外文翻譯---循環(huán)流化床煤灰綜合建筑應(yīng)用 (2)

1、<p><b>　　附錄</b></p><p><b>　　外文文獻及翻譯</b></p><p>　　Utilization of CFB Fly Ash for Construction Applications</p><p>　　R. E. Conn and K. Sellakumar</p>

2、;<p>　　Foster Wheeler Development Corporation</p><p>　　Livingston, NJ</p><p>　　A. E. Bland</p><p>　　Western Research Institute</p><p>　　Laramie, WY</p>&l

3、t;p><b>　　ABSTRACT</b></p><p>　　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 ge

4、nerating 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 significa

5、ntly limit their potential utilization. Many uses of CFB ash are being investigated by Foster Wheeler, which can provid</p><p>　　Construction applications have been identified as one of the major uses for CF

6、B 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 s

7、pecifications 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</p><p&

8、gt;　　INTRODUCTION</p><p>　　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

9、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

10、shown that the environmental impact from CFB ashes is less than those from p.c. ashes and should not </p><p>　　Traditionally, p.c. fly ash has often been sold for use as an admixture in the production of Por

11、tland 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, construct

12、ion applications and waste treatment.</p><p>　　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

13、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 c

14、onsidered for different construction applications. Depending upon the specific application, different degrees of compres</p><p>　　The specific objectives of this work were:</p><p>　　CHARACTERIZA

15、TION OF FLY ASHES</p><p>　　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%

16、) bituminous coal, petroleum coke (5.0% S), and RDF (0.3% S). </p><p>　　The fly ashes had significantly different chemical compositions as would be expected considering the types of fuels being fired. The bi

17、tuminous 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,

18、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</p><p>　　Phase analyses of the ashes by x-ray diffraction (XR

19、D) 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 petr

20、oleum coke ash was composed principally of sorbent derived compounds and minor amount of silica (3.0%). </p><p>　　The fly ashes were all relatively fine with greater than 80% passing a 200-mesh screen (74_m)

21、. 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

22、 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 lo</p><p>　　type of in

23、organics. 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.</p><p>　　

24、EXPERIMENTAL PROCEDURES</p><p>　　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

25、. (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 samp

26、les were cured under saturated conditions at 23oC for 3, 7 and 28 days. The compressive strength of the hyd</p><p>　　This procedure was intended to simulate the actual construction uses in which cement paste

27、s 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

28、ASTM C-109. As a result, the compressive strengths may differ somewhat from those obtained by the ASTM test.</p><p>　　In most cases no other materials were mixed with the fly ashes except water. Strength dev

29、elopment 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 ide

30、al 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 s</p><p>　　CONSTR

31、UCTION USES FOR CFB ASHES</p><p>　　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) s

32、tructural fills, (3) road base, (4) synthetic aggregate, and (5) soil stabilization. </p><p>　　CONCRETE AND CEMENT PRODUCTION</p><p>　　The potential also exists for using CFB ash for regulating

33、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). Qu

34、antitative 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</p><p&g

35、t;　　Type I cement clinker including:</p><p>　　94.5% clinker, 4.6% gypsum;</p><p>　　_ 94.5% clinker, 2.3% gypsum, 2.8% fly ash; and</p><p>　　_ 94.5% clinker, 5.5% gypsum.</p>

36、<p>　　The cements were ground in a batch ball mill and tested for compressive strength and time of set according to ASTM standards C-109 and C-191, respectively.</p><p>　　The results in Table 6 confir

37、med 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

38、28-day strength tests. Setting time was shorter for the experimental cements, but remained comfortably within standard limits. Test results would be expected to vary for cement clinkers of different compositions.</p&g

39、t;<p>　　STRUCTURAL FILLS</p><p>　　Natural soil borrow, granular fill, boiler slag, and other embankment or structural fill materials are typically tested to determine their shear strength (Brendal et

40、al, 1997). Cementitious materials such as fly ash, however, are more appropriately evaluated by the unconfined compressive strength test. </p><p>　　The two major types of structural fill materials are (1) fl

41、owable (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 designe

42、d 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

43、5</p><p>　　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, u

44、tility lines, or other locations that may need to be accessed. </p><p>　　Compressive strength results show that only the RDF fly ash would qualify as a flowable fill since its 28-day strength was 145 psi (10

45、0 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 reaso

46、n 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 h</p><p>　　The high-sulfur bituminous coal fly ash would quali

47、fy 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.

48、 As a result, it is currently being marketed as a component in permanent fill materials. </p><p>　　Free lime, particularly in combination with FAS components, is one of the key ash components that influence

49、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 pozz

50、olanic reactions with ferric oxide, aluminum oxide and silicon oxide</p><p>　　The low-volatile bituminous fly ash did not develop very high strength despite its moderate free lime content of 12.5%. The cause

51、 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 l

52、imited the formation of etrringite or gypsum. There is conflicting evidence as to the effect of high carbon content on the strength development of CFBC</p><p>　　Figure 1 also shows compressive strength data

53、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 a

54、sh 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 itcont

55、ains sufficien</p><p>　　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 consid

56、erable self-cementing properties. This low strength was a result of the formation of a porous hydrated ash as mentioned earlier.</p><p>　　The petroleum coke fly ash listed in Table 1 had high free-lime conte

57、nt, 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:</p>

58、<p>　　CaO + H2O _ Ca(OH)2 calcium hydroxide (5)</p><p>　　CaSO4 + 2H2O _ CaSO4_2H2O gypsum (slow) (6)</p><p>　　Insignificant pozzolanic reactions would be expected with this ash since it co

59、ntains 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 q

60、ualify 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</p><p&

61、gt;　　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 wa

62、s 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

63、increased the 28-day compressive strength to 750 psi (5.2</p><p>　　MP). Slightly higher amounts of lime addition should make the mixture suitable for use as a compacted fill.</p><p>　　The effect

64、 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 2

65、8 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

66、to qualify as potential compacted fills. Addition </p><p>　　possibly due to its lower FAS content.</p><p>　　Compressive strength is only one of the physical properties that fill materials must m

67、eet. 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

68、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 fo</p><p>　　The

69、 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 perme

70、ability 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

71、CFB fly ashes has been shown to range from about 10-5to 1</p><p><b>　　ROAD BASE</b></p><p>　　CFB fly ashes have the potential as substitutes for lime or fly ash in road base construc

72、tion or as a sole material. To provide strength, durability, and dimensional stability, the following criteria should be applied to CFB ash as road base:</p><p>　　The 7-day unconfined compressive strength wh

73、en cured under moist conditions at 70oF to73oF (21oC to 23oC), must be 400 to 450 psi (2.8 to 3.1 MP).</p><p>　　_ The strength of the mix must increase with time (GAI Consultants, 1992). The 28-day unconfine

74、d compressive strength should be at least 550 to 600 psi (3.8 to 4.1 MP).</p><p>　　_ Expansion requirements are not well established. However, it is suggested that linear expansion be restricted to between 0

75、.1 to 0.5% (Minnick, 1982).</p><p>　　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

76、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 moistu

77、re and compaction.</p><p>　　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 increa

78、se 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.</p><p>　　Ex

79、perience 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

80、ash and 30% bed ash. The hydrated material is compacted with heavy equipment and allowed to cure for several weeks. After curing, the ash is then reclaimed and sized for different uses. The pressure from the compaction p

81、robably helps yield a product with sufficient strength for</p><p>　　Mixtures using small percentages of CFB coal fly ash in aggregate-based mixes have been tested which developed adequate strength without ex

82、cessive expansion. Although mixtures of moderate proportions of CFB ashes may allow beneficial use of greater quantities of material, test road bases with moderate percentages of CFB ash experienced excessive expansion (

83、Minnick, 1982). The longterm formation of ettringite has been attributed to these expansion problems. The high-sulfur bituminous coal and RDF</p><p>　　expansion. The high-sulfur bituminous ash has a signific

84、ant SO3 content (7.4%), while the RDF ash contains considerable aluminum metal.</p><p><b>　　AGGREGATE</b></p><p>　　The aggregate market encompasses conventional aggregate products, s

85、uch as masonry units and ready-mix concrete. Also, with crushing, aggregates can be produced for use in asphalt paving, road base construction, and roller compacted concrete. Lightweight aggregate can also be used in man

86、y structural building products. As such, synthetic aggregate for construction application appears to be a major market for CFB ashes, as well as a method for storage of ash in the construction offseason. Prelimina</p&

87、gt;<p>　　There are three methods that can be used to form synthetic aggregate from CFB ash: mechanical agglomeration, briquetting, or forming large blocks. With mechanical agglomeration, the aggregates are formed

88、by adding water to the ash to form spheroids pellets in a mixer. This agitation process of agglomeration causes the individual particles to ball together as a consequence of mechanical and capillary forces and is free of

89、 external compacting forces. In the briquetting method, a machine uses moldi</p><p>　　Fresh pellets, briquettes or blocks/beams do not have enough strength for most aggregate uses. The strength requirement o

90、f synthetic aggregate must be achieved by curing. The bonding between individual particles is attained by cementing and pozzolanic reactions, which are accelerated at curing temperatures between 100oF to 200oF (38oC to 9

91、3oC).</p><p>　　As mentioned earlier, compaction of the ash during hydration may also be a means of increasing the strength of the final aggregate product. Another option would be to cure the hydrated ash at

92、a higher temperature to achieve the required strength. Compressive strength is only one of the tests which materials must pass to be suitable as aggregate. Los Angeles abrasion resistance (ASTM C-131) and soundness (ASTM

93、 C-88) test requirements must also be met.</p><p>　　Western Research Institute (WRI) has developed a proprietary process for CFBC ashes that makes them amenable for aggregate production (Bland, 1998). The SY

94、NTAGTM process is designed to reduce the expansion properties of the hydrated CFBC ashes (Figure 4). The SYNTAG process reduces the expansion for a medium sulfur coal derived CFBC ash to less than 0.1%. The untreated CFB

95、C ash exhibited expansion in excess of 0.8%. The SYNTAG process has been shown to produce an aggregate that meets the ASTM an</p><p>　　(AASHO) specifications including LA abrasion resistance and the soundnes

96、s.</p><p>　　SOIL STABILIZATION</p><p>　　The use of CFB ash for stabilization of soils is a potentially large ash use market. Soil stabilization can be defined as a means of permanently altering

97、soil to increase its strength and bearing capacity, and decrease its water sensitivity and volume change potential (National Lime Association, 1991). This ash use application is similar to the cement stabilization common