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4.3. Chemical And Leachate Characterization of Gasifier Slag

The primary environmental concern associated with disposal or utilization of coal utilization byproducts (CUBs) is acknowledged to be the potential for water contamination. Noteworthy toxic trace elements include arsenic, barium, cadmium, chromium, lead, mercury and selenium, the semi-volatile and volatile trace elements that preferentially deposit on fly ash. Without proper handling of CUBs, the potential hazard of ground water contamination has a multi-dimensional impact upon human and recreational health, croplands and aquatic life1. The public may consume contaminated water from wells, creating a human health hazard. Groundwater used to irrigate croplands may adversely affect sensitive crops and bioaccumulate through animal and plant products bought by the consumer. Runoff from irrigated fields can infiltrate into aquatic systems to become a threat to aquatic life and eventually again to human health.

The chemical characterization of fossil fuel CUBs is based on the total concentration of primary constituents of concern. Leachate characterization focuses on the results of leaching analyses of CUB materials. The primary analyses used are the Toxicity Characteristic Leaching Procedure (TCLP) and extraction procedure (EP) analysis. These were the analyses used to characterize waste leachate for use in the risk assessment portion of the U.S. Environmental Protection Agency’s (EPA) 1999 Report to Congress2. TCLP and EP toxicity are determined from laboratory procedures that simulate leaching from a disposal site under actual disposal conditions (as if the waste was disposed in a sanitary landfill and undergoing natural leaching processes). These results can then be compared with regulatory standards to ascertain if a waste is hazardous or non-hazardous. Applicable TCLP limits are RCRA standards or the Universal Treatment Standards (UTS) if the waste is classified as a “decharacterized” hazardous waste; however, it should be remembered that coal combustion wastes have generally been found to be non-hazardous.

Comparison of Wabash River integrated gasification combined cycle (IGCC) gasification slag with pulverized coal utility boiler slag in terms of their behavior when undergoing TCLP shows that the gasification slags outperform, with TCLP values of <0.682 mg/L (1997 average analysis) and <0.12 mg/L (1998 average analysis) compared to 1.28 mg/L for boiler slag. Barium was the only constituent that demonstrated leachable characteristics in the gasification slag; nevertheless, RCRA limits on barium are much higher than for other metals.

Table 2 presents detailed Wabash River IGCC coal slag analysis for specific metal constituents and organics, and compares the TCLP results with RCRA and UTS standards. The data indicates that the Wabash River slag has superior leachability characteristics. Data obtained from EPRI’s Cool Water Project tend to support the results obtained at the Wabash River Facility. These results are presented in Table 3.

Table 2. Wabash River Coal Slag Analysis Leachability Results for Trace Metals and Select Organics3,4

Antimony -- 2.1 < UTS
Arsenic 5.0 5.0 < UTS
Barium 100.0 7.6 < UTS
Beryllium -- 0.014 < UTS
Cadmium 1.0 0.19 < UTS
Chromium (Total) 5.0 0.86 < UTS
Cyanides (Total) -- 590 mg/g3 < UTS
Mercury (non WW) 0.2 0.2 < UTS
Mercury (all other) 0.02 0.025 < RCRA
Nickel -- 5.0 < UTS*
Selenium 1.0 0.16 ND
Silver 5.0 0.3 < UTS
Thallium -- 0.078 ND
Vanadium -- 0.23 < UTS*
Zinc -- 5.3 < UTS
1,1,1,2-Tetrachloroethane   6.0 ND
1,1,2,2-Tetrachloroethane   6.0 ND
Tetrachloroethylene   6.0 ND
2,3,4,6-Tetrachlorophenol   7.4 ND
Acenaphthylene   3.4 ND
Acenaphthene   3.4 ND
Acetone   160 ND
Acetonitrile   38 ND
Acetophenone   9.7 ND
2-Acetylaminofluorene   140 ND
Acrolein   NA ND
Aniline   14 ND
Anthracene   3.4 ND
Benzene   10 ND
Benz(a)anthracene   3.4 ND
Carbon Disulfide   4.8 mg/lTCLP < UTS
Diethyl phthalate   28 < UTS
Fluoranthene   3.4 < UTS
Toluene   10.0 ND
Vinyl chloride   6.0 ND
1,1,1-Trichloroethane   6.0 ND
1,1,2-Trichloroethane   6.0 ND
Trichloroethylene   6.0 ND

Single pass and recycle (* single pass only)
WW – Wastewater
ND –Not Detected

Table 3. RCRA Gasifier Slag Test Results for Cool Water Gasification Plant

Arsenic ND 5.0 (<0.06)
Barium 0.32 100.0  
Cadmium ND 1.0 (<0.002)
Chromiuma ND 5.0 (<0.005)
Lead ND 5.0 (<0.08)
Mercury ND 0.2 (<0.0004)
Selenium ND 1.0 (<0.08)
Silver ND 5.0 (<0.002)

ND = Not Detected
aChromium species not identified

These favorable slag characteristics permit its disposition as any coal combustion waste material that is classified as nonhazardous. This conclusion will generally apply to slagging-type gasifiers that consume coal or petroleum coke, but does not necessarily extend to plants that use other feedstocks, such as municipal solid waste (MSW), or non-slagging gasification processes. Also it should be remembered that even if slag is classified as non-hazardous, state or local regulations may require more stringency than federal guidance, and require disposal of slag in a different class of landfill. 

  1. Cherry, D., et al., “Review Of The Global Adverse Environmental Impacts To Ground Water And Aquatic Ecosystems From Coal Combustion Wastes,” Report prepared by Biology Department at Virginia Tech for Hoosier Environmental Council And Citizens Coal Council, March 2000.
  2. “Technical Background Document For The Report To Congress On Remaining Wastes From Fossil Fuel Combustion: Waste Characterization,” March 15, 1999.
  3. Amick, P., “Power Industry: Gasification-Based Repowering of a Coal Fired Plant The Wabash River IGCC,” Global Energy Presentation to GTC Gasification Workshop, September 12, 2001
  4. Global Energy data from independent lab tests sent to DOE/NETL, 12/4/2001

Solid Waste/Byproducts of Gasification


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