WO1987006605A1

WO1987006605A1 - Pressure influenced emission sorption system process

Info

Publication number: WO1987006605A1
Application number: PCT/US1987/000893
Authority: WO
Inventors: James J. Reuther; Herman Feldmann
Original assignee: Battelle Development Corporation
Priority date: 1986-04-21
Filing date: 1987-04-20
Publication date: 1987-11-05
Also published as: JPH01500040A; EP0268613A1

Abstract

A novel process which has a positive pressure dependence for the capture of noxious emissions during thermal combustion or oxidation wherein coal is impregnated with CaO at a molar ratio not exceeding 3:1, then thermally oxidized at preferably superatmospheric pressures, typically from 1 to 4 atmospheres, such that H2S is preferentially captured rather than SO2 resulting in a more efficient sulfur capture process.

Description

PRESSURE INFLUENCED EMISSION SORPTION SYSTEM PROCESS

Background of the Invention

1. Field of the Invention

The present invention relates to processes for the control of SO2 and NO_x emissions from carbonaceous fossil fuel sources, principally coal. The present inven¬ tion relates to calcium associated combustion of coal wherein the calcium is associated with the coal in a manner resulting in a positive influence of pressure on noxious emissions, principally the reduction of SO2 and NO_x.

2. Description of Related Art

The presence of sulfurous, nitrogenous, and other inorganic contaminants are inherent to coal. The threat of acid precipitation has increased the urgency for developing technologies with which to burn coal with a minimum of SO2 and NO_x emissions. Several technologies have been developed that either suppress N0_X production during combustion (staging) and/or capture SO2 immediately after its formation during combustion (limestone injection multistage burning). Each of these in situ technologies requires major modifications to conventional pulverized coal-burning practice, involving burner replacement, furnace refitting, and use of calcium agents such as limestone. Increased capital costs and increased operating costs result. Moreover, operational problems because of increased slagging and particulate loadings also occur because of the fuel-rich conditions and the added mineral matter. It would, therefore, be technologically and economically advantageous if a process were found that could effectively control NO_x and SO2 emissions with low levels of calcium addition while enabling firing of pulverized coal at excess air conditions. In 1984, G. A. Simons, A. Gannon, and A. Boni, in an article in Chem. Phys. Processes Combust. 33/1-33/4 (1984) reported the high pressure sulfur sorption by limestone. The Simons group addressed the capture of SO2 within the pores of a CaC03 particle co-injected with the coal particles. A slight positive pressure dependence was noted at calcium to sulfur molar ratios in excess of 3 and combustor residence times greater than 1 second. The Simons process at 5 atmospheres, a Ca/S = 3, and residence time 1 second, captures 25 percent of the sulfur.

The Simons process, however, has numerous draw¬ backs. It requires high levels of calcium addition (Ca/S ratios of 3 or higher) which creates attendant slagging problems. It requires long residence times, and also requires a calcination step. More seriously, the Simons process has low sulfur capture efficiency. A more effi¬ cient process would be an advance in the art.

This invention advances the art by disclosing process which does not require calcination (conversion of CaCθ3 to CaO) and which, for example, captures 80 percent of the sulfur at a Ca/S ratio of only 1 and in 0.2 seconds of residence time.

It is an object of the present invention to disclose a process for capture of sulfur during thermal combustion of coal having high sulfur capture efficiency.

It is a further object of the present invention to disclose a process for control of sulfur and NO_x emis¬ sions from coal combustion utilizing calcium oxide in impregnated coal.

It is an object of the present invention to disclose such a process which has a positive pressure dependence and utilizes low ratios of associated calcium to sulfur, Ca/S. Brief Description of the Drawings

Figure 1 presents graphical data on percent sulfur capture as a function of pressure for different types of firings. The positive pressure dependence of sulfur capture at low Ca/S molar ratios is graphically depicted.

Figure 2 presents graphical data on NO_x emissions as a function of pressure for different types of firings.

Summary of the Invention The present invention is a process for the capture of noxious emissions such as NO_x and sulfur from the thermal combustion of coal.

The invention involves dispersing coal in a water slurry of CaO to form a calcium impregnated coal. Calcium in the form of CaO or CaOH is thereby impregnated into the pores of the coal particle. Application of pressure is unnecessary as satisfactory impregnation occurs at ambient pressures. Advantageously, the water slurry of CaO can be mildly heated to from 100 F to 160 F, with 140 F preferred. The mild heating can enhance the rate of Ca impregnation into the coal pores.

When calcium impregnated coal is placed into the oxidizing atmosphere of a combustor at temperature, a positive pressure dependence for sulfur capture has been found at low Ca/S molar ratios (<3). By the invention, H2S liberated from coal during combustion is captured by the calcium inside the pores of the coal before the H2S can be exposed to the oxidizing atmosphere outside the coal particles which would convert H2S to SO2, a less reactive composition. H2S being more reactive enables higher efficiencies in capture. The positive pressure dependency of sulfur capture at low Ca/S molar ratios means lower Ca loading levels can be used since applied pressures can be utilized, when Ca/S ratios are less than 3, in order to increase sulfur capture efficiencies. This positive pressure dependence of sulfur capture at low Ca/S molar ratios is surprising. Also surprising is the fact that above a Ca/S molar ratio of 3, the posi- tive pressure dependence effect of increased sulfur capture ceases to exist. The effect is detectable from Ca/S molar ratios of 0.6 to 3.0. The positive pressure effect appears pronounced when CaO, in other words lime, is used as the Ca source. The effect appears not to be detectable or pronounced with limestones. Calcium oxide appears to be unique for showing this positive pressure effect on sulfur capture at low Ca/S molar ratios.

Description of the Invention

The present invention is a process for the capture of noxious emissions such as NO_x and sulfur, particularly sulfur, from combustion of coal. The present invention teaches that pressure can be made to influence the effi¬ ciency of sulfur capture when coal is impregnated with CaO at a Ca/S molar ratio not exceeding 3:1 and preferably from 1:1 to 2:1.

The invention involves impregnating coal with a water slurry of CaO such that not only is calcium intro¬ duced to the surface, but in addition is introduced to the internal pores of the coal particles. Impregnation is accomplished by simple dispersing or immersing of the coal in a water slurry of CaO. Preferably, the water slurry of CaO is mildly heated to about 140 F. Ambient pressures can be used, therefore the impregnation is simple and straight forward. When this coal, with calcium on the surface and in the internal pores, is placed in a thermal reactor, a reducing environment exists within the coal particles in contrast to the thermally oxidizing environment outside the particle. More significantly, sulfur is liberated within the coal particle, primarily as H2S. This H2S is more reactive than SO2 thus the invention enables more efficient capture of sulfur, as the H2S is reacted with the impregnated calcium prior to exposure to the oxidizing environment outside the coal particle which would drive the H2S to SO2 species; thus H2S is preferentially captured by the calcium as H2S which is more reactive than SO2 species.

Some surprising implications of being able to capture sulfur from H2S rather than the oxidized SO2, come to light from the higher reactivity of H2S. Through the inventive process, pressure it has been found, can influence the efficiency of sulfur capture. It is be¬ lieved, though not with absolute certainty, that the kinetics of the higher reactivity of H2S versus SO2 give rise for this pressure dependence on the efficiency of sulfur capture. The influencing of sulfur capture by pressure during coal combustion in the presence of calcium oxides has not been previously reported in relation to a high efficiency sulfur capture process. The amount of calcium that has been either mixed with or impregnated into a coal prior to combustion or injected into a coal flame after combustion is normally expressed as the molar calcium-to-sulfur ratio, Ca/S, where the sulfur is that amount in the raw coal. A Ca/S ratio of unity would be required if all the calcium were utilized in the capture of all the fuel-bound sulfur. This perfection is never the case. Because of this, non-stoichiometric Ca/S ratios of about 2-3 are typically required to capture about 90 percent of the sulfur during atmospheric pressure combustion. ⁽2,3⁾ Hence, the actual calcium utilization is more like 30-40 percent than 100. Atmospheric pressure (15 pounds per square inch absolute, psia) represents boiler conditions; super-atmospheric pressure (65 psia or greater) represents combustion turbine conditions. Figure 1 presents data on percent sulfur capture as a function of absolute chamber pressure for seven different situations: (1) raw coal (RC) firing at a natural Ca/S ratio of 0.13, (2-4) calcium oxide impregnated coal firing at a Ca/S ratio of either 1.16, 2.21, or 3.09, (5) a physical mixture (PM) of coal and lime (calcium oxide) firing at a Ca/S ratio of 3.04, (6) raw coal firing with downstream limestone injection (LI) at a Ca/S ratio of 3.00,(4⁾ _an_ (7) _ra coal and limestone firing in a pressurized fluidized bed combustor (PFBC) operating at a Ca/S ratio of 1.10.(5⁾ Data for the first five situa¬ tions illustrated in Figure 1 as solid circles were ob¬ tained by actual coal firings; data for the other two (LI, PFBC) were obtained from literature<⁴.⁵ respectively) and are illustrated as straight lines.

Table 1 summarizes the important features of the data plotted in Figure 1. The unexpected finding was that the sulfur capture efficiency of calcium oxide impregnated coal at low levels of calcium impregnation, that is, Ca/S ratios of 3 or less, exhibited a dramatic positive pressure dependence. It had been expected that the sulfur capture efficiency of BTC would be independent of pressure at all Ca/S ratios because the mechanism for in situ sulfur capture was thought^⁾ to be dominated by the following reaction:

CaO_(s) + H₂S_(g) = CaS_(s) + H₂0_(g) (1)

It is hypothesized that sulfur capture must take place inside the pore structure of the calcium oxide impregnated coal particle where reducing conditions domi- nate and where the calcium has been impregnated. Because this chemical reaction involves gas-solid interactions on both sides of the equation, sulfur capture would be expected to be independent of pressure for any amount of added calcium. As Figure 1 shows, sulfur capture by calcium impregnated coal is nearly independent of pressure at a high Ca/S ratio of 3.09, as is that for the raw coal. That the pressure dependence of sulfur capture by calcium impregnated coal would be a function of Ca/S ratio of less than approximately 3, therefore becomes a very unexpected result.

TABLE 1. COMPARATIVE SULFUR CAPTURE PRESSURE DEPENDENCIES

Sulfur Capture

Pressure Dependence

Fuel/Combustion Ca/S Relative Relative

Conditions Ratio Direction Magnitude Ref.

Raw Coal 0.13 None 0.0

Physical Mixture 3.04 None 0.0

Calcium Oxide 3.09 Negative -0.07

Impregnated Coal

Calcium Oxide 2.21 Positive +0.44

Impregnated Coal

Calcium Oxide 1.16 Positive +0.77

Impregnated Coal

Limestone Injection 3.00 Positive +0.60 4

Pressurized Fluidized 1.10 Positive +0.27 5

Bed Combustion

The fact that a pressure dependence for sulfur capture efficiency for calcium oxide impregnated coal exists and that it becomes more positive the lower the initial Ca/S ratio, is significant in that it means that very high sulfur capture efficiencies (>80 percent) can be achieved at low levels of calcium impregnation (Ca/S<3 and preferably Ca/S<2) by burning pulverized calcium impregnated coal at superatmospheric pressures (>65 psia), such as those in future coal-fired combustion turbines.

If less calcium is required to capture the same high percentage of sulfur at superatmospheric pressure as at atmospheric pressure, operating costs and problems associated with the use of calcium for sulfur capture are reduced. This is because (1) less calcium has to be purchased to accomplish effective sulfur removal,

05 thereby lowering process costs, (2) the raw coal experi¬ ences less of a heating value loss upon calcium impregna¬ tion because less inert mineral matter is added, thereby increasing the coal fuel's calorific value, (3) less ca5J_cdiuπr-σontaining particulate ash passes through the ff' comfru^taσr, thereby lowering maintenance costs -because of shutdowns caused by internal erosion and deposition, and (4) less particulate ash has to be removed from the exhaust gas stream, thereby lowering post-combustion cleanup costs. Another important feature is that compli-

15 ance sulfur removal (>90 percent) can be achieved during high-pressure turbine combustion under single-stage, excess-air conditions.

For limestone injection to be as efficient at sulfur capture as calcium oxide impregnated coal at

20 any Ca/S ratio and at any pressure, turbines would have to be operated in a two-stage, fuel-rich/-lean mode. ⁽4⁾ The first fuel-rich stage then gives rise to aggravated slagging problems because coal ash fusion temperatures are generally lower under reducing conditions. Such

25. problems are avoided completely with calcium oxide impreg¬ nated coal in the process of the invention since sulfur can be captured under excess air conditions. Moreover, the need for fluidized bed operation to effect sulfur removal is rendered unnecessary. Thus, calcium oxide

30 impregnated coals allow the use of more conventional and less-expensive equipment.

An effort was made to contrast the unexpected pressure dependence observed for calcium oxide impregnated coal combustion at low Ca/S ratios to that witnessed

35 for other coal-firing/sulfur-capture techniques. This information has been included in Figure 1 and Table 1 in the form of examples for limestone injection (LI) ⁽4⁾ and pressurized fluidized bed combustion (PFBC). ⁽5⁾ τ+ι_e data quoted are all readily available in a search of the open literature. Data for limestone injection are from a modeling effort and not experiments. ^{( 4 )} The first important observation that should be made is that while the relative direction of the pressure dependence is the same for limestone injection, fluid bed combustion, and calcium oxide impregnated coal combustion, the- magni¬ tude of the positive pressure dependence is significantly higher for calcium oxide impregnated coal combustion. Moreover, the Ca/S ratio dependence of the positive pres¬ sure dependences for limestone injection and fluid bed combustion are opposite to that for calcium oxide impreg¬ nated coal. Specifiically, (1) the positive pressure dependence is strong for limestone injection at a Ca/S ratio in excess of 3.00 whereas it is nonexistent (slightly negative) for BTC combustion with equivalent calcium and (2) the positive pressure dependence is weak for fluid bed combustion at a low Ca/S ratio of 1.1 whereas it is strong for calcium oxide impregnated coal combustion with equivalent calcium. This opposite behavior clearly indicates that the sulfur capture mechanism for calcium oxide impregnated coal combustion is different from that operating in either limestone injection or pressurized fluidized bed combustion. This is important because it has been well established that the overall sulfur capture mechanism in operation during limestone injection and fluidized bed combustion can be described by the following reaction:

CaO(_s) + S0₂(g) + ⁱ_0₂(g) = CaS0₄(_s) (2) As a gas-solid reaction overall, a positive pressure dependence for sulfur capture would be expected; as the information in Figure 1 and Table 1 show, this is observed. Observing a pressure dependence for sulfur capture that is more dramatic for a case where it is not expected than for one where it is expected is com¬ pelling evidence that an unusual phenomenon has been discovered. It is also indirect evidence for the differ¬ ences in chemical mechanism for sulfur capture. As the Ca/S ratio increases from 1.16 to above

3, approximately 3.09, the positive pressure dependence weakens and eventually appears to disappear. This is also consistent with numerically calculated trends that as pressure is increased from 15 to 65 psia, the rate of raw coal devolatilization decreases by a factor of 1.8(6)

For the case where calcium would be most limiting, that is, calcium oxide impregnated coal with a Ca/S ratio of 1.16, sulfur capture efficiency increases from 43 percent to 76 percent, or by a factor of 1.8, as pressure is raised from 15 to 65 psia.

In limestone injection and fluidized bed combus¬ tion technologies calcium is added externally to the coal particle, thus this effect of Ca/S ratio was never contemplated.

Another aspect or added benefit of the invention involves NO_x emissions during atmospheric and pressurized combustion. Measurement of NO_x during combustion of calcium oxide impregnated coal was made. Figure 2 presents the results. Within the uncertainty of the data, N0_X production from raw coal combustion is independent of pressure. This was somewhat expected because it is known from the literature that the conversion of fuel-bound nitrogen to NO_x during nitrogen-enriched oil combustion is independent of pressure under gas turbine-firing conditions. ^{( )} - li ¬ lt is also known from the literature that NO_x emissions decrease with increasing pressure for the case of fluidized bed combustion of raw coal and limestone; (^ • these data are also shown in Figure 2. Since the suspension-firing of finely pulverized calcium oxide impregnated coal (44 microns) typically takes place at hotter gas temperatures than the fluidized- bed firing of coarse raw coal and limestone (400+ microns), 2200 F versus 1600 F, respectively, NO_x emissions from the former would be expected to be higher because such emissions are known to increase slightly with temperature.(8) For example, for fluidized bed combustion, N0_X emissions increase from about 300 pp to about 500 ppm to about as the bed temperature is raised from 1600 F to 2100 F.(8) in both instances, the contribution of thermal-NO_x to the total-NO_x is expected to be insignificant because both calcium oxide impregnated coal combustion and fluid bed combustion take place at temperatures well below that for the onset of significant thermal-NO_x, 2800 F.

The results in Figure 2 show some unexpected differences and some unexpected similarities between pulverized calcium oxide impregnated coal combustion and coarse raw coal/limestone fluid bed combustion as a function of pressure. It should be noted that the NO_x data for calcium oxide impregnated coal combustion and fluid bed combustion were taken using an Illinois No. 6 coal with 1.2 weight percent nitrogen burning at an overall gas temperature of 2100 F or 1600 F, a Ca/S molar ratio of 1.16 or 1.35, and an excess oxygen content of 12 percent or 6 percent, respectively. Because the NO_x data have been normalized to zero percent oxygen, the only major difference in operating conditions is overall gas temperature; that for calcium oxide impregnated coal, combustion was 500 F hotter. What was unexpected first was the result that within experimental uncertainty, calcium oxide impregnated coal combustion at a Ca/S ratio of 1.16 produced considerably less N0_X than fluid bed combustion at atmospheric pressure, even though it was taking place at higher temperatures. The next unexpected finding was that as the Ca/S ratio of the calcium oxide impregnated coal increased from 1.16 to 2.21, N0_X emissions dramatically increased at atmospheric pressure. Finally, and most importantly, the third unexpected finding was that as pressure increased from 15 to 65 psia, NO_x emis¬ sions from calcium oxide impregnated coal combustion at Ca/S ratios of either 1.16 or 2.21 dramatically de¬ creased and did so under single-stage, excess air conditions. The significance of these results is that a low level of calcium impregnation, Ca/S = 1.16, not only results in NO_x emissions from calcium oxide impregnated coal combustion that are lower than those from fluidized bed combustion at atmospheric pressure and equivalent Ca/S ratio, but also imparts a negative pressure dependence to N0_X production that is absent for raw coal combustion. In other words, the pressurized suspension burning of pulverized calcium oxide impregnated coal has the attri¬ butes similar to the pressurized fluidized bed combustion of raw coal and limestone, that is, simultaneous and efficient NO_x and SO2 reduction. The critical and sur¬ prising difference, however, is that these attributes can be achieved at much higher overall gas temperatures. This translates into two novel applications. First, calcium oxide impregnated coal can be fired in a pres¬ surized fluidized bed combustor instead of raw coal and limestone using higher operating temperatures, thus achieving accompanying higher overall operating efficiencies with N0_X and SO2 control. This would allow pressurized fluidized bed combustors to be simpler in design, as heat transfer tubes inserted into the bed in order to improve thermal efficiency are rendered unnecessary, and can be eliminated. Hence, the capital and operating costs of pressurized fluidized bed combustion 5 is significantly reduced by the invention. Secondly, because of the extent of simultaneous NO_x and SO2 reduction, the direct, pressurized firing of pulverized calcium oxide impregnated coal at low Ca/S ratios with a conventional burner can replace pressurized fluidized CD bed combustion technology altogether because theoretically each coal particle operates like an entire fluidized bed combustor.

The invention has broader commercial implications since it can be extended to any coal conversion system 5 employing an oxidation step, particularly a pressurized ₄oxidation step. For example, in a pressurized fixed-bed gasifier, there is an oxidation zone at the bottom that operates at approximately the same temperature as a pul¬ verized coal combustor. It is therefore reasonable to 0 anticipate that the invention applied to combustion can also be applied to gasification.

Literature Cited to Illustrate the State-of-the-Art

The following references, though not essential to an understanding of the invention disclosed herein, 5 are disclosed for purposes of indicating the background of the invention and for purposes of illustrating the state-of-the-art.

1. Feldmann, H. F. , and Conkle, H. N. , "Coal Catalysis Expands Gasifier Application Options", Energy Progress 0 3:2, 105-109 (1983).

2. Merryman, E. L., and Levy, A., "In Situ Capture of Sulfur in Combustion", Central States Section Combus¬ tion Institute, CSS/CI-82-10 (1982). 33.. Reuther, J. J. , Conkle, H. N. , and Feldmann, H. F. ,

"Evaluation of Calcium Impregnated Coal as a Fuel for Turbine Combustors", Proceedings of the Second

Annual Heat Engines Contractors Review Meeting,

05 Morgantown Energy Technology Center, U.S. Department of Energy, April 1985 (in press). 4. Boni, A. A., Garman, A. R. , Johnson, S. A., and Simons, G. A., "Mineral Matter Deposition and Sorption of Fuel-Bound Contaminants", ibid.

ID 5. Scott, R. L. , "Fluidized-Bed Combustion: Pressurized Systems", Proceedings of the First Annual Heat Engines Contractors Meeting, Morgantown Energy Technology Center, U.S. Department of Energy, F. W. Crouse, Chairman, DOE/METC/84-31 (1984).

15 6. Phouc, T. X., and Durbetaki, P., "Heat and Mass Trans¬ fer Analysis of Coal Particles Undergoing Pyrolysis", American Flame Research Committee International Flame Research Foundation International Symposium on Alterna¬ tive Fuels and Hazardous Wastes, Paper 3:1-26 (1984).

20 7. Decorso, S. M. , Pillsbury, P. W. , Bauserman, G. , Mulik, P. R., and Stein, T. R. , "Gas Turbine Combustor Performance on Synthetic Fuels", Final Report from Westinghouse Electric Corporation to the Electric Power Research Institute, EPRI AP-1623 (1980).

25 8. Theonnes, C. M. , "The General Electric CFCC Pressurized Fluidized-Bed Combined Cycle Power Generation System" , Proceedings of the Pressurized Fluidized Bed Combustion Technology Exchange Workshop, U.S. Department of Energy, CONF-7906157 (1980).

30 The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms

35 disclosed, since these are to be regarded as illustrative rather than restrictive variations and changes can be made by those skilled in the art without departing from the spirit and scope of the invention.

Example A coal-fired combustion turbine simulator was constructed by converting a continuous high-pressure reactor from a gasifier to a combustor. This device is capable of simulating the following combustion turbine conditions: • Superatmospheric pressure: 4 atmospheres.

• Maximum coal flame temperature: 2600 F.

• Maximum firing temperature: 1900-2100 F.

• Maximum liner temperature: 1000-1500 F.

• Primary fuel/air ratio: 20 percent excess air. • Overall fuel/air ratio: 50-100 percerit excess air.

• Overall residence time: 150-300 milliseconds.

• Carbon conversion efficiency: 98+ percent.

To begin a combustion experiment, four electrical heaters (Lindberg) were used to raise the internal wall temperature of the combustion chamber to about 1400 F.

This preheating shortened the time required to reach steady-state operating conditions.

Pulverized coal was then pneumatically conveyed from a pressurized hopper to the pressurized combustion chamber using compressed primary air. Primary air amounted to 20-30 percent of the total air required for combustion at 20 percent excess air overall. Secondary air made up the remainder. The pulverized coal feeder was of the rotating disk type. Feed rates of pulverized coal were typically 2-3 pounds per hour (20-30,000 Btu/hr), ±5%. This feed rate produced a combustion intensity of about 20,000 Btu/ft^-hr. Once the pulverized/feed was stabilized, a premixed methane/air pilot flame was then:, used to ignite the coal feed at atmospheric pressure. The pilot flame input was about 10,000 Btu/hr.

The coal burner was a simple arrangement of concentric nozzles, one for the primary air and coal

05 premixture, and the other the secondary air. These streams were nozzle-mixed prior to entry into the combustor chamber proper. Combustion turbine temperatures were monitored using. Inconel-sheath Chrome1-Alumel thermocouples. Temperatures were continuously monitored down the vertical

350- axϋa σf^" the combustor at single positions in the pre—flame, flame-,- firing, and post-flame zones. This positioning allowed a near-field combustion temperature profile to be obtained, including the peak temperature and the "firing temperature". Firing temperature was defined as the

15 temperature achieved after combustion products have been mixed with the tertiary, or cooling, air. Wall or liner temperatures could also be determined down the furnace axis.

Firing temperature was adjustable over a 200 F

20 range to about 2000 F by either varying the amount of tertiary dilution air introduced or by oxygen enriching the secondary air to various levels at low tertiary air levels. The overall fuel-air ratio was varied over 50-100 percent excess air.

25. The combustion test plan consisted of experiments designed to determine the comparative in situ sulfur capture efficiency, under simulated combustion turbine conditions of:

• Two raw pulverized (30-50 microns) bituminous 30 coals.

• Pulverized BTC prepared from these raw coals.

• Physical mixtures of the raw pulverized coal and pulverized lime (of different quality) with the same amount of calcium

35 added as for the CaO impregnated coal. In order to increase the statistical significance of the data resulting from the coal-fired combustion turbine simulation experiments, two lots of high-sulfur raw coal from the same seam, Illinois No. 6, were selected for study.

Inspection of survey data reveals that there are over 20 distinctly different Illinois No. 6 coals. Of all the components of the ultimate and proximate analyses for Illinois No. 6 seam coal, the one that varies the most is total sulfur. The weight percent of total sulfur in Illinois No. 6 seam coal varies from 1.16 to 8.2, with the average and standard deviation being 3.9 ± 1.7.

A lot of Illinois No. 6 coal possessing this average total sulfur content was selected for combustion turbine simulation. Henceforth, it shall be referred to as Raw Coal 1, or RC1.

The ratio of organic-to-inorganic sulfur can vary from 0.5 to 2.6. For the 10 different Illinois No. 6 seam coals that are within ±10 percent of the average total sulfur content of 3.9 percent, the organic-to-inorganic sulfur ratio ranges from 0.5 to 1.8, and averages 1.0 ± 0.49. It should be noted that the first lot of coal selected for testing not only had the average sulfur content of Illinois No. 6 seam coal, but also a near-average organic-to-inorganic sulfur ratio of 0.9 (that is, 47 percent of the total sulfur was organic sulfur) .

Because of this variability in the ratio or organic-to-inorganic sulfur, a second lot of Illinois No. 6 coal was selected for simulated turbine test firing. It was chosen so as to have the same total sulfur as average Illinois No. 6 coal (about 3.9 weight percent), but with different (higher) organic sulfur content (RC2). There were several reasons why an organic sulfur- enriched Illinois No. 6 coal was selected for test firing.

The first reason was that it allowed the effect of sulfur form on sulfur capture to be studied directly. Second, it allowed comparisons to be made among sulfur control techniques. Physical coal beneficiation might be directly competitive with CaO impregnated coal in terms of reduction of SO2 emissions if the coal being cleaned had a high fraction or inorganic sulfur. This is because physical beneficiation can only remove part of the inorganic sulfur fraction. If the bulk of the sulfur contained by a coal were of the organic type, physical beneficiation would be incapable of removing much sulfur at all. While chemical beneficiation could reduce the total sulfur content of an organic sulfur-rich coal, its economics are still rather exhorbitant and its commercialization still to be accomplished.

Third, because it had been experimentally demon¬ strated that CaO impregnated coal could capture about 90 percent of the sulfur emissions from a flame of the average Illinois No. 6 coal, it was deduced that such sulfur reduction was independent of sulfur form. By experimentally demonstrating a similar high level of sulfur capture for an Illinois No. 6 CaO impregnated coal, naturally rich in organic sulfur, this deduction would be proven directly.

The in situ sulfur capture efficiency of calcium- doped coals was studied under simulated combustion turbine conditions as a function of the following parameters: • Raw coal sulfur form (organic vs. inorganic).

• Molar calcium-to-sulfur ratio (Ca/S = 1, 2, or 3).

• Calcium source (technical or commercial-grade lime) . Listed in parentheses are how these parameters were varied. TABLE 2. SIGNIFICANT RESULTS OF TESTS TO STUDY THE EFFECT OF ORGANIC SULFUR CONTENT AND CALCIUM SOURCE ON SULFUR CAPTURE AS A FUNCTION OF PRESSURE, CALCIUM LOADING TECHNIQUE, AND Ca/S RATIO

Sulfur Capture

05 Organic Efficiency

Arbitrary Total Sulfur Ca/S

Sample Sample Code Sulfur (%, Total Calcium Ratio @15 PSIA @70 PSIA

Number Designation (wt-%, DAF) Sulfur) (wt-%, DAF) (Molar) (%) (%)

1 RCl-RWC-O-N 4.50 47 0.52 0.09 17 ± 9 13 ± 6

10 2 RC2-RWC-0-N 4.75 72 0.45 0.08 12 ± 5 18 + 9

3 RCl-CTC-1-T 4.76 47 6.93 1.16 43 ± 4 76 ± 5 i

4 RC1-CTC-2-T 4.25 47 11.75 2.21 55 ± 5 79 ± 5

5 RC1-CTC-3-T 4.36 47 16.88 3.09 94 + 1 87 + 1 «^>

6 RCl-CTC-1-C 4.85 47 7.97 1.31 60 + 5 81 + 6 1

15 7 RC1-CTC-3-C 4.34 47 22.30 4.11 83 + 7 95 ± 3

8 RC2-CTC-1-T 4.87 72 7.02 1.15 68 ± 2 82 + 2

9 RC2-CTC-3-T 4.53 72 18.34 3.23 86 ± 1 96 ± 2

10 RC2-CTC-1-C 5.01 72 7.08 1.13 61 + 12 80 ± 3

11 RC2-CTC-3-C 4.77 72 20.90 3.50 75 + 4 92 + 3

20 12 RC1-PM-3-T 4.57 47 17.37 3.04 58 ± 7 67 + 7

13 RC2-PM-1-C 5.25 72 6.34 0.97 35 ± 10 57 ± 18

14 RC2-PM-3-C 4.89 72 18.96 3.10 59 ± 7 74 ± 3

RWC = raw coal PM = physical mixture of coal & lime

25 RC1, RC2 = sample of Illinois coal #6 CTC = CaO impregnated coal

Two types of calcium loading techniques were directly compared in this program:

• Calcium oxide impregnation with a water slurry of CaO. • Physical mixtures of raw coal and lime (PM).

Calcium loading via the CaO impregnation process resulted in far better in situ sulfur capture than the simple physical mixing of coal and lime at an equivalent molar calcium-to-sulfur ratio. Samples of each type of calcium-loaded coal were prepared from the two Illinois No. 6 seam coals. The difference between the two seams was that one's (RC2) total sulfur (4.75%, DAF) was predominantly organic (72%), whereas the other's (RCl) consisted of a near-equal split between organic and inorganic sulfur. The two raw coals had near-equivalent total sulfur (4.50%, DAF) content, however.

To produce CaO impregnated coal, coarse raw coal (RCl or RC2) , 100 percent minus 20 mesh (850 microns), was first immersed in a water slurry of CaO at about 140 F to increase its molar Ca/S ratio. Excess water was decanted. Either technical-grade lime (99 percent CaO, Fisher Scientific) or commercial-grade lime was used as the source of calcium. These technical-grade and commercial-grade limes will be designated as TGL and CGL, respectively.

The difference between these calcium sources is purity and cost, with the commercial grade being less pure and less expensive. Within the uncertainty in the data in Table 2, the following observations can be made:

• Atmospheric pressure, raw Illinois No. 6 coals naturally captured about 15 percent of their fuel-bound sulfur. This inherent capture is comparable to that determined elsewhere for high-sulfur eastern bituminous coals.

• Inherent sulfur capture by raw Illinois No. 6 seam coal appears to be independent

05 of sulfur form, i.e., organic vs inorganic.

• At atmospheric pressure, the sulfur capture efficiency of CaO impregnated coal with a Ca/S ratio of about 3 was 85 ±8 percent. This sulfur capture is almost 5 times greater

Lff than that of the raw coal under similar conditions, and is independent of calcium oxide source.

• Sulfur capture by Illinois No. 6 seam coals treated via the calcium impregnation appears

15 to be independent of sulfur form and grade of lime at both low and high Ca/S ratios.

• Sulfur capture at low levels of calcium impregnation (Ca/S<2) exhibits a positive pressure dependence over the entire range

20 of pressures studied.

• At atmospheric pressure, the physical mixtures of raw pulverized coal and lime with a Ca/S ratio of about 3 captured about 58 percent of their fuel-bound sulfur. This level

25 of sulfur retention is comparable to that observed by others elsewhere when cofiring pulverized coal and limestone under atmo¬ spheric pressure, staged-combustion conditions.

30 • Sulfur capture by CaO impregnation made from Illinois No. 6 seam coal is at least 1.5 times more efficient than that achievable upon physically mixing raw coal with lime.

Claims

1. A process for capture of noxious emissions during thermal oxidation of coal comprising: establishing a water slurry of CaO,

05 impregnating coal particles with the water slurry of CaO such that Ca is introduced to the surface and into the internal pores of the coal particles and such that the Ca to S molar ratio of the coal particles dues not exceed 3:1,

ID. decanting the water slurry from the coal, parti¬ cles after impregnation of the coal particles such that the impregnated coal particles are essentially dry particles, introducing the calcium impregnated coal parti-

15 cles having a Ca to S molar ratio that does not exceed 3:1 into a thermal oxidation zone of a reactor, applying heat and a pressure greater than atmo¬ spheric to the calcium impregnated coal particles such that a reducing atmosphere is created within the coal

20 particles sufficient to liberate H2S and a thermally oxidizing atmosphere on the outside of the coal particles, substantially capturing the sulphur of the liberated H2S within the coal particle by the calcium impregnated to the coal particles through formation of

25 calcium sulfide before thermal oxidation of the coal particle is completed.

2. The process according to claim 1 wherein the calcium to sulfur molar ratio is from 0.6 to 3.

3. The process according to claim 1 wherein 30 the calcium to sulfur ratio is from 1:1 to 2:1.

4. The process according to claim 1 wherein the applied pressure is from one to four atmospheres.

5. The process according to claim 1 wherein the heat applied in the thermal oxidation zone of the

35 reactor is from 1600 F - 2600 F.

6. The process according to claim 1 wherein NO_x emissions are captured by the calcium in addition to sulfur.

7. The process according to claim 1 wherein the water slurry of CaO is mildly heated to not more than 160 F.

8. The process according to claim 7 wherein the water slurry of CaO is heated to 140 F.