Enzyme treatment of coal for mercury remediation

Abstract

Emissions of mercury, NOx, and/or SOx are reduced by enzyme treating coal before combustion, optionally with further treatment of the coal with certain non-bromine containing powder sorbents. y using the steps together, mercury can be reduced by 40% or more, and NOx by 20% or more. Advantageously, no bromine is introduced with the remediation steps.

Claims

1. A method of burning coal to reduce emissions of mercury and nitrogen oxides (NOx), comprising: treating coal with an enzyme composition comprising an enzyme and a pH buffer by applying the enzyme composition onto the coal and reacting for a time period to make an enzyme treated coal; combusting the enzyme treated coal in the presence of a single powder sorbent composition comprising silica, alumina, and calcium in the furnace of a coal burning facility to create heat energy and flue gases to reduce mercury emissions by at least 40% relative to mercury emissions and NOx emissions by at least 20% relative to NOx emissions achieved when coal is combusted without the enzyme treatment and without using the single powder sorbent composition, wherein the single powder sorbent composition comprises one or more components selected from the group consisting of: an aluminosilicate clay, calcium-containing aluminosilicate minerals, Portland cement, cement kiln dust (CKD), clinker, cement kiln feed, transition cement, clinker grind outs, limestone, lime, lime kiln dust, sugar beet lime, slag, calcium oxide, calcium hydroxide, montmorillonite, sodium montmorillonite, kaolin, and combinations thereof, wherein the single powder sorbent composition is the only sorbent composition used before, during, or after the combusting and wherein no additional halogens or halogen-containing compounds are added; measuring the amount of mercury in the flue gases; and adjusting the treating step if the measured amount of mercury is above or below a target level, wherein the enzyme composition comprises water and the enzyme at a pH at which the enzyme is active.

2. The method according to claim 1, wherein adjusting the treating step comprises applying a greater amount of the enzyme composition, applying a lesser amount of the enzyme composition, reacting for a shorter time, or reacting for a longer time.

3. The method according to claim 1, wherein the enzyme composition comprises an oxidoreductase classified as EC 1 in the EC number classification of enzymes.

4. The method according to claim 1, wherein the enzyme composition comprises an oxidase that catalyzes an oxidation reduction reaction involving molecular oxygen as electron acceptor.

5. The method according to claim 1, wherein the enzyme composition comprises one or more components of the pyruvate dehydrogenase complex.

6. The method according to claim 1, wherein the enzyme composition comprises NAD, NADP, or FADP.

7. The method according to claim 1, wherein the enzyme comprises laccase, pyruvate dehydrogenase, dihydrolipoyl transacetylase, or dihydrolipoyl dehydrogenase, and the cofactor comprises coenzyme A, CoA-SH, thiamine pyrophosphate, lipoic acid, flavin adenine dinucleotide, or nicotinamide adenine dinucleotide.

8. The method according to claim 1, further comprising applying the single powder sorbent composition onto the coal before combustion or feeding the single powder sorbent composition into the furnace along with the coal as the coal is fed to the furnace for combustion.

9. The method according to claim 8, wherein the single powder sorbent composition comprises an aluminosilicate clay and one or more of Portland cement and cement kiln dust.

10. The method according to claim 9, wherein the single powder sorbent composition comprises less than 0.5% by weight Na.sub.2O, less than 0.5% by weight K.sub.2O, or less than 0.5% by weight chlorine.

11. The method according to claim 10, wherein the single powder sorbent composition comprises less than 0.1% by weight Na.sub.2O, less than 0.1% by weight K.sub.2O, and less than 0.5% by weight chlorine.

12. A method for reducing mercury and nitrogen oxides (NOx) emissions from a coal burning plant, comprising: enzyme treating the coal by applying an enzyme and pH buffer; and applying a single powder sorbent composition comprising silica, alumina, and calcium to the enzyme treated coal; combusting the enzyme treated coal in the presence of the single powder sorbent composition to reduce mercury emissions of the coal burning plant by at least 40% as compared to mercury emissions and NOx emissions by at least 20% relative to NOx emissions achieved when coal is combusted without the enzyme treating and without using the single powder sorbent composition, wherein the single powder sorbent composition comprises one or more components selected from the group consisting of: an aluminosilicate clay, calcium-containing aluminosilicate minerals, Portland cement, cement kiln dust (CKD), clinker, cement kiln feed, transition cement, clinker grind outs, limestone, lime, lime kiln dust, sugar beet lime, slag, calcium oxide, calcium hydroxide, montmorillonite, sodium montmorillonite, kaolin, and combinations thereof and the single powder sorbent composition is the only sorbent composition used before, during, or after the combusting and wherein no additional halogens or halogen-containing compounds are added.

13. The method according to claim 12, wherein the one or more components comprises an aluminosilicate clay.

14. The method according to claim 12, wherein the one or more components comprises Portland cement or cement kiln dust.

15. The method according to claim 12, wherein the single powder sorbent composition comprises less than 0.5% by weight Na.sub.2O, less than 0.5% by weight K.sub.2O, or less than 0.5% by weight chlorine.

16. The method according to claim 12, wherein the single powder sorbent composition comprises less than 0.1% by weight Na.sub.2O, less than 0.1% by weight K.sub.2O, and less than 0.5% by weight chlorine.

17. The method according to claim 12, wherein the single powder sorbent composition additionally comprises a calcium powder.

18. The method according to claim 1, wherein the enzyme composition further comprises a cofactor.

19. A method of burning coal to reduce emissions of mercury and nitrogen oxides (NOx), comprising: treating coal with an enzyme composition comprising an enzyme, a pH buffer, and water, the enzyme composition having a pH at which the enzyme is active and applied onto the coal at greater than or equal to about 1 to less than or equal to about 10 part per million (ppm) of coal and reacting for a time period to make an enzyme treated coal; combusting the enzyme treated coal in the presence of a single powder sorbent composition comprising silica, alumina, and calcium added to the enzyme-treated coal at a rate of greater than or equal to about 0.1 to less than or equal to about 1% by weight in the furnace of a coal burning facility to create heat energy and flue gases to reduce mercury emissions by at least 40% relative to mercury emissions and nitrogen oxides (NOx) emissions by at least 20% relative to NOx emissions achieved when coal is combusted without the enzyme treatment and without using the single powder sorbent composition, wherein the single powder sorbent composition comprises one or more components selected from the group consisting of: an aluminosilicate clay, calcium-containing aluminosilicate minerals, Portland cement, cement kiln dust (CKD), clinker, cement kiln feed, transition cement, clinker grind outs, limestone, lime, lime kiln dust, sugar beet lime, slag, calcium oxide, calcium hydroxide, montmorillonite, sodium montmorillonite, kaolin, and combinations thereof and the single powder sorbent composition is the only sorbent composition used before, during, or after the combusting and wherein no additional halogens or halogen-containing compounds are added; and measuring the amount of mercury in the flue gases; and adjusting the treating step if a measured amount of mercury is above or below a target level.

20. The method according to claim 19, wherein the enzyme is selected from the group consisting of: laccase-isozyme, pyruvate dehydrogenase, dihydrolipoyl transacetylase, or dihydrolipoyl dehydrogenase.

21. The method according to claim 19, wherein the enzyme comprises laccaseisozyme.

Description

EXAMPLES

Example 1

Fuel Preparation

(1) Bituminous coal was a sample obtained from the coal pile at an electric utility.

(2) The as-received feedstock coal was inspected for surface moisture on receipt and floor-dried as necessary. The air-dried sample was crushed to -inch top size and fed to a hammer mill pulverizer, creating a size distribution of approximately 70 wt % passing 200 mesh for use during testing.

(3) The pulverized coal was split into a feedstock sample and two coal samples that were processed into refined coals. The refined coals were prepared by laying out weighed quantities (about 500 lb) on the floor of the coal preparation facility. Weighed quantities of an enzyme solution, ammonium acetate, and powder sorbent were applied to the coal, which was periodically mixed while the sorbents were applied. The powder sorbent was distributed directly by hand, making several passes over the extent of the coal pile, with mixing of the fuel after each pass. The aqueous enzyme solution and ammonium acetate were placed in a small pressurized metal spray canister such that the spray canister nozzle produced a mist that was applied to the exposed surface of the pile. Treatment required several passes to completely distribute the sorbents. After each pass, a rake was used to turn the pile over, exposing new surface for the next treatment pass. In each case, several small portions of the sorbents were distributed over the coal pile, followed by mixing until the specified treatment rate was achieved. After treatment, the refined coals were homogeneous products that are comparable to that produced at refined coal facilities.

(4) Each of the samples (feedstock coal and refined coals) was transferred to storage hoppers for use in the pilot-scale testing described below. These storage hoppers sit directly above the coal feed hopper during testing. A rotary valve is used to transfer the feedstock and refined coal samples from the storage hoppers to the feed hopper. The storage hoppers and feed hopper are cleaned with a dilute acid solution after each test to remove any trace of the treated fuel.

(5) The as-fired feedstock coal (with no powder sorbent and no enzyme treatment) had a heating value of 12,964 Btu/lb at a moisture content of 3.05 wt %. Moisture-free heating value and ash content were determined to be 13,371 Btu/lb and 9.56 wt %, respectively. The sulfur content was determined to be 1.10 wt % (1.635 lb SO.sub.2/MMBtu) on a moisture-free basis. The Hg content was analyzed at 0.0867 g/g (6.484 lb Hg/TBtu, dry basis), with a chlorine content of 921 g/g (dry basis).

(6) The as-fired coal 1 (enzyme treated only) had a heating value of 13,071 Btu/lb at a moisture content of 2.54 wt %. Moisture-free heating value and ash content were determined to be 13,412 Btu/lb and 9.34 wt %, respectively. The sulfur content was determined to be 1.10 wt % (1.653 lb SO.sub.2/MMBtu) on a moisture-free basis. The Hg content was analyzed at 0.0824 g/g (6.144 lb Hg/TBtu, dry basis), with a chlorine content of 915 g/g.

(7) The as-fired Refined coal 2 (powder sorbent and enzyme treated) had a heating value of 13,026 Btu/lb at a moisture content of 2.77 wt %. Moisture-free heating value and ash content were determined to be 13,396 Btu/lb and 9.63 wt %, respectively. The sulfur content was determined to be 1.11 wt % (1.653 lb SO.sub.2/MMBtu) on a moisture-free basis. The Hg content was analyzed at 0.0849 g/g (6.144 lb Hg/TBtu, dry basis), with a chlorine content of 943 g/g.

Example 2

Operating Conditions

(8) The early-morning hours were used to establish baseline emissions from the combustion of the feedstock coal fired at an average rate of 42.46 lb/hr (0.550 MMBtu/hr) to achieve an average FEGT of 2169 F. Excess oxygen was controlled to an average of 3.06% (about 17.03% excess air) at the furnace exit, with tertiary air utilized at an average 21.71% of total combustion air.

(9) The Refined coal 1 fired during the late-morning and early-afternoon hours was treated with 0.0010 wt % (10.0 ppm) enzyme and 0.00083 wt % (8.3 ppm) ammonium acetate. This coal was fired at an average rate of 42.65 lb/hr (0.557 MMBtu/hr), achieving an average FEGT of 2160 F. at an average excess oxygen level of 3.03% (about 16.87% excess air) at the furnace exit, with tertiary air utilized at an average 21.97% of total combustion air. During this refined coal test period, the NO.sub.x emission reduction goal was not achieved, while the Hg emission reduction goal was achieved.

(10) The Refined coal 2 fired during the late-afternoon hours was treated with 0.0010 wt % (10.0 ppm) enzyme, 0.00083 wt % (8.3 ppm) acetate, and 0.25 wt % powder sorbent. This coal was fired at an average rate of 44.07 lb/hr (0.576 MMBtu/hr), achieving an average FEGT of 2151 F. at an average excess oxygen level of 2.99% (about 16.60% excess air) at the furnace exit, with tertiary air utilized at an average 21.97% of total combustion air. During this refined coal test period, both NO.sub.x and Hg emission reduction goals were achieved.

Example 3

Emissions

(11) Flue gas emissions sampled by continuous emissions monitors (CEMs) were obtained from the duct at the outlet of the furnace and wet scrubber (WS). Hg emissions sampled by continuous mercury monitors (CMMs) were also collected at the WS outlet. The stack emissions (WS outlet) were used to calculate the reductions in NO.sub.x and total Hg and were corrected to 3.0% O.sub.2 to account for system air in-leakage at the WS outlet. All reductions reported here were calculated on a mass basis (lb/MMBtu or lb/TBtu).

(12) A summary of the CTF operating conditions and mass emission levels for feedstock and refined coals at the WS outlet is provided in Table 1 with the resultant emission reduction levels at the WS outlet noted during the coal test periods provided in Table 2.

(13) TABLE-US-00002 TABLE 1 Summary of CTF Operating Conditions and Mass Emission Levels Fuel Identification: Feedstock Coal Refined Coal 1 Refined Coal 2 Start of Test 0650 1121 1521 End of Test 1040 1446 1742 Reporting Period, hr:min 1:00 1:00 1:30 Refined Coal Treatment, wt % Enzyme 0.0 0.0010 0.0010 Acetate 0.0 0.00083 0.00083 Powder Sorbent 0.0 0.0 0.25 Fuel Feed Rate, lb/hr 42.46 42.65 44.07 Firing Rate, MMBtu/hr 0.550 0.557 0.576 FEGT, F. 2169 2160 2151 Tertiary Air, scfm 22.90 22.90 23.63 Tertiary Air, % 21.71 21.97 21.97 Wet Gas Flow, scfm 119.37 119.07 121.63 Dry Gas Flow, scfm 110.00 109.80 112.11 Flue Gas Analysis Furnace WS Furnace WS Furnace WS O.sub.2, % 3.06 3.70 3.03 3.73 2.99 3.59 CO.sub.2, % 15.83 15.26 15.79 15.17 15.82 15.30 SO.sub.2, ppm 810 40 790 25 770 36 NO.sub.x, ppm 305 293 340 305 236 226 CO, ppm 18 12 18 11 21 13 Excess Air, % 17.03 16.87 16.60 Corrected to 3.0% O.sub.2 Furnace WS Furnace WS Furnace WS NO.sub.x, ppm 306 305 340 318 236 234 SO.sub.2, ppm 812 42 792 26 770 37 Hg.sub.(T), g/dNm.sup.3 0.165 0.090 0.082 Mass Emission Rates WS Out WS Out WS Out NO.sub.x, lb/hr 0.231 0.240 0.182 NO.sub.x, lb/MMBtu 0.420 0.431 0.316 SO.sub.2, lb/hr 0.044 0.028 0.040 SO.sub.2, lb/MMBtu 0.080 0.050 0.069 Hg.sub.(T), lb/hr 6.81 10.sup.8 4.28 10.sup.8 3.44 10.sup.8 Hg.sub.(T), lb/TBtu 0.124 0.066 0.060
NO.sub.x Emissions

(14) There is a time delay between initial firing of the refined coal and the moment at which NO.sub.x reduction is calculated, and sufficient time was allotted to achieve the desired NO.sub.x reduction before calculations were made. Because the sorbents used to produce the refined coal have no heating value and emission reductions are reported on a lb/MMBtu basis, there is no dilution in emissions attributable to the sorbents.

(15) Corrected NO.sub.x emissions obtained during combustion of the feedstock coal averaged 305 ppm (0.420 lb NO.sub.x/MMBtu) at the WS exit. Corrected NO.sub.x emissions sampled during the firing of the Refined coal 1 treated with an enzyme concentration of 0.0010 wt % (10.0 ppm) and 0.00083 wt % (8.3 ppm) acetate increased to an average of 318 ppm (0.431 lb NO.sub.x/MMBtu), representing an increase of 2.62% from feedstock levels.

(16) Corrected NO.sub.x emissions sampled during the firing of the Refined coal 2 treated with 0.0010 wt % (10.0 ppm) enzyme, 0.00083 wt % (8.3 ppm) acetate, and 0.25 wt % S-Sorb were reduced to an average of 234 ppm (0.316 lb NO.sub.x/MMBtu), representing a 24.76% reduction from feedstock emission levels.

(17) Hg Emissions

(18) A CMM was installed at the WS outlet during each test period. Sampling activities characterized the feedstock Hg emissions and those obtained during combustion of the refined coal. For each test period, the individual Hg data points were corrected to a constant flue gas O.sub.2 concentration and also for the CO.sub.2 concentration that was removed during each reporting period. Because the sorbents used to produce the refined coal have no heating value and emission reductions are reported on a lb/TBtu basis, there is no dilution in emissions attributable to the sorbents.

(19) TABLE-US-00003 TABLE 2 Summary of NO.sub.x and Hg Emission Levels - WS Outlet and BH Outlet NO.sub.x Results: NO.sub.x, NO.sub.x, ppm NO.sub.x, NO.sub.x O.sub.2, % ppm corrected to 3.00% O.sub.2 lb/MMBtu Reduction, % Feedstock Coal 3.70 293 305 0.420 Refined Coal 1 3.73 305 318 0.431 2.62 Refined Coal 2 3.59 226 234 0.316 24.76 Hg.sub.(T) Results: Hg.sub.(T), g/dNm.sup.3 Hg.sub.(T), Hg O.sub.2, % CO.sub.2, % corrected to 3.00% O.sub.2 lb/TBtu Reduction, % WS Outlet: Feedstock Coal 3.70 15.26 0.165 0.124 Refined Coal 1 3.73 15.17 0.089 0.066 46.77 Refined Coal 2 3.59 15.30 0.082 0.060 51.61 BH Outlet: Feedstock Coal 3.70 15.26 1.239 0.972 Refined Coal 1 3.73 15.17 0.823 0.719 42.83 Refined Coal 2 3.59 15.30 0.758 0.664 47.79
SO.sub.2 Emissions

(20) For the test series documented here, 90%-95% WS efficiency was specified and was determined from the data collected by the CEMs at the furnace exit and the WS outlet.

(21) The corrected SO.sub.2 emissions during firing of the feedstock coal averaged 812 ppm (1.557 lb SO.sub.2/MMBtu) before and 42 ppm (0.080 lb SO.sub.2/MMBtu) after the WS system, which equates to a 94.86% removal efficiency. Corrected SO.sub.2 emissions resulting from combustion of the Refined coal 1 averaged 792 ppm (1.493 lb SO.sub.2/MMBtu) before and 26 ppm (0.050 lb So.sub.2/MMBtu) after the WS system, which equates to a 96.65% removal efficiency. Corrected SO.sub.2 emissions resulting from combustion of the Refined coal 2 averaged 770 ppm (1.446 lb SO.sub.2/MMBtu) before and 37 ppm (0.069 lb SO.sub.2/MMBtu) after the WS system, which equates to a 95.23% removal efficiency.

Example 4

Fly Ash Analysis

(22) Fly ash was collected by the BH during both the feedstock and refined coal test periods. Average BH inlet temperature was approximately 300 F. during the tests. After completion of the feedstock and refined coal test periods, the BH was back-pulsed, and the fly ash was collected and submitted for bulk inorganic elemental oxide composition by XRF analysis and for Hg, Cl, and ash carbon content. A summary of these analyses and the BH operating conditions is documented in Table 3

(23) TABLE-US-00004 TABLE 3 Fly Ash Analysis-BH Hopper Catch Fuel Identification: Feedstock Coal Refined Coal 1 Refined Coal 2 BH Inlet Temperature, F. 302.30 303.17 302.83 XRF Ash Analysis, wt % As-Rec. SO.sub.3-Free As-Rec. SO.sub.3-Free As-Rec. SO.sub.3-Free SiO.sub.2 50.80 50.95 51.23 51.36 50.33 50.60 Al.sub.2O.sub.3 28.70 28.78 28.89 28.96 28.49 28.64 Fe.sub.2O.sub.3 11.48 11.52 11.13 11.16 11.06 11.12 TiO.sub.2 2.17 2.17 2.14 2.15 2.10 2.12 P.sub.2O.sub.5 0.20 0.20 0.19 0.19 0.21 0.21 CaO 1.16 1.16 1.12 1.12 2.24 2.25 MgO 1.33 1.33 1.29 1.30 1.30 1.30 Na.sub.2O 0.34 0.34 0.34 0.34 0.34 0.34 K.sub.2O 3.53 3.54 3.41 3.42 3.41 3.43 SO.sub.3 0.29 0.26 0.52 Hg, g/g, dry 0.89 1.20 1.29 Cl, g/g, dry 65.2 48.8 60.9 Carbon, wt % 8.00 8.19 6.38

(24) The bulk inorganic chemistry of the fly ash samples obtained from the BH during the feedstock and refined coal test periods indicated that the feedstock and Refined coal 1 fly ash are compositionally similar. The Refined coal 2 fly ash sample is enriched in CaO and SO.sub.3 with a corresponding depletion in SiO.sub.2 and Fe.sub.2O.sub.3 relative to the feedstock fly ash. Hg content of the feedstock fly ash was determined to be 0.89 g/g, with a chlorine content of 65.2 g/g. In the Refined coal 1 fly ash, the Hg content was 1.20 g/g, with a chlorine content of 48.8 g/g. In the Refined coal 2 fly ash, the Hg content was 1.29 g/g, with a chlorine content of 60.9 g/g. Fly ash carbon content was determined to be 8.00 wt % in the feedstock ash, 8.19 wt % in the Refined coal 1 ash, and 6.38 wt % in the Refined coal 2 ash.

Example 5

Toxicity Characteristic Leaching Procedure

(25) Toxicity characteristic leaching procedure (TCLP) was conducted on the feedstock and refined coal ash samples to determine the concentrations of total Resource Conservation and Recovery Act (RCRA) contaminant metals (arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver). TCLP is a sample extraction method for chemical analysis employed as an analytical method to simulate leaching through a landfill. The TCLP procedure is generally useful for classifying waste material for disposal options. The concentrations of pollutants in the TCLP leachate are analyzed for substances appropriate to the protocol; for the purposes of this report, the RCRA contaminant metals. The TCLP analysis results for the fly ash generated during combustion of the feedstock and refined coal are given in Table 4.

(26) TABLE-US-00005 TABLE 4 Summary of TCLP Results for Total RCRA Metals - BH Ash Drinking Fuel Feedstock Refined Refined RCRA Water Description: Ash Ash 1 Ash 2 Limit, g/L MCL,.sup.1 g/L Analyte, g/L Arsenic 42.0 49.5 60.0 5000 50 Barium 278 228 254 100,000 1000 Cadmium 14.5 13.6 15.3 1000 5 Chromium 55.6 54.0 56.0 5000 50 Lead 11.2 12.6 9.6 5000 15 Mercury <0.1 <0.1 <0.1 200 2 Selenium 90.9 92.9 75.3 1000 50 Silver <0.5 <0.5 <0.5 5000 50 .sup.1Maximum contamination level

(27) Burning of the Refined 1 CAPP bituminous coal (enzyme treated only) resulted in a reduction of less than 20% in NO.sub.x emissions and at least 40% in Hg emissions as compared to the burning of the feedstock coal after adjustment for the reduced thermal energy of the refined coal as required by Section 45 of the Internal Revenue Code. Burning of the Refined 2 CAPP bituminous coal resulted in a reduction of at least 20% in NO.sub.x emissions and at least 40% in Hg emissions as compared to the burning of the feedstock coal after adjustment for the reduced thermal energy of the refined coal as required by Section 45 of the Internal Revenue Code.

(28) The Refined coal 1 was treated with 0.0010 wt % (10.0 ppm) enzyme and 0.00083 wt % (8.3 ppm) acetate, resulting in an average NO.sub.x emission increase of 2.62% and an average total Hg emission reduction of 46.77% at the WS outlet and 42.83% at the BH outlet when compared with feedstock coal NO.sub.x and Hg emission levels. The Refined coal 2 was treated with 0.0010 wt % (10.0 ppm) enzyme, 0.00083 wt % (8.3 ppm) acetate, and 0.25 wt % powder sorbent, resulting in an average NO.sub.x emission reduction of 24.76% and an average total Hg emission reduction of 51.61% at the WS outlet and 47.79% at the BH outlet when compared with feedstock coal NO.sub.x and Hg emission levels.