Brown coal properties

In 1963 a classification of coals by rank (differing from the ECE scheme) was pubUshed by the International Committee for Coal Petrology (Table 2) (9). This includes a classification of brown coal that correlates a number of important properties including the percent reflectance of vitrinite in the coal. This is a simpler version of that used in German practice, which further subdivides soft brown coals into foHaceous and earthy. Most brown coals belong to the latter group. 

Because of the wide variation in composition and properties of brown coal (see Table 3), efficient combustion of these fuels caimot be accomphshed by a single system. The moisture content limits combustion efficiency because some chemical energy is required to convert Hquid water to steam in the flue gases. The steam then increases the dew point of the gases, requiring higher temperatures to avoid condensation in the stack. For fuels up to 25% moisture content, 80% efficiency can be achieved. As the moisture content increases to 60%, the efficiency decreases to 70% and efficiency continues to decline about another 1% for each additional 1% moisture to 70%. 

R. A. Durie, ed.. The Science of Victorian Brown Coal Structure, Properties and Consequencesfor Utilisation, Butterworth Heinemann, Oxford, 1991. An excellent reference not only for Victorim Brown Coal, but for lignitic coals of the world. 

In some blackpowder type mixtures, especially those for mining purposes, charcoal is partially or wholly replaced by carbon black, brown coal, pitch from coal tar, coal tar itself, naphthalene, paraffin, wood bark, cellulose or wood meal, starch, resin etc. Thus mixtures are obtained with properties similar to blackpowder. Generally, however, they bum more slowly and are more difficult to ignite. 

The development and adaptation of modern analytical techniques for analysis of Victorian brown coal was pioneered jointly in the 1960 s by the Commonwealth Scientific and Industrial Research Organization and the State Electricity Commission of Victoria. As a result, the total coal analysis time was halved and the determination of the ash forming constituents directly on the coal took one sixth of the time of conventional ash analysis. More importantly brown coal analysis was put onto a rational basis taking its unique properties into account, thereby providing more pertinent information concerning the genesis, occurrence and use of Victorian brown coal. 

In addition to the variation of chemical properties within coal seams, significant variation also occurs between different coal fields in Victoria. An extensive research program in which this variation was investigated has been conducted by the State Electricity Commission of Victoria on behalf of the Victorian Brown Coal Council. 

As a result of the Brown Coal Evaluation Program, the understanding of the variability of Victorian brown coals and its implications for utilization have improved substantially. It has become apparent that certain chemical properties can have important consequences for utilization of the coal for power generation, liquefaction and other applications. 

Correlations Between Petrographical Properties, Chemical Structure, and Technological Behavior of Rhenish Brown Coal... 

Subsequent to the petrographical coal analysis, both a chemical and a chemo-physical investigation were carried out. Figure 7 shows the chemical and physical properties of the investigated brown coal lithotypes. 

These investigations on correlations established between the raw material properties and briquettability of Rhenish brown coal led to the following results ... 

Numerous publications (10 to 12) have appeared, principally from the GDR, on the required quality properties of brown coal and their influence on the quality characteristics of formed coke. Since the Rheinische Braunkohlenwerke AG is not engaged in formed coke production at present, raw material quality and coking behaviour are of interest only for the production of fine coke using the rotary hearth furnace (13> 14). 

For the purpose of an assessment with a view to refining, the petrographical, chemical and physical properties of lithotypes of Rhenish brown coal were established and compared with one another. 

For the gasification process no usable quantitative relations were established between the petrographical coal properties and the gasification behaviour. It is possible without any material problems to convert nearly all the Rhenish brown coal types into gaseous hydrocarbons or synthesis gas. 

The membrane wall s life was estimated to be over 25 years. The process can handle a wide variety of solid feedstocks, including lignite, brown coal, subbituminous coal, bituminous coal, anthracite, and petroleum coke. This gasifier can handle different coal types and is relatively insensitive to the size, condition, or other physical properties of the raw coal. 

The slagging and fouling tendencies of coals largely differ according to the deposit but also differ within the deposit because of widely varying coal composition. Ash properties are so complex that evaluation of coal performance is extremely difficult. The performance of coals in furnaces are further complicated by the processes which control deposition on heat transfer surfaces in boilers. Based on these complications, we must neither deceive ourselves as to the possibility of reliable estimates nor deny the possibility of making a useful estimate of the performance of brown coal in boilers. 

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ordering of the crystallographic structure of carbon, which is of significant importance in the case of modification of carbon deposits contained in the carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure. Thus the surface properties of hydrothermally modified complex adsorbents are defined by the course of three processes ... 

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