experimental investigation of o2-co2 firing during pulverized coal combustion

Institut für Verfahrenstechnik und Dampfkesselwesen Institute of Process Engineering and Power Plant Technology Direktor: Prof. Dr.-techn. G. Scheffknecht Pfaffenwaldring 23 · D-70550 Stuttgart · Tel.: 0711-685-3487 · Fax: 0711-685-3491 Universität Stuttgart

Master Thesis No. 2762

�Experimental investigation of O2/CO2 firing during pulverized coal

combustion�

Kashif Imtiaz Choudhry Matriculation No 2071972

Stuttgart, 28.02.2005

Start date: 01.06.2004 Submission date: 28.02.2005 Supervisor: Bhupesh Dhungel

This work is dedicated to my

Parents

Their prayers are always behind me

I

Acknowledgement I must firstly thank my supervisor Engr. Bhupesh Dhungel who does deserve my greatest thanks, since he provided me with incredible support, encouragement, and guidance for my thesis research work. I am thankful to Stefan Kiening & Patrick Mönckert for their generous assistance during this time. I also thank my colleagues of the MSc WASTE Process Engineering students 2003 (entry) class in universität Stuttgart Germany of for sharing experiences and knowledge during the time of study. Finally, I take this opportunity to express my profound gratitude to my beloved parent, my brothers and sisters for their moral support and patience during my study in Universität Stuttgart Germany. Engr. Kashif Imtiaz Choudhry M.Sc Engineer

II

Abstract Global warming is one of the largest environmental challenges of the present time. Increased

carbon dioxide level in the atmosphere is the dominating contributor to increased global

warming. Carbon dioxide emitted to the atmosphere through combustion of fossil fuels in

power plant, automotive engines for industrial use and for heating purposes. Three main

options for reducing CO2 emissions from fossil fuel based energy conversion are 1) increase

the fuel conversion efficiency 2) switching to a fuel with lower fossil carbon contents and 3)

capturing and storing the CO2 emitted from fossil fuel.

In order to reduce emissions of carbon dioxide from large point sources Oxy fuel combustion

technique can be used in pulverized coal fired power plants with CO2 capture. The fuel is

burnt in oxygen and recycled flue gas, yielding a high concentration of CO2 and low

emissions like NOx and SOx in the flue gas. The aims of the study are;

• Carrying out experiments for the determination of coal particle temperature profile

along the reactor axis with different O2/CO2 concentration and comparing it with base

line air condition for ignition behaviour.

• Evaluating the behaviour of two-colour pyrometer used for particle temperature

measurement at different reactor conditions and CO2 concentration

• Measuring the concentration of different gases along the reactor length during oxy fuel

combustion with different concentration of O2/CO2 and comparing it with base line air

condition for combustion behaviour.

III

Contents ACKNOWLEDGMENT .................................................................................................��.� ..I ABSTRACT�........................................................................................................�� ... .II CONTENTS ��..�� III LIST OF FIGURES ��. IV LIST OF TABLES ��... VI CHAPTER 1 ��.�...1 INTRODUCTION ��..�� ..1 1.1 Coal & its classification ��.�� ..1 1.2 Emissions from coal combustion��.��.. ..3 CHAPTER 2��..�� ..5 INTRODUCTION TO OXY FUEL COMBUSTION ..�� ..5 2.1 CO2 free power generation �...��. ..5

2.1.1 Pre combustion ��.��.. ..5 2.1.2 Post Combustion capture from conventional plants ��. ..6 2.1.3 O2/CO2 (Oxy fuel) combustion ��.� ..7

2.2 Air separation �� 11 2.2.1 Cryogenic distillation �� 11 2.2.2 Membrane separation �� 11 2.2.3 Pressure swing adsorption �� 12

2.3 CO2 sequestration �� 12 2.4 Development in Oxy fuel combustion ��. 13 2.5 Ignition Mechanism ��. 16 CHAPTER 3 .............................................................................................................�. 20 DESCRIPTION OF EXPERIMENTS��.�. 20 3.1 Test facility ��.. 20 3.2 Temperature measurement equipment ��. 21 3.3 Flue gas analysis ��.. 23 3.4 Characterisation of the fuel ��.. 24 3.5 Parameters for ignition tests ��. 24 3.6 Parameters for combustion tests�� 25 CHAPTER 4 ...............................................................................................................� 26 IGNITION RESULTS��...��. 26 4.1 Influence on particle temperature for different conditions during air combustion.. 26 4.2 Ignition during Oxy fuel ��... 31 CHAPTER 5 ...........................................................................................................�� 38 COMBUSTION RESULTS��...��.� 38 CHAPTER 6 ...........................................................................................................�.... 43 CONCLUSION AND FUTURE WORK.....��.��.�.. 43 6.1 Conclusion �� 43 6.2 Future work ��... 44 6.3 Future Challenges �� 44 BIBLIOGRAPHY �� 46

IV

List of Figure Figure 1.1 Kinds of coal ............................................................................................. 2 Figure 2.1 Pre -combustion capture �� 6 Figure 2.2 Post combustion capture �� 6 Figure 2.3 Oxy fuel combustion capture ��. 7 Figure 2.4 Schematic of Oxy fuel combustion �� 8 Figure 2.5 Effect of Oxygen enrichment on flame temperature ��.. 9 Figure 2.6 Ratio of the required volume of gas in the case of combustion with

recycled flue gas over the required volume in the case of combustion in air �� 10

Figure 2.7 Main processes in coal combustion ........................................................... 17 Figure 2.8 Stages in pulverized coal conversion ��. 18 Figure 2.9 Variation of concentration of oxygen and fuel with distance from the

fuel surface. Ignition occurs at some definable concentration ratio .......... 19 Figure 3.1 Schematic of pulverised coal test facility and arrangement for coal

particle temperature measurement using two-colour pyrometer with movable coal injection probe & for combustion experiments ��... 20

Figure 3.2 Optical set up of the pyrometric measurements at flow reactor �� 21 Figure 3.3 Time dependence of primary and reference signals when fuel particle

passes through the FOV of the optical probe �� 22 Figure 4.1 Influence of Nitrogen/Air (Carrier gas) on coal particle temperature,

Lausitz, WT= 1100°C .............................................................................. 27 Figure 4.2 Influence of coal feeding rate, Lausitz, WT=1100°C, 90-125µm,

Carrier gas = N2��. 28 Figure 4.3 Influence of particle size on coal particle temperature, Lausitz,

WT=1100°C ............................................................................................. 29 Figure 4.4 Dispersion of particles along the tip of dosing unit ��. 30 Figure 4.5 Influence of Oxy fuel on Coal particle temperature, Lausitz

WT=1100°C, fraction = 212-315µm ........................................................ 31 Figure 4.6 Influence of Oxy fuel on Coal particle temperature, Klien Kopje,

WT = 1100 & 1300°C, fraction = 150 � 212 µm ..................................... 32 Figure 4.7 Particle temperature behaviour under air and Oxy fuel conditions

at reactor temperature 1300°C, fraction 212-315µm (Klien Kopje) ��. 33 Figure 4.8 Particle temperature behaviour under air and Oxy fuel conditions

at reactor temperature 1300°C, fraction 90� 150µm (Klien Kopje)��... 34 Figure 4.9 Particle temperature behaviour under air and Oxy fuel conditions

at reactor temperature 1300°C, fraction 90-150µm (Elcerejon)��. 35 Figure 4.10 Particle temperature behaviour under air and Oxy fuel conditions

at reactor temperature 1300°C, fraction 150 � 212µm (Elcerejon) ��... 36 Figure 4.11 Particle temperature behaviour under air and Oxy fuel conditions

at reactor temperature 1300°C, fraction 212-315µm (Elcerejon) ��. 36 Figure 4.12 Relation between detected particles over the distance from burner ��. 37

V

Figure 5.1 Emission concentration of NO, CO, SO2 and CO2, Lausitz,

WT=1100°C, O2excess= 3 % ��.. 38 Figure 5.2 Emission concentration of NO, CO, SO2 and CO2, Lausitz,

WT=1300°C, O2excess= 3 % �� 39 Figure 5.3 Concentration of NO [ppm] in flue gas at 1100°C & 1300°C, Lausitz � 40 Figure 5.4 Concentration of SO2 [ppm] in flue gas at 1100°C & 1300°C, Lausitz � 41 Figure 5.5 Emission concentration of NO, CO, SO2 and CO2, Klien Kopje,

WT=1300°C ��. 41

VI

List of tables Table 1.1 Coal elemental and proximate analysis �� 2 Table 1.2 Specific CO2 emissions of various fuels ��.. 4 Table 3.1 Analyser employed for flue gas analysis with their ranges ��. 23 Table 3.2 Proximate and ultimate analysis of fuels ��.� 24 Table 3.3 Parameters for ignition tests �� 24 Table 3.4 Parameters for combustion test ��... 25

Introduction

1

1 Introduction

1.1 Coal & its classification

Coal is a solid, brittle, combustible, carbonaceous rock formed by the decomposition and

change of vegetation by compaction, temperature, and pressure. It varies in colour from

brown to black. Coal is a readily combustible rock containing more than 50 percent by

weight of carbonaceous material. Coal still provides a large portion of the world's energy

requirements, accounting for about 40 % [1] of the total worldwide electricity generation.

The use of coal is bound to increase in the future because world reserves are large and

widespread and coal is a low-cost alternative to other fuels

Coal is formed by the physical and chemical alteration of peat by processes involving

bacterial decay, compaction, heat, and time. Coal is an agglomeration of many different

complex hydrocarbon compounds. Peat deposits are actually quite varied and contain

everything from plant parts (roots, bark etc.) to decay plants, decay products, and even to

charcoal if the peat caught fire. Peat deposits typically form in a waterlogged environment

where plant rubbish is accumulated.

In order for the peat to become coal, it must be buried by sediment. Burial cause

compaction of the peat and, water is squeezed out during the first stages of burial.

Continued burial and the addition of heat and time cause the complex hydrocarbon

compounds in the deposit to start to break down. The gaseous alteration products (methane

is one) are typically expelled from the deposit and the deposit becomes more and more

carbon-rich. The stages of this trend proceed from plant debris, peat, lignite, sub-

bituminous coal, bituminous coal, anthracite coal, to graphite (a pure carbon mineral).

The kinds of coal are lignite (brown coal), sub-bituminous, bituminous, and anthracite.

These classes are further divided into subclasses based on their degree of alteration.

Introduction

2

Table 1.1: Coal elemental and proximate analysis Coal Elemental Composition Coal Proximate Analysis

C 50 � 95 % Char 20 � 70 %

H 2 � 7 % Ash 5 � 15 %

O < 25 % Water 2 � 20 %

S < 10 % Volatiles 20 � 45 %

N 1 � 2 %

Coal consists of a complex range of materials and varies greatly in quality from deposit to

deposit, depending on the varying types of vegetation from which the coal originated, the

depth at which the deposit was formed, the temperatures and pressures exerted at that

depth, and the length of time the coal has been forming.

Figure 1.1: Kinds of coal

Coal is classified according to:

• The degree of transformation of the original plant material into carbon, ranging

from anthracite (the hardest) to bituminous, sub-bituminous and lignite

• Moisture content: coals high in carbon and low in moisture are ranked the highest

• Composition: coal is predominantly carbon but also contain hydrogen, oxygen,

nitrogen and varying amounts of sulphur.

Peat Lignite Bituminous Anthracite Graphite

Time Temperature

Introduction

3

1.2 Emissions from coal combustion

During coal combustion, carbon is converted to carbon dioxide (CO2), hydrogen is

converted to water vapours (H2O) and sulphur to sulphur dioxide (SO2) and a small amount

of trioxide (SO3). If the temperature of the furnace is high enough some of the nitrogen in

the coal and in the air is converted to nitric oxides (NOx).

Both SO2 and NOx combine with oxygen and water in the atmosphere to form acids that

return to the ground in the form of acid rain with very negative environmental impact. For

example,

SO2 + 1/2 O2 + H2O → H2SO4

When coal containing a high quantity of sulphur is burned, the sulphur dioxide produced

must be reduced to meet emission standards set by local government regulations. Another

environmental hazard in burning coal is fly ash, which consists of particles left over after

combustion. Two types of ash are produced from coal combustion. Heavier particles, called

bottom ash because they are removed from the bottom of the furnace and typically flushed

with water to a settling pond outside the plant. Fly ash, on the other hand, consists of finer

(lighter) particles that are carried away with the furnace flue gas (CO2, H2O, etc). Fly ash is

a pollutant, which like the SO2 and NOx gases must be removed from the flue gas before it

is vented to the atmosphere

Basically, the main drawback of fossil fuels is pollution. Burning any fossil fuel produces

carbon dioxide, which contributes to the greenhouse effect, and is blamed for global

warming. Burning coal produces carbon dioxide (CO2), carbon monoxide (CO), sulphur

dioxide (SO2), nitrous oxides (NOx) and particulate in exhaust gases. Their levels vary

widely, depending on fuel composition, combustion system and operating conditions. For

example, the flue gas of a coal-fired power plant may contain 300�3,000 ppm SOx, 100�

1,000 ppm NOx, and 1,000�10,000 mg/m3 particulate matter. Natural gas firing

significantly lowers the contaminant levels to less than 1 ppm SOx, 100�500 ppm NOx, and

around 10 mg/m3 particulate matter [3]. CO2 contributes to greenhouse effect associated

with global warming while SO2 and NOx contribute to acid rain. Acid rain continues to

damage sensitive lakes. NOx emissions from coal-fired power plants contribute the ground

Introduction

4

level ozone, also known as smog. Coal fired power plants are responsible for most of the

sulphate particles that cause haze and reduce visibility.

When burned in a relatively uncontrolled fashion, coal can cause a lot of damage due to

pollutants. With modern technologies and strict controls it is possible to remove most of the

SO2 and NOx before they are emitted from a power plant, however coal contains more

carbon and less hydrogen than other fossil fuels such as oil and natural gas and produces

more CO2 per unit of electricity produced than any other fuel. Burning of lignite emits 80

% [33] more carbon dioxide emissions with respect to the energy contents than burning of

natural gas. Due to this coal fired power plants have been targeted the most by

environmental agencies.

Table 1.2: Specific CO2 emissions of various fuels [33] Fuel Emissions (kgCO2/kWh)

Peat 0.38

Lignite 0.36

Hard coal 0.34

Diesel 0.27

Gasoline 0.25

Natural gas 0.20

One of the methods for capture of CO2 is oxy fuel combustion; in this method we prevent

nitrogen from being mixed with the combustion products, particularly CO2. For instance,

the concentration of CO2 in flue gas can be increased greatly by using oxygen instead of air

for combustion. Having captured the CO2 it would need to be stored securely for hundreds

or even thousands of years, in order to avoid it reaching the atmosphere.

Introduction to oxy fuel combustion

5

2 Introduction to Oxy fuel combustion

Presently coal is one of the most widely used fuels in the power generation, accounting for

around 40 % [1] of the power produced worldwide. However coal requires extensive

emission control strategies, primarily due to the NOx, SOx, Hg, Particulates and CO2

emissions. To meet the increasingly stringent emission control standards, Pulverized coal

fired plants are now required to be equipped with a variety of flue gas treatment systems.

For a power plant these devices represent a significant cost increases. Oxy-fuel combustion

is considered to be one of the effective methods to improve thermal efficiency and reduce

NOx, SOx and CO2 emission for high temperature furnaces. A major benefit of Oxy

combustion is the fact that the flue gas exhaust flow rate is significantly reduced because of

re-circulation of flue gas, can lead to significant cost savings.

Flue gases emitted from medium to large point sources are generally at or slightly above

atmospheric pressure. They typically contain 3�15% (by volume) of carbon dioxide. For

example, flue gas from a coal-fired power plant typically contains about 14% CO2, 5 % O2

and 81% N2. Flue gas from a natural gas turbine is even leaner in CO2 but higher in O2 with

a typical composition of 4% CO2, 15% O2 and 81% N2. [3].

In the past decade there has been a growing concern about greenhouse gas emission (CO2,

CH4 and N2O) and its potential impact on climate change. Since coal fired power plant

account for high percentage of CO2 emission worldwide, it has a significant impact on the

global greenhouse gas effect. To reduce CO2 emission, it is necessary to develop clean and

efficient combustion technologies for existing and new coal fired power generation plants.

2.1 CO2 free power generation

Three system approaches are possible for CO2 capture from power systems:

2.1.1 Pre-combustion capture Carbon is captured in the form of CO2 from the fuel before combustion. The fossil fuel is

first transferred (via gasification or reforming) to hydrogen and CO2 for CO2 capture.

Afterwards, power production is achieved.


6

Figure 2.1: Pre -combustion capture

2.1.2 Post Combustion capture from conventional plants

Carbon is captured in the form of CO2 from the flue gasses after combustion. The energy

content of the fossil fuel is converted to electricity. Afterwards, CO2 is separated form the

flue gas. The power production efficiency will be lower by 7 % - 29 % and power

production cost will be rise by 1.2 � 1.5 times for separating CO2 [13]. The main reason

that causes the raising cost is difficult to separate CO2 from lower concentration of CO2

flue gas that the main component is N2.

Figure 2.2: Post combustion capture

Flue gas Gasification /

reforming

CO shift H20 + CO => H2 + CO2

CO2 Separation

Combustion

Air/O2 /H2O CO2 Air

Fuel

Energy Electricity CO2

Flue gas

Fuel

Air Power Process

CO2Separation


7

2.1.3 O2/CO2 (Oxy fuel) combustion The principle of this method is to prevent nitrogen from being mixed with the combustion

products, particularly CO2. For instance, the concentration of CO2 in flue gas can be

increased greatly by using oxygen instead of air for combustion.

C + O2 => CO2

Due to oxy fuel technique CO2 concentration becomes high, resulting in easier and

economical separation.

Figure 2.3: Oxy fuel combustion capture

O2/CO2 Combustion involves burning of the fuel in an atmosphere of oxygen and recycled

flue gas instead of air. Figure below shows a schematic layout of an O2/CO2 coal power

plant. The mixed flow of oxygen and recycled flue gas is fed to the boiler together with

fuel. A part of flue gas is separated and mixed with new oxygen. The remaining part of flue

gas is treated, compressed, transported for another application or sequestered.

Recirculated flue gas Water

Electricity

CO2

Flue gas

Air Fuel

Oxygen Air

Separation

Power Process Condenser

N2


8

Figure 2.4: Schematic of Oxy fuel combustion

The amount of required oxygen can be produced in an air separation unit. The oxygen is

diluted with recycled flue gas in order to attain combustion conditions. Concentration of

oxygen in the feed gas can be varied from pure oxygen to lower concentration. This mean

that it is possible retrofit an existing boiler plant to O2/CO2 combustion, even if a design of

a new power plant is more preferable, since it opens for optimisation of the oxygen

concentration in the feed gas which should yield a higher combustion efficiency. To use

pure oxygen we need material that can withstand higher temperature. With the use of pure

oxygen, combustion efficiency can be highly increased and reduce the furnace size over the

conventional furnace for the same energy output. At normal combustion of coal in the air

the concentration of carbon dioxide in the flue gas is approximately 14 %. This mean an

expansive process is necessary to increase the concentration of the carbon dioxide in order

to gain the concentrated carbon dioxide in the flue gas.

With Oxy fuel combustion system, it is possible to make CO2 concentration in the flue gas

up to 98 % [4] by separation of nitrogen from the combustion air in advance and using

oxygen and recycled flue gas, most of which is CO2. Carbon dioxide concentration in the

Air Nitrogen

Coal

Flue gas~97% CO2

Boiler

Drier

Water

Air Separation

Compressor

G

TurbineGenerator

Feed Pump

Oxygen

Flue Gas (~ 90% CO2) Recycle ~75%


9

flue gas depends directly on the oxygen concentration in the feed gas, the higher the oxygen

concentration in the feed gas, the higher the carbon dioxide concentration in the flue gas.

Effect of Oxygen enrichment on flame temperature

1900200021002200230024002500260027002800

20 30 40 50 60 70 80 90 100

OXYGEN %

TE

MP

(C)

Figure 2.5: Effect of Oxygen enrichment on flame temperature [5]

Advantages

• In Oxy fuel combustion, Concentration up to 98 % carbon dioxide in the dry flue

gas may be possible, allowing direct sequestration.

• The reduced volume involved due to combustion at higher oxygen concentration,

and thus with less inert gas volume. Advantages to this reduced volume are lower

dry gas energy losses, higher plant efficiency and lower energy gas loss for gas

cleaning and separation. In figure 2.6 shows the ratio of the required volume of gas

in the case of combustion with recycled flue gas over the required volume in the

case of combustion in air


10

Figure 2.6: Ratio of the required volume of gas in the case of combustion with recycled flue gas over the required volume in the case of combustion in air [4].

We can see from the figure 2.6, 5% excess oxygen situation requires lower volume

of gas than for the 1% excess oxygen case. In fact the difference between these two

cases is in the volume of pure oxygen brought into the system. For the 5% excess

oxygen case, less volume of pure oxygen is needed because more oxygen is brought

back to the system through re-circulation of the flue gas than in the case of 1%

excess oxygen. Another advantage with the higher oxygen concentration is the

ability to minimize unburned carbon.

• NOx and SO2 formed during Oxy fuel combustion is lower than from conventional

combustion system [4]

Disadvantages

• Because much oxygen is required, a large amount of parasitic power is required for

producing oxygen, resulting in high cost

• Due to recycle of exhaust gas, corrosive components in the exhaust gas become

high in concentration.


11

2.2 Air separation

The difference between oxy fuel combustion plants from conventional plant from retrofit

point of view is air separation unit, where we separate Oxygen and Nitrogen. There are

many possibilities for air separation. Details of some methods are given below

2.2.1 Cryogenic distillation

Cryogenic Separation is a distillation process that occurs at temperatures close to -170° C.

At this temperature, air starts to liquefy. Before separation can occur, there are specific

operation conditions that must be achieved. Distillation requires two phases, gas and liquid.

Air must be very cold for this to happen. For this instance, at one atmosphere, nitrogen is a

liquid at -196° C. A pressure 8-10 time�s atmospheric pressure is required for this process.

These conditions are achieved via compression and heat exchange; cold air exiting the

column is used to cool air entering it. Nitrogen is more volatile than oxygen and comes off

as the distillate product. A cryogenic air separation plant is expensive and large; the

distillation column is several stories high and must be well insulated. Consequently, it only

becomes economically feasible to separate air this way when a large amount is needed.

Cryogenic separation is also capable of producing much purer nitrogen than either of the

other two processes because the number of trays in the distillation column can be increased.

2.2.2 Membrane separation

Membrane separation of air is primarily a physical process, based upon specific

characteristics of each molecule, such as size and permeation rate. The molecules in air can

be separated to form mostly pure forms of nitrogen, oxygen, or both. In a membrane

system, there is a hollow tube filled with thousands of very thin membrane fibres. Each

membrane fibre is another hollow tube in which air flows. The walls of the membrane fibre

are porous and are specially made so oxygen molecules can permeate through the wall at a

faster rate than nitrogen, allowing a nitrogen-rich stream to flow out the other end of the

fibre. Meanwhile, the air outside the fibre, in the hollow tube, is now oxygen-rich and can

be collected somewhere else.

The purity of the nitrogen generated depends primarily on two factors: the flow rate and air

pressure. If we have high air pressure, the oxygen molecules have greater incentive to


12

permeate through the fiver wall. If our flow rate is slower, then oxygen has more time to

permeate through the fibre wall. We can easily adjust both of these factors to allow a

system operator to vary the amount and purity of the nitrogen generated in a very short

amount of time.

2.2.3 Pressure swing adsorption

Pressure Swing Adsorption units separate air using a special sieve that adsorbs oxygen

preferably to nitrogen. When high-pressure air flows through the sieve, oxygen molecules

are caught while nitrogen molecules pass on. The sieve continues to adsorb oxygen until a

saturation point is reached. After that, the entering air stream is cut off and the oxygen is

able to leave the tank at low pressure. In a PSA unit, two connected tanks and containing

sieves, work together to produce a near-continuous stream of nitrogen. When one tank has

become saturated and is releasing adsorbed oxygen, the entering air stream is switched to

the other tank for oxygen adsorption.

2.3 CO2 sequestration

Carbon dioxide collected from carbon dioxide removal technologies can be compressed and

transportation to the point of sequestration. Carbon dioxide sequestration can be

accomplished either by an offset (indirect sequestration) or by reduction in the emissions

from generation facility (direct sequestration) [6].

Direct CO2 sequestration involves capturing CO2 at its point of generation before it is

released to the atmosphere. The CO2 is then put in long-term (hundreds to thousands of

years) environmentally sound storage, usually in a deep geological formation. Removing

CO2 from the exhaust streams of factories and electric plants and storing it deep

underground would be an example of direct sequestration Direct Sequestration technologies

includes:

• Injection into oil and gas reservoirs

• Injection into deep, unmineable coal seams.

• Injection into saline aquifers.

• Injection of liquid carbon dioxide into deep Ocean.


13

Indirect CO2 sequestration involves capturing CO2 that has already been released to the

atmosphere. CO2 is removed from the atmosphere through intake by plants or by fixing

carbon in the soil. The CO2 captured by indirect sequestration occurs through a variety of

means, both natural and anthropogenic.

2.4 Development in Oxy fuel combustion

In the power generation there are tremendous efforts due to economical and ecological

reason to reduce the emission without reducing the efficiency or increasing the capital costs

significantly. There are lot of research works going on to get the desired goals. Some of the

information regarding those research works towards betters efficiency, economic and

emission reduction is given below

A retrofit oxygen fired plats with different capacities of 30, 100, 200, and 500 MW power

output [1], consisting of Air separation unit, boiler, recalculated flue gas, DeNOx system,

Hg removal system, Electrostatic precipitator, flue gas desulphurisation, carbon dioxide

conditioning and sequestration, and stack. It was found that full oxy combustion will lower

the required heat transfer areas by ~ 50 %, making the boiler more compact and less capital

intensive NOx removal efficiency 80 � 95 % and particulate removal efficiencies of 99 �

99.9 % is achieved. The analysis shows that, under the assumed conditions, the total

annualised cost of the oxy-fired plants is comparable to those of the air-fired cases. For new

plants, which would also include advanced, compact, full oxy fired boilers; the total costs

of the oxy-fired plants are lower than those of the air fired plants. Due to economic scaling

factors, the oxy-fired plants are more economically viable at smaller sizes. Finally it is

noted that oxy-fired operation generates a flue gas rich in carbon dioxide, which may be

easily captured in order to sequester.

In the enriched oxygen coal fired combustion scheme [4] the carbon dioxide concentration

in the flue gas depends directly on the oxygen concentration in the feed gas, the higher the

oxygen concentration in the feed gas, the higher the carbon dioxide concentration in the

flue gas and up to 98 % dry volume, carbon dioxide concentration can be achieved in the

flue gas. Research shows that O2/CO2 recycle system reduced volume involved due to

combustion at higher oxygen concentration, and thus with less inert gas volume, like 5 %

excess oxygen situation requires lower volume of gas than 1 % excess oxygen case. All the


14

experiments performed at a firing rate of 0.2 MW for Eastern bituminous coal, 5 % vol dry

excess oxygen in flue gas and oxygen in the feed varies between 28 and 42 % vol on a dry

basis and with oxygen purity of 100 % and 90 %. The flame temperature increases with the

concentration of oxygen in the feed gas, the temperature rises from about 1300°C at 28 %

oxygen to about 1500°C at 42 % Oxygen. Results shows that SO2 chemistry is not affected

by the presence of high concentration of carbon dioxide and oxygen, at least in the range of

28 to 35 % for the oxygen. At 28 % and 35 % with flue gas re-circulation shows much

lower SO2 emission rates than non-recycle runs. CANMET energy technology centre

mentioned in an research paper � Oxy fuel combustion research at CANMET energy

technology centre� [24] that 30 % to 35 % oxygen in recycled flue gas (dry or wet) is the

most feasible retrofit option since temperature profile and heat transfer matches to

conventional air combustion.

Combustion test facility [10] with vertical cylinder type furnace and combustion capacity of

1.2 MWt (equivalent to coal firing rate of 150 kg/h). The fuel ratio of coal used in this

study was 0.8, 1.3 and 1.6. Pulverized coal used with the grain size from 53 to 63 µm. With

O2 / RFG pulverized coal combustion system, it is possible to make the CO2 concentration

in the exhaust gas 95 % or highest by separation nitrogen from combustion air in advance

and using oxygen and recycled flue gas, most of which is CO2.

A pilot scale demonstration of oxy combustion with flue gas re-circulation in a pulverized

coal fired boiler [16] had an experimental set-up for 1.5 MW pulverized coal boiler with re-

circulated flue gas (without flue treatment). The operating condition were primary Oxidant

was 15 to 20 % of total, secondary oxidant was 50 to 85 of total, Tertiary oxidant was 0 to

30 % of the total and outlet oxygen concentration was 3 %. Only 80 % of the air stream was

replaced by recycled flue gas. It was found NOx reduction up to 76 % during staged oxy

combustion when compare to un-stage air firing and 50 % reduction of Hg during oxy

firing. During oxy firing unburned carbon were 22 % lesser without staging and 48 % lesser

with staging when compared to non-staged air firing.

A joint research was carried out from Air Liquide, Illinois Clean Coal Institute, Illinois

Department of Commerce & Economic Opportunity and Babcock & Wilcox Company [17]

on 1.5 MW pulverized coal boiler. They used FLUENT as a simulation tool to optimise the

amount, location and injection of oxygen in the boiler. All the experiments were carried out


15

in different conditions without staging (Base air firing, Oxygen enrichment case) and with

air staging and oxy enriched staging. With the simulation, it was found that NOx emission

was 15 to 21 % lower than base air firing during oxygen enrichment.

A 1.2 MW Boiler [20] used to analysis of the flame formed during oxidation of pulverized

coal by O2 � CO2 mixture. Oxygen was supplied in two streams, one was mixed with re-

circulated flue gas and the second was injected directly into the furnace. It was found that

the temperature of gases during O2 � CO2 firing near the burner zone was 200°C lower than

air firing and the concentration of oxygen in the combustion gas was never more than 27 %.

Drying of re-circulated flue gas transportation increased the temperature near the burner

zone by about 150°C indicating better ignition stability.

A 44 MW wall fired boiler [21] test facility with bituminous coal was investigated for NOx

reduction. It was found that loss on ignition was very small when stoichiometric ratio was

reduced from 0.9 to 0.85, when small amount of oxygen (< 5 %) was added from staged

stream. Oxygen addition (< 5 %) during staging resulted almost 40 % NOx reduction

compared to base line air staged combustion without any restriction on loss on ignition and

burner stability.

The research paper �Development of the CO2 recovery type pulverized coal fired power

plant applied oxygen and recycled flue gas combustion� [22] consists of micro gravity test,

combustion test and design & simulation. In the micro gravity test, spherical combustion

camber (200 mm inner diameter) with micro gravity field to form uniform coal particle

cloud was used. The used fuel rations were 0.8, 1.3 and 1.6 with the particle size of 53 to 63

µm for the investigation. It was found that flame propagation velocity was higher for more

volatile coals, the velocity and brightness of flame were much lower in CO2/O2

environment. The maximum velocity was found at the concentration when the coal particles

were at the distance of 10 times the particle size.

In a combustion test, 1.2 MW (inner diameter 1.3 m & height of 7 m) boiler with the

Bituminous coal as fuel. It was found SO2 and SO3 emissions were higher in O2/RFG

combustion. Dust concentration was 2 times higher than air combustion and higher

desulphurisation rate was observed for O2/RFG combustion with increasing Ca/S molar

ratio.


16

In the design and simulation, experimental results were used for the simulation of 1000

MW pulverized coal fired power plant and after simulation it was found optimum oxygen

purity of 97.5 % at which total power consumption becomes least for both CO2 liquefy

fraction and gas recovery. Due to high cost of oxygen production and CO2 treatment, net

thermal efficiency decreased to 30 % from 39 % with conventional plant.

A promising technology for CO2 Capture �Oxy combustion process in pulverized coal fired

boilers� [26] investigation set up consists of boiler with capacity of 1.5 WM, video camera

for flame picture/ temperature mapping with the flue gas recycle (FGR) from 80 to 95 %

and mass flow rate was constant. It was noted that furnace exit temperature was lower for

O2/RFG firing than for baseline air firing. NOx reduction of nearly 70 % achieved with

O2/RFG. CO2 concentration in the flue gas was 80 % due to air infiltration.

2.5 Ignition Mechanism

Coal particles ignite homogeneously, heterogeneously, or through a combination of both

mechanisms. Ignition of coal particles can be either homogeneously, i.e. prior pyrolysis and

subsequent ignition of the volatiles, or heterogeneously, i.e. direct oxygen attack on the

whole coal particle. The ignition mechanism depends on volatile matter content of coal,

flammability of volatiles and their transport from the particle. Generally, larger coal

particles ignite homogeneously and smaller particles ignite heterogeneously with a

separating boundary at a particle diameter, depending on the ambient conditions. Despite of

the fact, that pulverized coal particles are known to be irregular shaped


17

Figure 2.7: Main Processes in Coal Combustion [37]

Homogenous ignition is a 2 step process; primary step is the initial ignition of the volatiles.

Following this, the combustion of the volatile; secondary ignition, of the char, then occurs

as pyrolysis terminates.

Heterogeneous ignition can involve 3-stages. The primary ignition is by direct attack of the

reactant gas on the solid. This solid is the whole coal, not just a char, and the heterogeneous

reaction removes material that would otherwise be expelled as volatiles. As a parallel to

homogeneous case, this reaction can sometimes be quenches as pyrolysis becomes

appreciable, even if the volatiles do not burn. Secondary ignition, when it occurs, is that

volatiles, and this may be followed in due course by a re-ignition of the char at the end of

pyrolysis. One step ignition, of the course, is when only the first ignition process occurs in

either case.

Coal particle D= 30 �70µm

Devolatilization

Volatiles

Char

Homogeneous Combustion

Heterogeneous Combustion

tchar=1-2sectvolatiles=50-100mstdevolatile=1-5ms

t


18

Figure 2.8: Stages in pulverized coal conversion [36]

Ignition temperature is an invariant of the fuel, two major factors that can influence the

ignition source temperature, in addition to concentration and particle size, are the speed of

the cloud past the ignition source and the size of the ignition source. The homogenous

ignition temperature is inversely proportional to particle size and oxygen concentration.

Oxygen and volatiles are diffuse into each other, ignition will occur in a narrow region

where the gas temperature, oxygen, and volatiles concentration reaches the flammability

limits.

1 Coal dust

+ Air

2 Radiative Preheat 3

Conductive heating

4Pyrolysis

onset

5Radiation

+ Pyrolysis

10 Radiation

9 Gaseous reaction

8Gaseous diffusion

7Volatiles oxidation

6Volatilesdiffusion

CO, H2

CO

H

CO + O ! CO2H + OH ! H2O

PRE HEAT ZONE FLAME ZONE POST FLAME ZONE

Distance

Particle Temperatur


19

Homogeneous ignition occurs first at low oxygen concentration since sufficient oxygen is

not available for heterogeneous reactions. At higher oxygen concentration however

heterogeneous ignition occurs first and heterogeneous ignition temperature decreases with

the increase of the particle diameter.

CO

NC

ENTR

ATI

ON

DISTANCE

Figure 2.9: Variation of concentration of oxygen and fuel with distance from the fuel surface. Ignition occurs at some definable concentration ratio.

Ignition Zone

O2

Fuel

Description of experiments

20

3 Description of experiments 3.1 Test facility

The pulverised coal combustion test facility at Institute of Process Engineering and Power

Plant technology (IVD) Universität Stuttgart Germany was used for oxy fuel combustion

investigation. The test facility [figure 3.1] consists of electrically heated ceramic tube with

reaction zone of 2500 mm length and 200 mm diameter. Electrical heating is in order to

adjust a constant wall temperature. Wall temperature can be varied from 1100°C to 1400°C

during the experiments.

For ignition experiments, the pulverised coal is fed into the reactor by means of a water-

cooled feeding probe, which can be moved in vertical direction enabling particle

temperature measurement at different positions from the burner. The combustion gases are

injected through an annular clearance between burner and top section of reactor and are

divided into primary and secondary streams.

Figure 3.1: Schematic of pulverised coal test facility and arrangement for coal

particle temperature measurement using two-colour pyrometer with movable coal injection probe & for combustion experiments.


21

For the combustion experiments, the pulverised coal is fed by carrier gas to the top

mounted burner through which it is injected into the combustion chamber. The feeding

system consists of a gravimetric conveyor and screw feeder. The combustion gases are

injected through an annular clearance between burner and top section of reactor and are

divided into primary and secondary streams. Flue gas sampling probe are mounted at the

bottom of reactor from where flue gas is extracted to obtain a gas sample for the emission

analysis.

For oxy fuel tests independent mixing device were installed with facility to adjust gas

composition of O2 and CO2. For temperature measurement of single particles, two-colour

pyrometer was used. A process control system was used to monitor all the relevant process

data including the gas analysis data and to control the whole facility. The flue gas is

extracted at the final section of the heated reaction tube for the emission analysis of O2,

CO2, CO, NO, Nox, HCN, SO2 and char particles.

3.2 Temperature measurement equipment

For the detection of the surface temperature of single particles a two-colour pyrometer was

used. Pyrometer measures the temperature of objects without touching them. Every object

whose temperature is above absolute zero (-273.15 °C) emits radiation. This emission is

heat radiation and is dependent upon temperature. The term infrared radiation is also in use

because the wavelengths of the majority of this radiation lie in the electro-magnetic

spectrum above the visible red light, in the infrared domain. Temperature is the determining

factor of radiation and energy. Infrared radiation transports energy. This radiated energy is

used to help determine the temperature of a body being measured.

Types of Pyrometers

• Spectral Band Pyrometers (Narrow and Broad band)

• Total band Pyrometers

• Two-colour Pyrometers


22

Working principal of two-colour Pyrometer

The optical set up of the pyrometric measurements in the reactor is show in the [figure 3.2].

The radiation emitted by a coal particle is collected through the lens system and focused to

the end of an optic fibre bundle. In the middle of the bundle is one large fibre, called the

primary fibre. This fibre is used for pyrometry. The primary fibre is surrounded by smaller

fibres, called reference fibres. They are used for particle discrimination. The radiation

entering the primary fibre is measured in a radiometric units over two separate wavelength

bands, and the radiation entering the reference fibres is measured collectively in a separate

unit over a single wavelength band.

Figure 3.2: Optical set up of the pyrometric measurements at flow

reactor [31].

Figure 3.3: Time dependence of primary and reference signals when fuel particle passes through the FOV of the optical probe [31].

When a coal particle hotter than the background passes through the field of view (FOV) of

the primary fibre, a corresponding increase is detected in the pyrometric single levels.

Figure 3.3 shows the trajectory of the particle image in the plane of the end face of the fibre

bundle and the corresponding signals as function of time. The particle temperature is solved

using the ratio of the pulse height measured at the two wavelength bands. Temperature of

Particle Tp can be solved from the equation below[34].


23

(R1 � R01) / (R2 � R02) = [ F1(Tp) � R01] / [ F2(Tp) � R02]

Where Ri � R0i is the pulse height at wavelength band λ i (i = 1,2), R0i is the system response

when no particles are in the FOV (i.e., a signal value between the peaks), and Fi(T) is the

system response calibrated against a blackbody radiator at temperature T.

Advantages The main advantages of the two colour Pyrometers are firstly, it is a non-contact

temperature measurement instrument so that it does not affect the target and the material of

the equipment. Secondly it has high response speed for temperature fluctuations which in

not possible with contact measuring equipments. Because of the Pyrometer�s quick

response time; temperature of the moving object can be measured accurately. Finally, the

temperature reading is independent of the particle area and emissivity fluctuation. Five

factors contributing to uncertainty of measured particles are noise, accuracy of calibration,

the emissivity of the particle, the accuracy of the temperature determination and solid angle

between particle and collecting pyrometer optics

3.3 Flue gas analysis

For the measurements of flue gas components, flue gas sampling probe is mounted at the

bottom of reactor from where flue gas is extracted to obtain a gas sample. The measuring

ranges of analysers and techniques are listening below [Table 3.1]. Nox is calibrated in the

CO2 environment during oxy fuel test to avoid quenching effect.

Table 3.1: Analyser employed for flue gas analysis with their ranges Gas Component Measuring technique Measurement range

O2 Paramagnetism 0 � 50 % vol

CO2 Non dispersive infra red 0 � 100 % vol

CO Non dispersive infra red 0 � 5,000 ppmv

NO, NOx Chemiluminence 0 � 10,000 ppmv


24

3.4 Characterisation of the fuel

The coals used for this investigation were low volatile Bituminous coal (Klien Kopje),

medium volatile Bituminous coal (Elecrejon) and high volatile brown coal (Lausitz). The

fuel was analysed for it proximate and ultimate analysis. The proximate analysis divided

the fuel into the components water, volatile matters, fixed C and ash, the ultimate analysis

into carbon, hydrogen, nitrogen, sulphur and oxygen [Table 3.2].

Table 3.2: Proximate and ultimate analysis of fuels Brown coal

(Lausitz) High volatile

Bituminous Coal (Elcerejon) Medium volatile

Bituminous Coal (Klien Kopje) Low volatile

Volatiles [%, waf] 57.36 40.30 27.76

Ash [%, wf] 5.46 8.11 19.29

Moisture [%, ar] 10.20 06.30 03.60

Carbon [%, waf] 66.78 81.07 83.93

Hydrogen [%, waf] 6.60 6.16 5.01

Nitrogen [%, waf] 0.65 1.51 1.67

Sulphur [%, waf] 0.72 0.44 0.36

Oxygen [%, waf] 25.25 10.82 9.03

3.5 Parameters for ignition tests

Table 3.3: Parameters for ignition tests Wall temperature 1100°C, 1300°C

Pressure Atmospheric

Coal Particle 90-125µm*, 90-150µm, 150-212µm & 212-315µm

Inlet oxygen concentration 21 %, 27 %, 35 % and Air as base line

Distance from Burner (cm) 2.5, 5, 10,15,20,25,30 and 40

Primary stream Air or O2/CO2 mixture, (3.5 m3/h)

Secondary stream Air or O2/CO2 mixture, (5.3 m3/h)

Carrier stream Same as primary and secondary stream, (100 l/h) * Old fraction 90-125µm used for some experiments for comparison with new fraction results

Note: Whenever the volumetric fraction of O2 is mentioned, remaining portion in the mixture is always CO2.


25

3.6 Parameters for combustion tests

Table 3.4: Parameters for combustion test Wall temperature 1300°C

Pressure Atmospheric

Coal Particle D50 ≈ 60µm

Inlet oxygen concentration 21 %, 27 %, 35 % and Air as base line

Primary stream Air or O2/CO2 mixture, (3.5 m3/h)

Secondary stream Air or O2/CO2 mixture, (5.3 m3/h)

Carrier stream Same as primary and secondary stream, (1.5 m3/h) Note: Whenever the volumetric fraction of O2 is mentioned, remaining portion in the mixture is always CO2.

Ignition results

26

4 Ignition results

Experiments were conducted to investigate the influence of furnace temperature, coal

particle size, carrier gas (e.g. Air, N2), coal feeding rate and oxy fuel on coal ignition. The

experimental combustion facility has electric furnace with water-cooled movable single

particle dosing probe. Coal particles are transported in the inner part of the probe with the

carrier gas and primary gas introduced from the outer part of probe. Due to movable probe

it gives the possibility to measure the coal particle temperature at different positions in the

combustion chamber. All experiments were performed without flue gas re-circulation and

CO2 is directly supplied for combustion. Coal particle temperature was measured by 2-

colour pyrometer installed at a collinear optical port at 1.55 m as shown in figure 3.1.

Three different types of coals named as Brown coal, high volatile bituminous coal and

medium volatile bituminous coal with a fraction of 90-150 µm, 150-212 µm and 212-315

µm were investigated. Furnace temperature was maintained at 1100 °C and 1300 °C during

experiments with the variation of Oxygen supply from 21 % to 35 % for Oxy fuel

behaviour and air was used as a base line for comparison.

4.1 Influence on particle temperature for different conditions during air

combustion

The figure 4.1 shows the influence of nitrogen/air as carrier gas on coal particle

temperature. The result shows that nitrogen, as carrier gas does not give a significance

change in the coal particle temperature as compared to air. The nitrogen concentration

(carrier gas) is very low as compared to total gas in reactor to show any significant changes.

Ignition results

27

1600

1650

1700

1750

1800

1850

1900

0 5 10 15 20 25 30 35 40 45

Distance from Burner [cm]

Part

icle

Tem

pera

ture

[°C

]

150-212µm CG=air150-212µm CG=N290-125µm CG=air90-125µm CG=N2

Figure 4.1: Influence of Nitrogen / Air (Carrier gas) on coal particle temperature, Lausitz, WT= 1100°C

Figure 4.2 shows the effect of coal feeding rate on the coal particle temperature. During the

experiments the behaviour of coal particle temperature under the feeding rate from 5 Hz

(~7g/h) to 50 Hz (~160 g/h) with air and nitrogen as carrier gas was observed.

Coal feeding rate shows strong influence on particle temperature. Results have indicated

that the higher the coal-feeding rate the lower is the particle temperature. During the

experiments, oxygen concentration around 21% at exit was always maintained, so for high

feeding rate, one cannot claim that oxygen was not enough for ignition. The reason could

be the working principle of two-colour pyrometer. In two-colour pyrometer the coal particle

temperature is the function of radiative energy of particle (E1) and radiative energy of

background, which in this case is the reactor wall (E2).

Temperature =f (E2E1 )

Ignition results

28

1550

1600

1650

1700

1750

1800

1850

0 5 10 15 20 25 30 35 40 45

Distannce from Burner [cm]

Part

icle

Tem

pera

ture

[°C

]

5Hz10Hz20Hz50Hz

Figure 4.2: Influence of coal feeding rate, Lausitz, WT=1100°C, 90-125µm, Carrier

gas = N2

During higher particle feeding rate complete flame was formed, which could be the reason

to increase the value of background radiative energy (Reactor wall) E2 and decreasing the

ratio of E1 / E2, which shows the low temperature of the coal particles. Another reason for

this lower temperature with higher feeding rate could be intra particle energy loss as

particles are much closer to each other in this case.

The rate of reaction between coal and oxygen is affected by particle size, for the smaller

particle size the coal becomes much more reactive. For smaller particles, the rate of heat

generation is higher for two reasons. First, the effectiveness factor is closer to 1, leading to

greater oxidation rate per unit volume. Second, the mass transfer coefficient increases with

decrease in the particle size, from figure 4.3 behaviour of particle temperature over the

particle size in the presence of air as carrier gas can be seen.

Ignition results

29

1600

1650

1700

1750

1800

1850

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]

air 212-315µm

air 150-212µm

air 90-125µm

Figure 4.3: Influence of particle size on coal particle temperature, Lausitz, WT=1100°C

Ignition/combustion of coal particle can be generally regarded as a two-step process. First

the surrounding/surface ignites due to release of volatiles, and then inner surface ignites.

First step is strongly influenced by availability of oxygen and release of ignitable volatiles.

Second step is influenced by the rate of diffusion of oxygen inside the coal particle for

combustion of char. Therefore larger particle theoretically should have higher temperature

in the beginning due to larger surface area that comes in contact with oxygen and lower

temperature afterwards since diffusion of oxygen inside the particles takes longer time. This

is exactly what the trend of the curves for particle fraction of 90-125µm and 150-212µm

shows. However the largest fraction do not show such tendency. This is most probably by

too much release of volatiles resulting in oxygen deficiency or longer time required for

release of volatiles due to longer time required to heat up larger particles.

The most distinguishable and understandable trend among three particle fractions is the

temperature peaks. The peak of particle temperature for smallest fraction is nearest from the

burner (around 5cm if the curve follow the same trend) confirming that smaller the particle,

the faster it ignites. Peaks for other two larger fractions are further away from the tip of

burner, indicating longer ignition time. For the clear understanding of the relation between

Ignition results

30

particle diameter and particle temperature, it was observed that temperature of the particle

decreases with the increase in the particle diameter.

As we move nearer to the burner the density of particle cloud increases as shown in figure

4.4, since the particles dispersal has approximately a form of conical shape. So when we

move nearer to burner, for pyrometer it�s difficult to measure a single particle and take

many particles as a particle cloud and this can influence the accuracy of coal particle

temperature measurements.

Figure 4.4: Dispersion of particles along the tip of dosing unit

Ignition results

31

4.2 Ignition during Oxy fuel

Ignition behaviour of the particles in the presence of different oxygen concentration for

different types of coals were investigated. The reaction between coal and oxygen is affected

mainly by two factors: first one involves the chemistry such as carbon type, active sites etc.,

and the second one covers physical characteristics, such as specific surface area, pore or

surface diffusion, etc. For different coals, the physical properties differ, so that activation

energy and reactivity change with the type of coal. Activation energy depends on the

chemical structure of the coal and individual particle temperature. The reactivity, which

represents the effective number of collision between carbon and oxygen molecules,

depends heavily on coal properties.

As we know from the literatures, higher concentrations of oxygen will result in higher

temperature and early ignition. In Lausitz 1100°C Curve (figure 4.5) for 35% oxygen case

indicates the most rapid ignition rate, which is clearly visible from the steep slope of the

curve after 15cm and in case of 21 % oxygen the ignition delay is maximum and giving

maximum temperature at the distance of 30 cm from burner. It was observed that curve of

27 % oxygen and air for the fraction size of 212-315 µm have a similar trend.

1650

1700

1750

1800

1850

1900

1950

2000

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]

21% O227% O2,35% O2Air

Figure 4.5: Influence of Oxy fuel on Coal particle temperature, Lausitz

WT = 1100°C, fraction = 212-315µm

Ignition results

32

In the figure 4.6, effect of the reactor wall temperature of 1100°C and 1300°C for the

fraction of 150 to 212 µm (Klien Kopje) at the 21 % oxygen concentration and air can be

observed. In case of Air, ignition delay is greater for 1100°C as compare 1300°C.

In case of 21 % oxygen with the wall temperature of 1100°C, no particles were detected

from 10 cm to 40 cm for the burner distance. Which means the temperature of particles

were too low for detection by two-colour pyrometer or ignition delay was so high, on other

hand particle detection can be seen for 10 cm to 30 cm distance from the burner at 1300°C.

On the basis of the results that no particles were detected for 21% O2 at 1100°C, and the

realistic furnace temperature for hard coals are ~ 1300°C, It was decided to carry out

experiments only at 1300°C.

1400

1450

1500

1550

1600

1650

1700

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]

Air (1100°C)Air (1300°C)21% O2 (1300°C)

Figure 4.6: Influence of Oxy fuel on Coal particle temperature, Klien Kopje, WT = 1100 & 1300°C, fraction = 150 � 212 µm

Figure 4.7 show the particle temperature behaviour under air and oxy fuel conditions at

1300°C reactor temperature for the fraction of 212-315µm for Klien Kopje. The trends of

the graph indicate that at a distance of more than 10 cm, burnout phase of ignition is

already reached and to see the heat-up and pyrolysis of ignition, measurements at distances

less than 10cm is necessary which was not possible during the time when the experiment

Ignition results

33

was carried out due to the length of the fuel feeding probe. To see these phases of ignition,

a new fuel probe was constructed enabling measurements at nearer distances.

1500

1550

1600

1650

1700

1750

1800

1850

1900

1950

2000

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]

Air35% O227% O221% O2

Figure 4.7: Particle temperature behaviour under air and Oxy fuel conditions at

reactor temperature 1300°C, fraction 212-315µm (Klien Kopje)

With the new fuel-feeding probe, experiments were carried out for the coal particle

temperature measurements up to 2.5 cm from the burner distance. Figure 4.8 shows the

particle temperature behaviour under air and oxy fuel conditions at 1300°C reactor

temperature for the fraction of 90-150µm (Low volatile Bituminous coal = Klien Kopje).

The temperature measurements were carried out for different distances starting from 40 cm

away from the burner. The maximum temperature of coal particle was observed at 35 % O2.

In case of 27 % O2 the coal particle peak temperature can be seen at the distance of 7.5 cm

from burner as compare to 5cm for air, but the trend of coal particles temperature profile

were similar for both air and 27 %. Particle fraction 90-150µm were detected only up to 30

cm from the burner as compare to particle fraction of 212-315µm indicating rapid burnout

due to small particle size.

Ignition results

34

1400

1500

1600

1700

1800

1900

2000

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]

Air

21% O2

27% O2

35% O2


reactor temperature 1300°C, fraction 90� 150µm (Klien Kopje)

Figure 4.9 shows Particle temperature behaviour under air and oxy fuel conditions at

reactor temperature 1300°C for fraction 90-150µm (medium volatile bituminous coal

=Elcerejon). Pyrometer detected particle temperature up to 25 cm for oxy fuel conditions

and 30 cm in case of air, as the fraction size was smallest and the fuel was more volatile.

Coal particle temperature was maximum for the 35 % O2 and minimum for 21 % O2.

Although sampling was carried out up to the distance of 2.5 cm near the burner but peak of

coal particle temperature was not appeared except 27 % O2 in feed gas at 5 cm away from

burner.

Ignition results

35

1400

1500

1600

1700

1800

1900

2000

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]Air21% O227% O235% O2


reactor temperature 1300°C, fraction 90-150µm (Elcerejon)

Figure 4.10 shows Particle temperature behaviour under air and oxy fuel conditions at

reactor temperature 1300°C, fraction 150 � 212µm (Elcerejon). It was observed that

particle detection was up to 30 cm away from the burner in case of oxy fuel and 40 cm in

case of air. Maximum Coal particles temperature was observed at 35 % O2 concentration at

2.5 cm away from burner.

Particle temperature behaviour under air and oxy fuel conditions at reactor temperature

1300°C, fraction 212-315µm (Elcerejon) is shown in figure 4.11. It was found that

pyrometer detected coal particles up to 40 cm away from burner for both air and oxy fuel

test, because of bigger fraction of coal as compared to 25 cm for the smallest fraction of 90-

150µm. No clear peak of coal particles temperature was detected for air or oxy fuel tests

but highest temperature was observed for 35 % and lowest for 21 % with air and 27 % O2

laying in between them and showing similar trend.

Ignition results

36

1400

1500

1600

1700

1800

1900

2000

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]Air21% O227% O235% O2

Figure 4.10: Particle temperature behaviour under air and Oxy fuel conditions at reactor temperature 1300°C, fraction 150 � 212µm (Elcerejon)

1400

1500

1600

1700

1800

1900

2000

0 5 10 15 20 25 30 35 40 45


Part

icle

Tem

pera

ture

[°C

]

Air21% O227% O235% O2


reactor temperature 1300°C, fraction 212-315µm (Elcerejon)

Ignition results

37

Figure 4.12 shows the relation between number of detected particles for certain amount of

time over the distance from burner during tests. It was found that as feeding probe distance

decrease from 10 cm to 2.5 cm to the burner, the detection of coal particles by pyrometer

decreased considerably.

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35 40 45


Part

icle

s D

etct

ed

21% O2

27% O2

35% O2

Figure 4.12: Relation between detected particles over the distance from burner

It was observed that at such close distance some particles were fully ignited, some were

about to ignite and some were not ignited at all. Temperature was calculated on the basis of

average of total detected particles and since the two-colour pyrometer detected only those

particles which were already ignited, the average temperature of particles kept on

increasing. In reality the average particle temperature should have decreased at distance less

than 10 cm for this case indicating the heat up phase of ignition but due to the detection

limit of pyrometer, the average particle temperature kept on rising.

Combustion results

38

5 Combustion results

Combustion experiments too were carried out with air as well as oxy fuel. For experiments

in O2/CO2 mixture, the nitrogen is replaced by CO2. Every experiment is done at ~ 3 %

excess oxygen in the flue gas. Oxygen gas concentration varies in feed gas between 21 %,

27 % and 35 % vol on a dry basis for oxy fuel combustion tests. Concentration of the

oxygen in flue gas was controlled by variation of supplied coal in reactor.

Figure 5.1 shows the emission concentration of NO, CO, SO2 and CO2 in flue gas at

1100°C for Lausitz coal. The maximum achieved concentration of carbon dioxide was ~ 95

% in the flue gas in case of oxy fuel combustion, reactor was operated at positive pressure.

Trend of emissions concentration in flue gas shows, higher the oxygen concentration in the

feed gas leads to increase the NOx and SOx. As we have seen from ignition results, increase

the oxygen concentration in feed gas leads to higher temperature, leads to increase in NOx

emission in flue gas.

0

400

800

1200

1600

2000

Air 21 27 35Oxygen [vol %]

NO

, CO

, SO

2 [pp

m]

0

20

40

60

80

100

O2,

CO

2 [vo

l %]

Coal [g/h] SO2 CO NO O2 CO2

Figure 5.1: Emission concentration of NO, CO, SO2 and CO2, Lausitz, WT=1100°C

O2excess= 3 %

Combustion results

39

It was found that NOx concentration in the flue gas was higher in case of oxy fuel (27 %

and 35%) than air because of higher conversion rate of fuel N to NO due to higher fuel

feeding rate and oxygen concentration. Similarly higher concentration of SO2 in flue gas

because of higher fuel sulphur (S) contents for combustion with higher feeding rate of coal.

In case of combustion in air and 21 % O2, when all the input parameters were same during

the experiments, we found reduction in the NOx emissions in flue gas. An increase in CO

concentration can be seen in the flue gas due to dissociation of CO2, normally it can be

observed at temperature greater than 1267°C and at atmospheric reaction pressure [35].

CO2 → (1 � x) CO2 + x CO + (x/2) O2

CO concentration was observed on the online gas analyser although coal feeding was not

yet started for combustion, which confirms that temperature was not uniform in the reactor.

Figure 5.2 shows the emission concentration of NO, CO, SO2 and CO2 in flue gas at

1300°C for Lausitz coal for 3 % excess Oxygen in flue gas. Approximately 94 % CO2 was

found in the flue gas for oxy fuel combustion at 1300°C.

Figure 5.2: Emission concentration of NO, CO, SO2 and CO2, Lausitz, WT=1300°C O2excess= 3 %

0

400

800

1200

1600

2000


NO

, CO

, SO

2 [pp

m]

0

20

40

60

80

100

O2,

CO

2 [vo

l %]


Combustion results

40

Results showing continuous increase in the emission of NO and SO2 with the increase of

oxygen concentration in feed gas. Maximum concentration of NO and SO2 in the flue gas

was found at 35 % O2 case. With the comparison of results of Lausitz for 1100°C &

1300°C, higher concentration of NO was found at higher temperature (825.9 ppm at

1300°C & 703 ppm at 1100°C) for the 35 % O2 in feed gas and 3 % excess O2 in flue gas

condition. Trend of the graph for air and 27 % O2 shows, similarity in case of concentration

of emissions in the flue gas. Behaviour of the NO, SO2 and CO emission were same like

previous result.

Figure 5.3 and 5.4 shows the relationship between temperature and emissions concentration

(NO, SO2). It was clearly found, increase of emission concentration (NO, SO2) in the flue

gas with the increase of temperature from 1100°C to 1300°C for the 3 % excess oxygen

condition in the flue gas. Very small difference in the concentration of NO in flue gas was

found between combustion in air and 27 % O2 at 1300°C

200

400

600

800

1050 1100 1150 1200 1250 1300 1350

Temperature [°C]

NO

[ppm

]

Air 21% O2 27% O2 35% O2

Figure 5.3: Concentration of NO [ppm] in flue gas at 1100°C & 1300°C, Lausitz

Combustion results

41

400

600

800

1000

1200

1050 1100 1150 1200 1250 1300 1350

Temperature [°C]

SO2 [

ppm

]Air 21% O2 27% O2 35% O2

Figure 5.4: Concentration of SO2 [ppm] in flue gas at 1100°C & 1300°C, Lausitz

0

400

800

1200

1600

2000


NO

, CO

, SO

2 [pp

m]

0

20

40

60

80

100

O2,

CO

2 [vo

l %]


Figure 5.5: Emission concentration of NO, CO, SO2 and CO2, Klien Kopje, WT=1300°C

Combustion results

42

Figure 5.5 shows the emissions concentration in the flue gas for the combustion at

temperature of 1300°C, low volatile Bituminous coal (Klien Kopje) and for 3 % excess O2

in flue gas. It was found that the concentration of NO and SO2 were maximum at 35 % O2

and lower in 21 % O2 in feed gas as compare to Air as base line. Same trend of emissions

concentration can be noticed from this graph like previous results, where the concentration

of emission were lower in case of 21 % O2 and higher for 35 % O2 but have similarity in

case of combustion in 27 % O2 and air.

Conclusions and future work

43

6 Conclusions and future challenges

6.1 Conclusions

This experimental investigation of O2/CO2 firing during pulverised combustion was done to

observe ignition behaviour of coal particle, behaviour of two-colour pyrometer for particle

temperature and emission concentration in the flue gas during combustion.

It was found from ignitions experiments that coal particle temperature decrease with the

increase in coal feeding rate. The results show that coal particle temperature is inversely

proportional to the particle size and smaller particles are faster to ignite. Increasing the

amount of oxygen in the feed gas leads to increase the coal particle temperature and early

ignition. It was found that coal particle temperature profile for 27 % O2 concentration in the

feed gas showing similar trend as air.

During the investigation of pyrometer behaviour for coal particle temperature measurement,

it was found that for higher coal feeding rate / near to the burner, its difficult for pyrometer

to measure single and take many particles as a particle cloud and this can influence the

accuracy of coal particle measurements.

It was observed that emissions concentration CO2, NO, SO2 and CO leads to increase with

the increase of oxygen concentration in the feed gas. The maximum achieved concentration

of carbon dioxide was ~ 95 % in the flue gas in oxy fuel combustion conditions. NO

emission trend shows the similarity between 27 % O2 in feed gas to combustion in air.

The ignition and combustion analysis shows that, under the assumed conditions, oxy fuel

fired plants operated at 27 % O2 in feed gas are comparable to those of the air-fired cases.

Finally, it is noted that the oxy fuel fired operation generates a flue gas rich in carbon

dioxide ~ 95 %, which may be easily captured in order to be sequestered. Thus, the boiler

operation can be efficiently converted to a zero emission plant.


44

6.2 Future work

For the future work, following points are suggested to get better results and better

understanding on oxy fuel firing experiments.

Improvement is needed in measurement method near the burner to see better coal particle

temperature profile. Because it was found that as feeding probe distance decrease from 10

cm to 2.5 cm to the burner, the detection of coal particles by pyrometer decreased

considerably.

Instead of air, pure oxygen and CO2 is fed to the reactor and the effect of water is

neglected, so in future water stream in the feed gas is to be added to see the effect of water

in O2/CO2 firing.

We have to consider the effect of HNO3 to get better results.NO2 dissolves into the water

and forms HNO2 and HNO3 (Nitrous and Nitric acid )

2NO2 + H2O → HNO2 + HNO3

But HNO2 is unstable and will be changed into HNO3 finally

2HNO2 + O2 → 2HNO3

Experiments were done at constant 3 % excess oxygen in the flue gas which means the air /

fuel ratio was different in case of 21 %, 27 % and 35 % oxygen in the feed gas. For future

investigation, it is suggested to observe the emission concentration in the flue gas at

constant air / fuel ratio. 6.3 Future challenges

Combustion of coal in an O2/CO2 atmosphere has been investigated to increase the

knowledge of combustion characteristic.

Unlike the N2 molecules, the CO2 and H2O molecules are emitters of thermal radiation,

meaning that when N2 is substituted with CO2 in the plant, the heat transfer characteristics

will change. There will be a need for verification and validation of reliable heat transfer

models that include the changed thermal radiation characteristics.


45

Combustion of coal in pure oxygen gives a high temperature, which leads to increase Nox.

The solution can be re-circulation of flue gas.

Higher the CO2 concentration in the flue gas means heat flux to the wall will be higher and

higher temperature corrosion is therefore likely occur more rapidly in an O2/CO2

combustion boiler than in an air-fired boiler, corrosion testing is there necessary.

Future challenges concerning oxygen production technology for the O2/CO2 combustion of

coal.

Bibliography

46

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CANMET Energy Technology Centre (CETC) 5 Oxy Fuel Techniques www.airproducts.com/Markets/Glass/oxy-fuel-techniques.htm

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system By Hideki WAKU and Masanori INOUE Energy Vol. 22, No. 2/3, pp.249-255, 1997 9 Experimental investigation of natural gas combination in oxygen / Exhaust gas

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By Fehmi Akgün, University of Sakarya, Faculty of Engineering, Department of Mechanical Engineeing, 54188 Adapazari, Turkey And Ahmet Arsoy, Technical University of Istanbul, Faculty of Mechanical Engineering, 80191 Gumussuyu, Istanbul, Turkey

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