NOx Emissions from Cement Mfg - US Environmental Protection ...

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NOxemissions will form. The heater exhaust gas could be combined with the kiln exhaust gas prior to the SCR unit. The SNCR processes may also increase CO concentrations in the flue gas, although this increase for urea-based systems is apparently much less than that due to combustion modifications such as overfire air and sub-stoichiometric combustion used in utility boilers.7 On the other hand, one reference claims that ammonia injection has no effect on CO emissions.8 Interestingly, the intentional addition of CO in the reaction zone of the process broadens the operating temperature window for urea-based systems, even at CO concentrations as low as 500 ppm, although it increases N2O emissions.9 At a demonstration of the Nalco urea based SNCR system, CO and SO2 emissions were unaffected when the NSR was less than 0.7 or when the preheater exit oxygen was greater than 2.3%. At lower O2 and at NSR's greater than 0.7, both SO2 and CO increased.6 The SCR employs catalysts that are composed of various active materials, such as titanium dioxide, vanadium pentoxide, and tungsten trioxide, as well as inert ingredients to impart structural strength and ease of forming.5 Most concerns over hazards have centered on vanadium pentoxide, which is a toxic compound. After deactivation, these catalysts must be disposed of properly. Some catalyst manufacturers may take back any spent catalyst for reactivating and recycling. Where the spent catalyst cannot be recycled, it must be disposed in an approved landfill.7 7.2 ENERGY IMPACTS OF NOx CONTROLS 7.2.1 In-combustion NOx Control Systems Combustion control techniques for NOx reduction rely on reducing excess air and excess burning (or high flame temperatures) for optimum conditions. Both of these factors will likely increase the energy efficiency of the cement-making process. Thus by intuition, NOx reduction measures based on combustion process controls are also likely to reduce the energy requirement per ton of clinker produced. Techniques such as secondary firing allow combustion of low grade fuels such as 7-10
solid wastes with high heating value. These fuels are much cheaper than the primary fuels and are likely to result in significant cost savings. Secondary firing in the calcining zone results in efficient utilization of energy and is likely to reduce the energy requirement per unit amount of clinker produced. The NOx reduction potential of the secondary firing technique, has, however, not been confirmed experimentally. Quantitative information is not available regarding the impact of low NOx burners on the energy consumption in cement kilns. 7.2.2 Postcombustion NOx Control Systems Both the SCR and SNCR involve high operating costs associated with the ammonia or urea reagent used and their delivery systems. Since there are no catalysts involved, SNCR process is not susceptible to particulate fouling and, thus, may be implemented within the kiln at an appropriate location. The catalysts used in the SCR process may, however, be susceptible to SO2 and particulate fouling. Thus, the SCR system may need to be installed downstream of a particulate control device. Since the SCR process requires gas temperature to be about 300 to 425 EC (575 to 800 EF), the gas stream may need to be reheated causing an additional energy cost. The energy penalty associated with reheating of gases for SCR application may be determined approximately as shown below: In cement kilns, heat consumption of 1 MM Btu/hr produces about 350 std ft3/min of flue gas. Assuming the flue gas temperature at the exit of a particulate control device of 300 EF, an SCR reactor temperature of 850 EF, molecular weight of flue gas of 30, and an average specific heat of the flue gas during reheating of 0.255 Btu/lb-EF the heat requirement for reheating the flue gas is about 246,000 Btu/hr. Thus, with no heat recovery in the flue gas reheater, approximately 24.6 percent of additional energy would be required for flue gas reheating. With an efficient, energy recuperative type process heater an energy recovery of 60 percent may be possible. With 60 percent energy recovery in a flue gas reheater, the energy 7-11
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EPA-453/R-94-004 ALTERNATIVE CONTRO
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ALTERNATIVE CONTROL TECHNOLOGY DOCU
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Chapter Page 4.1 MECHANISMS OF NO F
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TABLE OF CONTENTS (con.) Chapter Pa
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LIST OF TABLES Number Page 2-1 UNCO
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CHAPTER 1 INTRODUCTION Congress, in
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CHAPTER 2 SUMMARY This chapter pres
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2.2 NO EMISSION CONTROL TECHNOLOGIE
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TABLE 2-2. ACHIEVABLE NO REDUCTIONS
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TABLE 2-4. CAPITAL AND ANNUAL COSTS
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TABLE 2-5. COST EFFECTIVENESS OF NO
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eductions ranged from 470 tons/year
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CHAPTER 3 INDUSTRY DESCRIPTION 3.1
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Type II. Moderate-heat-of-hardening
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3 a TABLE 3-2. UNITED STATES CEMENT
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Figure 3-1. U.S. Portland cement pl
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Table 3-3. (con.) Rank Clinker 3 (1
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5 cement processes: wet process and
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Figure 3-3. New technology in dry-p
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substantial levels of silica and al
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are achieved by clinker coolers of
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126,000 to 963,000 tons/year with a
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such as crushing and grinding opera
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4.1.1 Thermal NO Formation x Therma
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temperatures are on the order of 16
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4-6
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expected to produce more fuel NO th
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introduce a large proportion of com
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will also mean a somewhat lower sec
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ILC systems: In these systems, the
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which indicates an average and the
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4-18
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Uncontrolled NO x emissions Plant c
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from 2 to 9 lb/ton of clinker, and
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Capacity Heat in NO x emissions Met
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TABLE 4-3. (con.) Capacity Heat in
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in estimating uncontrolled and cont
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13. Letter from Sheridan, S.E., to
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44. Chemecology Corporation Source.
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73. Letter and attachments from Smi
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NO control approaches applicable to
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5.1.2.2 Kiln Fuel Changing the prim
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oxygen content of the primary air m
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tend to recirculate hot combustion
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e about 15 to 38 percent depending
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combustion system shown in Figure 5
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5-14
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Figure 5-3. Schematic of hazardous
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5-18
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higher than typical cement kiln flu
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window is in the middle of a kiln.
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Figure 5-4. Application of the sele
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in Section 5.1.2 is usually difficu
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5.4 REFERENCES 1. Helmuth, R.A., F.
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25. Letter from Wax, J., Institute
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TABLE 6-1. CEMENT KILN MODEL PLANTS
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6-4
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6-6
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The total capital cost is the sum o
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TABLE 6-3. ANNUALIZED COST ELEMENTS
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6-12
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6.1.3.7 Administrative Charges. The
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At this time only three installatio
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6-18
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The annualized costs for low NOx bu
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No additional costs for utilities o
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20 percent PEC cost for contingenci
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TABLE 6-7. ANNUALIZED COSTS FOR RET
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6-28
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PEC for model plant 300,000 ' Heat
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Page 150 and 151: TABLE 6-10. CAPITAL COSTS FOR A URE
Page 152 and 153: TABLE 6-11. ANNUAL OPERATING COSTS
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Page 156 and 157: TABLE 6-12. CAPITAL COSTS FOR AN AM
Page 158 and 159: TABLE 6-13. ANNUAL OPERATING COSTS
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Page 162 and 163: costs range from $9.9 million for t
Page 164 and 165: The estimated annualized costs for
Page 166 and 167: 6.3 COST EFFECTIVENESS OF NOx CONTR
Page 168 and 169: emoved. Cost effectiveness were det
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Page 174 and 175: TABLE 6-18. COST EFFECTIVENESS OF M
Page 176 and 177: TABLE 6-19. COST EFFECTIVENESS OF U
Page 178 and 179: 6.3.4 Selective Catalytic Reduction
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Page 182 and 183: TABLE 6-21. COST EFFECTIVENESS OF S
Page 184 and 185: 6-70
Page 186 and 187: 11. Letter and attachments from Nov
Page 188 and 189: Tables 7-1 7-2
Page 190 and 191: TABLE 7-2. REDUCTION IN NO x EMISSI
Page 192 and 193: model plants described in Section 6
Page 194 and 195: temperature and excess air may be u
Page 198: equirement for the flue gas reheati
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