1. Background

The 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 power company 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 few grams sample of a coal with an attractively low price which would typically be situated 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 boiler operation, that would result from adding that coal, in various proportions, to the pulverised 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 achieve this goal.

The IFRF Combustion Handbook aims to clarify the options for pulverised (solid) fuel characterisation in a cluster of linked Combustion Files. These will be updated from time to time as industry requirements change and analytical capabilities advance.

2. Concepts

Based 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 heating process 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 process can 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:
  • The simplicity of the sub-characterisation technique;
  •  The speed with which data can be gained;
  • The cost of generating the fuel characterisation data.
• Human experience regularly plays an essential role in fuel characterisation process process.

3. An example

From the foregoing paragraphs, it is clear that it is difficult to generalise fuel characterisation. 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.

The
sub-processes themselves are identified in Figure 1, which illustrates a conceptual wall 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 its saleability 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 cement manufacturing 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:

• [GLOSS]Macerals[/GLOSS] Analysis – [GLOSS]Maceral Analysis[/GLOSS] can give information from which reactivity can be estimated;
• Drop tube furnace analysis (e.g. The IFRF Isothermal Plug Flow Reactor – [GLOSS]IPFR[/GLOSS] 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 Characterisation

There are two levels of characterisation.

4.1 Basic Pulverised Fuel Characterisation

There is a series of tests, which have been entered in the glossary terms in the glossary database, for example, [GLOSS]Proximate Analysis[/GLOSS], 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:

• [GLOSS]Fouling index[/GLOSS]
• [GLOSS]Fuel[/GLOSS]
• [GLOSS]Grindability index[/GLOSS]
• [GLOSS]Hardgrove grindability index[/GLOSS]
• [GLOSS]Heavy metals analyses[/GLOSS]
• [GLOSS]Particle size distribution[/GLOSS]
• [GLOSS]Macerals[/GLOSS]
• [GLOSS]Maceral Analysis[/GLOSS]
• [GLOSS]Proximate analysis[/GLOSS]
• [GLOSS]Slagging index[/GLOSS]
• [GLOSS]ThermoGravimetric Analysis[/GLOSS]
• [GLOSS]Ultimate analysis[/GLOSS]

Examples of analyses are given in CF24 and CF120

4.2 Advanced Pulverised Fuel Characterisation

There 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/temperature history 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 be fired.

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 theme

It 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).