRACT 1
2. OBJECTIVE 2
3. INTRODUCTION 3
4. METHODOLOGY 6
4.1 . Pre-heater (HiTAC) 7 4.2 . Biomass feeding system 8 4.3 . Gasifier 11 4.4 . Gas combustor 13 4.5 . Monitoring and measuring devices 14
5. EXPERMENTATION 17
5.1. Gasification process 17 5.2. Biomass characterization 17 5.3. The Preheater burner Temperature and pressure 19 5.4. Performance influencing parameter 20
6. RESULTS AND DISCUSSION 21
6.1. CASE 1 21 6.1.1 Temperature distribution along the reactor 6.1.2 Synthetic Gas (Syngas)
6.2. CASE 2 24 6.2.1 Temperature distribution along the reactor 6.2.2 Synthetic Gas (Syngas)
6.3. CASE 3 27 6.3.1 Temperature distribution along the reactor 6.3.2 Synthetic Gas (Syngas)
6.4. H2/CO Ratio 30 6.5. Lower Heating Value 31 6.6. CO/CO Ratio 32
7. CONCLUSION 33 8. FUTURE WORK 34 9. REFERENCE 35
APPENDIX A: Start up and operation procedure
APPENDIX B: Nomenclature
LIST OF FIGURES
1. Figure 1: Schematic diagram of Continuous type up-draft
2. Figure 2: Picture of the facility Updraft HTAG
3. Figure 3: HTAG Facility in the up draft configuration at continuous operation of the biomass.
4. Figure 4: Preheater (HiTAC) by NFK
5. Figure 5: Preheater and controlling pad of the preheater
6. Figure 6: Feeding system photographs; Left: top of the gasifier; right top: transport pipes to
the feeders; right bottom: Fuel input channels to the gasifier
7. Figure 7: Graphs illustrates the relation between the feed rate of biomass with the
frequency
8. Figure 8: Diagram of the HTAG updraft gasifier
9. Figure 9: Shape and look of the grate (Kanthel Steel)
10. Figure 10: Gas combustor 11. Figure 11: Type S thermocouple
12. Figure 12: Schematic diagram of Distribution of thermocouple in the gasifier
(thermocouple’s distribution inside the reactor left: along the axial right)
13. Figure 13: Pressure measurement device (Digital Precision Manometer DM 9200)
14. Figure 14: Biomass inside the feeder tank 15. Figure 15: The recordings of the burner are independent on the amount of biomass added
to the gasifier.
16. Figure 16 : Temprarure distribution inside the gasifier (CASE 1)
17. Figure 17: Percentage composition of the Synthesis gas and pressure difference (CASE 1).
18. Figure 18: Temperature distribution inside the gasifier (CASE 2)
19. Figure 19: Percentage composition of the Synthesis gas (CASE 2)
20. Figure 20 : Temprarure distribution inside the gasifier (CASE 3)
21. Figure 21: Percentage composition of the Synthesis gas and pressure difference (CASE 3)
22. Figure 22: LHV in each case study
23. Figure 23: Hydrogen /carbon monoxide ratio in each case study
24. Figure 24: Carbon monoxide/carbon dioxide ratio in each case study
Appendix A: Start up and operation procedure Quick Manual – 0.5 MWth HTAG plant
1. Main flue gas fan ON (in the gas storage room)
2. Water valves ON – both for cooling biomass feeder and for water sprays in the chimney
3. Water sprays control cabinet ON (switch levers in A position)
4. Cooling fan for NFK preheater valves ON (behind HTAC furnace)
5. Natural gas line ON
6. NFK preheater ID fan and FD fan ON
7. Start NFK preheater following its own manual
4
6
11
9
13
11
3
8. When temperature reaches the level start safety burner at combustor
9. Secondary air ON
10. Open bottom damper at main biomass hopper
11. Biomass feeder ON
12. Observe negative pressure in the reactor – if diminishes try to gradually close secondary air
ducts
13. If steam needed, activate boiler switch and boiler itself in advance (1 hour)
14. When steam flow activated, drain the steam tubes first
………………………………………………………………………………………………………………………………………..
15. After experiment empty biomass feeding line (shut off damper at the bottom of biomass
hopper)
16. Burn any combustible material deposited in the reactor – observe gas analyzer until no CO and
>20% O2 is detected
17. Shut off safety burner (but not its fan!)
18. Shut off NFK preheater burners (but not its fans!)
19. Close natural gas valves
20. Wait until temperatures in NFK and in reactor are <500C
21. Shut off safety burner fan
22. Shut off NFK preheater ID and FD fans and cooling fan for its valves
23. Wait until system is further cooled down to about 200C and switch off main flue gas fan.
17
8
APPENDIX B: NOMACLATURE A Equivalent cross sectional area of the sum of holes (m2)
A0 Cross section area of the gasifier, (m2)
D Equivalent diameter of the sum of holes (m)
D0 Diameter of the gasifier
dp Diameter of pellets
d single hole diameter
ER Equivalence ratio (mol/mol)
gi Mass fraction of the species
h Gas production rate (Nm3/kg)
lp Length of pellets
mF Mass flow rate of the fuel
mFG Mass flow rate of feeding gas (kg/h)
mH2O Mass flow rate of steam (kg/h)
PB Pressure difference between the top and bottom of the gasifier (mmH2O)
Pressure difference between the feeder and atmospheric pressure (mmH2O)
Pressure difference between the feeder and top of the gasifier (mmH2O)
Re Reynolds number
Syn gas Synthesis gas
S/F Steam to fuel ratio (kg/kg)
SV Superficial velocity (m/s)
Ti Number of the thermocouple
VFG Volume flow rate of the feeding gas (Nm3/h)
VG Volume flow rate of the producer gas (Nm3/h)
xi Molar/volumetric fraction of the species
Greek letters
Conversion of the carbon to gas
Density of the feeding gas (kg/m3)
Viscosity of the feeding gas (Pa*s)
cold Cold gas efficiency
1
1. Abstract
Biomass is an important source of energy and the most important fuel worldwide after coal, oil
and natural gas. Biomass does not add carbon dioxide to the atmosphere as it absorbs the same
amount of carbon in growing as it releases when consumed as a fuel. Its advantage is that it can
be used to generate electricity with the same equipment or power plants that are now burning
fossil fuels. However, the low energy density of the biomass requires developments and
advances in conversion technologies in order to increase process efficiency and reduce
pollution. One of the most promising converting methods for treatment of biomass and waste
feedstock is gasification. In this study a highly preheated air/steam of temperatures >800oC is
introduced to the gasifier which is fed with wood pellets’ feeding rate 40-50 kg/h.
The system is redesigned to work as a continuous type updraft HTAG. The aim of the
studies was to test the performance of an Updraft configuration in various operating conditions
using Biomass (wood pellets) as the feedstock, and facing primarily technological difficulties
and process limitations. Determining the Temperature distribution along the reactor and
synthesis gas composition of the process are reported for various operating parameters.
During the experiment it is observed that the introduction of more steam flow rate
increases the LHV (lower heating value) of the synthesis gases. Three case studies (Case1,
Case2, and Case3) are conducted, each case having different biomass feeding rate, steam flow
rate and process air flow rate. The result show that the amount of LHV of gas varied from 3 to
4.2 MJ/Nm3, the H2: CO ratio is between 0.5-0.9 and the CO/CO2 ratio has range 1.0-1.7. Case
3, in which 40 kg/h biomass feeding rate and 80 kg/h Steam flow rate is maintained gives High
LHV, high H2/CO ratio and more CO/CO2 ratio among the rest case studies.
Further improvement can be done within the reactor, increase in retention time and
variation of more parameters can examine, in order to get the optimum result in future.
2
2. Objective
The objective of this thesis is to provide the following information
Study the results and outcomes of the gasification experimentation i.e. LHV, synthesis
gas emission values.
Influence of steam flow rate and process air on the composition of syngas in an updraft
gasifier.
Make HTAG gasifier run smoothly in a continuous biomass feeding system.
In future in order to meet the environmental constraints, the power and heat generation will
have to be CO2 neutral or at least minimum exhaustion of CO2. To cope this goal one way of
controlling the emission is to switch from fossil fuels to renewable energy resources e.g.
biomass fuels are cost-effectively justified for heat and power generation plants.
3
3. Introduction
Energy conservation goes beyond that into both bigger ways to conserve energy and finding
other productive sources of energy. To conserve effectively our existing energy supply it is
imperative to first fully develop a diverse blend of alternative energies (biomass, wind energy,
solar energy etc.). The pollution is a large factor with the effects of carbon dioxide causing
global warming and the greenhouse effect.
Biomass is one of the sources of renewable energy that could be a good alternative for
declining fossil fuels resources and increasing demand for energy. Nevertheless, the low energy
density of the biomass requires developments and improvements in conversion technologies in
order to increase process’ efficiency and reduce pollution. One of the most promising
converting methods is ‘gasification’ in which the Biomass and waste feedstock is converted in
to Synthesis (syn) gas. In this process highly preheated agent (air/steam) up to 800oC is
introduced into the ‘gasifier’. This is also known as ‘High Temperature Air/Steam Gasification’
(HTAG) is a method in which a preheated Air/steam is used as the oxidizer (gasification agent),
as it takes heat from the exhaust gases inside the preheater. This HTAG process follows the
advancement in the High Temperature Air Combustion (HiTAC), which has revealed to be better
in energy cutback and shown less pollution compared to the old conventional combustion
method. [1, 2]
This system is on scaled-up to updraft pilot plant of a capacity of 0.7MW [3, 5]. The use of
highly preheated agent results in high conversion of fuel to gas, higher LHV and relatively lower
tar content compared to conventional gasification. The preheated Air/steam oxidizer supplies
additional amount of energy into the gasification process which as a result enhances the
thermal decomposition of the biomass feedstock [2].
This Preheating of Air/steam is realized by means of the modern ‘High cycle regenerative
Air/steam preheater’.
4
Consequently, the HTAG increases both the calorific value of the producer gas, and the cold
gasification efficiency. In this work, the advantages of the HTAG processes is presented by
considering performance influencing parameters that include materials quality, oxidizer type,
equivalence ratio (ER), gasification temperature, and bed additives.
During experiments it was observed that, the position of the grate has a strong contribution on
the performance of the gasifier and the producer’s gas composition. This has an influence on
superficial pressure and thus on the whole process.
Figure 1: Schematic diagram of Continuous type up-draft
gasifier
Feedstock
Silo
Preheater
Raw
Producer
Gas
5
Figure 2: Picture of the facility Updraft HTAG
b) Updraft HTAG system
(NEW DESIGN)
6
4. Methodology
The experimental setup of the HTAG system comprises of these integrated units:
1. Pre-heater (HiTAC)
2. Biomass Feeding system
3. Gasifier
4. Synthesis Gas combustor (after burner)
5. Monitoring and measuring devices
The schematic drawing of the system is presented in Fig 3.
Figure 3: HTAG Facility in the up draft configuration at continuous operation of the biomass.
7
The performance of an updraft configuration is observed in various operating conditions using
wood pellets as the feedstock, and facing primarily technological difficulties and process
limitations. Temperature and pressure distribution along the gasifier reactor, gas composition,
gas production yield of the process are monitored [4].
High Temperature Air/steam Gasification (HTAG) test facility is equipped with the following
main devices (figure 4):
4.1 Pre-heater (HiTAC)
A compact high temperature air generator provides the supply of high-temperature air or
mixture of air and steam [3, 11]. Figure 7 shows a conceptual figure of a highly preheated gas
generator. While the regenerator located in the bottom of a combustion chamber is heated up
by combustion gas, the highly preheated gases go to exit through another chamber (Figure 8)
Heat storage and heat release in the regenerators are repeated periodically when combustion
gas and low temperature gas are alternately provided by on-off action of switching valve
located on the low temperature side. The preheated gas continuously discharges from each exit
nozzle at left hand side section, and combustion gas exhaust from right hand side section.
Figure 4: Preheater (HiTAC) by NFK
8
The High Temperature Air Combustion (HiTAC) preheater (Figure 5) is used to preheat air or
steam in the higher temperature range. [5] New Energy and Industrial Technology Development
Organization, NEDO (Japan). The fruitful collaboration with Nippon Furnace Kogyo Kaisha Ltd,
(NFK) Japan
Figure 5: Preheater and controlling pad of the preheater
4.2 Biomass feeding system:
The Feedstock feeding system consisting of three parts: Main hopper, feeding Hopper,
Transport ducting (figure 6):
i. Biomass main hopper: The hopper is a tank of cuboid-ended-in-the-pyramid shape and
the capacity of 2m3. The solid fuel is fed from the top and gravimetrically falls down to
damper. When the damper is opened the biomass can enter to the transport screw.
ii. Feeding hopper: In a tank of approximately 80 liter (approx.) capacity biomass, the
channel is connected with a stirrer powered by electrical motor and the other end of the
channel is connected to the feeding screw view transparent pipes.
9
iii. Transport ducting: Transport duct with a transporting screw conveyer powered by the
motor m1 of …kW. The pipe of diameter 100mm is made of polypropylene. The motor
of the feeding system doesn’t run smoothly at low frequency, hence only ONE channel
(among four channels) is used for the feeding of biomass. Other three channels were
manually block in order to get the desired feeding rate ranging between 40-50 kg/h
Figure 6: Feeding system photographs; Left: top of the gasifier; right top: transport pipes to the
feeders; right bottom: Fuel input channels to the gasifier
Before starting the experiment it is very importer to calibrate the devices which may affect the
parameters of the result. The feeding rate initially required for the gasification is 40 kg/hour.
And in order to fix this feeding rate the frequency of the feeder must have determined. To
calculate and examine the relation between frequency of the engine and feeding rate, numbers
of trails were conducted with the biomass. In which data was collected by varying the
frequency at a certain time interval and finally weighting the biomass. Below is the table (1)
which shows the variation of Feeding rate (kg/h) with changing the frequency of the feeding
engine.
10
Frequency [Hz] Feeding rate
[kg/5 min]
Feeding rate
[kg/h]
20 1.98 23.76
25 2.3 27.6
30 2.8 33.6
35 3.25 39
36 3.35 40.2
40 3.7 44.4
Table 1: Relation between the frequency (Hz) of the feeding engine and feeding rate (kg/h)
Figure 7: Graphs illustrates the relation between the feed rate of biomass with the frequency
20
25
30
35
40
45
50
55
60
16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
[kg/h]
[Hz]
1 engine
Linear (1 engine)
11
The feeding system is attached with two engines, 1, which controls the feeding rate of the
biomass to the gasifier and 2(second engine) which controls the biomass rate from the storage
facility of the biomass to the feeder tank. The feeding rate of the feeder is predetermined
before the experiment. The feeder works with a specific frequency and this frequency is
correlated with the feeding rate. As shown the graph above, the yellow spot represents the
point where we get the 40kg/h and 50 kg/h feeding rate for the gasification at the frequency of
36 Hz and 45.5 respectively (figure 7).
4.3 Gasifier
It is a Continuous type, co-current and the up draft fixed-bed gasifier. Fixed-bed gasifier, is a
vertical cylindrical reactor, which consists of six sections (see Figure below): [11]
− Top section of feedstock feeder
− GPP - gas phase part, fuel gas outlet section
− WB - wind box
− BP - bed part, feedstock (fixed bed) section
− PB - grate and pebble bed part
− SB - slag box serving as slag collector
12
Figure 8: Diagram of the HTAG updraft gasifier
− Top section of feedstock feeder
− GPP - gas phase part, fuel gas outlet section
− WB - wind box
− BP - bed part, feedstock (fixed bed) section
− PB - grate and pebble bed part
− SB - slag box serving as slag collector
13
4.3.1 Grate size:
The grate was made from the material called Kanthal steel. The Khantal was the name of
company who introduced this steel.
Figure 9: Shape and look of the grate (Kanthel Steel)
The grate is ordered specially for this purpose. A highly perforated grate solution is developed.
With an appropriate grate which have a minimal resistance for the flow, high density of holes
(low density of restricted for flow area). A diameter of the grate is D=385 mm and is thin
compare to the previous grate in order to avoid clogging of ash in tunnels (Figure 9).
4.4. Synthesis Gas combustor:
Synthesis Gas combustor or afterburner (shortly called afterburner) to burn completely the
produced fuel gas. The total volume is around 0.45 m3. Afterburner has one inlet for the fuel
gas from the gasifier and one outlet for the flue gas. Afterburner is also equipped with a pilot
burner and set of additional air nozzles to assure complete combustion of the fuel gas produced.
(Figure 10)
14
Figure 10: Gas combustor
4.5 Monitoring and measuring devices:
4.5.1 Temperature measurements
Temperatures were measured via thermocouples - Types S (figure 12(a)) located in several
points of the gasifier. The distribution of thermocouples is displayed in figure 12(b). The
thermocouples indicate a vertical temperature gradient along the reactor height. It was
assumed that the horizontal gradient of temperature was that not significant maintaining
homogenous temperature profile around the cross section slides of the gasifier.
15
Figure 11: Type S thermocouple
Figure 12: Schematic diagram of Distribution of thermocouple in the gasifier (thermocouple’s
distribution inside the reactor left: along the axial right)
16
4.5.2 Pressure measurements
The pressure measurements have been taken inside the gasifier, below the grate and above the
grate. Digital Precision Manometer DM 9200 has been used to obtain the pressure readings.
DM 9200 has measuring range: ± 75 hPa (mbar).
Figure 13: Pressure measurement device (Digital Precision Manometer DM 9200)
4.5.3 Gas sampling
A dry and cleaned gas was afterwards analyzed using continuously working gas analyzers (GA) and
periodically gas chromatograph (GC).
17
5. Experimentation:
5.1 Gasification process
The gasification of the biomass has been done at around 800-900 oC to get the optimum result.
The process involves three stages. [Appendix A]
1. Reaching up to 800 oC using oxygen/fuel in the preheater. At this stage the ratio of air-
oxygen has been balanced with the fuel input. Until the temperature of the preheater
reaches up to 1200-1300 at the exit of the preheater the process continues.
2. When the Gasifier is heated up and the temperature of the preheated Air/steam
reaches up to 800 oC, Biomass is introduced inside the gasifier. The feeding rate of the
biomass is predetermined
3. To control the feeding rate of the biomass, a feeding system is used which electronically
controls the feeding rate. The feeder has been installed on top of the gasifier where the
temperature can go up to 1100-1200 so it is connected with the water cooling system.
These feeding were predetermined before the experiment. The feeder works with a specific
frequency and this frequency is correlated with the feeding rate i.e. at 36 Hz the feeding rate is
40 kg/h. Furthermore in order to calculate the % Syngas, measurements are done by Gas
chromatography (GC). As the temperature is very high so safety precaution has to be taken in
consideration among the people participating in the process.
5.2 Biomass characterization
The biomass used for the investigation was wood pellets of diameter 8 mm and an average
ration of length to diameter l/d=4, manufactured by BooForssjö Energi AB (figure 14), the LHV
value of the biomass is 17.76 MJ/kg.
The table 2 shows the characterization of the biomass which is used in the gasification process.
18
Proximate analysis
Moisture content at 105°C 8 %
Ash cont. at 550°C 0.4-0.5 % (dry)
LHV 17.76 MJ/kg (as received)
Volatile matter 84 % (dry)
Density 630-650 kg/m3
Ultimate analysis
Sulphur S 0.01-0.02 % (dry)
Carbon C 50 % (dry)
Hydrogen H 6.0-6.2 % (dry)
Nitrogen N <0.2 % (dry)
Oxygen O 43-44 % (dry)
Ash fusion temperatures (oxidizing conditions)
Start of melting, IT >1400 °C
Corner round off, ST 1400-1500 °C
Half-sphere, HT 1500 °C
Melting complete, FT 1500-1550 °C
Table 2: Biomass characterization
Figure 14: Biomass inside the feeder tank
19
5.3 The Preheater burner Temperature and pressure
In the High Temperature Air Combustion (HiTAC) preheater is used to preheat air or steam and
to this HiTAC is has automatic monitoring controller. Using its controlling pad temperature and
pressure differences of preheated air/steam inside is monitored. The (figure 15) graph shown
shows the completed data of the temperature and pressure changes within the preheater
burner. This data also indicates the amount of fuel, cooling air, process gas, flue gases from the
burners, steam flow rate.
(Note: The Three red, green, blue arrows show the case studies, which is discussed later in
Results and Discussion part of the report)
Figure 15: The recordings of the burner are independent on the amount of biomass added to the
gasifier.
-200
0
200
400
600
800
1000
1200
1400
1600
Rre
lati
ve s
cale
Timeline
Preheater burner data
Comb „A” T1[oC]
Comb „A” T2[oC]
Comb „A” T3[oC]
Comb „A” P [kPa]
Comb „B” T1[oC]
Comb „B” T2[oC]
Comb „B” T3[oC]
Comb „B” P [kPa]
Pilot fuel [m3/h]
Main fuel [m3/h]
Cooling Air [m3/h]
Comb Air [m3/h]
Flue gas P P [kPa]
Case 1 Case 2 Case 3
20
5.4 Performance influencing parameter: In general the performance influencing the parameters of HTAG are [1, 5, 10]:
1. Quality of the Biomass materials (moisture content, size, shape, type, LHV etc.)
2. Oxidizer type of Oxidizer (Air, steam, oxygen)
3. Air/Steam ratio.
4. Temperature of Gasification.
5. Bed additives.
6. Residence time.
On the other hand, gasifier output performance parameters forming bases for the comparative
are: carbon conversion efficiency, yield and product gas composition, product gas quality (tar,
particulate dust), and product gas calorific value. Insulation is also important to control the
energy loses and to make the experiment more effective. All the monitoring devices like
Thermocouple, probes, barometer, Gas analyzing apparatus has to be ready before the start of
experiment. Checking all the connection and the supply line of gas, cooling water, fuel, and
ventilation pipes are very essential. As the temperature is very high so safety precaution has to
be taken in consideration among the people participating in the process.
The HTAG process has shown features in terms of product gas yield, gas composition and
heating value. The effect of its high reaction temperature is to sustain the gas phase reactions
that are dominant at elevated temperatures of over 1000 oC. While the low temperature
gasifier can process biomass feedstock with moisture content up to 50% only, HTAG can handle
higher level of moisture content. The presence of moisture in the HTAG biomass feedstock
increases combustible gas yield and its heating value since the moisture takes part in the
secondary reduction and steam reforming reactions that are responsible for the formation of
more CO and H2 gases. [8]
21
6. Results and Discussion
The experiment was conducted with different case parameter. The feeding rate, steam flow
rate and the process air flow is changed. From all those cases with much variation in
parameters, three cases which give the better result are selected. Table 3 shows the case
studies which are being monitored in this experimentation:
Case study Biomass
[kg/h]
Steam flow
rate [kg/h]
Process Air
[Nm3/h]
Case 1 50 0 60
Case 2 40 60 10
Case 3 40 80 0
Table 3: Experiment Parameters of three Case studies
Prior to the experiment all the equipment has to be monitoring thoroughly because of high
temperature it can affect the results. Each Case is described separately, determining each Case
study with respect to the temperature distribution along the reactor and synthesis gas
composition of the process are reported.
6.1 CASE 1:
The parameter of Case 1 is shown in the table 4 below. The feeding rate of the Biomass is fixed
to 50 kg/h shows and Process air flow rate of 60 Nm3/h. In this case steam has not been used.
Case study
Biomass [kg/h]
Steam flow rate
[kg/h]
Process Air [Nm3/h]
Case 1 50 0 60
22
Table 4: Case 1 parameters
6.1.1 Temperature distribution along the reactor:
Figures below show the temperature profiles for analyzed cases. The charts indicate influences
of grate and flows of biomass and preheated gases on temperature distribution insider the
gasifier.
Figure 16 : Temprarure distribution inside the gasifier (CASE 1)
As illustrated in the Figure 16, the doted blue line represents CASE 1 and the temperature
values are on the Y axis. T1 which is below the grate temperature remained close to (T1) 800 oC.
Above the grate the temperature rises till 1000 oC. This rise in temperature is because of the
partial combustion of biomass inside the gasifier. Largely the high temperature air/steam
mixture converts the solid biomass in to the gas. As the solid biomass converts into the gas it
0
200
400
600
800
1000
1200
1400
Tem
pre
ture
oC
Gasifier - Temperature distribution with biomass (Case 1)
T1 below
T2 above
T3 above
T4 above
T5 above
T6 above
T7 above0:00 0:20 0:40 01:00 1:20 1:40 2:00 2:20
Timeline
23
consumes energy, but due to the partial combustion in the gasifier the temperature on top of
the grate raises [9]. Temperatures on top of the grate T2, T3, T4, T5, T6 are not very much
different. The minor difference in temperature can be because of the thermocouple placement.
Temperature at T7 is the exit temperature from the gasifier. This graph also indicates that the
gasification of biomass occurred above 800oC.
6.1.2 Synthetic Gas (Syngas):
Syngas composition has been measured of Case 1 which is illustrated in the graphs (Figure 17).
Figure 17: Percentage composition of the Synthesis gas and pressure difference (Case 1).
From the graph below the percentage Synthesis gas illustrates the variation of gas with time.
Overall the percentage of CO2 remained constants between 10-12% throughout the process,
along with O2 (1.6%) and CH4 (0.3%)
Table 5 shows the percentage volume of different gases produced for the gasification process
and the Lower Heating value (LHV) in each observation (section 6.6). The LHV depends on the
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
0:00 0:20 0:40 1:00 1:20 1:40 2:00 2:20
Axi
s Ti
tle
Axis Title
Synthesis Gas composition and pressure
CH4
H2
CO
CO2
O2
N2
ΔP (Gasifier)
24
average percentage of the H2, CH4, CO2 and CH4, as the fraction of these gases increases the
LHV is 3.2 MJ/Nm3. But at the start of the experiment this LHV value remained low because of
less H2 and CO fraction in the syngas.
CH4 (%vol.)
H2 (%vol.)
CO (%vol.)
CO2 (%vol.)
O2 (%vol.)
N2 (%vol.)
ΔP (Gasifier)
H2/CO Ratio
CO/CO2 Ratio
LHV [MJ/Nm3]
0.3 9.4 13.5 11.3 1.6 63.3 -0.6 0.7 1.2 3.2
Table 5: Syngas composition percentage of Case 1
6.2 Case 2:
In this case the biomass feeding rate has been decreased from 50 kg/h to 40kg/h. Whereas the
steam flow rate has been increased to 60 kg/h and in this case but process air rate has been
reduced to 10 Nm3/h involved as shown below in the table.
Case study Biomass [kg/h]
Steam flow rate [kg/h]
Process Air [Nm3/h]
Case 2 40 60 10
Table 6: Case 2 parameters
6.2.1 Temperature distribution along the reactor
The temperature below the grate remained constant at 900 oC. As form the graph the
temperature above grate was increased initially when the steam flow rate is suddenly increases
to 60kg/h (without any process air). But when the gasification becomes stable the temperature
came down slowly around 1050 oC. (Figure 18)
25
Figure 18: Temperature distribution inside the gasifier (CASE 2)
6.2.2 Synthetic Gas (Syngas):
The Syngas composition graph of Case 2, show very linear behavior, the percentage of CO2 and
CO almost remains constant, while H2 percentage increases slowly from 10.5% to 13.2%.
(Figure 19)
0
200
400
600
800
1000
1200
1400
Tem
pre
ture
oC
Gasifier -Temperature distribution
T1 below
T2 above
T3 above
T4 above
T5 above
T6 above
T7 above
Timeline
CASE 2
26
Figure 19: Percentage composition of the Synthesis gas Case 2
The Syngas composition has shown LHV value ranging from 3.6MJ/Nm3 (table 7) in Case 2. If
Case 2 is compare with Case 1, it can be said that increase of steam flow rate and reducing of
process air resulted in increase in percentage of the LHV value. The addition of Steam into the
gasifier increases the amount of hydrogen molecules inside the gasifier, and this increase may
have increase the H2, CO, CH4 volume fraction in the synthesis gas. As the increase in the
percentage fraction of H2, CO, CH4 and C02 also enhances the LHV of syngas.
CH4 (%vol.)
H2 (%vol.)
CO (%vol.)
CO2 (%vol.)
O2 (%vol.)
N2 (%vol.)
ΔP (Gasifier)
H2/CO Ratio
CO/CO2 Ratio
LHV [MJ/Nm3]
0.5 11.2 15.6 11.4 1.0 60.2 -0.7 0.7 1.4 3.6
Table 7: Syngas composition percentage of Case 2
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
0:00 0:20 0:40 1:00
Vo
l. f
ract
ion
Time(h)
Syngas Composition and pressure differnece (CASE 2)
CH4
H2
CO
CO2
O2
N2
ΔP (Gasifier)
ΔP
27
6.3 Case 3:
The biomass feeding rate remained is 40kg/h as in Case 2 while the steaming rate has been
increased from 60 kg/h to 80 kg/h and no process air has been used.
Case study Biomass [kg/h]
Steam flow rate [kg/h]
Process Air [Nm3/h]
Case 3 40 80 0
Table 8: Case 3 parameters
6.3.1 Temperature distribution along the reactor
The Steam flow rate again changed from 60 kg/h to 80 kg/h. A slight decrease in temperature
has been observed, this decrease can be an indicator of less partial combustion. As the steam
flow rate is increase so the biomass is converting into the gas with less combustion. Hence this
drop in temperature shows that the conversion of biomass to Syngas occur effectively as
compare to the previous case studies. High steam flow rate may have increased the gasification
(meaning less combustion occurred) which resulted in the temperature drop the variation in
the temperature graph can also because of the pressure changes in the gasifier.
28
Figure 20: Temperature distribution inside the gasifier (CASE 3)
In order to maintain the Air/stream temperature and pressure, it is essential to monitoring the
components which indirectly affect the process in preheater. The suction from the gas
combustor might have changed the pressure inside the gasifier, which can result in changing
the overall temperature. It is also important to regulate and control the preheat burner, as the
changes inside the preheater can also have effect on the changes inside the gasifier. Comparing
with the preheater graph (Figure 20) there is a small drop in the preheated steam temperature
and it can be related to this drop in temperature in the gasifier.
6.3.2 Synthetic Gas (Syngas):
From the figure 20 and the Table 9, the recording of the Case 3 are shown.
0
200
400
600
800
1000
1200
1400
Tem
pre
ture
oC
Gasifier -Temperature distribution
T1 below
T2 above
T3 above
T4 above
T5 above
T6 above
T7 above
Timeline
CASE 3
29
Figure 21: Percentage composition of the Synthesis gas and pressure difference in Case study 3
With the increase in the stream flow rate, it is observed that the percentage composition of H2
and CO was increased significantly (meaning High LHV). The value of H2 was 14.7 % whereas
the 17.2% CO is obtained. This in total is 32 % of the overall gas composition. .
CH4 (%vol.)
H2 (%vol.)
CO (%vol.)
CO2 (%vol.)
O2 (%vol.)
N2 (%vol.)
ΔP (Gasifier)
H2/CO Ratio
CO/CO2 Ratio
LHV [MJ/Nm3]
0.5 14.7 17.2 11.3 0.3 56.0 -0.4 0.9 1.5 4.2
Table 9: Syngas composition percentage of Case 3
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
0:00 0:20 0:40 1:00
volu
me
fra
ctio
n
Axis Title
CASE 3
CH4
H2
CO
CO2
O2
N2
ΔP (Gasifier)
ΔP
30
6.4 Hydrogen/Carbon monoxide ratio:
Figure 22: Hydrogen /carbon monoxide ratio in each case study
The ratio of hydrogen to carbon monoxide has been in the range 0.6-0.9 for all cases as it is
seen in graph (figure 22). There is an increase of H2/CO ratio with decrease in the process air
flow. As steam flow rate is increase the H2/CO ratio raised. In Case 3 where the steam flow rate
around 80 kg/h, the H2/CO ratio of 0.9 is recorded. In Case 3 the H2/CO ratio is higher compare
to other cases and this increase can be because of the exothermal oxygen-carbon reaction
which also increased the temperature inside the gasifier [6, 7]. Less H2/CO can be because of
the increase in combustion inside the reactor or unconverted carbon from the biomass
resulting in inefficient gasification process.
0
0.2
0.4
0.6
0.8
1
Case 1 Case 2 Case 3
H2/ CO ratio
H2/ CO ratio
31
6.5 Lower heating value (LHV):
Figure 23: LHV in each case study
The lower heating value (LHV) of the synthesis gas depends on the volume fraction of CH4, H2,
CO and C02 content. The percentage volume of H2 and CO in Case 3 is higher compare to Case
1 and Case 2 (Figure 23). The reason could be the increase in steam flow rate which has
resulted in the rising the amount of hydrogen in the gasifier. The initial LHV value of the
wooden pallet was 17.76 MJ/kg. Hence the LHV values determined during these case studies
are lower. Overall the LHV is quite low this is because of very less retaining time of biomass
inside the gasifier. In order to improve the LHV, the retaining time of the biomass has to be
increased by putting larger amount of biomass.
0
1
2
3
4
5
Case 1 Case 2 Case 3
LHV [MJ/Nm3]
LHV [MJ/Nm3]
32
6.7 CO/CO2 Ratio:
Figure 24: Carbon monoxide/carbon dioxide ratio in each case study
The ratio of carbon monoxide to carbon monoxide has been in the range 1.0-1.7 for all cases as
it graph (figure 24) is showing the average ration in each case study. The gaseous products of
gasification of biomass are CO, CO2, and H2O. Methane (CH4), hydrogen (H2), and other low-
molecular hydrocarbons are released as the temperature rises. The CO content is the profound
indicator of the early stages of biomass oxidation [12], CO2 production increases when the
combustion occurs inside the gasifier. CO/CO2 ratio indicated the efficiency of the gasification
inside the gasifier. Increase in CO/CO2 ratio means clean gasification, whereas decrease in this
ratio shows the increase CO2 content. The increase in the amount CO2 is a result of combustion
process inside the gasifier. This combustion process is also profound by observing the
temperature graphs in each case. High value of temperatures >1000oC above the grate
indicated this increase in combustion process. Case 3 show better result of CO/CO2 ratios than
the other two cases.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
CASE 1 CASE2 CASE 3
CO/CO2 Ratio
CO/CO2 Ratio
33
7. Conclusion:
A new Updraft HTAG system was redesigned and experiments are conducted at Royal Institute
of Technology, Stockholm Sweden. The preheated air/steam is introduced to the Updraft
gasifier from the bottom and the synthesis gas exhausts from the top. The new grate was made
from the material called Kanthal steel has been used for the better conversion of biomass.
Three case studies (Case1, Case2, and Case 3) are conducted, each case having different
biomass feeding rate, steam flow rate and process air flow rate. Results from the three cases
were obtained, each having different parameter:
The amount of LHV of gas varied from 3 to 4.2 MJ/Nm3. Case 3, in which 40 kg/h
biomass feeding rate and 80 kg/h Steam flow rate is maintained gives the maximum LHV
of 4.2 MJ/Nm3. It is observed that the increase in Steam flow rate increases the LHV of
the synthesis gas.
The H2/CO ratio is recorded between 0.5-0.9. In Case 3 the H2/CO ratio is higher
compare to other cases and this increase can be because of the exothermal oxygen-
carbon reaction which also increased the temperature inside the gasifier
CO/CO2 ratio indicated the efficiency of the gasification inside the gasifier. The CO/CO2
ratio has the range from 1.0 to 1.7. High value of CO/CO2 ratio shows less combustion
and Case 3 showed the high CO/CO2 ratio among the other case studies.
Increasing the residence time between and biomass of steam as the gasifiying agent is highly
recommended. By increasing the retaining time the LHV value of the syngas can be improve
significantly.
34
8. Future work:
Investigating of different biomass feed i.e. Size of the pallet, chemical composition, and
the physical structure and shape etc.
Study of Tar and exhaust gases for better environmental aspects.
Increasing the residence time between and biomass of steam as the gasifiying agent
Grate Material characteristics and changes at high temperature gasification
Economics and ecological aspects of biomass gasification
35
10. Reference
1. S. Kalisz, D. Szewczyk, C. Lucas, R. Abeyweera, High Temperature Air/steam Gasification
(HTAG)- Technical Report No 2:, Royal Institute of Technology, Stockholm 2003, ISRN
KTH/MSE -- 03/61 –SE+ENERGY/TR
2. Lucas C, High temperature air/steam gasification of biomass in an updraft fixed bed batch
type gasifier, Doctoral Thesis in Energy and Furnace Technology Stockholm, Sweden 2005
. A. Ponzio, Thermally Homogenous Gasification of Biomass /Coal / Waste for Medium or
High Calorific Value of Syngas Production. Doctoral Thesis, KTH, Stockholm 2008.
4. Kalisz S, Abeyweera R, Szewczyk D, Jansson A, Lucas C and Blasiak W: Energy balance of
high temperature air/steam gasification of biomass in updraft, fixed bed type gasifier, IT3
Conference, May 10-14, 2004, Phoenix, Arizona.
5. A. Ponzio, S. Kalisz, W. Blasiak, Effect of operating conditions on tar and gas composition
in high temperature air/steam gasification (HTAG) of plastic containing waste, Fuel
Processing Technology, 87, (2006), 223 – 233
6. I.I. Ahmed, A.K. Gupta.: Pyrolysis and gasification of food waste: Syngas characteristics
and char gasification kinetics. Applied Energy 87 (2010) 101–108.
7. Jangsawang W, Klimanek A, Gupta A.K; Enhanced yield of hydrogen from wastes using
high temperature steam gasification, J. Energy Resources Technology, 22 July 2005.
8. K. Umeki, Modelling and simulation of biomass gasification with high temperature steam
in an updraft, fixed-bed gasifier, doctoral thesis at Tokyo Institute of Technology, (2009)
Tokyo, Japan
9. P. Basu, P. Kaushal, Modeling of Pyrolysis and Gasification of Biomass in Fluidized Beds: A
Review, Chemical Product and Process Modeling, v.4, 1 (2009), 21.
10. Chih-Lun Hsi, Tzong-Yuan Wang,, Chien-Hsiung Tsai, Ching-Yuan Chang Chiu-Hao Liu, Yao-
Chung Chang, and Jing-T. Kuo, Characteristics of an Air-Blown Fixed-Bed Downdraft
Biomass Gasifier, Energy & Fuels (2008), 22, 4196–4205.
11. B.V. Babu, Pratik N. Sheth, Modeling and simulation of reduction zone of downdraft
biomass gasifier: Effect of char reactivity factor, Energy Conversion and Management 47
(2006) 2602–2611
36
12. Liming Yuan, Alex C. Smith, CO and CO2 emissions from spontaneous heating of coal
under different ventilation rates, National Institute for Occupational Safety and Health,
2011
