GB2104796A - Reduction of NOx emission from fluidized bed combustion systems - Google Patents

Abstract

A nitrogen-containing fuel, e.g. coal, is burnt in air-fluidized bed (20) of non-catalytic material under substoichiometric conditions; the gases pass to a secondary combustion zone (46), where carbon monoxide (CO) and unburned hydrocarbons, both present in the first stage off-gas, are combined with additional air admitted at the air inlet (40). CO levels in the final effluent are reduced with little or no increase in NOx emissions. <IMAGE>

Description

SPECIFICATION Fluidized bed combustion systems The invention relates to fuel burning fluidized bed combustion systems. More particularly, the invention relates to an apparatus and method for the reduction of NOx emissions from a fluidized bed combustion system.

In U.S. Patent No: 4,054,407, a fluidized catalyst bed is taught (Col. 6, lines 29 to 30) for use in a method of two stage combustion employing a fuel containing nitrogen and a catalyst. The method described in this Patent comprises forming a first mixture of the fuel and an amount of air substantially less than the amount needed for complete combustion of all the combustible components in the fuel, and combusting this first mixture in a first combustion zone in the presence of a catalyst, having an operating temperature below a temperature that would result in any substantial formation of oxides of nitrogen or other fixed nitrogen compounds from atmospheric nitrogen present in the mixture, to form a first effluent.The first effluent is mixed with an additional amount of air at least sufficient for complete combustion of all combustible components remaining in the first effluent to form a second mixture, which is combusted in a second combustion zone below a temperature that would result in any substantial formation of oxides of nitrogen from atmospheric nitrogen.

In U.S. Patent No: 4,021,186, there is disclosed a method and apparatus for reducing NOx from non-fluidized bed furnaces. NOx produced by combustion of nitro-containing fuels is reduced by a forced draft burner operating with below stoichiometric mixtures of air and fuel in a primary combustion chamber, combustion being completed by controlled injection of secondary air near the outlet of the chamber.

The present invention resides in a fluidized combustion system for the reduction of nitrogen oxides (NOx) in the emissions from a fluidized bed combustion system burning a nitrogen-bearing fuel. The fluidized bed combustion process has already demonstrated the ability to produce lower NOx emissions than conventional combustion systems. Typically, NOx emission levels from both pilot- and demonstration-scale units have been in the 275 to 600 ppm range while burning a 1% nitrogen coal (calculated at 3% 02).

Currently proposed standards in the state of California, United States of America, for process heaters (model rule) will require an NOx emissions level of less than 100 ppm by volume calculated at 3% 02. The apparatus and process of this invention meet and surpass these stringent limits and thus will permit the use of coal for these applications.

Over the course of several years, we have found that the existence of more than trace amounts of carbon monoxide in the off-gas from a fluidized bed or the operation which promoted CO in the off-gas was effective in the reduction of NOx emissions from the bed. These observations were sometimes dramatically demonstrated when coal feeding problems resulted in a fuel-rich mixture and therefore substoichiometric operation. Under these conditions, emissions of the oxides of nitrogen were observed to fall to levels under 20 ppm by volume. The CO and hydrocarbons emissions which resulted were, of course, far higher than could be tolerated in continuous operation. We believed that these could be lowered with overfire air using staged combustion.While it has never been determined whether CO was entering into a reduction reaction with the NOx of whether the reduced emissions were due to a low oxygen partial pressure, the results were extremely beneficial. Utilization of this phenomenon in a fluidized bed for NOx reduction had never been seriously considered because of the adverse effect of partially reducing conditions on sulphur capture.

Simply stated, the system of the invention is one of staged combustion. In it, the fluidized bed is operated with very little residual 02 in the off-gas (for example, 0.40.8%). On an as-fired basis, the actual coal and air rates result in a substoichiometric mixture. CO emissions are of the order of 0.8% with trace amounts of methane and other hydrocarbons also present. While the actual mechanism is unclear, the above operating conditions produce extremely low NOx emissions. When calculated at 3% residual 02, they amount to 45-81 ppm of NOx by volume. To reduce CO and methane to acceptable levels, overfire air is then added just below the upper convection bundle. The rate is adjusted to increase the residual 02 in the off-gas to approximately 1%.This results in a large reduction of the CO with little or no increase in NOx emissions.

Addition of air near the convection bank permits reaction between the CO and 02 but limits the gas temperature, thereby preventing formation of additional NOx. The additional air also results in true excess air on an as-fired coal and air rate basis.

The process has been demonstrated at several superficial velocities and bed temperatures with similar results. There appears to be no degradation of the sulphur capture performance of a limestone bed, if in use.

The invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a side elevational view of a pilot plant combustion system constructed in accordance with the principles of the present invention. The lower portion is shown in partial vertical section; Figure 2 is a partial vertical sectional view taken along line 2-2 in Figure 1; Figure 3 is a graph showing the NOx and SOx emission data for run 1 of a pilot plant combustion system constructed and operated according to the principles of the invention; and Figure 4 is a graph showing the NOx and SOx emission data for run 2 of a pilot plant combustion system constructed and operated according to the principles of the invention.

Referring to Figures 1 and 2 of the drawings, reference numeral 10 refers in general to a pilot plant atmospheric fluidized bed combustor unit chosen for the purpose of illustrating the principles of the invention. The application of the principles of the invention to a commercial combustion system; (e.g., a boiler or steam generator or similar type device) is considered to be part of the invention and would be routine to a person of ordinary skill in the art given the description therein.

The combustor 10 has a front wall 12, a rear wall 14, and two sidewalls 1 6. Each wall is lined with a refractory material 18, it being understood that a commercial combustion system embodying the invention would have a means for heating water or generating steam, such as a water wall enclosure, in place of the refractory material 1 8.

A non-catalytic, but chemically reactive, fluidized bed of particulate material, maintained at approximately atmospheric pressure, is shown in general by reference numeral 20. The bed 20 is disposed within a primary combustion chamber 22 and is supported by a perforated distribution plate 24 extending horizontally in the lower portion of the unit. The bed 20 can consist of a mixture of discrete particles of inert material, fuel material such as coal, and an adsorbent; (e.g., limestone particles) for the capture of sulphur oxides (SOx) released during combustion of the fuel material. Other fuels which may be used are medium to high nitrogen fuels such as sewage sludge, certain types of municipal waste, and residual fuel oil.

An air plenum 26 is provided immediately below the plate 24 and an air inlet 28 is provided through the rear wall in communication with the plenum 26. Air for primary combustion of the fuel and for fluidizing the bed 20 is provided to the plenum 26 from an external source such as a compressor or pump (not shown) through an air inlet 28. An air flow regulating valve 30 is suitably mounted in the inlet 28 and acts to vary the effective opening in the inlet and thus control the flow of air into the plenum 26.

In place of the valve 30. air dampers of a conventional design could be employed. A bed light-off burner (not shown) is mounted through the front wall immediately above the plate 24 for initially lighting off the bed 20 during startup.

An in-bed fuel feeder pipe 32 (Figure 2) is provided in communication with a feeder port 34 in one sidewall 16. The feeder pipe 32 receives relatively fine particulate coal from an external supply and is adapted to feed the relatively fine coal particles through the feeder port 34 into the bed 20 in a conventional manner such as by pneumatic conveyance. The quantity of fuel supplied to the bed 20 through the feeder pipe 32 is controlled by a metering device shown schematically as 36 suitably mounted in the feeder system. It is understood that feeder pipes 32 can also be provided through one or more of the front wall 1 2, the rear wall 1 4 and the other sidewall 1 6. Optionally, a plurality of over-bed feeders may be provided in lieu of the in-bed feeders.A drain pipe 37 is provided through the plate 24 in communication with the bed 20 to withdraw the spent fuel material, primarily in the form of ashes, from the bed.

Disposed within the bed 20 are multiple in-bed cooling coils 38 extending inwardly through the wall 14, through which water, or other fluids, are passed to remove heat from the bed 20.

A horizontally extending overfire air distribution pipe 40 is provided in the combustor 10 at an elevated position relative to the bed 20. The pipe 40 extends through a sidewall 1 6 and has a flow regulating valve 42 for controlling the volume of air supplied by an external source (not shown) such as an air pump or compressor. The pipe 40 has a plurality of perforations extending therethrough for passage of the air into a secondary combustion zone 46 which extends from an area immediately adjacent the pipe and upwardly to the upper portion of the combustor 1 0. It is understood that, as an alternate to the air distribution pipe 40, a distribution grid, a manifold of sparger pipes, or a plurality of wall jets could be used for injecting an adequate quantity of air into the secondary combustion zone 46.

As will be explained in detail later, the additional quantity of oxygen in the air injected by the pipe 40 chemically combines with the CO and other combustible gases in a second stage combustion or oxidation, process.

A bank of convection coils 48 is provided in the upper portion of the secondary combustion chamber 46 for passing water, or other fluids, to be heated and a freeboard zone 58 is disposed between the primary combustion zone at the location of the fluidized bed 20 and the secondary combustion zone 46. As a result, the off gases passing from the fluidized bed 20 are cooled before they enter the secondary combustion zone 46.

The combustor 10 includes a roof 52 through which an outlet 54 is provided for exhausting the exhaust gases from the secondary combustion zone 46 to a stack or other external equipment.

To confirm the validity of the present invention, a relatively lengthy test protocol was chosen.

Primary objectives of this protocol were: 1) confirmation of the validity of NOx reduction results obtained during batch tests in a test where chemical equilibrium is approximated; 2) documentation of the stoichiometry requirements of each stage of the process; 3) documentation of combustion efficiency, sulphur capture, and gasesous emissions; and 4) determination of the effect of the process on bed chemistry.

The protocol test was run using a perforated 1-inch stainless steel schedule 40 pipe as the air distributor pipe 40. The precise location of the air distributor pipe 40 was determined by conventional heat balance methods. As the off-gas from fluidized bed 20 passes up through the freeboard zone 50, it is cooled by radiation and convection. There are temperature limits below and above which the desired second stage oxidation reaction of CO will not occur, with a preferred range being approximately 1300 to 1 6500 F. To speed the installation and to reduce measurement problems, 120 psig compressed air was used to supply the overfire air distributor pipe 40.

A low sulphur coal and commercially-available limestone were used in this test. The coal had the following analysis: COAL ANALYSIS C 68.51 (% by weight) H 5.19 0 12.61 N 1.16 S 0.66 Ash 6.92 H2O 4.95 Higher heating value: 12,270 BTU/lb.

A superficial velocity of 4 fps and an average bed temperature of 1 5500F were chosen as conditions for the staged combustion protocol test. The bed temperature should be within the range of approximately 1500 to 16500F.

The 50-hour test protocol was divided into periods of approximately 20 hours duration for startupstabilization and 30 hours for testing. Operating conditions were then varied to obtain the optimum results with respect to NOx, SOx, and CO emissions.

The general procedure for testing each set of conditions consisted of the following steps: first, combustor 10 was stabilized at a given superficial veiocity and residual 02 (nominally 4 fps and 2% 02).

Next, the coal feed rate was siowly increased to a point where NOx was reduced to the target level (60--70 ppm). Normally, this resulted in a residual 02 concentration of 0.4% and a CO concentration of 0.61.2%. Finally, the overfire air distributor pipe 40 was turned on until the CO emissions reduction was maximized without exceeding the NOx maximum emissions target level (100 ppm). The best of these quickly screened conditions was then maintained for the 30 hours of stabilized testing.

In order to perform complete combustion efficiency calculations on the staged combustion process, two combustion evaluation runs were made during the 30-hour test. Results from these combustion checks (Runs 1 and 2), which were of 8- and 6-hours duration, respectively, appear in Tables 1,2, and 3, below, and in Figures 3 and 4.

Table 1 presents a summary of the operating conditions maintained during the staged combustion test and the average emissions which resulted. To permit direct comparison of the results to target limits, reported values have been corrected to 3% residual 02. Examination of these results will show that the average value maintained during both runs for both NOx and SOx was below target limits. As shown in Figure 3, instantaneous values of SOx in Run 1 did, in some cases, exceed the 60 ppm target limit. This problem was eliminated by slight changes in the operating conditions as can be seen in Figure 4 showing the results from Run 2.

TABLE 1 STAGED COMBUSTION TEST RESULTS

EMISSIONS CORRECTED TO 3% OXYGEN Combustion Average Superf'l Efficiency SOx (ppm) NOx (ppm) % CO Bed Temp Velocity Based on Run &num; (OF) (Ft/s) Carbon % Avg. Low High Avg. Low High Avg. Low High 1 1556 4.06 96.7 33 0 99 69 53 89 0.06 0.02 0.13 2 1549 3 99 3.99 94.9 8 0 18 76 54 89 0.08 0.02 0.23 The distribution of combustion air between the primary air injected in bed 20 and the overfire air injected by overfire air distributor pipe 40 is tabulated in Table 2. below. In each case, an initial stoichiometry of approximately 85% was necessary to provide the required reduction in NOx emissions.

Measurements of CO prior to introduction of overfire air showed that the resultant CO emission was 0.6% - 1.2% by volume with an average value of 0.8%. Optimum reduction of CO emissions without large increase in NOx emissions occurred at an overall stoichiometry of approximately 104%. This resulted in a residual 0, level of 0.8% and 0.7%, respectively. During the original batch tests, equivalent NOx emissions reductions were accomplished with a higher overall excess air rate which resulted in lower CO emissions.

TABLE 2 STAGED COMBUSTION AIR DISTRIBUTION

Coal Primary % % CO prior Overfire Cumul. % % Oxygen Flow Air Stoich. to Overfire Air Stoichiometric Run &num; (Ib/h) (Ib/h) Air Air (Ib/h) air Off-gas 1 111.25 801 85.30 0.8 178.70 103.90 0.80 2 108.40 793 86.39 0.8 156.58 103.45 0.70 A better idea of the NOx and SOx reduction picture can be obtained in Figures 3 and 4. These graphs present the actual measured NOx and SOx concentrations at 1 0-minute intervals for Runs 1 and 2.

Reference lines representing the target limits for these pollutants have been corrected to the residual 0, maintained during the test. As can be seen in all cases, the NOx emissions are well within the target limits. Similarly, the SOx emissions for Run 2 also meet the limits by a wide margin.

As shown in Table 1 above. combustion efficiency determined during the two combustion checks (Runs 1 and 2) were found to be 96.7% and 94.9% respectively. based upon carbon combustion. Table 3, below, presents an accounting of the distribution of the carbon combustion efficiency losses for each run. As might be expected. the cyclone fines represent the largest single loss amounting to 66.16% and 76.21%. respectively, of the total carbon loss in each run.

TABLE 3 CARBON LOSS ACCOUNTING Combustion Efficiency Run 1 % Loss Total Loss Baghouse Dust 0.647 19.42 Cyclone Dust 2.206 66.16 CO in Off-Gas 0.480 14.42 3.333 100.00 Run 2 Baghouse Dust 0.639 12.65 Cyclone Dust 3.881 76.21 CO in Off-Gas 0.563 11.14 5.053 100.00 Limestone feed rates maintained during the runs appear in Table 4, below. Even though the Ca/S mole ratios of 6.03 and 4.75 appear a little high, the actual lime usage rate is well within commonly encountered bounds for higher sulphur coals. This is more easily observed by examining the ratio of lime feed/coal feed. As an example, a 4% sulphur coal requiring a Ca/S mole ratio of 3:1 would require a lime/coal weight ratio of 0.38.Thus, although the mole ratio may look high, because of the low sulphur content of the coal, the actual lime usage rate is within limits experienced in atmospheric fluidized bed steam generators.

TABLE 4 STATED COMBUSTION MASS FLOW FLOW (lb/h) Run 1 Run2 Coal 111.3 108.4 Limestone 37.0 28.6 Cyclone 25.9 23.8 Baghouse 3.2 5.0 Ib Lime/lb Coal 0.33 0.26 Ca/S Mole Ratio 6.03 4.75 Of great concern during the staged firing tests was the formation of calcium sulphide (CaS) in the partially reducing zones of the bed. This compound, whose formation is favoured over calcium sulphate (CaSO4) in reducing atmospheres, decomposes in water, producing hydrogen sulfide gas (H2S) and calcium oxide (CaO). Thus, if present in large quantities in the waste bed material, it can create disposal problems.

To determine the severity of this potential problem, hot samples were removed from the bed and cooled in a sealed container prior to analysis. Similarly, samples from the cyclone and baghouse were also collected for analysis. The results of these analyses appear in Table 5, below.

TABLE 5

ANALYSIS OF SULPHUR COMPOUNDS (% BY WEIGHT) IN COMBUSTOR EFFLUENT STREAMS CYCLONE SAMPLE BAGHOUSE SAMPLE BED SAMPLE Total Sulphate Sulphide Total Sulphate Sulphide Total Sulphate Sulphide Sulphur Sulphur Sulphur Sulphur Sulphur Sulphur Sulphur Sulphur Sulphur Run &num; (%) (%) (%) (%) (%) (%) (%) (%) (%) 1 * * * * * * 4.89 3.44 1.45 2 1.91 1.60 0.31 1.66 0.96 0.70 3.89 2.66 1.23 * Analysis not available As expected, all three samples contained varying amounts of calcium sulphide. Of greatest concern is the composition of the bed material, which will make up the majority of the waste stream.

Sulphide contents of the bed material samples were found to be approximately 30% of the total sulphur captured. However, they amounted to only 1.45% and 1.23% on a total basis.

A solution to this problem is described and claimed in our copending application entitled "A Fluidized Bed Combustion System Utilizing Sulphide Conversion" (LAZKK Docket No: A-1 2,894), Serial No: , filed April ', 1 980.

Claims (22)

1. A non-catalytic two stage fluidized bed combustion system for reducing emissions of nitrogen oxides (NOx) produced during combustion of nitrogen-containing fuel, comprising: (a) a primary combustion chamber; (b) a non-catalytic bed of particulate material, including a nitrogen-containing fuel, disposed within the primary combustion chamber; (c) first introducing means for introducing air to the bed, to fluidise the bed and to promote the combustion of the fuel; (d) second introducing means for introducing fuel to the bed; (e) means for regulating at least one of the introducing means to achieve combustion of the fuel in the primary combustion chamber under substoichiometric conditions; (f) a secondary combustion chamber located above the primary combustion chamber and in communication therewith for receiving the combustion off-gas from the fluidized bed; and (g) a second air introducing means for introducing additional air to such combustion off-gas in the secondary combustion chamber to effect combustion under stoichiometric conditions.

2. A system according to Claim 1 wherein the oxidizable components of said combustion off-gas are oxidized in the secondary combustion chamber.

3. A system according to Claim 1 or Claim 2 including means for passing water in a heat exchange relation to the combustion chambers to add heat to said water.

4. A system according to any preceding Claim wherein the temperature in the non-catalytic fluidized bed is within the range of 1 500 to 1 6500 F.

5. A system according to any preceding Claim wherein the secondary combustion chamber is located in a spaced relation to the primary combustion chamber to effect cooling of said off-gas as it passes between the chambers.

6. A system according to any preceding Claim wherein the non-catalytic fluidized bed is maintained at approximately atmospheric pressure and includes an inert material and an adsorbent for the capture of sulphur oxides (SOx).

7. A system according to any preceding Claim wherein the nitrogen-containing fuel is coal.

8. A system according to any preceding Claim wherein the first introducing means comprises a perforated distribution plate for supporting the fluidized bed, and a plenum chamber disposed under the plate for receiving air and directing the air through the plate and into the bed.

9. A system according to any preceding Claim wherein the second introducing means for introducing fuel comprises an in bed feeder means.

10. A system according to any preceding Claim wherein the second air introduction means comprises a perforated conduit.

11. A system according to any preceding Claim wherein the temperature in the secondary combustion chamber is within the range of 1300 to 16500 F.

12. A two stage fluidized bed combustion system substantially as described herein with reference to the accompanying drawings.

13. A non-catalytic two stage combustion process for reducing emissions of nitrogen oxides (NOx) produced during combustion of nitrogen containing fuel, comprising the steps of: (a) providing a bed of non-catalytic particulate material including a nitrogen-containing fuel without catalyst; (b) introducing air into the fuel to fluidize the bed and promote the combustion thereof in a primary combustion stage; (c) introducing additional fuel to the bed to replenish the combusted fuel; (d) controlling at least one of the fuel feed rate and the air feed rate to achieve combustion in the primary combustion stage under substoichiometric conditions; and (e) introducing air into the combustion off-gas above the fluidized bed to produce combustion under stoichiometric conditions in a second combustion stage.

14. A process according to Claim 1 3 wherein the heat produced in the combustion stages is used to add heat to water through a heat exchange relation.

1 5. A process according to Claim 13 or Claim 14 wherein the nitrogen containing fuel is coal.

16. A process according to any of Claims 13 to 1 5 wherein the temperature in the non-catalytic fluidized bed is in the range 1 500 to 1 6500 F.

1 7. A process according to any of Claims 1 3 to 1 6 wherein the non-catalytic fluidized bed is maintained at approximately atmospheric pressure and includes an inert material and an adsorbent for the capture of sulphur oxides (SOx).

1 8. A process according to any of Claims 1 3 to 1 7 wherein the temperature in said secondary combustion stage is within the range of 1 300 to 1 6500 F.

19. A process according to any of Claims 13 to 15 wherein the residual oxygen (0,) in the off-gas just above the non-catalytic fluidized bed is in the range of 0.4 to 0.8%.

20. A process according to any of Claims 13 to 19 wherein the NOx emissions level is less than 100 ppm by volume calculated at 3% oxygen.

21. A process according to any of Claims 13 to 20 including the step of passing said off-gas through a cooling zone before it passes to the second combustion stage.

22. A non-catalytic two stage combustion process substantially as described herein with reference to the accompanying drawings.