Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
2000-08-14
2002-05-21
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S169000, C422S170000, C422S171000, C422S173000
Reexamination Certificate
active
06391266
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for injecting limestone into a furnace to produce a highly reactive lime which is available for downstream flue gas desulfurization processes.
Increased concerns as to the adverse environmental impact of sulfur dioxide emissions and stricter regulations have increased the need for efficient processes for removing sulfur dioxide from the flue gas streams of furnaces utilizing sulfur-containing fuel such as coal or oil. It is well known to utilize lime or hydrated lime to remove sulfur dioxide from furnace flue gas streams.
U.S. Pat. No. 4,609,536 to Yoon et al. discloses a process wherein the lime, utilized as a sorbent, is produced by calcining limestone in a separate reactor. The resultant lime is injected into the flue gas stream downstream of the furnace where it reacts with sulfur dioxide to form solid calcium sulfate and calcium sulfite which is separated from the flue gas stream. The separated solid is innocuous and may be utilized as a construction material or buried in a landfill without adverse environmental effects. As noted in the background of U.S. Pat. No. 5,002,743 to Kokkonen et al., it is also known to inject lime directly into a fluidized bed furnace to reduce the sulfur dioxide content of the flue gas in the furnace.
The Kokkonen '743 patent, U.S. Pat. No. 5,246,364 to Landreth et al. and U.S. Pat. No. 4,788,047 to Hamala et al. disclose two-step processes in which finely ground limestone, calcium carbonate, is injected into a furnace under conditions which result in the calcination of the limestone to lime and the reaction of the resultant lime with sulfur dioxide in the flue gas stream. Due to several factors including the relatively short retention time of the lime in the furnace and inefficient lime utilization in these processes, only a portion of the lime reacts with the sulfur dioxide therein.
Additional sulfur dioxide removal from the flue gas stream may occur in various downstream wet, semi-dry or dry flue gas desulfurization processes. For example, in some processes, the unreacted lime is hydrated in a slurry in a downstream reactor under conditions which favor reactions between the hydrated lime and sulfur dioxide to form calcium sulfite. Hydrated lime is generally more efficient at sulfur dioxide removal than lime which is not hydrated. Alternatively, in semi-dry processes the solid hydrated lime particles are wetted to form a liquid film on the surface of the particle comprising calcium hydroxide in solution which then reacts with dissolved gaseous sulfur dioxide to form calcium sulfite.
Although the basic chemistry of the two-step furnace limestone injection calcination and sulfur removal process appears relatively simple, the overall process chemistry and physics are quite complex.
FIG. 2
(derived from “Furnace Dry Sorbent Injection for SO
2
Control Pilot and Bench Scale Studies” prepared for EPRI, Rept. No. 2533-09, November 1992 by Southern Research Institute, Fossil Energy Research Corp. et al.) provides a visual reference for some aspects of the process chemistry and physics.
At elevated temperatures, limestone (calcium carbonate) decomposes to form lime (calcium oxide) and carbon dioxide as represented by the following formula:
CaCO
3
→CaO+CO
2
.
The minimum temperature at which limestone decomposition or calcination occurs in a furnace is dependent upon several factors including the pressure in the furnace and the concentrations of carbon dioxide and water vapor in the combustion gasses. As used herein, the minimum calcination temperature or the calcium carbonate decomposition temperature, generally refers to the temperature at which the rate of limestone calcination is in equilibrium with the rate of recarbonation of the lime to limestone. In conventional furnace applications, the minimum calcination temperature typically ranges between about 1365° Fahrenheit (F) and 1430° F., but can extend as low as 1200° F., if the furnace operates under negative pressure and the concentration of carbon dioxide in the flue gas is low.
The rate of calcination in the furnace is dependent upon temperature, pressure, carbon dioxide and water vapor concentrations in the furnace gas and the size and quality of the limestone particles injected into the furnace. Smaller particles are heated to their core quicker which increases the overall rate of calcination of the particles. Further, carbon dioxide more readily escapes from smaller particles thereby reducing internal carbon dioxide vapor pressures which would otherwise reduce the rate of reaction.
In the second step of the process, which occurs almost simultaneously with calcination, the resultant lime reacts with sulfur dioxide and oxygen in the flue gas stream to form calcium sulfate as represented by the following formula:
CaO+SO
2
+{fraction (
1
/
2
)}O
2
→CaSO
4
.
This reaction may be referred to as quicklime sulfation. The rate at which sulfur dioxide reacts with lime in the furnace is a function of the temperature and pressure in the furnace and the concentration of sulfur dioxide in the flue gas. As used herein, the effective quicklime utilization/sulfation temperature window or envelope refers to the temperature range at which quicklime sulfation occurs at a rate sufficient to result in an appreciable amount of quicklime sulfation in the furnace. The lower end temperature of the effective quicklime utilization/sulfation temperature window, which may also be referred to as the minimum effective quicklime utilization/sulfation temperature, refers to the temperature below which the rate of quicklime sulfation is sufficiently slow to result in a negligible amount of calcium sulfate formation on the particles in the furnace taking into consideration the retention time of the flue gas in the furnace. The relatively short retention time of the flue gas in a pulverized coal type boiler or similar boiler and the relatively high quench rate drives up the minimum effective quicklime utilization/sulfation temperature which typically ranges between about 1600 to 1800° F. The amount of resultant lime conversion typically significantly decreases below about 1800° F. The upper end temperature of the effective quicklime utilization/sulfation temperature window corresponds with the decomposition temperature of calcium sulfate which in a pulverized coal type boiler or similar boiler ranges from about 2,200° to 2300° F.
In existing processes of injecting limestone into a furnace for calcination and quicklime sulfation, the limestone is injected into the furnace at temperatures within the effective quicklime utilization/sulfation temperature window and generally at the higher temperatures thereof all of which exceed the minimum calcination temperature.
Injection of limestone into the furnace for calcination and quicklime sulfation therein provides a relatively inexpensive source of lime for desulfurization, as compared to purchasing and injecting commercially available lime or constructing a separate calcination reactor for providing lime on site. However, the conditions under which the limestone particles are calcined in known furnace limestone injection and calcination processes results in lime particles with reduced reactivity due to sintering, core plugging and complex calcium compound formation from impurities, all of which result in inefficient lime utilization and therefore increased reagent costs and increased downstream auxiliary power requirements.
The efficiency of lime utilization is generally dependent upon the molar percentage of the resultant lime (or calcium ions) which is exposed to sulfur dioxide (i.e. moles of exposed lime divided by the moles of lime). By decreasing the size of the injected limestone particles, and therefore the size of the resultant lime particles, the total surface area, and therefore, the percentage of exposed lime is increased. As carbon dioxide is released from the limestone particles during calcination, pores are formed in the particles which exponentially increase the surf
Black & Veatch Corporation
Shughart Thomson & Kilroy P.C.
Tran Hien
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