Patent Publication Number: US-4059060-A

Title: Method and apparatus for coal treatment

Description:
BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates to the burning of fuel products, and more particularly is directed to rendering a combustion process in compliance with an applicable SO X  :Btu standard. The invention is particularly concerned with the upgrading of subbituminous coal and/or lignite fuel products so that they comply with such a standard. 
     At the present time, the Federal Government and many states have imposed SO X  :Btu standards for the burning of fuels. As noted in Report on Sulfur Oxide Control Technology, published in 1975 by the U.S. Department of Commerce, Commerce Technical Advisory Board, The Environmental Protection Agency (EPA) has adopted a national standard which limits SO 2  emissions by each &#34;new&#34; coal-fired (i.e., installation commenced after August 1971) stationary source to 1.2 lbs. of SO 2  per million Btu of heat input. The panel that published this report considered the following five potential near-term options for continuously meeting emissions standards of coal-fired electricity generating plants: 
     1. Use of low sulfur coal (Eastern &amp; Western) 
     2. Use of coal beneficiation (cleaning) 
     3. Use of sludge-producing flue gas desulfurization (F.G.D.) 
     4. use of coal beneficiation combined with flue gas desulfurization (F.G.D.) 
     5. use of regenerative F.G.D. 
     None of these alternatives gave serious consideration to the use of subbituminous coal and/or lignite fuel relatively high in total moisture content. In fact, as pointed out in the publication Burning the Sub-bituminous Coals of Montana and Wyoming in Large Utility Boilers, by R. J. Gray and G. F. Moore, published by The American Society of Mechnical Engineers (1975), the lower heat content of lignites and subbituminous coals &#34;creates a problem, primarily because it takes more coal for a given heat input.&#34; As also noted, because of their low calorific values, some of these coals will not meet the Federal air pollution requirement of 1.2 lbs. of SO 2  per million Btu heat input, even though they are low in sulfur content. 
     Electric utility companies have accordingly found that they are unable to use some western subbituminous coal and lignite fuel products as they are presently produced. 
     On the other hand, some electric utility companies have decided in order to use some western subbituminous coal and lignite fuel products that they will install stack gas scrubbing systems in order to comply with EPA regulations related to sulfur dioxide. 
     The present invention is directed to the use of such fuel products, and specifically to upgrading them from a condition in which they are non-complying with the applicable SO X  :Btu standard to a condition in which the product is in compliance during the combustion process. This invention provides such upgrading by treatment at the combustion site. The invention is particularly concerned with such upgrading of a fuel product without an associated high cost. 
     The present invention proceeds on the basis that an applicable SO X  :Btu standard may be met raising sufficiently the heating value of the fuel product without necessarily resorting to sulfur removal processes so that the SO X  :Btu standard will be met during the combustion process. Thus this invention primarily approaches the problem from the standpoint of raising the heating value rather than lowering the sulfur content. 
     Recognizing that total moisture is the major cause of relatively low heating values of western subbituminous coals and lignite fuel products, the present invention contemplates drying a fuel product at the combustion site to lower significantly the total moisture content of the fuel product and to concomitantly raise it sufficiently so that the SO X  :Btu standard will be met during the combustion process. Additionally, the product may be &#34;cleaned&#34; i.e., made to undergo particle weight separation so that heavier weight ash particles containing sulfur are removed. In this fashion some sulfur reduction is achieved as well as increasing the heating value of the fuel product, all to contribute toward the lowering of the SO X  :Btu ratio so that it is in compliance with the applicable emission standard during the combustion process. 
     Fuel drying has taken place at the combustion site in the past. Many utilities recognize the fact that increased pulverizer efficiencies may be obtained if the fuel product is dried prior to introduction to the pulverizer. To this extent, fuel drying is presently being practiced by many utilities. However, there is no appreciation of the fact that a non-compliant fuel product (non-complying with applicable SO X  :Btu standards) may be upgraded so that it is a compliant fuel product by the use of fuel drying. Kreisinger et al U.S. Pat. No. 1,608,699 is typical of a prior art system utilizing drying ahead of a pulverizer to increase pulverizer efficiency. A system such as shown in that patent would be suitable for the processing of subbituminous coal and lignite fuel products that need only be dried in order to meet the applicable SO X  :Btu standard. Kreisinger, however, does not provide for cleaning. 
     In the practice of the present invention, waste heat from the combustion process itself may be utilized in a recycling thereof for the purpose of carrying out the drying operation. Additionally, since the drying of the fuel product increases the efficiency of the pulverizing unit, advantage may be taken of this increased efficiency by utilizing pulverization at different stages during the treatment process, for example, providing an initial pulverizing operation, followed by a separation of heavier weight sulfur-containing ash particles, followed by a final pulverizing operation. 
     The invention will be more completely understood by reference to the following detailed description, when taken in conjunction with the appended drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 to 3 illustrate, in block diagram form, the practice of the invention. 
    
    
     DETAILED DESCRIPTION 
     Before a detailed description of the invention is given, it will be best to consider some various relationships regarding coal and the burning thereof. As noted above, emission standards for SO X  emissions are expressed in terms of pounds of SO 2  per million Btu of heat input. The present national standard adopted by the EPA is that SO 2  emissions may not exceed 1.2 lbs. of SO 2  per million Btu of heat input. This relationship is expressed as follows: ##EQU1## 
     It is well known that the pounds of sulfur dioxide emitted from a fuel product may be approximated from the percentage of sulfur in the dry product as follows: 
     
         lbs. SO.sub.2 = S% × 10.sup.-2 × 2             1b 
    
     In equation (1b) the sulfur percentage, S%, is in terms of the dry product, as is conventional. 
     Relation (1a) may accordingly be rewritten as follows by substitution from equation (1b): ##EQU2## 
     Coal is normally defined in terms of its heating value, namely, the Btus of heat input per pound of coal. Thus, the factor &#34;million Btu&#34; in relation (1c) may be expressed in terms of the actual heating value, and relation (1c) may be rewritten as follows: ##EQU3## 
     The terms in relation (1d) may be rearranged to provide the following relation: 
     
         S% ≦ heating value x emission standard × 5 × 10.sup.-5 (1e) 
    
     In relation (1e), the percent sulfur is expressed as a dry product percentage. Relation (1e) thus indicates the upper limit of that sulfur percentage in a fuel product in order to comply with applicable emission standards. 
     Another relationship that is important is as follows: ##EQU4## 
     Equation (2) defines the well known relationship between heating values of the same basic fuel product with different moisture and ash contents. In equation (2), the moisture and ash content is expressed as a fraction or as a decimal. It is apparent from equation (2) that heating value varies directly with the moisture and ash content, and for each percentage decrease of moisture and ash content the heating value will increase by a corresponding percent. 
     Equation (2) may be rewritten as follows: ##EQU5## 
     Equation (3) expresses the heating value.sub.(2) of a particular fuel product having a moisture and ash percentage.sub.(2) in terms of the heating value.sub.(1) of the same fuel product and of a corresponding moisture and ash percentage.sub.(1). 
     Relation (1e) may thus be rewritten in terms of equation (3) as follows: ##EQU6## In relation (4) the left-hand portion thereof is a ratio that is termed herein the &#34;Sulfur Factor&#34;. Thus the Sulfur Factor is the ratio of percent sulfur in a fuel product to the percent of non-moisture and ash components of that fuel product (100% less the moisture and ash percentage). AS noted above, the sulfur percentage is expressed on a &#34;dry product&#34; basis, and hence does not vary. The moisture and ash percentage, on the other hand, is on an &#34;as received&#34; or overall product basis, and hence may vary as the fuel product is treated. Relation (4) indicates that the Sulfur Factor must be less than a value dependent upon the initial heating value, the initial non-moisture and ash percentage in that fuel product and the emission standard. The initial heating value, i.e., the heating value.sub.(1) appearing in the right-hand portion of relation (4) is typically the &#34;as received&#34; heating value of the fuel product, e.g., as received by a utility and including the usual moisture and ash content accompanying that product. 
     It will be noted, that for any &#34;as received&#34; fuel product, the right-hand portion of relation (4) is constant. A portion of that constant is known as the moisture and ash free heating value of the fuel product. Specifically, the heating value.sub.(1) divided by the non-moisture and ash content [100% -- moisture and ash%.sub.(1) ] represents the heating value of the fuel product on a moisture and ash free basis. That moisture and ash free heating value is thus multiplied by a constant, including the applicable emission standard, to produce a constant factor representing the right-hand portion of relation (4). 
     In other words, relation (4) may be expressed as ##EQU7## in which the heating value MAF  is the moisture and ash free heating value of the fuel product and K is a constant dependent upon the applicable SO X  :Btu emission standard. 
     Relation (5) is useful in examining any &#34;as received&#34; fuel product and determining the upper limit of the Sulfur Factor. In particular, consider the following Table I which characterizes two representative fuel products, namely, Wyoming Powder River Coal Basin (Anderson Seam) subbituminous coal and North Dakota lignite. The data for the table were taken from the Gray and Moore publication cited above, page 2, and Review of Wyoming Coalfields, 1975, by Gary B. Glass and published by Geological Survey of Wyoming, page 9. 
     
                                           TABLE I                                 
__________________________________________________________________________
                          Wyoming Powder River Coal                       
                          Basin (Anderson Seam)                           
                          Subbituminous Coal                              
                                         North Dakota Lignite             
__________________________________________________________________________
Moisture (Total)          29.5%          37.6%                            
Volatile Matter, fixed                                                    
 carbon &amp; ash             70.5%          62.4%                            
 Sulfur (dry basis)       0.52%          0.8%                             
 Ash                      6.5%           7.0% (11.2% dry)                 
Moisture &amp; Ash (as received)                                              
                          36%            44.6%                            
Non-Moisture &amp; Ash (as received)                                          
                          64%            55.4%                            
Heating VAlue (Btu/lg.)                                                   
as received               7,979          6,664                            
 &#34;                                                                        
dry           11,300      10,680                                          
 41                                                                       
moisture &amp; ash free                                                       
              12,450      12,050                                          
Sulfur Factor.sub.max*    0.00747        0.00723                          
lbs. SO.sub.2 /million Btu (as received)                                  
                          1.30           2.40                             
__________________________________________________________________________
 *for SO.sub.x :Btu less than 1.2 lbs. SO.sub.2 /million But as an emissio
 standard                                                                 
 
    
     Each of the fuels is characterized by a total moisture percentage, with the remaining percentage of product being constituted by volatile matter, fixed carbon, and ash. The sulfur content is given as a percentage on a dry basis, while the ash content is given as a percentage of the total product. The moisture and ash in the &#34;as received&#34; product is given as a percentage figure, as well as the non-moisture and ash portion. The &#34;as received&#34; heating value is specified, as well as the computed heating values of the dry product (considered as if total moisture were completely removed) and the moisture and ash free product. Based upon the moisture and ash free heating value [see relation (5)], the maximum permissible Sulfur Factor is designated. The figures developed are for an SO 2  :Btu emission standard of no more than 1.2 lbs. of SO 2  per million Btu of heat input. In this case, the value of k from relation (5) is equal to 6 × 10 -7 . 
     TAble I also specifies, for the &#34;as received&#34; fuel product, the computed ratio of pounds of SO 2  per million Btu. It will be noted that the ratio is 1.30 for the Wyoming subbituminous coal and 2.40 for the North Dakota lignite, both are non-compliant with the presently existing EPA emission standard of 1.20. Wyoming subbituminous coal may be treated prior to the combustion process, at the combustion site, by undergoing a drying process in which the total moisture is reduced to about 5%. Reductions below about 5% total moisture are not desirable because of the dust condition that is created, producing a highly explosive product. Table 2 tabulates the results of the treatment. 
     
                                           TABLE 2                                 
__________________________________________________________________________
                 Wyoming Powder River Coal                                
                 Basin (Anderson Seam)                                    
                 Subbituminous Coal                                       
                                North Dakota Lignite                      
__________________________________________________________________________
Moisture (total) 5%             5%    5%                                  
Volatile matter, fixed                                                    
 carbon &amp; ash    95%            95%   95%                                 
 Sulfur (dry basis)                                                       
                 0.52%          0.8%  0.6%                                
 Ash             8.8%           10.7% 5%                                  
Non-Moisture &amp; Ash%                                                       
                 86.2%          84.3% 90%                                 
Sulfur Factor    0.006          0.0095                                    
                                      0.0067                              
Heating value (Btu/lb.)                                                   
                 10,750         10,150                                    
                                      10,850                              
lbs. SO.sub.2 /million But (as treated)                                   
                 0.97           1.57  1.10                                
__________________________________________________________________________
 
    
     In Table 2, the total moisture is indicated as being reduced to 5%. The remainder, namely, volatile matter, fixed carbon, and ash, constitutes 95%. The sulfur percentage of the product does not change, and remains at 0.52% for the Wyoming subbituminous coal. The ash percentage increases as a percentage of the total product from 6.5% (Table I) to 8.8% (by virtue of the reduction in total moisture). The non-moisture and ash percent is at 86.2%, and the Sulfur Factor is computed from relation (5) to be 0.006. Since the maximum value of that sulfur factor from Table I is 0.00747, it is apparent that this moisture removal treatment of the Wyoming subbituminous coal is sufficient to render that coal product in compliance with the applicable EPA emission standard. Note also from Table 2 that the computed heating value for the coal product following treatment is elevated to 10,750 Btu per pound, significantly higher than the heating value of 7,979 Btu per pound in the &#34;as received&#34; condition. The pounds of SO.sub. 2 emitted for each million Btu heat generated has been computed to be 0.97 for this Wyoming subbituminous coal following treatment to reduce the total moisture to 5%. This is well within the upper limit of 1.2 constituting the EPA emission standard. 
     This Wyoming subbituminous coal is an example of a fuel product which may be rendered in compliance with the applicable EPA emission standard simply by reducing the total moisture content of the fuel product. Many other fuel products cannot be upgraded simply by moisture reduction. The North Dakota lignite is an example of such a fuel product. Referring to Table 2, the first column of figures for North Dakota lignite, assume that the total moisture content is reduced to 5%. The sulfur percentage, on a dry basis, remains at 0.8%. The ash percentage increases to 10.7% from the 7.0 percentage in the &#34;as received&#34; condition as noted in Table I. The non-moisture and ash percentage of this product is 84.3%. The Sulfur Factor computes to be 0.0095, and hence this product is still non-compliant with the EPA emission standard requiring a Sulfur Factor of no more than 0.00723 as noted in Table I. This is confirmed by the computed SO 2  :Btu ratio of 1.57 which exceeds the EPA limit of 1.2 pounds of SO 2  per million Btu. 
     Assume, therefore, that the North Dakota lignite product is &#34;cleaned&#34; to remove heavier weight ash particles in addition to the reduction of total moisture to 5%. In Table 2 it is assumed, from the second column of figures under North Dakota lignite, that the ash percentage is reduced to 5%. Because pyritic sulfur will be removed along with the ash, it is assumed that the sulfur percentage (on a dry basis) in the fuel product is lowered to 0.6%. The non-moisture and ash percentage of the fuel product is thus 90%, and the Sulfur Factor computes to be 0.0067. This Sulfur Factor is less than the upper limit of 0.00723 (from Table I), and hence the North Dakota lignite fuel product is rendered in compliance with the applicable EPA emission standard by this moisture reduction and ash removal treatment. The heating value of the treated product computes to be 10,850 Btu per pound, and the SO 2  emission per million Btu computes to be 1.10, less than the upper limit of 1.2 constituting the EPA standard. 
     Refer now to FIG. 1 of the drawing, which shows in block diagram form a procedure at the combustion site for upgrading a fuel product so that the burning thereof will be in compliance with an applicable SO x  :Btu emission standard. Coal, typically in particle form, is applied to a drying unit 10 which is conventional. A number of suitable coal drying units are referred to in Coal Preparation, published by the American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. (New York 1968), edited by Joseph W. Leonard and David R. Mitchell. Following the drying step, the coal is applied to a conventional pulverizing unit 12 within which it is pulverized and thereafter burned in a boiler. Waste heat from the combustion process (not stack gas containing sulfur) may be utilized in a recycling operation to assist in the pulverizing and drying operations. The arrangement shown in FIG. 1 is suitable for the handling of coal such as the Wyoming Powder River Basin subbituminous coal discussed above in connection with Tables I and 2. Such subbituminous coal may be upgraded so that it is in compliance with the applicable EPA emission standard simply by undergoing drying, as noted. 
     If a coal product such as North Dakota lignite referred to in Tables I and 2 above is to be burned, then a system as shown in FIG. 2 is required. In this case, pulverized coal is applied to a drying and separating unit 14. Such a unit may constitute an &#34;airflow cleaner&#34; as shown and described on pages 11-6 to 11-8 in Coal Preparation cited above. In such a unit, not only is the coal dried, but heavier weight particles of coal are separated and removed. Thus, heavier weight ash particles containing pyritic sulfur (as distinguished from inherent sulfur) are removed. The product that emerges from the drying and separating unit 14 thus has its moisture content significantly reduced as well as its sulfur content, both sufficiently so that the product is in compliance with the applicable EPA emission standard. The product is applied to a pulverizing unit 16 and thence is burned. Again, waste heat recycling can be utilized for the drying and separating operation as well as the pulverizing operation. 
     FIG. 3 shows another form of system suitable for the treatment of coal to upgrade it, in which the product is initially dried in a dryer 18. Following drying, the coal product is then pulverized. The initial drying of the product increases the efficiency of the pulverizer and improves its operation, as is well known. Following pulverization in the pulverizer 20, the product is applied to a separator 22 which separates out heavier weight ash particles containing pyritic sulfur. The product from the separator is applied to a final pulverizing unit 24, from which it is applied to a furnace for burning. As in the other arrangements described, waste heat recycling may be employed in the operation of the various units. With an arrangement such as shown in FIG. 3, it is possible to utilize a lesser pulverizing capacity in the entire system as might otherwise be required. By separating the pulverizing operations into initial and final pulverizing, the final pulverizer 24 has less of a product to handle than the initial pulverizer 20 in view of the separation out of the heavier weight ash particles. By utilizing some pulverizing following separation, less of a total pulverizer capacity is required than if all pulverizing were done prior to the separating step. 
     By upgrading fuel products as noted, significant cost savings can be realized. In particular, it is possible to install drying and separating units below the coal bunker now found in an electric utility generating station, for example. Little extra space is required for the installation of such units. Additionally, expensive stack gas scrubbing units are completely avoided. 
     The present invention thus approaches the problem of air pollution from the standpoint of upgrading the heating value of the fuel product rather than from the more expensive flue gas treatment proposed by others in the field. It is apparent that the presently preferred embodiments described above are subject to modification. Accordingly, the invention should be taken to be defined by the following claims.