Ph.D. Thesis: Characterization, Stabilization, and Utilization of Waste-to-Energy Residues in Civil Engineering Applications
By Yixi Tian
Advisors: Prof. A.C. (Thanos) Bourtsalas, Prof. Nickolas J. Themelis, and Prof. Shiho Kawashima
Department of Earth and Environmental Engineering
Fu Foundation School of Engineering & Applied Science
Research was sponsored by Covanta Energy, the Earth Engineering Center and the Department of Earth and Environmental Engineering of Columbia University
About 27 million metric tons of municipal solid waste are used annually as fuel in U.S. Waste-to-Energy (WTE) power plants, which annually generate seven million tons of bottom ash (BA) and fly ash (FA). In the U.S., bottom ash and fly ash residues are mixed to “combined ash” (CA) in the approximate ratio of 6 to 1, and are disposed in landfills after metal separation. The disposal of WTE ash is a significant cost and land use item of waste management.
This dissertation aims to (i) comprehensively understand the characterization and properties of WTE ash; (ii) provide practical and economic stabilization technologies to reduce the leachability of heavy metals in WTE ash and assessing whether it can be further beneficially used as secondary materials; (iii) utilize the stabilized/processed WTE ash as secondary construction materials in civil engineering applications, thus diverting materials from landfills and contributing to the circular economy.
The Characterization section provides a comprehensive assessment of WTE bottom ash, fly ash, and combined ash, including chemical composition (XRF, ICP-OES, IC), mineral composition (X-ray diffraction-XRD quantification), thermogravimetric analysis (TGA), particle size distribution, and scanning electron microscopy (SEM). The physical properties of WTE residues were also investigated, including moisture, bulk density, specific gravity, void content, and water absorption. Leaching Environmental Assessment Framework (LEAF) Method 1313 of the U.S. Environmental Protection Agency (EPA) was used to understand the effect of eluate pH on the leachability of heavy metals. Combination of the above methods was applied to quantify the crystalline and amorphous phases present in WTE residues and produced specimens.
In the U.S., WTE BA is discharged from the combustion chamber into a water tank. The BA includes 50-70% mineral fraction, 15-30% glass and ceramics, 5-13% ferrous metals, 2-5% non-ferrous metals, and 1-5% unburned organics. This thesis received the BA samples after ferrous and non-ferrous metal recycling. The major chemical composition includes SiO2 (34%), CaO (21%), Al2O3 (9%), and Fe2O3 (11%). According to XRD quantification results, BA consists of 76% amorphous phases (glass and metastable minerals), and the dominant crystalline mineral is quartz (SiO2, 12%). The calcium silicate (aluminate) hydrates (C-S-(A)-H) gel formed during the water quenching process embeds fine particles in the amorphous phases.
The U.S. WTE air pollution control systems commonly include semi-dry scrubbers, with a few plants using dry scrubbers. FA consists of two kinds of particles: the furnace particles carried in the process gas and the newly-formed particles in the scrubber. The major chemical composition in FA includes CaO (40%), Cl (15%), SO3 (8%), CO2 (8%), and activated carbon/organic matter (3%) due to the injection of absorbents (hydrated lime and activated carbon) and the effects of flue gas scrubbing. The empirical formulae of the constituent crystalline (40-50%) and amorphous (50-60%) phases were derived. The excess water in semi-dry scrubbers improved the hydration reaction between newly-formed particles and furnace particles and resulted in the transformation of amorphous phase to calcium silicate hydrates (C-S-H) phase. The hydration products of semi-dry FA immobilized some heavy metals and reduced their leachability to below the levels of the Resource Conservation and Recovery Act (RCRA) by Toxic Characteristic Leaching Procedure (TCLP) test, as compared to dry scrubber FA, which exceeded the limits of RCRA.
The U.S. combined ash can pass the TCLP test and comply with the RCRA standards for non-hazardous landfill disposal. The Stabilization section examines the effects of processing combined ash. CA undergoes water washing, crushing, and size separation processes to three fractions: coarse (27%, CCA, 9.5-25 mm), medium (37%, MCA, 2-9.5 mm), and fine (25%, FCA, < 2 mm), identified by particle size distribution results. The by-products of the washing process are extra fine filter cake ash (EFFCA, 8% of CA) collected from the water treatment system and ash dissolved in the wastewater (3% of CA). The characterization (chemical composition, mineral composition, and leachability) of ash the fractions (CCA, MCA, and FCA) showed that their mineral changed during the processing and exhibited significantly lower leachability (LEAF Method 1313-pH dependence), in comparison to as-received CA. The processed ash fractions with reduced leachability of heavy metals, can be further beneficially used as secondary materials.
The effect of pH of the washing agents (water, acid and alkaline solutions) on the chemical/mineral transformation and the heavy metals leachability of the FA, BA, and CA was assessed. A novel technique of determining the distribution of various elements in washed ash (product), filter cake (by-product), and wastewater (dissolution) during ash processing was developed to compare the effectiveness of the washing process, which is dominated by dissolution and precipitation reactions. As-received FA, BA, and CA contained 50-75% of amorphous phases in metastable status, which are transformed to crystalline phases during the washing process. It was concluded that water washing is the most practical method for transforming WTE CA to construction material.
The Utilization section examined the use of WTE ash in civil engineering applications, i.e.
(i) Using the CCA and MCA fractions as stone aggregate substitute in structural concrete;
(ii) Using FCA as sand substitute or using the milled FCA (MFCA) powder as mineral addition in cement mortar;
(iii) Using FCA and EFFCA powder as metakaolin substitute in artificial aggregate;
(iv) Using FA and phosphate FA (PFA) as cement substitute in cement mortar.
In conclusion, the CA size fractions, i.e., MCA and CCA, are suitable for use as aggregate substitutes in the production of structural concrete. Up to 100 wt.% of stone aggregate in concrete can be substituted by MCA and CCA. The compressive strength of the optimal products exceeds 28 MPa after 28 days of curing, which is comparable to commercial concrete products using natural stone aggregate. The optimum concrete mixture composition was 40 wt.% of MCA or CCA, 30 wt.% sand, 20 wt.% cement, 10 wt.% water, and superplasticizer, with compressive strength of 28-30 MPa and elastic modulus of 6,300-6,600 MPa. The optimal products complied with stringent leaching standards, and the properties of the final products were comparable to the conventional civil engineering materials.
All FCA or MFCA products were effectively stabilized/solidified and transformed to non-hazardous material that can be used in construction. The main challenge in the utilization of FCA or MFCA in cement mortar is the cementitious phase expansion due to the metallic aluminum present in FCA or MFCA. It was concluded that up to 50 vol.% of sand in cement mortar can be directly substituted by FCA, and up to 25 vol.% of MFCA can be utilized as mineral addition to replace cement in the production of cement mortar.
In the production of artificial aggregates, up to 15% of FCA or up to 10% of EFFCA can replace metakaolin by volume. The produced samples indicated crushing strength of 4 and 1.5 MPa, respectively. The specific gravity and water absorption of optimal ash aggregate is 1.3 and 30%. The FCA and EFFCA aggregates exhibited good chemical stability and reduced the cracks observed in the fire resistance test. The ash aggregates can be used as a lightweight aggregate for non-structural applications. FCA can improve the workability of the metakaolin mixture and extend the setting time, which is beneficial for geopolymer aggregate manufacturing. The heavy metals from FCA and EFFCA can be effectively stabilized/solidified in artificial aggregate.
Phosphoric acid can effectively stabilize the as-received FA, so that the dry scrubber FA passes the TCLP test and complies with the RCRA standards. The mineral transformations of individual ash and ash-cement paste were investigated by the XRD quantification analysis. FA and PFA enhanced the hydration degree of cement, and received higher mechanical performance than reference in 0-25 vol.% cement replacement. The leachability of heavy metals was effectively reduced in a wide leaching range (eluate pH 0-12.5), realized the stabilization/solidification purposes under restricted non-hazardous landfill standards.