FIELD OF THE INVENTION
This invention relates to a method for the classification of coal or blend of coals. This invention further relates to a method for the classification of coal or blend of coals based on sintering characteristics of coal ash for power plant applications. In pulverized coal combustion, slagging and ash deposition are a set of serious operational problems associated with power station boilers. Numerous parameters are used for judging the quality of coal with respect to slagging and ash deposition. Most of the existing coal classification, based on slagging indices derived from chemical analysis of ash. The indices established for particular type of coals are not necessarily reliable when their use is extended to other type of coals. A new method of coal classification based on ash sintering/fusion profile for predicting the quality of coal with respect to deposition is proposed.
BACKGROUND OF THE INVENTION
Coals are an important source of energy and are also used as feed stocks for other industrial process such as the production of gaseous fuels and synthesis gas. However coals are complex, heterogeneous solids that vary widely in their properties and hence in their suitability for particular applications.
Irrespective of the way coal is looked upon, there has always been a need to classify it. The nature of a classification system will depend upon the particular application for which the system is to be employed. Classifications for coal may be subdivided into those that are scientific/genetic and those that are technical or commercial, for immediate practical or applied use.
In 1826, Karsten introduced a classification of coal on the basis of the composition of the residue left when coal is heated; he made a distinction between "Sand Kohle" (Sand Coal), "Sinter Kohle" and "Backende Kohle" (Caking coal).

In the USA, Fraser (1877), following the work of Rogers (1858), devised a system based on the so-called fuel ratio - the ratio between carbon residue and the volatile matter content.
The terminology in this system is still being used in America. In 1928, Parr elaborated a suggestion by Campbell (1906) that not only the content of volatile matter, or the fixed carbon content, but also the calorific value should be used as a classification parameter. This suggestion has resulted in the US standardized coal classification system.
In Great Britain, Seyler published a system in 1899 which, after being extended and refined in 1931, and again in 1938, may still be regarded a masterpiece of scientific coal classification. Seyler's system is entirely based on the elementary composition of coal, which may be the reason why it has never met with ready acceptance in commercial and technical circles.
Around 1980, B. Alpern developed an international petrographic classification for coals. Alpern took the light reflectance of the main petrology of coal viz. vitrinite, as the rank parameter for all coals; this being more suitable than the volatile matter.
Most of the existing coal slagging indices use laboratory ash to measure the properties of interest [4], and since they have been established for particular types of coal properties, their use is not necessarily reliable when extended to other type of coal.
For decades, the ash fusion test (AFT) and cone melt down testing have been employed in evaluating the slagging behavior of coal ash, yet the criterion did not yield a better tool for selecting good coals.
Coal ash chemistry is also used to predict coal slagging performance. Good coals have a high silicon oxide ratio (SR), 78% and above, or a low level of iron oxide (% Fe2O3), below 6% of the ash content [1, 5]. The ratio of basic (B) to acidic (A) oxides in the ash (B/A) also assists in distinguishing between good and bad coals. Increasing the B/A of the coal decreases its fusibility and hence

increases the slagging potential. Coals having B/A  0.11 are considered as good coals.
The empirical slagging index (Rs), which has been derived by multiplying the percentage of the dry sulphur content in the coal, S, by the ratio B/A was developed based on US and European coals, where iron is typically present in iron sulphite (FeS2) as pyrite . However, in low sulphur coals, where there is strong correlation between iron and carbonate (FeCO3) as siderite, the percentage of Fe203 in the ash is considered instead of sulphur content. If the slagging index of a coal, determined using the product of B/A and dry S, is less than 0.6, then the coal is said to have a low slagging potential. Since this index is based on particular coal properties, it may give misleading results when tested on other coals. Most of the existing coal slagging indices are based on the fusibility, the viscosity or the chemistry of the ash.
Ash slagging and fouling have always been major factors with regards to boiler design and operation, particularly as power stations now generally buy coals from all over the world. Coals from the world market have huge variations in total ash content and mineral composition.
Slagging deposits are normally located in the high temperature region of boilers with direct exposure to the combustion flame. They are commonly found in the radiant section of boilers where the formation process is associated with the sintering and fusion of ash particles on surfaces, at temperatures in excess of 1000° C.
Although furnace design and operating conditions may contribute to the occurrence of slagging and fouling, ash characteristics play a major role. Ash chemical and mineral compositions determine its melting characteristics and fusion temperatures.
The slagging and fouling propensity of a particulars solid fuel can be predicted using various approaches including the ash fusion test, dilatometry/shrinkage sinter strength test, viscosity measurements, various

empirical indices as well as pilot-scale trials. Each test has advantages and disadvantages which can include cost, reliability and complexity.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose a method for the classification of coals or blend of coals. It is a further object of this invention that relates to a method for the classification of coals or blend of coals, which is based on ash sintering/shrinkage or fusion pattern for predicting the quality of coal with respect to deposition.
Another object of this invention is to propose a method for the classification of coals or blend of coals, which is simple and cost-effective. These and other objects and advantages can include cost, reliability and complexity.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 : Plot of area shrinkage percentage due to sintering of the ash pellet vs. temperature.
SUMMARY OF THE INVENTION
According to the invention, a method is developed to create a shrinkage/sintering/fusion profile of ash pellet, while heating the ash pellet from ambient to 1400°C. According the present invention, the area shrinkage pattern of the ash pellet is continuously captured as image during heating in a furnace and the reduction/expansion of area shrinkage compared to original area at ambient is plotted against temperature. From the shape of the profile related to temperature, the coals are classified into "severe deposition", high deposition, moderate deposition, less deposition and no deposition while firing the coal in pulverized-fuel system.

DETAILED DESCRIPTION OF THE INVENTION
Thus, according to this invention is provided a method for the classification of coals or blend of coals.
In accordance with this invention, the as-received solid fuels are passed through a sieve and the samples are dried to a constant weight and crushed to pass through a smaller sieve. The samples passing through this sieve are used for analyses of proximate and calorific value. From proximate analysis and calorific value estimation, ash quantity in kg per million kilo calories is to be calculated. Adequate quantity of ash of each fuel is generated as per proximate analyses at 750°C for further analyses.
For sintering and ash fusion measurement, ash pellets of predetermined shape and size are prepared by pressing of ash powder in a die without using any binder. The ash pellet is then air dried and introduced at room temperature into a furnace of the microscope. The furnace is heated to 1000°C at a constant heating rate and further heated to 1400°C with reduced heating rate.
This sintering/fusion test runs are performed in air. The images of the ash pellet are continuously captured using camera at pre-selected time intervals, during the whole heating process. This dimensional changes of the ash pellet namely height, corner angle, area and shape factor are also measured from the stored images of the specimen. The area shrinkage% = (A0 -AT) 100/A0 where A0 and AT are the projected area of the ash pellet and area after each interval of temperature "T" respectively are calculated from room temperature to final temperature of each run. The area shrinkage percentage due to sintering of the ash pellet vs. temperature is plotted for all the ash samples. (Fig. 1 & Fig. 2). The drawings are pertaining to particular samples used for the sintering/fusion test and the texts marked in the drawings are the reference details for the samples.
From the plot, the maximum shrinkage by % area reduction Rmax and the corresponding temperature related to Rmax are to be noted as shown. Since the

ash deposition in the pulverized fuel combustion system is the function of sintering/shrinkage/fusion behaviors of ash, the temperature at which the maximum shrinkage occurs and ash loading, all the above three data are used in the proposed new system of coal classification as shown in the below Table 1.

The invention will now be explained with the help of the following non-limiting
example.
EXAMPLE
The solid fuel is passed through No. 4 mesh sieve (4.75 mm). The sample is then air dried until the loss in weight is not more than 0.1% per hour. The air dried sample is again crushed to pass through No. 72 mesh sieve (212m).

The 72 mesh sample is used for analysis of proximate and calorific value. For proximate and calorific value estimation, ash quantity in kg/million kilo calories is to be calculated. Adequate quantity of ash of each fuel is generated as per proximate analysis at 750°C for further analysis. For sintering and ash fusion measurement, ash pellets (Specimen) of shape (Truncated cone of 4 mm height, diameters of 3 mm - bottom and 1.5 mm - top) were prepared by uni¬axial pressing of ash powder in a die without using any binder. The ash pellet was than air dried. The ash pellet was introduced at room temperature into a furnace of the microscope. As per the heating program, the furnace was heated to 1000°C at a constant heating rate of 10°C per min and further heated to 1400°C with reduced heating rate of 8°C per min. This sintering/fusion test runs are performed in air and the image of the ash pellet was continuously captured using camera at time intervals of 10s during the whole heating process. The dimensional changes are measured and the area shrinkage % is calculated and plotted. From the plot the maximum shrinkage by % area reduction Rmax and the temperature corresponding thereto, are noted. In this method the coals are classified into three major categories based on maximum shrinkage observed. These three categories are again classified into three sub groups based on the maximum temperature at which the maximum shrinkage is observed. The sub groups are again classified into three types of deposits based on the kilogram ash obtained for each million kilo calories generated in a boiler. If the following results are available for a coal i.e. maximum shrinkage, maximum temperature at maximum shrinkage, ash obtained for every million kilo calories of energy, then from this data the coal is classified into a type such as severe deposition, high deposition, moderate deposition, less deposition or no deposition.

We Claim:
1. A method for the classification of coal or blend of coals comprising
passing the fuel sample through a sieve and air drying the sieved sample,
crushing the air-dried sample to pass through a smaller sieve and
subjecting the same to ash formation,
forming said ash into pellets and drying the same, introducing said ash
pellet into a furnace,
heating said furnace to 1000°C at a constant heating rate and further to
1400°C at a reduced heating rate and obtaining images of the pellet at
regular intervals, measuring the dimensional changes of the pellet from
stored images of the pellet,
calculating the area shrinkage % from the formula (AO-AT) 100/AO where
Ao - the projected area of the ash pellet and
AT- the area of the ash pellet after each interval of temperature T
Plotting temperature to area shrinkage % on sintering of the ash
obtaining the Rmax, and classifying the coal based on the Rmax,
temperature and ash loading.
2. The method as claimed in claim 1, wherein the coals are classified into
any of the types such as severe deposition, high deposition, moderate
deposition, less deposition, no deposition.