Acid washed montmorillonite clays for improving electrostatic precipitation of dust particles
INTRODUCTION 
     Description of the Prior Art 
     A conventional way of separating dust particles from a gas stream in which the particles are entrained is by the use of an electrostatic precipitator. This apparatus utilizes the corona discharge effect, i.e., the ionization of the particles by passing them through an ionization field established by a plurality of discharge electrode wires suspended in a parallel plane with a grounded collecting electrode plate. The ionized particles are attracted to the collector plate from which they may be removed by vibrating or rapping the plate. Examples of this type precipitator are found in Cummings&#39; U.S. Pat. No. 3,109,720 and Pennington U.S. Pat. No. 3,030,753. 
     Dust particles have different characteristics depending upon their source. One characteristic is resistivity which is measured in ohm-centimeters. For example, where the source of particles is a coal-fired boiler, there is usually a predictable relationship between the type of coal burned and the resistivity of the particles. Typically, low sulphur coal, i.e. less than 1% sulphur, produces particles having high resistivity, i.e. 10 +13  ohm-centimeters resistance; coal with 3-4% sulphur produces particles having 10 +8  -10 +10  ohm-cm. resistance; and, poorly combustible coal produces particles having 10 +4  -10 +5  ohm-cm. resistance. 
     It has been found that most efficient separation or precipitation of the particles occurs when their resistivity is about 10 +8  -10 +10  ohm-centimeters. When the resistivity is higher than this, the precipitation process is encumbered because the particles tend to hold their charge; particles collected on the plate in a layer tend to remain negatively charged and particles subsequently charged in the gas stream are not attracted to the plate with a resultant loss of efficiency. Conversely, when the resistivity is lower than this, the low resistivity particles lose their charge rapidly upon contact with the collector plate thereby being difficult to retain thereon; re-entrainment then occurs with a resultant loss of efficiency. However, when the particles are of the preferred resistivity, a balance is achieved between the tendency to have either overcharged or undercharged particles with a resultant increase in precipitation efficiency. Thus, the problem which existed until now was to provide a means for reducing the resistivity of high-resistivity particles and increasing the resistivity of low-resistivity particles. 
    
    
     THE INVENTION 
     The electrostatic removal of high-resistivity particles entrained in a gas stream can be improved by the addition to such gas stream of pre-selected amounts of an acid washed montmorillonite clay. The preferred acid washed montmorillonite clay is an acid washed bentonite clay. 
     Dosage 
     The amount of the acid washed montmorillonite clay that is effective in decreasing the resistivity of the dust particles may vary. Generally it is used in an amount ranging from 0.1 up to about 6 weight percent based on the weight of the particles present in the gas stream. In a preferred embodiment, the dosage ranges between 0.5-3% by weight. 
     A convenient method of dosing the acid washed montmorillonite clay is to add 45-1250 grams per metric ton of coal burned to form the gas. 
     Application Temperature 
     Most large coal-fired boilers are composed of a number of regions. These regions, starting with the combustion flame and ending with the electrostatic precipitator which, in most instances, is located prior to the exhaust gas stack, form a series of progressively cooler gas temperature zones. For purpose of simplification, these zones and their respective temperatures are set forth below in a simplified manner: 
     
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Location              Temperature                                         
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In the flame          2500° - 3500° F.                      
In the furnace-radiant section                                            
                      2000° - 2500° F.                      
After super heater    100° - 1600° F.                       
After economizer      500° - 750° F.                        
After air heater      250° - 350° F.                        
Up the stack          250° - 350° F.                        
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     The acid washed montmorillonite clay is added to the gas stream at a temperature in excess of about 250°, but, preferably, at a temperature greater than 600° F. In certain instances, they can be employed at temperatures as high as 1800° F. 
     The Acid Washed Montmorillonite Clays 
     The acid washed montmorillonite clays include the minerals, montmorillonite, hectorite, saponite, nontronite, volkonskoite, and sauconite. A preferred commercial species of this clay is an acid washed bentonite which is readily available from deposits in Wyoming. The acid washing is conducted in accordance with known methods and procedures. Any mineral acid can be used to treat the clays. As a general rule, the washing treatment is such whereby at least 20% of the alkali metal ions or other metal ions contained in the clay are exchanged for the hydrogen of the acid. 
     A typical acid-washing procedure which was used to prepare the clays hereafter tested is as follows: 
     For acid-washing the clays, 50 g. of the clay was slurried with 500 ml. of HCl at 0.1 N. After 20 minutes of mechanical stirring, the sample was vacuum filtered. The 500 ml. of 0.1 N acid provides enough protons to exchange with all of the cations on 50 g. of our nominally most active clay. 
     The clays after or before acid washing are preferably commuted by grinding or other methods to render them in the form of a fine powder having a particle size less than 10 microns. They are preferably applied as an aqueous slurry although the finely divided powdered acid washed montmorillonites may be sprayed into the hot gas stream as a powder. 
     Evaluation of the Invention 
     To evaluate the effectiveness of the treatment chemical as a gas treating aid to improve electrostatic precipitator performance, the following test method was used. 
     ASME Power Test Code 28, which is described in the December, 1972 issue of Power Engineering in an article by W. E. Archer, was one test method utilized for determining fly ash bulk electrical resistivity. Briefly, this test entailed: 
     a. placing a treated ash sample in a conductivity cell maintained at approximately 300° F. and at about 8% humidity; 
     b. lowering an electrode onto the surface of the ash sample; 
     c. applying 2 kv/cm at a constant field to the cell and measuring current through the ash sample; 
     d. calculating the resistivity of the ash sample by relying on the voltage and current readings; 
     e. applying increased voltages to the cell while observing the current through the ash sample until electrical breakdown of the sample layer occurred; and 
     f. calculating resistivity by relying on the voltage and current readings in the range of 85-95% of the breakdown voltage. 
     The treated ash sample was prepared by slurrying the fly ash in a small amount of water, adding the treatment chemical and heating to drive off the water. The treatment level was 2% of acid washed montmorillonite clay based on the weight of the fly ash. 
     The fly ash used in the test was obtained from a midwestern utility. Its characteristics are set forth below in Table I. 
     
                       TABLE I                                                     
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                 As      Baked Down                                       
                 Received                                                 
                         With DI Water                                    
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Bulk Density (g/cm..sup.3)                                                
                    1.19      1.30                                        
Resistivity, Ambient (ohm-cm.)                                            
                   1.7 × 10.sup.10                                  
                             7.5 × 10.sup.9                         
Resistivity, 300° F.(ohm-cm.)                                      
                   1.2 × 10.sup.12                                  
                             9.3 × 10.sup.12                        
Resistivity, 300° F. moist (ohm-cm.)                               
                   9.7 × 10.sup.11                                  
                             9.3 × 10.sup.12                        
Resistivity, breakdown (ohm-cm.)                                          
                   6.0 × 10.sup.11                                  
                             3.9 × 10.sup.12                        
Dielectric Strength (kv./cm.)                                             
                   15.00     23.30                                        
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     The results of treating the fly ash with non-acid washed bentonite vs. bentonite are set forth below in Tables II and III. 
     Also in Table III is presented the resistivity reduction brought about in the fly ash by using 4% acid-washed bentonite. Greater amounts decrease resistivity in similar fashion. 
     
                       TABLE II                                                    
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Resistivity Data for Bentonite                                            
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Bulk Density (g/cm.sup.3)                                                 
                         0.85                                             
Resistivity, ambient (ohm-cm.)                                            
                         2.1 × 10.sup.7                             
Resistivity, 300° F. (ohm-cm.)                                     
                         3.9 × 10.sup.12                            
Resistivity, 300° F., moist (ohm-cm.)                              
                         3.7 × 10.sup.12                            
Resistivity, breakdown (ohm-cm.)                                          
                         1.8 × 10.sup.12                            
Dielectric Strength (kv./cm.)                                             
                         23.8                                             
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                       TABLE III                                                   
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Resistivity Data for Acid-Washed Bentonite                                
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Bulk Density (g/cm.sup.3)                                                 
                         0.75                                             
Resistivity, ambient (ohm-cm.)                                            
                         3.0 × 10.sup.7                             
Resistivity, 300° F. (ohm-cm.)                                     
                         2.2 × 10.sup.7                             
Resistivity, 300° F., moist (ohm-cm.)                              
                         3.9 × 10.sup.7                             
Resistivity, breakdown (ohm-cm.)                                          
                         1.7 × 10.sup.8                             
Dielectric Strength (kv./cm.)                                             
                         21.1                                             
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Addition of 4%/wt. of Acid-Washed Bentonite to Fly Ash                    
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Bulk Density (g./cm.sup.3)                                                
                         1.23                                             
Resistivity, ambient (Ω-cm.)                                        
                         1.5 × 10.sup.8                             
Resistivity, 300° F. (ohm-cm.)                                     
                         2.7 × 10.sup.11                            
Resistivity 300° F. moist (ohm-cm.)                                
                         2.3 × 10.sup.11                            
Resistivity, breakdown (ohm-cm.)                                          
                         7.0 × 10.sup.10                            
Dielectric Strength (kv./cm.)                                             
                         16.9                                             
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     In addition to the above experiments, other clays, both with and without acid washing, were tested at the 2% level. The results of these tests indicated little decrease in resistivity.