Samples of flue gas desulfurization sludge were characterized chemically and mineralogically. X-ray diffraction analysis indicated that the major crystalline components in the sludge samples were calcite, hannebachite, gypsum, and quartz. Identification of major compounds was confirmed through morphological and chemical analyses using scanning electron microscopy with energy dispersive X-ray analysis. In a related study, equilibrium solubility measurements of these samples corresponded with known solubilities of calcite, hannebachite, and gypsum.

Significant concentrations (∼6%, as CaS) of sulphides or other reduced-sulphur species in solid residues from a small-scale circulating fluidized bed (CFB) combustor have been reported in the literature. The presence of sulphides in similar quantities in residues from a utility-scale combustor would present significant difficulties with handling, disposal or utilization of the residues. This paper discusses the preliminary findings of an investigation of sulphur capture in a small-scale circulating fluidized bed combustion (CFBC) unit using a limestone bed and burning a high-S, high-Fe, Eastern Canadian coal. Data are presented on sulphur capture and chemical speciation for residue samples taken from a number of locations in the circulating bed during operation. The results are discussed in terms of probable mechanisms for the formation of sulphur compounds of reduced oxidation state in the bed and the combustion-gas cleaning system.

Over the last five years, the Kentucky Energy Cabinet (KEC) and the Tennessee Valley Authority (TVA) have developed and demonstrated the production of concrete from atmospheric fluidized bed combustion (AFBC) spent bed (SB) ash, and pulverized fuel ash (PFA). This AFBC concrete contains no cement and relies on the reaction of residual lime in the SB ash to react with the pozzolan PFA to form cementitious products. The SB ash is prehydrated in order to reduce exothermic lime hydration reactions and minimize molar volume expansion. Laboratory tests were conducted to establish the performance characteristics of AFBC concretes relative to conventional concrete. AFBC concretes exhibit slower strength gain characteristics, but long term (60 day), unconfined compressive strengths of 5,000 psi have been documented. This slow strength development is typical of pozzolanic concretes. AFBC concrete is more flexible and less brittle than conventional Portland cement concrete, as evidenced by its much lower modulus of elasticity. Setting times for AFBC concretes are extended, requiring the use of accelerators under certain applications. Field demonstrations of the AFBC concretes in ready mix concrete, masonry units, and road base applications have indicated excellent workability and finishing characteristics and confirm the laboratory performance characteristics.

The paper describes the results of the testing program with emphasis on the ash chemistry/conditioning, the performance characteristics and field demonstrations.

The effects of furnace sorbent injection on waste characteristics and the performance of electrostatic precipitators (ESPs) have been studied under conditions simulating full scale operation at Ontario Hydro's 640 MJ/h Combustion Research Facility. Twice as much waste was produced as with coal ash; the waste was found to contain high levels of calcium compounds (CaO, CaSO4, etc.). In situ resistivity and impactor data were obtained using various sorbents injected either as a dry powder or in the form of a slurry while burning coals with sulphur content ranging between 1.4 to 2.8%. Resistivities were found to increase 2- to 3-orders of magnitude from a baseline level of 108 ohm.cm, and the size distribution of particles suspended in the flue was slightly finer than the baseline ash. Experiments with flue gas conditioning showed that the resistivity of the ash/sorbent mixture produced from limestone injection could be reduced by 1- to 2.5-orders of magnitude during SO3 conditioning and 3-orders of magnitude during water conditioning. Both agents improved the performance of the ESP and reduced particulate emissions by a factor of 1.2 to 4.

Canadian utilities are investigating the use of Furnace Sorbent Injection (FSI) for control of acid gas emissions from thermal generating stations. Typically, limestone or slaked lime is injected into the furnace [1,2] and is calcined to calcium oxide (CaO), which reacts with sulphur dioxide (SO2) and oxygen, forming calcium sulphate:

or

The by-product is removed with the fly ash, using back-end particulate control equipment. The resulting solid waste usually contains substantial amounts (e.g., 20 to 40% w/w) of unreacted CaO, which can cause handling and disposal problems because of its vigorous exothermic reaction on contact with water:

The solid waste can set prematurely in the handling equipment. It can also produce a harmful dust and/or yield a highly alkaline leachate that could affect local ground and/or surface water supplies.

A stabilization process for coal cleaning and coal combustion-related wastes has been developed that uses the energy derived from the fuel contained in the coal cleaning wastes. The wastes are pulverized, when necessary, formed into granules in a rotary pan agglomerator, and then fired to a sintering temperature. The result is a readily disposable product that is highly resistant to environmental degradation. Granules of refuse only, and of refuse/fly ash and refuse/FGD sludge mixtures have been investigated. About 90 wt% of the sulfur in the refuse/sludge mixtures is evolved as SO2 during the combustion/sintering portion of the process. Studies using a microreactor system have shown the generation of SO2 to be a function of temperature and to parallel the decomposition of the sulfur-containing compounds present. Based on laboratory tests, SO2 concentrations of 4–8 vol% are predicted in the SO2-rich off-gases. By-product sulfur recovery from the off-gases appears practical and attractive using current technology that is proven and well established.

A protocol for relatively inexpensive and rapid semi-quantitative x-ray diffraction analysis of fly ash mineralogy by the Reference Intensity Ratio (RIR) method is described. RIR's for the common crystalline phases in fly ashes derived from low rank and bituminous coal are given. The method is semi-quantitative for some phases because of unavoidable overlaps of the stronger peaks suitable for quantitation. Use of the protocol is illustrated with the four fly ash Standard Reference Materials supplied by the National Institute of Standards and Technology. Recommendations for implementation of this protocol in other laboratories and for improvements in quantitation of fly ash mineralogy are given.

A suite of three fly ashes, National Bureau of Standards* Standard Reference Materials (SRMs) 2689, 2690 and 2691, was studied by transmission electron microscopy and by analytical electron microscopy (aem): sample preparation techniques are described. The chemical compositions of the glassy phase, analyzed by aem, showed a wide dispersion of values although the mean composition of the glass remained close to that of the whole fly ash.

The microstructure of these materials is complex: besides crystalline inclusions, the aluminasilica glasses have undergone phase separation on a nanometer scale, and consist of two glassy phases. Since liquid-liquid phase separation has also been found in British fly ashes, it appears to be a characteristic feature of the glassy phase in Class F ashes and is also encountered in occasional Al-Si rich particles in Class C ashes. High iron content glass was found in fly ash SRM 2689, and its phase separation and crystallization were investigated.

The hydration of cementitious materials is an exothermic process which results in significant temperature increases in large masses of these materials. The thermal environment under these conditions is nearly adiabatic. The heat of hydration of cementitious aluminosilicate blend materials incorporating fly ash and granulated blast-furnace slag was calculated from the adiabatic temperature rise. Additional reaction of siliceous fly ash with alkaline solutions results in the formation of zeolitic reaction products. Studies were carried out to characterize the hydration products by X-ray diffraction and through analysis of solutions. In addition, zeolitic reaction products were prepared by reacting the pure Class F fly ash at 90°C with NaOH and different anions. This study has implications for general thermal properties of hydrating cementitious materials, and for costeffective immobilization of radioactive and chemical waste cations and anions.

The fly ash present in concrete mixtures is readily identified with qualitative electron microscope techniques. Quantification of the fly ash present in samples is more difficult. Visual analysis of a representative sample volume can be accomplished in a reasonable amount of time through use of image analysis techniques.

Our research efforts have focused on the development of an image analysis technique which uses particle shape and composition to quantify the fly ash present in samples. The resultant binary image is filtered to eliminate the smallest particles and pieces of particles. Shape factor analysis distinguishes spherical fly ash particles from irregularly shaped grains of unhydrated cement clinker. The volume percent of nearly spherical particles is calculated and adjusted to compare the percentage of spherical particles in the matrix of fly ash-containing specimens to the percentage in a control specimen with no fly ash.

There is a 2.2-fold increase in the number of spherical particles associated with the substitution of Portland cement by 60 wt% fly ash. Proportional changes in this ratio should be observable in samples with varying amounts of fly ash. The most obvious limitation of the method is related to the requirement that one knows the percentage of high atomic number particles in the fly ash beforehand.

Following dissatisfaction with the existing calibration factor for standard fly ash used in Australia to AS 1129–1971, “Fly Ash for Use in Concrete,” a joint investigation was made by Amatek Limited and Construction Technology Laboratories Inc. Test results for five reference materials and two Australian fly ashes from tests to ASTM C 430 on both electroformed and woven wire sieves were examined and “corrected” by several means.

The ASTM standard method of sieve correction is challenged, both in the use of Standard Reference Material (SRM) cement and the basis of sieve correction for coarse samples. Preferred bases of correction are suggested.

Detailed mineralogical characterization by X-ray diffraction, scanning electron microscopy and electron microprobe analysis has been performed on a suite of residues and on three samples of unaltered overburden excavated from a 1983 underground coal gasification (UCG) test conducted at a mine site near Centralia, Washington. The residues were classified into nine zones based on their appearance and location in the burn cavity. Because they were isochemical with the overburden, six of these zones could be identified as altered overburden and proved to be particularly useful for correlations of mineralogy with the temperature and redox atmosphere prevailing in these materials during the UCG process. A previously unrecognized, moderate temperature, reaction between reduced iron phases and clay minerals to form an Al-rich hercynite-spinel solid solution (Al-spinel), (Fe,Mg)Al2O4, was identified. This reaction is a precursor to the well-known formation of Fe-cordierite, (Mg,Fe)2 Al4Si5O18 from SiO2 and the Al-spinel at higher temperatures. The mineralogies of altered overburden were analogous to high-temperature, low pressure, metamorphic rocks of the sanidinite facies, and to thermally altered rocks from natural coal combustion. Modeling of the mineral phase assemblages was successfully performed using a phase diagram for the system (FeO+MgO)-Al2O3-SiO2 constructed from literature diagrams. Melting relations were also modeled with this diagram. Deviations from the assemblages predicted by this diagram were attributed to oxidizing conditions.

The individual particle compositions of three circulating fluidized bed combustion (CFB) ashes and one Type C fly ash were studied to determine differences in composition existing between the individual particles within a single ash. The methods used for the investigation included optical microscopy, XRD and XRF analysis. It was found that all the ashes studied consisted of several types of particles that differed in morphology, color and composition.

Direct acid leaching (DAL) with HCl to recover Al and other metal values from fly ash has been examined extensively by the Electric Power Research Institute (EPRI) under RP2422. Large scale utilization of the solid residues from the DAL-process (DAL-residues) could significantly influence the economics of resource recovery. This paper describes a study of the potential use of DAL-residue as a pozzolan in the concrete industry.

Samples of six ashes from a range of coal types were beneficiated to remove magnetic particles and particles larger than 45 gim. The beneficiated fractions were leached with 6M HCl at 105°C to determine metals recovery for Al, Fe, Ca, Mg, Na, K, and Ti. The corresponding DALresidues were characterized by chemical composition, morphology and mineralogical content, and examined as potential concrete pozzolans by standard ASTM tests. One DAL-residue from a highrank coal was examined more extensively in Portland cement mortars.

The greatest yields of Al were from low-rank coal ashes, but were obtained at the ‘cost’ of substantial salt-cake production and high acid consumption. The DAL-residues from all ash types showed high-Si content: those from high-rank coals being spherical pseudomorphs of the original ash particles, and those from low-rank coals containing substantial quantities of gel-like materials.

All of the residues were pozzolanic, but those from low-rank coals caused an unfavorable increase in water demand in mortars. DAL-residues from beneficiated high-rank coal ashes are enhanced pozzolans conforming to standard specification requirements with pozzolanic indices in excess of 100%.

One of the major problems currently restricting the utilization of high-calcium (ASTM Class C) fly ashes in Iowa is lack of adequate storage space. High-calcium fly ashes are self-cementitious and are generally not economically reclaimable once they have been exposed to water (i.e., the environment). Since the on-site silo storage capacity of a given power plant is normally only about one or two weeks of overall generating capacity (assuming nearly full load), the fly ash industry is hard-pressed to meet the demand for fly ash during the peak construction months.

This paper presents some early research findings concerning the storage alternatives available for these high-calcium fly ashes. A simple pan agglomerator, requiring only the addition of a water spray was used to produce fly ash pellets. Since the method required minimal energy input the process would be expected to be economically feasible for field production. Laboratory produced pellets appeared strong and durable and would be expected to withstand field handling without significant degradation. The pelletization process did not appear to result in a loss of reactivity for either the air-dried or water-cured pellets. This is highly important for potential use in Portland cement concrete or soil stabilization.

Because of their high SO3 content (∼11%), sulfo-calcic fly ashes cannot be used as a cement admixture without pretreatment. The fly ashes contain mainly quicklime (CaO), anhydrite (CaSO4) and larnite (BC2S). At ambient temperatures and at normal consistency, their hydration leads to the formation of ettringite, slaked lime and gypsum. As a result, cracking and splitting of the hydrated product quickly occurs due to the crystallization of the ettringite.

The ability of these fly ashes to produce Portlandite, Ca(OH)2, led the authors to investigate pozzolanic binders by the addition of artificial pozzolans — siliceous fly ashes, slag and clay minerals — which had been calcined at moderate temperatures.

The sulfo-calcic fly ashes are ground, then slaked either in ambient or hydrothermal conditions (2 or 6 hours at 130°C). In the case of slaking under ambient conditions, ettringite is quickly produced with a definitive crystallographical structure. As a result, no volume change is observed after re-hydration of the material. By comparison, autoclave slaking does not lead to this phenomenon, and only slaked lime is produced.

The rate of hydration is studied through the development of compressive strength and by analyses of the products by differential thermal analysis (DTA) and X-ray diffraction (XRD) at various hydration times. The best results are obtained using calcined laterite or blast furnace slag.

Tests have been performed with Class F fly ashes used for the production of interground 20% fly ash cement. Unground and ground fly ashes were mixed with a reference high fineness Portland cement and tested for strength in mortar (ISO/CEN method). The fly ashes were also tested for chemical composition, bulk density and specific gravity, fineness (Blaine) and particle size distribution (laser method). There are considerable variations in the specific weights of the asdelivered fly ashes. It is shown that even a small amount of grinding increases the specific gravity significantly, and improves and homogenizes the particle size distributions, resulting in higher strength. The strength increase corresponds to the decrease in air void content.

Intergrinding of fly ash and clinker in large scale mills reduces the power consumption, giving the same 28-day strength as mixed fly ash cement with higher fineness.

In the first part of this study, fly ash has been used as a main component in developing glass compositions which are cellulated through carbonization and oxidation processes. The resulting foam glasses have highly uniform closed pore structure and negligible water permeability with densities ranging from 140 kg/m3 to >800 kg/m3. Foam glasses with open pore structure have also been produced and characterized. In the second part, mechanical and thermal properties are discussed. These include the stressing rate effect on the fracture strength in compressive and flexural loading for different densities. Variations of thermal conductivity and overall strength with density and pore size are also described.

Fly ash technology has been very effective in providing stability in roadway base courses composed either of shale or aggregate materials, and also in partly replacing Portland cement in concrete. X-ray diffractometry and scanning electron microscopy observations indicate that there are certain similarities among these three types of mixes concerning the hydration process; on the other hand, there is evidence of distinct differences in the hydration products which are found to act either as a filler, a chemical agent, or both. Fly ash suppresses the intensity of the clay minerals in shale, speeds up the hydration process in concrete and acts partly as a filler in aggregate mixes. The net practical result is strength development which varies not only in terms of the maximum level attained, but also in regard to its rate. The conversion of ettringite to monosulfoaluminate proceeds at a rate which is considered high in concrete, moderate in aggregate mixes, and moderate to low in shale. X-ray diffraction analyses help to identify other dissimilarities in the minerals produced. The modification which takes place in the fabric and the matrix of the mixes is morphologically the same; in contrast, the growth of crystallites at the “particle”/fly ash interface is explicitly different. Preliminary quantification of matrix changes resulting from new hydration products is also explored.

Refuse-derived-fuel (RDF) consisting mainly of waste paper and plastics is a viable fuel source for the production of power. An experimental test burn partially substituting coal with RDF was undertaken by the Public Service of New Hampshire at the Merrimack Power Station.

Five percent and ten percent RDF were substituted, on a BTU basis, for coal in the test bums. The chemical and physical properties of the resulting fly ash were determined. Twelve test burn days were run with 4 days of 5% RDF and 8 days of 10% RDF. Emphasis was placed on investigating the effect of the RDF fly ash on Portland cement concrete.

Most of the chemical and physical properties of the coal-RDF fly ash were found to be comparable with ordinary coal fly ash except for the amount of cadmium and lead, the pozzolanic activity index and the compressive strength of fly ash concrete. Cadmium and lead were at average levels of 5.1 ppm and 102.6 ppm for the 5% RDF, and 7.8 ppm and 198.3 ppm for the 10% RDF, respectively. Although the pozzolanic activity index of coal-RDF fly ash increases over normal coal fly ash, preliminary results show that the 28-day compressive strength of concrete with direct replacement of cement and sand decreases by up to 30%. Leaching tests on crushed concrete were conducted to evaluate the environmental effect of acid rain.