mining combustion and fuels laboratory finalfluid.wme.pwr.wroc.pl/~spalanie/dydaktyka/... ·...

Project co-financed by European Union within European Social Fund

EUROPEAN UNION EUROPEAN

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THE DEVELOPMENT OF THE POTENTIAL AND ACADEMIC PROGRAMMES OF WROCŁAW UNIVERSITY OF TECHNOLOGY

Mining and Power Engineering

dr inż. Tomasz Hardy (PhD Eng.)

Combustion and fuels - laboratory

Project co-financed by European Union within European Social Fund 2


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Topics of the laboratory classes

1. Structure of gaseous flames 2. Combustion of liquid fuels 3. Combustion of pulverized solid fuels 4. Combustion of biomass 5. Catalytic burning of CO and CH 6. Pyrolysis of solid fuels

Materials for theoretical preparation 1. Spalanie i paliwa, ed. by W. Kordylewski, Oficyna Wydawnicza Politechniki

Wrocławskiej, Wrocław, 2008 (ed. V) or 2005 (ed. IV) (in Polish) 2. Presentations for the lecture Combustion and fuels available online at

www.spalanie.pwr.wroc.pl (in English) 3. Handbook of combustion, Vol. 1-5, ed. by M. Lackner, F.Winter, A.K. Agarwal, Wiley-

VCH Verlag GmbH & Co. KGaA, Weinheim, 2010 (in English)



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1. Structure of gaseous flames 1.1. Introduction 1.1.1. Basic definitions Combustion – an exothermic chemical reaction between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. Combustible mixture – a mixture of a fuel and an oxidizer in which combustion is developing after an ignition source vanishes. Ignition – the initiation of the combustion process in the combustible mixture. There are two types of ignition:

- external ignition, caused by a local source introduced into the combustible mixture (e.g. spark, pilot flame, etc.) -self-ignition, caused by an even rise of temperature in the whole volume of the combustible mixture above a certain value.

Flame – the zone where the combustion process takes place. Considering the preparation of a combustible mixture, there are two principal types of flames

- premixed flame in which the oxidizer has been mixed with the fuel before it reaches the combustion zone, and the flame speed is determined by rules of chemistry - diffusion flame in which the oxidizer combines with the fuel by diffusion and the flame speed is limited by the rate of diffusion.

Fig 1.1. Premixed and diffusion flame

1.1.2. Structure of a premixed flame In the premixed flame three zones can be distinguished (fig 1.2): • Heating and reaction initiation zone where the oxidation process is initiated by heat and radicals

coming from the main reaction zone. This zone ends at the point where the oxidation process takes place spontaneously, which roughly corresponds to the bend point on the temperature curve.

• Main reaction zone (flame front) where intense reactions of fuel oxidation take place and the intermediate and final products of combustion (including radicals) are formed. This zone is very thin and large gradients of concentration and temperature occur there.

• After-flame zone where the temperature drops to ambient temperature and the concentration of combustion products tends to reach the background level .

When the amount of an oxidizer is not sufficient, a secondary diffusion flame is formed where the products of a partial combustion (e.g. carbon monoxide) are afterburned.

F – fuel (gas), A – oxidizer (air)



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Fig 1.2. Concentrations (C) and temperature (T) versus position (x) in premixed laminar flame: 1 –

fuel, 2 – flue gas, 3 – oxidizer, 4 – temperature, 5 – premixed flame, 6 – sec. diffusion flame 1.1.3. Structure of a diffusion flame The diffusion flame is formed in the boundary layer between a flammable gas and an oxidizer. Before combustion can occur, gases must be mixed by diffusion. The diffusion can be of molecular or turbulent nature. The intense reaction zone (flame front) of the diffusion flame is located where the ratio of air to fuel is stoichiometric.

Fig. 1.3. Laminar diffusion flame:

1 – gas, 2 – flue gas, 3 – air, 4 – temperature, 5 – flame 1.1.4. Gas burners A gas burner is a device used to form a flame using a gaseous fuel. The basic function of gas burners are:

- to prepare a combustible mixture with a proper air to fuel ratio; - to ensure a continuous ignition of the mixture; - to ensure total and complete combustion of fuel; - to stabilize the flame front; - to provide a desired shape and size of flame; - to provide desired thermal power of flame.



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The following types of gas burners can be distinguished: • Considering the preparation of a combustible mixture :

- premixing burners in which air and fuel are mixed before the outlet of the burner; - diffusion burners in which air and fuel are fed separately and mix at the outlet of the burner

or in the combustion chamber; - premix/diffusion burners in which some air is mixed with fuel before the outlet of the burner

and the rest of air is fed separately and mixes at or after the outlet. • Considering construction :

- jet-ejector burner – a premixing burner in which air is sucked by gas flowing from the nozzle;

- nozzle-mix burners - a premixing or diffusion burner where air is supplied by an additional device (e.g. a fan)

• Considering gas flow shaping: - jet burner; - swirl burner.

1.2. Aims of the laboratory

1. To become acquainted with the phenomenon of the combustion of gaseous fuels. 2. To observe a gas burner in operation. 3. To measure the temperature and content of the gas inside a flame in order to determine its

structure. 1.3. Diagram of the experimental setup

1.4. Results processing

1.4.1. Calculation of the content of the combustible gas in the mixture

airgas

gas

qq

qr

+=

digital temp. meter

[ t ]

probe positioning

[ x ]

sample of mixture from flame for chromatographic analysis

[ c ]

sample of fresh air/fuel mixture for chromatographic analysis

[ c100% ]

combustible gas

air

rotameters

flame

gas probe

thermocouple

burner



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where: r – content of the combustible gas qgas – volumetric flow rate of gas (dm3/h) qair – volumetric flow rate of air (dm3/h).

Because the volumetric flow rate of gas is measured with a rotameter scaled for air, the correction of the result is necessary (using the formula below)

gas

airgasgas qq

ρρ′=

where: qgas – real volumetric flow rate of gas (dm3/h)

q’gas – measured volumetric flow rate of gas (dm3/h) ρair – density of air (kg/m3) ρgas – density of gas (kg/m3).

Check if the calculated content of the gas is within the flammability limits (see textbook for reference). 1.4.2. Calculation of the burnout degree of gas

%100*1%100

−=

c

cw

where: w - burnout degree

c – the content of combustible gas in a certain point of flame, measured by chromatographic analysis of sample taken from the flame c100% – the content of combustible gas in fresh air/fuel mixture, measured by chromatographic analysis of a sample taken before the burner.

1.4.3. Preparation of the charts:

a) the burnout degree of gas (w) versus the distance from the centre of the flame (x) b) the temperature of a flame (t) versus the distance from the centre of the flame (x)

1.5. Values to be measured

1. Volumetric flow rates of air qair, dm3/h & gas q’gas, dm3/h 2. Content of unburned gas in fresh air/ fuel mixture c100% 3. Results of probing of flame (see table blow)

distance from the centre of a flame

content of unburned gas

temperature of flame

x, mm c t, ºC 0 1 …



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2. Combustion of liquid fuels 2.1. Introduction 2.1.1. Combustion of oil droplets Combustion of liquid fuels is most often based on their atomization (spraying) and then combustion of droplets. Therefore the mechanism of the single droplet combustion (fig. 2.1.) is important. The combustion of liquid fuels can be divided into two phases: evaporation, and then burning vapours. Consequently, the rate of combustion of liquid fuels is determined by the following factors:

- the rate of liquid evaporation, depending on the heat flux transferred into the fuel; - the rate of mixing between air and fuel vapours; - the chemical kinetics of fuel oxidation.

Fig. 2.1. Diffusion model of single oil droplet combustion:

a) flame geometry, b) temperature (T) and concentration of fuel (εP) and oxygen (εU) Combustion of heavy oil is more complicated because it forms bigger droplets and contains heavier hydrocarbons. First, the lighter fractions evaporate and burn, then the heavier fractions are decomposed and burned and finally the char residue burns. 2.1.2. Structure of sprayed oil flame

Fig. 2.2. Structure of sprayed oil flame: 1 – stream of sprayed fuel, 2– zone of evaporation, 3 – zone of ignition,

4 – zone of combustion of individual droplets, 5 – boundary of flame



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2.1.3. Atomization of liquid fuels Atomization of liquid fuels into small droplets provides a high intensity of evaporation and therefore intense combustion. Proper atomization is essential for the quality and efficiency of combustion. An improper spraying causes, among others, worse burnout (manifested by increased CO and soot levels). The disintegration of a liquid jet into a number of filaments and then into small droplets, requires the surface tension forces of liquid to be overcome. It may happen in three ways: - by the surface tension between moving liquid jet and steady air which destabilises the jet and

causes its disintegration; - by centrifugal forces of swirled liquid jet; - by outer mechanical and electrostatic forces and by supersonic acoustic. Liquid fuel can be sprayed using the following forms of energy (fig. 2.3.): - the energy of pressurized liquid ; - the energy of pressurised additional gas (air or steam); - the mechanical energy of rotation. An atomiser is characterized by several parameters, among which the most important are (fig. 2.4.): - output, kg/s; - the angle of dispersion; - droplets distribution in the sprayed stream.

Fig. 2.4. Characteristics of an atomizer

Fig. 2.3. Fluid atomization with different forms of energy



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2.2. Aims of the laboratory 1. To become acquainted with the phenomenon of combustion of liquid fuels. 2. To observe oil burners in operation. 3. To obtain the characteristics of combustion with respect to the excess air.

2.3. Diagram of the experimental setup

1. Combustion chamber 2. Oil burner 3. Oil filter

4. Oil tank 5. Measurement of sooting level (Bacharach’s method) 6. Probe of flue gas analyser

2.4. Results processing Calculation of the excess air coefficient

λ21

21 O2− where: λλλλ – excess air coefficient

21 – content of oxygen in air (percents) O2 – content of oxygen in flue gas (percents).

2.4.2. Calculation of the normalized values of carbon monoxide content and nitrogen oxide content (reference level of oxygen = 3%)

2

%3

21

321

OCOCO

−−⋅=

2

%3

21

321

ONONO

−−⋅=

flue gas analyser

[O2] [CO] [NOx]

measurement of flame

temperature

measurement of flue gas

temperature

2 1

4

3 5

6

flue gas

exhaust



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where CO3% – normalized value of carbon monoxide content (ppm) NO3% – normalized value of nitrogen oxide content (ppm) CO – measured value of carbon monoxide content (ppm) NO – measured value of nitrogen oxide content (ppm) 21 – content of oxygen in air (percents) 3 – reference content of oxygen in flue gas (percents) O2 – content of oxygen in flue gas (percents).

2.4.3. Preparation of the charts:

a) the normalised content of carbon monoxide (CO3%) and the normalized content of nitrogen oxide (NO3%) versus the excess air coefficient (λλλλ)

b) the temperature of a flame (tfl) and the temperature of flue gas tfg versus the excess air coefficient (λλλλ)

c) the sooting level (S) versus the excess air coefficient (λλλλ) 2.5. Table of values to be measured

temperature composition of flue gas

No flame tfl

flue gas tfg

O2 CO NO

sooting level

S

- ˚C % ppm ppm ˚B 1 2 …



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3. Combustion of pulverized solid fuels 3.1. Introduction 3.1.1. Composition of coal Basically coal consist of three components: - combustible organic matter, containing carbon, hydrogen, oxygen, sulphur, nitrogen and traces

of other elements; - moisture; - mineral matter (ash). Composition of coal is described by: - technical analysis, which determines the content of moisture (W), ash (A) and volatile matter, it

also determines the caloric value of coal; - elemental analysis, which determines the content of C, H, O, N, S and other elements.

Fig. 3.1. Composition of coal

3.1.2. Combustion of a coal particle After getting into a flame, a coal particle is heated and dried, then the evaluation and combustion of the volatile matter takes place and finally the burning of char (fig. 3.2.). The length of each stage depends on the particle size, the conditions of the combustion, and the properties of coal (composition, structure).

Fig. 3.2. Stages of single coal particle combustion (with a smudge photography of a burning coal particle)



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Processes occurring during the combustion of a single coal particle can be divided into two groups: • physical, such as:

- evaporation of water (drying), - swelling (dilation) of coal particles, - formation of porous structure of char, - physical transformation of mineral matter;

• chemical, such as: - pyrolisis of coal, - combustion of volatile matter, - combustion of char, - chemical transformation of mineral matter.

3.1.3. Basic coal combustion systems The most common systems for coal combustion are:

- a furnace with a grate (steady or moving); - a fluidized bed furnace (bubble or circulating bed); - a pulverized coal furnace.

Others systems are:

- a cyclone combustion chamber; - a retort furnace; - a rotary furnace.

Fig. 3.3. System for coal burning:

a) fixed bed, b) moving grate, c) fluidized bed, d) pulverized coal burner



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3.1.4. Pulverized coal burners and furnaces The following types of pulverized coal burners can be distinguished: • Considering the air flow and mixing with coal (fig 3.4.):

a) a jet burner, designed for the combustion of ‘lean’ hard coal with a low content of volatile matter (<20%) or lignite and young hard coal with a high content of volatile matter;

b) a swirl burner designed for the combustion of ‘fat’ hard coal with 17-40% volatile matter;

a) b)

Fig. 3.4. Pulverized coal burners • Considering the location of the burners in the furnace (fig. 3.5.)

a) wall-placed, b) roof-placed, c) corner-placed (tangential firing)

Fig 3.5. Location of pulverised coal burners in the furnace



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3.2. Aims of the laboratory 1. To become acquainted with the phenomenon of combustion of solid fuels. 2. To observe a dust burner in operation. 3. To observe a fluidized bed furnace in operation. 4. To measure pollutants emissions from combustion of solid fuel.

3.3. Diagrams of the experimental setups

Task to do: Compare the temperatures of the flame measured with a thermocouple and a pyrometer and explain why they are different.

measurement of temp. with

pyrometer

°°°°C

burner

to fume extractor

measurement of temperature

with thermocouple

outlet of fume samples

air +

dust

dust container

air gas

fluidized bed

furnace, electrically

heated

fluidizing air

fuel carrying air

fuel feeder

cyclonic separator

to fume extractor

flue gas analyser

[O] [CO] [NOx] [SO]

power supply for heaters

with temp. control

flue gas probe

°C A

return of dust from separator

electric air heater

power supply for feeder

A setup for the observation of combustion in the fluidized bed furnace

A setup for the observation of a dust flame

and measurements of temperature



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3.4. Results processing 3.4.1. Calculation of the excess air coefficient

λ21

21 O2− where: λλλλ – excess air coefficient

21 – content of oxygen in air (percents) O2 – content of oxygen in flue gas (percents).

3.4.2. Calculation of the normalized values of carbon monoxide content and nitrogen oxide content (reference level of oxygen = 6%)

2

%6

21

621

OCOCO

−−⋅=

2

%6

21

621

ONONO

−−⋅=

where CO6% – normalized value of carbon monoxide content (ppm)

NO6% – normalized value of nitrogen oxide content (ppm) CO – measured value of carbon monoxide content (ppm) NO – measured value of nitrogen oxide content (ppm) 21 – content of oxygen in air (percents) 6 – reference content of oxygen in flue gas (percents) O2 – content of oxygen in flue gas (percents).

powietrze wtórne

rozdrobnione paliwo stałe (węgiel, biomasa)

analizator spalin

separator pyłu

Palnik pyłowy

8 8 8

8 8

wyciąg spalin

dust burner

power supply (with temp. control) for the heaters of the furnace

elec

tric

ally

hea

ted

dr

op fu

rnac

e

fuel (in form of dust) and primary air

secondary air

dust separator

to fume extractor

flue gas analyser

A setup with the drop

furnace for measurements of emissions



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3.4.3. Preparation of charts showing the normalized content of carbon monoxide (CO6%) and the normalized content of nitrogen oxide (NO6%) versus the excess air coefficient (λλλλ) 3.5. Table of values to be measured

composition of flue gas No

mas

s flo

w

of fu

el

vol.

flow

of

air

O2 CO NO

- g/s ℓ/h % ppm ppm 1 2 3 …



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4. Combustion of biomass 4.1 Introduction

The recent years have brought about some climate changes, which may be caused by the constant growth of the amount of carbon dioxide in the air. CO2 as a greenhouse gas plays the main role in the absorption of heat radiation. This phenomenon may find its reason in a constant growth of fossil fuels consumption. In order to reduce the future growth of CO2 concentration, the use of renewable energy should be increased. One possibility could be increasing the participation of biofuels in the general fuel balance, with a special emphasis on producing heat energy in small holdings. Biomass absorbs carbon dioxide during growth, as such biofuels are regarded as neutral in CO2 emission. The main components of wood are: cellulose (45-55wt%), hemicellulose (12-20wt%) and lignins (20-30wt%). Moreover, it includes resins, tannins, fats, proteins and mineral substances. The main components of wood are carbon (50%), oxygen (43%) and hydrogen (6%). Wheat and rape straws contain slightly less carbon, 45% and 47% respectively. Biomass contains small amounts of sulphur compounds, max. 0.5% (wt% of dry fuel). In comparison the dried coal substance contains 0.5-7.5% (wt% of dry fuel). In the ash from biomass combustion the amount of K2O content (4-48wt% of dry ash) was noted to be significantly lower compared to coal burning (2-6 wt% of dry ash). The amounts of the Al2O3 and Fe2O3 were noted to be approximately twice lower. Biomass contains also chlorine - while wood contains little of it (approx. 0.01wt% of dry fuel), straws contain more (rape straw 0.63 and wheat straw 0.477 wt% of dry fuel). The high content of chlorine is the reason for exploitation problems of boilers, which is chloride corrosion. Even if biomass does not contain chlorine, high levels of potassium in ligninocellulose biomass ash cause increased slugging. This may intensify the corrosion and proper boiler functions may be impaired.

Similarly to other solid fuels, biomass combustion consists of three stages: • drying and preheating of a fuel; • pyrolytic release of volatile flammable gases; • combustion of pyrolytic gases and solid residue – tar and char.

Biomass thermal decomposition starts above 220°C. Individual components subject to decomposition at 220-320°C for hemicellulose, 320-370°C for cellulose and 320-500°C for lignin . Combustion technology and conditions, furnace construction and fuel quality affect the composition of pollutants emitted during wood combustion. Biomass generally has a lower heating value (14-21 MJ/kg) than coal (23-28 MJ/kg) due to its higher moisture. The higher amount of organic matter content in biomass causes lower ignition temperature (for biomass 145-153°C and for coal 217-223°C). Biomass combustion and co-combustion with coal helps to reduce most of the gaseous pollutants emissions such as CO, CO2, NOx and SO2.

Fig 4.1.1 Phases of wood combustion



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Sources and use of biomass According to Directive 2001/77/EC, biomass is “the biodegradable fraction of products, waste and residues from agriculture (including vegetal and animal substances), forestry and related industries, as well as the biodegradable fraction of industrial and municipal waste.” Biomass can be obtained from:

- agriculture and agri-food industry residues: straw, hay, husks, shells, seeds, pomace, etc.; - energy crops (willow, rose, sunflower, miscanthus, prairie cordgrass and many others); - forest wood and residues from wood, furniture and paper industry; - residues from gardening and other wood or plants residues; - residues from animal breeding (manure) and meat industry (fat, bones, etc.).

Biomass can be used for energy generation in the following ways:

- by direct combustion (often after drying and fragmentation) also in a compressed form (pellets, briquettes)

- by gasification and then combustion of gas - by fermentation and then combustion of gas (bio-gas) - by conversion into liquid fuels (oils, esters, alcohols) and their combustion.

4.2. Aims of the laboratory

1. To become acquainted with the phenomenon of the combustion of biomass. 2. To observe a biomass-fired boiler with a retort furnace in operation. 3. To obtain characteristics of combustion with respect to the excess air.


1) stack; 2) expansion vessel; 3) pressure gauge; 4) exhaust pipe; 5) boiler controller; 6) fuel container; 7) air heater; 8), 9) water thermometers; 10), 11) pressure gauges;12) pump;13) boiler; 14) air fan;

15) screw feeder; 16) motor; 17) exhaust gas analyzer; 18) exhaust gas thermometer; 20) flow meter



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4.4. Results processing 4.4.1. Calculation of the efficiency of the boiler

where: ηb – efficiency of the boiler (percents) mw – mean mass flow of water (kg·s-1) Cw – specific heat capacity of water (kJ·kg-1·deg-1) tw1 – mean temperature of return water (deg) tw2 – mean temperature of outgoing water (deg) Qw – caloric value of fuel (kJ·kg-1) * Bb – mass flow of fuel (kg·s-1)

Bb = m/∆τ

m – mass of consumed fuel (kg) ∆τ – duration of the measurements (s).

*The kind of burned biomass and its properties will be given at the laboratory.

4.4.2. Calculation of the excess air coefficient

λ21

21 O2−

where: λλλλ – excess air coefficient 21 – content of oxygen in air (percents) O2 – content of oxygen in flue gas (percents).

4.4.3. Calculation of the normalized values of the carbon monoxide content and the nitrogen oxide content (reference level of oxygen = 10%)

2

%10

21

1021

OCOCO

−−⋅=

2

%10

21

1021

ONONO

−−⋅=


NO10% – normalized value of nitrogen oxide content (ppm) CO – measured value of carbon monoxide content (ppm) NO – measured value of nitrogen oxide content (ppm) 21 – content of oxygen in air (percents) 10 – reference content of oxygen in flue gas (percents) O2 – content of oxygen in flue gas (percents).



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4.4.4. Preparation of the charts: a) the normalised content of carbon monoxide (CO10%) and the normalised content of nitrogen

oxide (NO10%) versus the excess air coefficient λλλλ; b) the temperature of a flame (tfl)and the temperature of flue gas (tfg)versus the excess air

coefficient (λλλλ). 4.5. Table of values to be measured

temperatures composition of flue gas

water No volum. flow

of water outgoing return

flue gas tfg

flame tfl

O2 CO NO

- m3/h ˚C % ppm ppm 1 2 …



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5. Catalytic burning of CO and HC 5.1.Introduction 5.1.1. Pollutants generation by internal combustion engines The major pollutants present in the exhaust gases from an internal combustion (IC) engine are CO, NOX, SO2, hydrocarbons (HC) and soot. The amount of the pollutants depends on the composition of the air/fuel mixture and the engine operating conditions (fig. 5.1.). For λ≈1,1 the content of CO and HC in the exhaust gas is minimal but then the amount of NOX reaches the maximum. For λ= 0,8-0,9 the amount of NOX is minimal, but the amounts of CO and HC reach their maximum. It means that these pollutants cannot be decreased simultaneously just by controlling the combustion in the engine. A solution to this problem is to apply a catalytic converter, in which NOx is reduced and CO and CH are oxidated.

Fig. 5.1. The relationship between the excess air and pollutants concentrations in a spark-ignition IC engine

Fig. 5.2. Car catalytic converter



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5.1.2. Construction of a catalytic converter (see fig 5.2.) 1. The core is often a ceramic honeycomb in modern catalytic converters, but stainless steel foil

honeycombs are also used. The honeycomb surface increases the surface area available to support the catalyst.

2. The washcoat (usually a mixture of silica and alumina), when added to the core, forms a rough, irregular surface, which has a far greater surface area than the flat core surfaces do. The catalyst is added to the washcoat (in suspension) before being applied to the core.

3. The catalyst itself is most often a precious metal. Platinum (Pt) is the most active catalyst and is widely used. Palladium (Pd) and rhodium (Rh) are two other precious metals applied. Platinum and rhodium are used as a reduction catalyst, while platinum and palladium are used as an oxidization catalyst.

5.1.3. Three-way catalytic converter (TWC) A TWC has three simultaneous tasks: 1. Reduction of nitrogen oxides: 2NO + 2CO = N2 + 2CO2 2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 = 2CO2 3. Oxidation of hydrocarbons to carbon dioxide and water: 2C2H6 + 7O2 = 4CO2 + 6H2O. To achieve the maximum efficiency of catalytic pollutants removal, the near stoichiometric aif/fuel ratio must be provided(fig.5.3.a)). It is achieved with a system which measures the content of oxygen in flue gas (lambda probe) and controls combustion in an engine (fig.5.3.b)). The efficiency of catalytic converters is >90%.

a) b)

Fig. 5.3. a) Optimal air/fuel ratio for catalitic conversion b) Contol system with lambda probe

5.2. Aims of the laboratory 1. To become acquainted with the phenomenon of catalysis. 2. To observe a catalytic converter in operation. 3. To measure the effectiveness of carbon monoxide postcombustion with respect to the

amount of additional air introduced into the catalytic converter.

conv

ersi

on e

ffici

ency



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5.4. Results processing 5.4.1. Calculation of the excess air coefficient λ

λ21

21 O2−

where: λλλλ – excess air coefficient 21 – content of oxygen in air (percents) O2 – content of oxygen in exhaust gas (percents).

5.4.2. Calculation of the normalized value of the carbon monoxide content (both for CObefore and COafter), reference level of O2 = 3%

2

%3

21

321

OCOCO

−−⋅=


CO – measured value of carbon monoxide content (ppm) 21 – content of oxygen in air (percents) 3 – reference content of oxygen in exhaust gas (percents) O2 – content of oxygen in gas (percents).

5.4.3. Calculation of the efficiency of the carbon monoxide postcombustion

%3

%3

1before

afterCO

CO

COE −=

where ECO – efficiency of carbon monoxide postcombustion

CO3%after – normalized content of CO after catalytic conversion (ppm)

CO3%before – normalized content of CO before catalytic conversion (ppm).



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5.4.4. Preparation of the charts (each chart should contain 3 curves – one for each load setting):

a) the efficiency of CO content reduction (ECO) versus the additional air flow (qair) b) the efficiency of CO content reduction (ECO) versus the excess air coefficient (λλλλ) c) the temperature of a catalytic converter (tcat) versus the additional air flow (qair)

5.5. Table of values to be measured

engine load

volumetric flow rate of additional air into catalytic

converter

content of oxygen in exhaust gas before catalytic

conversion

content of oxygen in exhaust gas after catalytic conversion

content of carbon monoxide in exhaust gas

before catalytic conversion

content of carbon monoxide in

exhaust gas after catalytic

conversion

temperature of catalytic converter

P qair O2before O2after CObefore COafter tkat W l/h % % ppm ppm ºC

0 500

1000

1500 … …



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6. Pyrolisis of solid fuels 6.1. Introduction 6.1.1. Pyrolisis Pyrolysis is a thermochemical decomposition of an organic material (coal, biomass, wastes, etc.) at elevated temperatures in the absence of oxygen. During this process volatile matter and liquids (tar) are released and the solid residue is called char. The process of coal pyrolisis is shown in figure 6.1. The process of pyrolisis and the resulting product depend on the conditions (temperature, pressure) and on the composition of a pyrolised material. The intensity of the volatile matter evaluation and the temperature of thermal decomposition vary strongly for different coals, as shown in figure 6.2.

Fig. 6.1. Process of coal pyrolisis Fig. 6.2. Volatile matter evaluation for different coals

Pyrolisis plays an important role in combustion as an independent fuel technology and also as a stage of combustion process or gasification process. It is also used in chemical industry. 6.1.2. Gasification Gasification is a process that converts organic materials (coal, biomass, wastes, etc.) into gas, by reacting the raw material with a gasifying medium (usually steam, oxygen, air). The resulting gas mixture, containing mainly carbon monoxide and hydrogen (and nitrogen if air was used), can be used as a fuel (for gas turbines, IC engines, boilers) or as raw material for a further fuel conversion or chemical processing. Gasification of most of solid fuels has two stages (fig 6.2):

- degassing of the solid fuel, - gasification of the char residue.

Fig. 6.3. Stages of gasification

high temperature

gasifying medium

gas mixture

fuel

char



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6.2. Aims of the laboratory 1. To become acquainted with the phenomenon of pyrolisis. 2. To observe the pyrolisis process for a selected solid fuel (coal or biomass). 3. To obtain characteristics of a pyrolisis process (temperature and mass loss versus time).

6.3. Diagrams of the experimental setups

1. Oven supply with a temperature controller 2. Oven for sample heating 3. Basket with a sample 4. Thermocouple

5. Inlet of gas for filling the oven 6. Sample mass measurement 7. Computer for control and result analysis

6.4. Results processing 6.4.1. Calculation of the speed of mass loss (a derivative of mass change with respect to time). 6.4.2. Preparation of a chart showing the change of quantities listed below with respect to time (each should have its own scale of values)

- the temperature of the process - the relative loss of mass - the speed of mass loss - the mass of the sample

6.4.3. Determining the content of volatile matter in a sample and the speed of volatile matter evaluation 6.5. Table of values to be measured

No time temperature mass loss of mass

- s oC g % 1 2 …

7

6

3

2

4

5

1

7

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