外文翻譯---循環(huán)流化床煤灰綜合建筑應(yīng)用

附錄 外文文獻(xiàn)及翻譯 Utilization of CFB Fly Ash for Construction Applications R. E. Conn and K. Sellakumar Foster Wheeler Development Corporation Livingston, NJ A. E. Bland Western Research Institute Laramie, WY ABSTRACT Disposal in landfills has been the most common means of handling ash in circulating fluidized bed (CFB) boiler power plants. Recently, larger CFB boilers with generating capacities up to 300 MWe are currently being planned, resulting in increased volumes and disposal cost of ash byproduct. Studies have shown that CFB ashes do not pose environmental concerns that should significantly limit their potential utilization. Many uses of CFB ash are being investigated by Foster Wheeler, which can provide more cost-effective ash management. Construction applications have been identified as one of the major uses for CFB ashes. Typically, CFB ash cannot be used as a cement replacement in concrete due to its unacceptably high sulfur content. However, CFB ashes can be used for other construction applications that require less stringent specifications including soil stabilization, road base, structural fill, and synthetic aggregate. In this study, potential construction applications were identified for fly ashes from several CFB boilers firing diverse fuels such as petroleum coke, refuse derived fuel (RDF) and coal. The compressive strength of hydrated fly ashes was measured in order to screen their potential for use in various construction applications. Based on the results of this work, the effects of both ash chemistry and carbon content on utilization potential were ascertained. Actual beneficial uses of ashes evaluated in this study are also discussed. INTRODUCTION CFB combustion has developed into a mature technology for burning a wide range of fuels, while still achieving strict air emissions requirements. Typically, fuels are burned in a CFB boiler with the addition of limestone to capture SO2 in a solid form. With larger CFB boilers being brought online, a greater emphasis has been placed on enhanced beneficial use of ash than in the past. Studies have shown that the environmental impact from CFB ashes is less than those from p.c. ashes and should not limit their utilization as marketable by-products (Conn and Sellakumar, 1997; Young, 1996). Traditionally, p.c. fly ash has often been sold for use as an admixture in the production of Portland cement. The utilization options for CFB ashes are somewhat more diverse than p.c. ash, due to the effect of sorbent (calcium) on the overall ash chemistry. These options include agricultural applications, construction applications and waste treatment. Beneficial use for construction purposes is one of the most common markets for CFB ash. These uses include soil stabilization, road base, structural fills, and synthetic aggregate. To qualify for these uses, the ash must have special properties and pass certain ASTM tests. Compressive strength is one of the most important physical properties a material must possess when being considered for different construction applications. Depending upon the specific application, different degrees of compressive strength are required. In this study, the unconfined compressive strength was measured for hydrated CFB fly ashes from boilers firing a wide range of fuels. These results provided a indication of potential construction uses for fly ashes with very different compositions. The specific objectives of this work were: CHARACTERIZATION OF FLY ASHES Nine fly ashes evaluated in this study were obtained from CFB boilers firing diverse fuels such as bituminous gob (0.5% S), low volatile bituminous coal (0.3% S), high sulfur (4.7%) bituminous coal, petroleum coke (5.0% S), and RDF (0.3% S). The fly ashes had significantly different chemical compositions as would be expected considering the types of fuels being fired. The bituminous gob fly ash was composed primarily of coal ash since it was taken from a boiler that does not use limestone injection for sulfur capture. The bituminous coal fly ash samples contained both coal ash and sorbent, with relatively high amounts of free lime. Fly ash from petroleum coke was composed mainly of sorbent compounds due to the low ash content of the fuel. Finally, the RDF fly ash had a composition fairly similar to that of a bituminous coal. However, most of the calcium in the fly ash was inherent in the RDF and not derived from sorbent. Some calcium in the RDF fly ash did originate from the semi-dry scrubber used to remove SO2 and HCl from the flue gas. Phase analyses of the ashes by x-ray diffraction (XRD) are shown in Table 2. The coal fly ashes were composed primarily of anhydrite (CaSO4), hematite (Fe2O3), silica (SiO2). Anhydrite was not found in the bituminous gob fly ash, since it did not contain sorbent. The petroleum coke ash was composed principally of sorbent derived compounds and minor amount of silica (3.0%). The fly ashes were all relatively fine with greater than 80% passing a 200-mesh screen (74_m). As a result, these ashes can readily be made into cement-type pastes without further milling. The poured bulk density of the fly ashes ranged from about 34 to 57 lb/ft3 (385 to 913 kg/m3); the compacted bulk density of the fly ashes were slightly higher and ranged from 53 to 74 lb/ft3 (849 to 1186 kg/m3). The specific gravity ranged from 1.8 to 3.0 for the fly ashes. The RDF fly ash had a relatively low specific gravity compared to the other ashes, probably since it contained a different type of inorganics. This ash was derived from fine inorganics in RDF, not limestone sorbent or coal minerals. Moisture was generally less than 1.0%, except for the RDF fly ash, which contained 1.6% moisture. EXPERIMENTAL PROCEDURES Unconfined compressive strength of the fly ashes was measured similar to ASTM C-109. A paste was prepared by mixing about 35% by weight water and 65% fly ash to form 0.75 in. (1.91 cm) pellets in a plastic mold. The bulk density of the ash in these pellets was about 60 lb/ft3 (960 kg/m3). For soil stabilization tests, fly ash (15% by weight) was mixed with clays to form a pellet. These samples were cured under saturated conditions at 23oC for 3, 7 and 28 days. The compressive strength of the hydrated samples was then measured using a compressive testing machine. This procedure was intended to simulate the actual construction uses in which cement pastes would be made from fly ashes. Considerably less water is used in the ASTM C-109 procedure compared to the hydration technique used in this study. In addition, the ash bulk density was less than that typically used for ASTM C-109. As a result, the compressive strengths may differ somewhat from those obtained by the ASTM test. In most cases no other materials were mixed with the fly ashes except water. Strength development resulted solely from the self-cementing properties of the ashes. No concrete-type mixtures incorporating sand or aggregate were evaluated in this study. The fine size distribution of the fly ashes makes them ideal candidates for producing pastes simply with the addition of water.Bottom ashes may also be suitable for some construction applications, but could require milling to a desired, much finer size distribution. CONSTRUCTION USES FOR CFB ASHES Laboratory tests were performed to address the use of different fly ashes in a number of construction applications including (1) cement replacement and manufacturing, (2) structural fills, (3) road base, (4) synthetic aggregate, and (5) soil stabilization. CONCRETE AND CEMENT PRODUCTION The potential also exists for using CFB ash for regulating the set time of Portland cement, instead of conventionally used gypsum (calcium sulfate dihydrate). Tests were conducted with petroleum coke fly ash that contained high concentrations of CaO and SO3 (calcium sulfate). Quantitative XRD analysis showed that this fly ash contained 66% CaSO4 and 30% CaO. To compare performance of cements with the fly ash and with commercial grade gypsum, three samples were prepared with a Type I cement clinker including: 94.5% clinker, 4.6% gypsum; _ 94.5% clinker, 2.3% gypsum, 2.8% fly ash; and _ 94.5% clinker, 5.5% gypsum. The cements were ground in a batch ball mill and tested for compressive strength and time of set according to ASTM standards C-109 a n d C - 1 9 1 , r e s p e c t i v e l y . The results in Table 6 confirmed the strength characteristics of the three cements exceeded the standard specifications of ASTM C-150. The cements using the petroleum coke fly ash slightly outperformed the control cement with conventional gypsum in 28-day strength tests. Setting time was shorter for the experimental cements, but remained comfortably within standard limits. Test results would be expected to v a r y f o r c e m e n t c l i n k e r s o f d i f f e r e n t c o m p o s i t i o n s . STRUCTURAL FILLS Natural soil borrow, granular fill, boiler slag, and other embankment or structural fill materials are typically tested to determine their shear strength (Brendal et al, 1997). Cementitious materials such as fly ash, however, are more appropriately evaluated by the unconfined compressive strength test. The two major types of structural fill materials are (1) flowable (or excavatable) and (2) compacted or embankment. Flowable fill is usually mixed in a ready-mix concrete truck, with mixing continuing during transport to prevent segregation. Although flowable fill may be designed for use under high loads, this material is typically designed for a compressive strength of 50 to 150 psi (345 to 1035 kP) at 28 days. (Note that this strength may continue to increase with time.) Strengths lower than 50 psi(345 kP) are insufficient for use as a structural fill. Strengths higher than 150 psi (1035 kP) at 28 days could result in fill materials which would not allow excavation. Compacted fills and embankments require materials with high strength for supporting heavy loads and should be considered permanent. These materials should not be considered for use around pipes, utility lines, or other locations that may need to be accessed. Compressive strength results show that only the RDF fly ash would qualify as a flowable fill since its 28-day strength was 145 psi (100 kP). It should also be noted that this ash showed considerable rapid expansion upon hydration, resulting in a very porous material. In fact, the hydrated ash pellets grew in volume by 50% in only ten minutes. The reason for this expansion is uncertain, but may be due to reaction of fine aluminum metal and Ca(OH)2 in the ash with water resulting in evolution of hydrogen gas. This reaction is similar to that used for autoclave cellular concrete (ACC). The high-sulfur bituminous coal fly ash would qualify as a permanent compacted fill and had a relatively high 28-day strength of near 1500 psi (10.3 MP). This high strength is not surprising since the ash nearly qualifies as a Class C pozzolan or self-cementing material. As a result, it is currently being marketed as a component in permanent fill materials. Free lime, particularly in combination with FAS components, is one of the key ash components that influence the strength of hydrated ashes. The compressive strength did correlate with the free lime content of most of the bituminous coal fly ashes. Free lime, once hydrated to calcium hydroxide, would be expected to undergo pozzolanic reactions with ferric oxide, aluminum oxide and silicon oxide The low-volatile bituminous fly ash did not develop very high strength despite its moderate free lime content of 12.5%. The cause of low strength development is unclear. The high carbon content of the ash (LOI = 18.9%) may have been responsible for limiting its strength development. On the other hand, the lower CaSO4 content of this ash may have limited the formation of etrringite or gypsum. There is conflicting evidence as to the effect of high carbon content on the strength development of CFBC ashes. Figure 1 also shows compressive strength data for a low volatile bituminous coal fly ash, which also had a relatively high LOI of 12%. This fly ash was obtained from a boiler firing a 4.5% S lowvolatile coal with significant inert carbon content. Although this fly ash had 12% LOI, it developed a 28-day strength of 1620 psi (11.2 MP), possibly due to its high free lime content of 22.5%.Consequently, it appears that high LOI may not limit the strength of hydrated ash, provided itcontains sufficient free lime and FAS to form pozzolanic reactions or soluble alumina and calcium sulfate to from ettringite or gypsum. The RDF ash was also very high in free lime content (16.3%) and almost qualifies as a Class C pozzolan. This ash developed low strength even though it would be expected to have considerable self-cementing properties. This low strength was a result of the formation of a porous hydrated ash as mentioned earlier. The petroleum coke fly ash listed in Table 1 had high free-lime content, yet moderate compressive strength 520 psi (3.6 MP) after 28 days. The petroleum coke ash developed this moderate strength due to hydration reactions of lime and calcium sulfate, not pozzolanic reactions: CaO + H2O _ Ca(OH)2 calcium hydroxide (5) CaSO4 + 2H2O _ CaSO4_2H2O gypsum (slow) (6) Insignificant pozzolanic reactions would be expected with this ash since it contains only minor amounts of FAS components (3% SiO2). Another petroleum coke fly ash (see Figure 1) developed considerably higher strength (820 psi/5.7 MP) but contained only 8.6% free lime. This strength would nearly qualify the ash as a suitable compacted fill, since it almost meets that required by the ASTM C-109 test. As a result, calcium sulfate content may be a better indication of strength development than free lime for hydrated petroleum coke ashes, since it may be the principal bonding mechanism. As shown in Table 4, the bituminous gob fly ash did not develop any significant strength since it contained little free lime (no self-cementing properties). The effect of lime addition on ash compressive strength was investigated. As shown in Figure 2, addition of only 10% lime (by weight) roughly doubled the 28-day strength (93 psi/641 kP) of the fly ash making it suitable for excavatable (flowable) fill use. Addition of 25% lime increased the 28-day compressive strength to 750 psi (5.2 MP). Slightly higher amounts of lime addition should make the mixture suitable for use as a compacted fill. The effect of different additives on the strength of hydrated petroleum coke fly ash was also investigated. As shown in Figure 3, addition of Portland cement and coal fly ash/Portland cement raised the compressive strength after 28 days to greater than 1500 psi (10.3 MP). This additional strength was partially a result of pozzolanic reactions of free lime with FAS components in the cement or fly ash. These mixtures would have sufficient strength to qualify as potential compacted fills. Addition of blast furnace slag to the petroleum coke ash did not result in as high as strength, possibly due to its lower FAS content. Compressive strength is only one of the physical properties that fill materials must meet. Other geotechnical tests must also be met such as expansion, swell and permeability. The expansion test is defined by specific ASTM standards C-157. Expansion of the fill material is undesirable and often occurs in hydrated coal ashes due to formation of ettringite. However, with coal ashes, the expansion generally occurs over a longer period of time (up to six months) compared to that mentioned earlier for the RDF ash. The permeability of an ash is a measure of the rate at which a fluid passes through a material and, along with leachate data, may be used to estimate possible impacts on groundwater quality. For comparison purposes, a permeability coefficient of 1 x 10-7 cm/sec or lower is often required for clay liners in landfills. A 1 x 10-6 cm/sec coefficient corresponds to a percolation rate of approximately 0.3 meters per year. Permeability data for CFB fly ashes has been shown to range from about 10-5to 10-9 cm/sec (Radian Corp., 1992). ROAD BASE CFB fly ashes have the potential as substitutes for lime or fly ash in road base construction or as a sole material. To provide strength, durability, and dimensional stability, the following criteria should be applied to CFB ash as road base: The 7-day unconfined compressive strength when cured under moist conditions at 70oF to73oF (21oC to 23oC), must be 400 to 450 psi (2.8 to 3.1 MP). _ The strength of the mix must increase with time (GAI Consultants, 1992). The 28-day unconfined compressive strength should be at least 550 to 600 psi (3.8 to 4.1 MP). _ Expansion requirements are not well established. However, it is suggested that linear expansion be restricted to between 0.1 to 0.5% (Minnick, 1982). As shown in Table 4, only the high-sulfur bituminous coal fly ash would meet the 7-day strength requirements for road base. The bituminous gob ash with 25% lime addition would also have suitable strength to be used as road base material. Probably lower strengths were obtained by the hydration of the ashes than would be obtained with less moisture according to the ASTM D698 optimum moisture and compaction. The petroleum coke fly ash had nearly sufficient 28-day strength, but not enough 7-day strength to qualify as a road base material. Addition of Portland cement was shown to increase the 7-day strength of the fly ash, such that it would qualify as a road base material (Figure 3). However, the potential might exist for long-term expansion due to the formation of ettringite. Experience with the petroleum coke CFB ash has shown that it can be used as a road base without expansion problems (Tharpe and Abdulally, 1997). Road base material is made batchwise from ahydrated mixture of about 70% fly ash and 30% bed ash. The hydrated material is compacted wi