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What is pulverised fuel

characterisation?

A Combustion File downloaded from the IFRF Online Combustion Handbook

ISSN 1607-9116Combustion File No: 48

Version No: 1

Date: 05-Nov-01

Author(s): Peter Roberts, Jenö Kovács

Source(s): Authors

Referee(s): Neil Fricker, Mikko Hupa

Status: Approved

Sponsor: IFRF

1. BackgroundThe phrase “Fuel Characterisation” can mean many different things to different people.This is not only due to the differences between scientific approaches and practical

requirements, but also due to the widely differing demands that can be placed on the

science and art of fuel characterisation.

Perhaps the most memorable (at least to first named author) view was that of a powercompany engineer who was concerned with imported coals for pulverised fuel (pf) firing.

He indicated that what he needed was:

•

For somebody to develop a “black box” analyser into which he could place a fewgrams sample of a coal with an attractively low price which would typically besituated in a ship, already on the high seas.

• The black box would analyse the coal and would quickly give him a print out from

which he would be able to estimate the impact upon all aspects of his boileroperation, that would result from adding that coal, in various proportions, to thepulverised coal blend presently fired in his particular boiler.

That indeed is the comprehensive view, as expressed by someone in industry, who must

face the day-to-day requirements of production, equipment operation and cost control.

It is the task of the combustion scientist to provide the “tools” with which to achievethis goal.

The IFRF Combustion Handbook aims to clarify the options for pulverised (solid) fuelcharacterisation in a cluster of linked Combustion Files. These will be updated from time

to time as industry requirements change and analytical capabilities advance.

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2. ConceptsBased on the background stated above:

• The object of fuel characterisation is to use laboratory analysis techniques to give

data from which to assess a fuel’s performance on full-scale industrial plant.

• The way in which a fuel is characterised is dependent on the industrial heatingprocess in which it is to be used, and upon the specific combustion equipment

employed.

• “Overall fuel characterisation” for the firing of a given industrial heating processcan comprise a number of “sub-characterisations”.

• Sub-characterisations generally incorporate emulation of a sub-process of the

overall industrial heating process. The experimental technique employed, generates

data from which it is possible at least qualitatively to assess change in performance,and possibly to predict performance quantitatively.

• Generally there is a trade-off between the accuracy of performance prediction and

value (to the plant operator), and:o The simplicity of the sub-characterisation technique;

o The speed with which data can be gained;

o The cost of generating the fuel characterisation data.

• Human experience regularly plays an essential role in fuel characterisation processprocess.

3. An example

From the foregoing paragraphs, it is clear that it is difficult to generalise fuelcharacterisation. Therefore it is proposed to develop the theme by use of an example.

As an example, the concept of adding a new coal to an existing blend in a pf fired power

station boiler is chosen to demonstrate the characterisation techniques associated with

sub-processes; summarised below in Table 1.

Table 1. An example of combustion sub-processes and associated

characterisation techniques.

Equipment Sub-process Impact assessment

requirement

Example of available

characterisation

techniques

Coal fields layoutFuels handlingsystems

Mixing/blending

System dependent

How will the addition of thenew coal affect theseprocesses?

Not required

Coal Mills Milling of solid fuels

System dependent

How will the addition of thenew coal affect the milling

process?

Grindability Index -Hardgrove Grindability

Index

Mill to burnerbelt-burnertransport

Fuel distribution.

System dependent

Will new coal modifydistribution characteristics?

Particle size distribution analysis

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transport

Burner

design/arrays

Ignition/stability.

System dependent

Will new coal modify

ignition/flame stabilitycharacteristics?

Drop-Tube Furnace (e.g.

IPFR generating “ignition”data)

Furnace/RadiantSection

Flame - combustioncharacteristics

How will the new coal affectthe general flame

characteristics?

Proximate Analysis,Ultimate Analysis, TGA,

Maceral analysisIPFR generating hightemperature volatile release

data


Flame to water wallheat transfercharacteristics

Will new coal modify flameradiation characteristics?[Effect on steam raising

performance]

Proximate -, UltimateAnalysis

Flame testing on combustion

rig


Conduction of heatfrom external surfaceto water/steam surface

Will new coal modify ashcharacteristics to promoteslagging in and above the

burner belt? [Effect steamraising characteristics]

Slagging Index

Super-heaters Conduction of heatfrom external surfaceto steam surface

Will new coal modify ashcharacteristics to promoteslagging on the super-heaters?

Will soot-blower efficiency beaffected?

Slagging Indices

Convection Banks Conduction of heat

from external surfacesto internal surfaces

Will new coal modify ash

characteristics to promotefouling on the convection banksections?

Fouling Index

Electro-Static

Precipitators/BagHouses

Filtering of fly ash Will new coal modify ash and

flue gas characteristics togive deterioration of fly ash

separation?

Heavy Metals analyses

Flue GasDesulphurisers

Flue gasdesulphurisation

Will new coal affect flue gascharacteristics to givedeterioration of this process?

Proximate -, UltimateAnalysis

NOx control CombustionModificationTechnology

Flue Gas deNOxing

Will new coal modify NOxemissions so as to affect NOxcontrol performance?

Ultimate analysis, IPFRgenerating N partitioningdata

The sub-processes themselves are identified in Figure 1, which illustrates a conceptualwall fired pf boiler and it’s associated equipment. This concept can apply equally well to

other types of pf-fired boiler.

In Table 1, only the effect of the new coal on the boiler and the ancillary equipment is

considered. No account of the effect of the new coal upon by-product quality and itssaleability and/or disposability is made. In the case of fly ash this is an essential

consideration. Fly ash is a by-product, typically with a market in the cementmanufacturing and other related industries, which forms a significant part of the total

economic picture. The carbon in ash level of the separated ash limits saleability. Tests

such as:

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• Macerals Analysis – Maceral Analysis can give information from which reactivity can

be estimated;

• Drop tube furnace analysis (e.g. The IFRF Isothermal Plug Flow Reactor - IPFR

generating char combustion rate data) can assist in the prediction of this factor.

Figure 1: Sub-processes associated with blended coal firing in a pulverised coal

fired boiler

4. Levels of CharacterisationThere are two levels of characterisation.

4.1 Basic Pulverised Fuel CharacterisationThere is a series of tests, which have been entered in the glossary terms in the glossarydatabase, for example, Proximate Analysis, which in this Combustion File, are linked

mainly from Table 1.

These are “traditional” techniques, which can be performed rapidly with relatively small

sample sizes. These analysis results give limited, but nevertheless valuable information.These can be described as “Basic” fuel characterisation techniques. The listing used

here is:

• Fouling index

• Fuel

• Grindability index

• Hardgrove grindability index

• Heavy metals analysis

• Particle size distribution.

• Macerals.

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• Maceral Analysis

• Proximate analysis

• Slagging index

• ThermoGravimetric Analysis

• Ultimate analysis Examples of analyses are given in: CF 24 CF 120

4.2 Advanced Pulverised Fuel CharacterisationThere are a further series of analysis techniques, some of which are identified in

Table 1, which go deeper into the characterisation process.

An example of these techniques is the IFRF Research Station, Isothermal Plug Flow

Reactor – IPFR, in which the pulverised coal particles experience a time/temperaturehistory more like that, to which they would be subjected in an actual boiler. This is an

example of an “Advanced” fuel characterisation technique.Another example is “flame testing in combustion rigs”, that is testing of fuels at a semi-industrial or pilot scale using typically scaled down industrial burners in experimental

furnaces designed to simulate the actual process conditions under which the fuels will befired.

There are other examples of Advanced Pulverised Fuel Characterisation techniques,which will be introduced in separate Combustion Files in due course.

4.3 Further development of the themeIt is the intention to extend this cluster of Combustion Files with clusters of more

detailed contributions, which will:• Exemplify the various fuel characterisation techniques available for different

fuels applied to different industrial heating processes, and;

• Presents data files tabulating fuel characteristics for fossil, biomass (Biofuels)and waste fuels (e.g. RDF-Refuse Derived Fuels).

Glossary termsFouling index – An index, which can be calculated from the ash analysis and which, gives

an indication of the propensity for that coal to cause fouling problems duringcombustion.

Fuel - A fuel is the generic term describing the material - solid, liquid, gaseous, emulsionetc. - which contains the carbon and/or hydrogen consumed in the industrial combustion

process.

Grindability index - Indicates the ease of pulverizing a coal in comparison to a

reference coal. This index is helpful in estimating mill capacity. The two most commonmethods for determining this index are the Hardgrove Grindability Method and Ball Mill

Grindability Method. Coals with a low index are more difficult to pulverize.

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Hardgrove grindability index - Indicates the relative grindability or ease of

pulverisation of a coal in comparison to coals chosen as standards. High values indicate a

coal easy to pulverise and low values indicate a coal hard to pulverise. Hardgrovegrindability index is rank dependent and increases as does rank, although anthracites

have low Hardgrove grindability indices.

Heavy metals analysis - Analysis that gives the heavy metal composition of the fuel.

Isothermal Plug Flow Reactor – IPFR – An IFRF Research Station pulverised fuel

characterisation facility in which the pulverised coal particles experience atime/temperature history more like that, to which they would be subjected in an actual

boiler.

Macerals – Macerals are the microscopically recognisable individual organic constituentsof coal. There are three maceral groups – Exinite, Inertinite and Vitrinite.

Maceral Analysis – Obtained by the microscopic examination of coal and is a volumetric

distribution of macerals in a coal sample. The maceral analysis is an important parameter

because vitrinite and inertinite are more reactive in both combustion and coke makingterms than other species in coal.

Particle size distribution – Analysis of a milled material product, using a differential

sieving technique (wet or dry), to produce information of the mass fraction of product

lying in a range of size ranges.

Proximate analysis – Analysis that gives the moisture, ash, volatile matter and (by

difference) fixed carbon contents of the fuel.

Slagging index – An index which can be calculated from the ash analysis and which gives

an indication of the propensity for that coal to cause slagging problems during

combustion.

TGA – Abbreviation - Thermogravimetric analysis - Instrumental technique thatmeasures the weight of a sample and how this weight diminishes as the sample reacts.

The sample temperature is controlled.

Ultimate analysis - Analysis that gives the elemental composition of the fuel.

Keywordsash fusion, coal, fouling index, fuel characterisation, Hardgrove grindability index, heavy

metal analysis, isothermal plug flow reactor, proximate analysis, slagging index, solidfuel, ultimate analysis.

Related Combustion FilesCF21 What is the relationship between the higher and lower calorific value of a fuel?

CF24 How do I make a basic combustion characterisation of Biofuel?

CF105 How do I estimate the higher calorific value of biomass fuels?

CF106 What data is available on solid biomass fuel characteristics?

CF120 What information do the basic fuel characterisation techniques provide?

Sources [To be proposed by author]Authors

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AcknowledgementsNone

File Placing[Power Generation]; [Fuels]; [Characterisation]

Access Domain[Open Domain] [Members Domain] – I think open with all the other Members?

The information contained in this Combustion File is derived from the IFRF Combustion

Handbook (http://www.handbook.ifrf.net)

IFRF 1999 - 2001

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