Professional Analytical and Consulting Services
GAED Determination of Activated Carbon in Fly Ash from Coal Fired Electric Power Plants
Henry Nowicki, George Nowicki, Barbara Sherman
There is a need for an accurate test method to determine activated carbon in fly ash. Activated carbon can spoil the sale of a power plants fly ash because it makes poor concrete products.
This communication provides the best available technology to determine the amount and type of activated carbon in fly ash. This BAT is called the Gravimetric Rapid Pore Size Distribution (GAED) determination. A typical GAED report is provided for four client fly ash samples. Please see "Full Characterization of Fly Ash for X-169, X-170, X-171, and X-172." using the GAED. This report is page 1 of 24.
Major environmental process changes at power plants result in contamination of fly ash with adsorptive activated carbons. In order to comply with nitrogen oxides emissions reductions changes in the coal burners now make small amounts of activated carbon. In order to comply with mercury emissions activated carbon injection (ACI).
Benefits of Using Coal Ash as a Portland Cement Replacement in Concrete
The use of fly ash in concrete is the highest-volume application for fly ash. Bottom ash is also used in concrete, but at a lower rate and primarily as an aggregate. Coal Combustion By-Products (CCB) use in concrete offers several benefits, including:
- Concrete requires less water when fly ash is used in place of cement, resulting in less shrinkage and cracking.
- CCB use improves water tightness (permeability) and corrosion in concrete.
- CCBs are easier to place, pump, work, and finish.
- CCBs have increased acid and sulfate resistance.
- CCBs offer long-term strength gain.
- CCB utilization saves virgin materials, energy, and landfill costs.
- Fly ash costs $15 per ton and Portland Cement costs $115 per ton
- Fly ash use is Green Chemistry
Replacement Rates
Replacement rates of coal fly ash for cement in the production of concrete are typically 15% to 35%, although coal fly ash-blended cements may range from 0% to 40% coal fly ash by weight, according to AST International C595, for cement Types IP and I (PM). EPA suggests that 15% is a more acceptable rate when coal fly ash is used as a; partial cement replacement as an admixture in concrete.
It is important to note that many state Departments of Transportation limit the addition of fly ash to 20% without addressing the issue of optimum performance. Higher additions (40% to 60%) of fly ash can generally result in a better-performing concrete. This is not true for Class F ashes that are not self-cementitious, but pozzolanic. Generally, a 20% addition of Class F fly ash is optimum.
Future GAED Instrument Developments and Publications
Dr. Nowicki will be happy to discuss the GAED instrument with conferees at the 2008 meetings in the references below and vendors who may have an interest in its use and the commercialization of the GAED technology.
Dr. Nowicki is a member of the Editorial Review committees for Filtration News and Water Condition & Purification.
Henry Nowicki, Ph.D. directs the day to day testing and consulting services for PACS. Dr. Nowicki regularly provides a 2-day short course on the fundamentals of activated carbon adsorption. He can be contacted at Henry@pacslabs.com or phone (724) 457-6576.
Barbara Sherman, B.S. directs the day to day short courses and conference services for PACS. PACS has some 60 short courses and 8 are focused on the needs of the sorbent industry. She can be contacted at Barb@pacslabs.com or phone (724) 457-6576.
George Nowicki, B.S. is a technician in the PACS activated carbon testing laboratory.
Web site: www.pacslabs.com
References:
- Filtration News "Modern Adsorption Testing with GAED" Jan/Feb 2008
- Pittsburgh Conference New Orleans March 2008 - 5 technical papers presenting different applications on the use of GAED including determination of activated carbon in fly ash from coal burning electric power plants.
- International Activated Carbon Conference Madrid Spain differentiating types and amounts of activated carbon in fly ash from coal burning electric power plants July 1-9, 2008
- International Activated Carbon Conference Pittsburgh PA Oct 1-14, 2008 Developing products and services for the coal burning electric power plants.
- Water Conditioning & Purification July 2008 "Application for GAED to reveal the location of chemical impregnants in activated carbons."
- Water Conditioning & Purification Aug 2008 "Application for GAED to differentiate unused and used activated carbon materials."
- Filtration News "GAED adds BET surface area and Trace Capacity Numbers for sorbent analysis in addition to standard isotherms."
Full Characterization of Fly Ash AES Beaver Valley samples
X-169, X-170, X-171, X-172 and X-173 using the GAED
PACS
October 7, 2007
Executive Summary
Five samples of fine mesh Fly Ash from AES Beaver Valley were fully characterized with GAED:
9-12-07 1440 hrs - (X-169)
9-13-07 1700 hrs - (X-170)
9-14-07 Reburn Ash - (X-171)
8-20-07 1535 hrs - (X-172)
8-22-07 1745 hrs - (X-173)
In order to show a more complete comparison of all five samples as well as the reference samples, two sets of figures, tables and graphs are contained in this report. The reference samples are two commercially available activated carbon samples:
CAL Coal-based Liquid phase and PCB coconut-based.
None of the samples lost more than 0.5% by weight on conditioning (heating to 240°C in argon and holding for 25 minutes) indicating all samples were exceptionally clean and dry. (Data Summary Table Appendix A). All samples were relatively low in adsorption potential. However, sample 9-14-07 Reburn Ash (X-171) and 8-20-07 1535 hrs (X-172) had 3 to 4 times more activity than the other Fly Ash samples but were only a fraction of the adsorption potential of the reference samples. For the application performance types, these same two samples did fairly well, but only in the type V, Trace Loading applications did they have about half of the adsorption potential of the reference carbons. The calculated BET surface area may be the quickest demonstration of the ranking of the samples. They are fairly low but relative to each other in that the order of activity is obvious (Data Summary Table Appendix A).
GAED Results:
The X-169 through X-173 samples were characterized by measuring the entire characteristic curve using the GAED.
The samples were labeled 9-12-07 1440 hrs (X-169), 9-13-07 1700 hrs (X-170), 9-14-07 Reburn Ash (X-171), 8-20-07 1535 hrs (X-172) and 8-22-07 1745 hrs (X-173). No other information was available.
These samples were then compared to two standard commercially activated carbon products made from a range of raw materials.
The samples were run in fine powder form. A summary of the actual test data and conditions used is listed in the data summary table at the end of the report in Appendix A. All five samples lost less than 0.5 weight percent on conditioning (heating to 240°C in argon and holding for 25 minutes). Losses of less than 8 percent indicate a well-stored sample that has been protected from the small amount of moisture pick-up from ambient air during handling and storage and was also fresh and not oxidized. All activities and adsorption capacities are calculated on a clean carbon basis. To observe these capacities in the field may require additional processing of the carbon on site.
The GAED runs were typical. The difference between the adsorption and desorption curves was minor throughout the experiment, therefore there was no hysteresis present, as is normal for commercially activated carbons. This report extends the comparison of these five Fly Ashes beyond just the presentation of the characteristic curves. The plots of the differential and cumulative characteristic curve data are presented in Figures 1 and 1b in a weight-based comparison. The specific run data and results are attached as Appendix A.
GAED Raw Data
The GAED (gravimetric adsorption energy distribution method) measures over 500 adsorption and desorption data points covering seven orders of magnitude in relative pressure (isothermal basis) and three orders of magnitude in carbon loading. The mass adsorbed was also divided by the carbon mass to generate a weight percent loading for easier comparison. The raw data is plotted in Figure 2. At 240°C the adsorbent gas, C134a or 1,1,1,2-tetrafluoroethane, is introduced and the loading increases. Note in Figure 2, the mass loading was plotted against temperature but the relative pressure was also changing. There are three variables affecting performance that change from point to point: vapor pressure, partial pressure, and temperature.
To make comparisons easier, the large data file of adsorption/desorption points at different temperatures and relative pressures was simplified. First the data was interpolated to get 30 evenly spaced points covering the entire data range. Next the adsorption and desorption results were averaged to get the equilibrium values (the difference between adsorption and desorption was minimal for this sample - no hysteresis). The y-axis is converted to pore volume measures, in cc liquid adsorbed or cc pores filled/100grams carbon, instead of weight percent. The average interpolated data for these characteristic curves is presented in Table 1, and Figure 1 and 1b.
Performance Prediction Models
These curves are the only carbon related information required to predict physical adsorption performance using the Polanyi Adsorption Potential theory. These single and multicomponent, gas and liquid phase, computer models are used to predict carbon performance and are available from PACS. To do performance predictions, the following polynomial describes these carbon samples:
Carbon name Characteristic curve polynomial - 3rd degree
X-169 y = 1.9885E-05x3 + 2.7301E-03x2 - 1.1432E-01x + 2.3305E-01
X-170 y = -6.3458E-05x3 + 4.9510E-03x2 - 1.0894E-01x + 7.0811E-02
X-171 y = -2.8433E-05x3 + 2.2034E-03x2 - 5.3158E-02x + 5.7912E-01
CAL Coal-base y = 3.5299E-05x3 - 1.8375E-03x2 - 4.0325E-02x + 1.6682E+00
PCB coconut-base y = 5.6334E-05x3 - 3.0968E-03x2 - 1.3312E-02x + 1.6731E+00
Carbon name Characteristic curve polynomial - 3rd degree
X-169 y = 1.9885E-05x3 + 2.7301E-03x2 - 1.1432E-01x + 2.3305E-01
X-170 y = -6.3458E-05x3 + 4.9510E-03x2 - 1.0894E-01x + 7.0811E-02
X-171 y = -2.8433E-05x3 + 2.2034E-03x2 - 5.3158E-02x + 5.7912E-01
X-172 y = 6.7645E-05x3 - 8.6713E-04x2 - 1.2146E-02x + 5.3784E-01
X-173 y = -2.2526E-05x3 - 1.6876E-03x2 - 7.3788E-02x + 3.7652E-02
In the equation, y is the common logarithm of pore volume in cc/100g carbon and x is the e/4.6V adsorption potential in cal/cc. Characteristic curve polynomials are also listed in Appendix A.
Performance in the Six Types of Applications
The simplest comparison of carbon for a specific application is to run the performance prediction calculations for specific conditions, concentrations, and components present in the application. However, our experience with years of carbon optimization and performance comparisons has found that all physical adsorption applications can be placed into six application types. The proof is part of a 16 hour/800 slide training course on carbon fundamentals given by PACS at least once a year.
The comparative results in Table 2b demonstrate the value of the different carbons for use in the different types of applications on a weight basis. For a given application type, the results are related to the amount of carbon required to get a certain level of performance. Therefore, a carbon with twice the cc/100g adsorption performance in an application type requires half the pounds of carbon to achieve a level of performance in that application type.
Unfortunately the apparent densities (AD's) were not known so we are forced to use Table 2b, which compares performance on a weight basis. These results can be multiplied by the AD when available for the volume analysis.
A series of two slides are attached as Appendix B that describes the 6 application types and the classification process to determine what is the application type. Wastewater applications tend to be Type II or Type III. Municipal water purification varies from Type III, Type IV or Type V applications. Removal limits are not low enough and analytical testing is not sensitive enough at this date for Type VI. (Purifying hydrogen of CO and N2 at room temperature is one of the few current Type VI applications). Municipal plants with surface water sources tend to be Type III or Type IV. Plants with ground water sources tend to be Type IV or V.
Table 2b gives the values of the comparative results for the sample carbons versus the performance for the standard commercial carbons for the six application types.
Trace Capacity Numbers
The characteristic curves were used to predict the values for the acetoxime trace capacity (TCN), gas-phase trace capacity number (TCNG) and mid capacity number (MCN). These results are presented at the bottom of the summary pages in Appendix A.
Adsorption Isotherms
The characteristic curves are also translated into adsorption isotherms using the programs mentioned above: Figure 3 for MTBE (weakly adsorbed material), Figure 4 for benzene (more strongly adsorbed species) and Figure 5 for phenol at pH=7 (quite strongly adsorbed material).
Pore Size Distributions
The Kelvin equation, modified by Halsey, can be used to convert the characteristic curve data to calculated BET surface areas or pore size distributions. This is not useful in terms of performance evaluations, but some audiences are more comfortable with the concepts of pore radius and a series of capillary sizes when thinking about activated carbon. Figure 6 shows the cumulative pore size distributions that we include but find of little use. The single and multi point BET surface area was calculated from these curves and is presented in the Summary Tables in Appendix A.
Interpretation of the GAED results:
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Five samples of fine mesh AES Beaver Valley Fly Ash were fully characterized with GAED:
(X-169) 9-12-07 1440hrs X-170 --- Fly Ash AES Beaver Valley 9-13-07 1700 hrs X-171 --- Fly Ash AES Beaver Valley 9-14-07 Reburn Ash X-172 --- Fly Ash AES Beaver Valley 8-20-07 1535 hrs X-173 --- Fly Ash AES Beaver Valley 8-22-07 1745 hrs No other information was given with the samples. -
This report contains two sets of figures, tables and graphs to cover the five samples comparing them to each other and comparing them to two commercially available activated carbon samples used as references: CAL coal-based liquid phase and PCB a coconut-based carbon.
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Each of the five samples lost less than 0.5% by weight on conditioning, indicating all samples were exceptionally clean and dry. (Conditioning entails heating the samples to 240°C in argon and holding for 25 minutes). (Data Summary Table Appendix A).
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Of the Five samples X-171 and X-172 were similar and had the highest adsorption potential with 3 to 4 times more activity than the other three Fly Ash samples (Table 1 and Figure 1).
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The two reference samples: CAL Coal-based Liquid phase and PCB, a coconut-based activated carbon, were about 14 times better than the best of the Fly Ash samples and 34, 47 and 65 times better than the activity of the lowest three (Table 1).
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In the type V application performance (Trace Removal) X-171 and X-172 performed the best of the five Fly Ash samples, but they were still only about half as good as the reference samples.
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The calculated BET surface area is fairly low but relative to each other the order of activity can easily be seen (Data Summary Table Appendix A).
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PACS Sample ID
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Client sample ID
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Calculated BET sq.meters/g.
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X-169
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AES Beaver Valley 9-12-07 1440 hrs
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21
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X-170
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AES Beaver Valley 9-13-07 1700 hrs
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15
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X-171
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AES Beaver Valley 9-14-07 Reburn Ash
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59
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X-172
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AES Beaver Valley 8-20-07 1535 hrs
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62
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X-173
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AES Beaver Valley 8-22-07 1745 hrs
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15
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Appendix A: GAED Summary Tables
Appendix B: The Six Application Types and Classifying an Application
For courses and conferences information, contact Barbara Sherman at:
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- 724.457.6576
- 800.367.2587
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For laboratory testing and consulting, contact Dr. Henry Nowicki at:
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