Emissions and their control from industrial installations using solid fuels
284 Pages

Emissions and their control from industrial installations using solid fuels


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Industrial research and development
Coal - hydrocarbons



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Language English
Document size 6 MB

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ISSN 1018-5593
European Commission
technical coal research
Blow-up from microfiche original European Commission
technical coal research
British Coal Corporation
Stoke Orchard
Cheltenham • Glos. GL52 4RZ
United Kingdom
Contract No 7220-ED/815
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Energy PARI EUROP. Biblioih.
N. (EUR 14894 EN 1994
CI. Published by the
Directorate-General XIII
Telecommunications, Information Market and Exploitation of Research
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information
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Catalogue number: CD-NA-14894-EN-C
1 ECSC — EC — EAEC, Brussels • Luxembourg, 1994
.1 III
In recent years there has been a growing awareness of the adverse environmental effects of
emissions from fossil fuel combustion. A major concern are emissions of sulphur dioxide (S02) and
nitrogen oxides (NO„). Legislation has been introduced in the UK and worldwide, requiring
reductions in emissions of both S02 and NOx. The main coal burning method in the UK industrial
sector is stoker-firing. Although, such plant are environmentally attractive because of their
inherently low emissions of particulates and NOx, there has been little research into methods for
the abatement of S02 emissions from such plant. It is considered that the availability of a cost
effective method of S02 abatement for stoker-fired plant would ensure that these plant remain the
favoured system for industrial scale applications. Without such a method the use of stoker-fired
plant would be deterred in favour of systems with demonstrated emissions control techniques such
as fluidised bed combustion or substitution by oil or gas.
The objective of this study was to identify and develop promising S02 abatement methods
applicable to stoker-fired plant which could be used in the short to medium term to meet emissions
legislation in the most cost-effective manner.
Following a literature survey two methods, both entailing the use of a dry limestone sorbent, were
selected for investigation:
On-grate addition of sorbent
ln-furnace injection oft
In the case of on-grate sorbent addition, the limited studies by other workers suggested that
sulphur removal performance was poor mainly due to unsuitable temperatures within the stoker fuel
bed. Thermodynamic and kinetic analyses of the relevant chemical reactions were conducted to
make a preliminary identification of factors likely to limit the effectiveness of the added sorbent.
These identified fuel bed temperature and poor sorbent/gas mixing as being the main limiting
factors. A series of preliminary commercial and laboratory scale scoping trials were then conducted
to assess methods to overcome these limitations. Sorbent was added to the fuel bed by two
Mixed with the lump coal prior to firing
Incorporated into a coal/sorbent pelletised fuel
It was considered that the incorporation of sorbent into a pelletised fuel would improve mixing.
Trials were also conducted using a range of fuel bed cooling techniques (water addition and flue
gas recirculation). Although the pelletised fuel showed some promise in these scoping trials, there
appeared to be little benefit from the application of fuel bed cooling.
Subsequently a series of detailed practical investigations were conducted on a purpose-built fixed
grate simulator in order to more clearly identify the factors limiting S02 abatement by this method.
The conclusions from these trials were:
i. Under operating conditions typical of industrial scale stoker-fired plant the maximum
sulphur removal obtainable by either method by the addition of limestone to the fuel bed
at a rate equivalent to a Ca:S molar ratio of 2:1 was equivalent to around 10% of the fuel
sulphur content. IV
ii. The principal factors limiting the effectiveness of sulphur removal by addition of limestone
to the fuel bed were:
a. the presence of both oxidising and reducing regions in the fuel bed which caused
a release of previously captured sulphur,
b. the presence of temperatures above the decomposition temperature of calcium
c. inadequate contact between limestone and sulphur bearing gases.
iii. Techniques to overcome the identified limiting factors were assessed with the following
a. the application of fuel bed cooling techniques to reduce peak temperatures had little
effect on sulphur removal,
b. operation under sub-stoichiometric conditions to encourage greater reducing regions
provided an increase in sulphur removal to around 25%,
c. the intimate mixing of limestone with coal achieved in the coal/limestone pelletised
fuel, showed increased reaction rates, but overall sulphur removal was similar to
that achieved with the direct mixing of limestone with the lump coal.
iv. Addition of limestone to the grate in the form of a pelletised fuel indicated three potentially
serious operational problems:
a. Reduced burning rate
b.d combustion efficiency
c. Excessive ash melting
The effects were not observed to the same extent when limestone was mixed with the
lump coal.
v. Neither of the on-grate limestone addition methods investigated in this study are considered
suitable for application to industrial stoker-fired plant due to their poor sulphur removal
performance and potentially detrimental effect on plant operation and performance.
Above-grate sorbent addition is a commercially available S02 abatement method. However
it was considered that there were a range of issues relating to the optimisation of operating
parameters and the effect on plant operation which required clarification to enable
confident application of this technology, particularly in retrofit applications. A series of
tests were conducted, using the fixed grate simulator, to resolve these issues. The
simulator was chosen for this testwork since it had been shown to be capable of
reproducing the above-grate conditions within operational stoker-fired plant. The
conclusions drawn from this testwork were:
vi. Under operating conditions typical of industrial scale stoker-fired plant the maximum
sulphur removal achievable by the addition of limestone and dolomite sorbents to the
furnace region, at a Ca:S molar ratio of 2:1, was equivalent to between 52 and 65% of the
fuel sulphur content.
vii. At similar mass addition rates there was little significant difference in the sulphur removal
performance of the limestone and dolomite. viii. A range of factors were identified as having a strong influence on the sulphur removal
performance of the sorbent:
Sorbent injection temperature
Furnace temperature profile
Sorbent reaction residence time
Flue gas S02 concentration
Sorbent type, particle size and addition rate
Many of these factors are inter-related making investigation of their individual influence on
sulphur removal difficult. For design purposes the most effective means of assessing
optimum operating conditions is likely to be an assessment of the specific applications
using a facility, such as the fixed grate simulator, which is capable of reproducing actual
plant conditions.
ix. Two major operational difficulties were identified:
a. a significant increase in flue gas particulate burden
b. deposition of injected sorbent in the boiler
It is considered that these problems can be satisfactorily overcome in many cases by the
use of appropriate particulate removal equipment and the careful design of facilities to
remove and collect deposited solids.
x. Of the two abatement methods investigated it is considered that in-furnace sorbent
injection offers the most suitable means of controlling S02 emissions from industrial stoker-
fired plant in the short to medium term. VII
Page No.
2.1 Coal Formation and Composition
2.2 Stoker Classification
2.2.1 Underfeed stokers 5
2.2.2 Reciprocating grate stokers 6
2.2.3 Travelling and chain grate stokers 7
2.2.4 Sprinkler stokers 8
2.2.5 Spreader stokers °
2.3 Combustion of Coal on a Fixed Grate 9
2.3.1 Reactions of coal in the fuel bed Devolatilisation Combustion under oxidising conditions 10 3 Reaction under reducings H General fuel bed conditions
2.4 General Operation 12
2.4.1 Combustion air distribution
2.4.2 Fuel bed thickness3
2.4.3 Excess air
2.4.4 Secondary or overfire air4
2.4.5 Thermal efficiency
2.4.6 Coal type
2.4.7 Grate and fuel bed temperatures 15
2.5 Uncontrolled Emissions from Stoker-Fired Plant
2.5.1 Emissions of sulphur dioxide
2.5.2s of nitrogen oxides
2.5.3 Particulate emissions 16
2.5.4 Carbon monoxide and hydrocarbon emissions 1
3.1 The Formation of S02 During Fuel Bed Coal Combustion 1
3.1.1 Devolatilisation 1
3.1.2 Reactions under reducing and oxiding conditions8
3.1.3 Release of sulphur from coal during stoker firing 19
3.2 The Chemistry of S02 Capture During Coal Combustion 20
3.2.1 Sulphur retention by coal mineral matter
3.2.2 S02 capture by sorbent addition to the fuel bed
3.2.3 S02e bytn above the fuel bed 22 VIII
Page No.
3.3 Techniques for the Control of S02 Emissions 22
3.3.1 Pre-combustion techniques3 Fuel switching 2 Coal cleaning
3.3.2 ln-furnace methods4 Natural retention by coal ash Sorbent addition to the fuel bed 25 Review of sorbent addition to the fuel bed 31 Sorbent addition above the fuel bed 3 Sorbent addition in-duct 34
3.3.3 Effect of S02 control techniques on emissions of other pollutants 3 Nitrogen oxides Particulate emissions5 Emissions of carbon monoxide and hydrocarbons 36
4.1 Study of On-Grate Sorbent Addition8
4.1.1 Strategy for the investigation
4.1.2 Selection of fuels and sorbents9
4.1.3n of operating conditions 41
4.2 Study of In-Furnace Sorbent Addition
4.2.1 Strategy for its investigation2
4.2.2 Selection of fuels and sorbents
4.2.3n of operating conditions3
5.1 Objectives and Scope 4
5.2 Reaction Between Sulphur Bearing Gases and Limestone
5.3 Thermodynamics of Reactions Between Limestone and Sulphur Bearing Gases
in a Stoker Fuel Bed5
5.3.1 The devolatilisation zone6
5.3.2 The combustion zone7
5.3.3 The cooling zone8
5.3.4 Destruction of the sulphate reaction product 4
5.3.5 Effect of fuel bed cooling techniques9 Flue gas recirculation 42 Water or steam addition 50 Staged combustion
5.4 Discussion 51