GB2545760A - Methods for removing contaminants from gas streams - Google Patents

Abstract

A method for the partial removal of contaminants from a process gas stream comprises the steps of separating a process gas stream into at least two process gas streams by means of a partition, baffle or damper. Feeding ozone (O3) into contact with at least one of the separated process gas streams to oxidize contaminants in the gas stream and feeding at least one of the process gas streams contacted by ozone to a scrubber for removal of the oxidized contaminants from the gas streams. The method may provide for a flexible ozone-based oxidation system to remove and capture a variable partial fraction of nitrogen oxides (NOx) contained in an exhaust gas. The variability can be in response to a source of nitrogen oxides contamination that continuously or step-wise varies with fuel or other operational parameters such as furnace load or changes in the required stack nitrogen oxides level.

Description

METHODS FOR REMOVING CONTAMINANTS FROM GAS STREAMS BACKGROUND OF THE INVENTION

[0001] Combustion and chemical processes generally generate gas streams containing contaminants that need cleanup before being exhausted to the atmosphere. Many industrial processes, power generating utilities, combustion processes, stationary and mobile sources such as engines, boilers, kilns and the like use solid fuels or low cost hydrocarbon fuels that contain sulfur, chlorine, nitrogen and metal compounds in hydrocarbons which result in exhaust gases that contain contaminants such as acid gases, particulate matter and heavy metals. To comply with stricter environmental rules mandated by legislation and a greater concern for the environment, combinations of scrubbing (wet or dry) and particulate capture devices such as electrostatic precipitators (ESP), wet ESP and bag houses are increasingly utilized for emissions control of acid gas and particulate matters.

[0002] Nitrogen oxides found in most combustion exhaust streams primarily are in the form of nitric oxide (NO), which is nearly insoluble in water and not very reactive. Nitric oxide is not removed to any significant extent by most wet or dry scrubber capture devices. To control nitrogen oxide emissions, therefore, the two major options are to lower nitrogen oxide formation at the source by modifying combustion or secondly treating nitrogen oxides in the exhaust gas stream using post combustion techniques.

[0003] Primary techniques used for reducing nitrogen oxide formation by modifying combustion are low nitrogen oxides burner (LNB), flue gas recirculation (FGR), staged combustion and overfire air (OFA). In most applications these technologies are not adequate for removing nitrogen oxides from combustion gas streams and post combustion techniques, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), become necessary to achieve the desired nitrogen oxide reduction levels.

Both SCR and SNCR processes realize good results but also have limitations. Ozone based oxidation technologies have recently gained success as an alternative post combustion technique, especially when an application is not suitable for SCR. Ozone based processes as described in U.S. Pat. Nos. 6,162,409; 5,206,002; and 7,303,735 provide multi-pollutant removal approaches.

[0004] Coal fired boilers with low nitrogen oxides burners and staged combustion often attain nitrogen oxides levels in the range of 0.25 to 0.4 Ib/MMBTU (one million British Thermal Units) cost effectively whereas regulations require nitrogen oxides emissions to be in the range of 0.1 to 0.15 Ib/MMBTU.

[0005] The ozone based processes for oxidizing nitrogen oxides described in U.S. Patent Nos. 5,206,002; 6,162,409; 6,649,132; and 7,303,735 are directed towards achieving high levels of nitrogen oxide removal (around 90%) and require the use of about 1.5 moles of ozone per mole of nitrogen oxide present in the exhaust gas stream. In the reaction time allowed in these methods, ozone reacts selectively with NOx forming higher oxides of nitrogen, especially the pentavalent form or higher which are very water soluble and readily removed by wet scrubbing. Configuring these processes to operate at lower levels of nitrogen oxide removal, however, causes both economic and process challenges.

[0006] The oxidation of NOx to N205 involves many reactions but for the sake of brevity, it can be simplified as follows: NO + 03—> N02+ 02 (1) N02+ 03—> NO3 + O2 (2) N02+ N03 — > N2Os (3) [0007] Reaction (1) is an order of magnitude faster than reaction (2). By the time reaction (2) starts to occur, most of the NO has already been oxidized to N02. At levels of NOx removal of 90% and higher the actual molar ratio of ozone to NOx removed that is required is close to the stoichiometric ratio indicated above. At low to moderate levels of NOx removal, however, not only is significantly more ozone required than the ideal stoichiometric amount, but also the NOx that is emitted from the stack is essentially in the form of the brown colored and acrid smelling N02.

[0008] U.S. Patent No. 8,965,098 describes an ozone-based method for the partial removal of contaminants from a process gas stream that overcomes the economic and process limitations of the prior art ozone NOx oxidation processes.

[0009] In these methods the process gas stream containing contaminants is divided into at least two process gas streams. Ozone is injected into a selected one or more of the process gas streams for mixing of the ozone with the contaminants including nitrogen oxides. The nitrogen oxides in the selected process gas stream or streams are essentially fully oxidized by ozone. Then the ozone treated selected process gas stream or streams that are now substantially free of un-oxidized nitrogen oxides are recombined with the remaining process gas stream containing contaminants. The oxidized nitrogen oxides are removed by a capture device either from the selected process gas stream or streams prior to recombination with the remaining process gas stream containing contaminants, or after recombination of the gas streams.

[0010] Essentially 100% of the NOx in the selected gas stream or streams is thereby removed utilizing close to the stoichiometric molar ratio of ozone to NOx removed. No ozone is utilized oxidizing NO to N02 in the un-treated stream and the NOx released from the stack remains mainly colorless NO. Both deficiencies in the earlier ozone NOx removal methods at low to moderate levels of NOx removal are hence addressed. In addition, since any small degree of ozone slip resulting from the high level of oxidation targeted in the one or more oxidized selected gas streams is immediately quenched by very reactive NO on recombination with the untreated gas stream.

[0011] The overall fraction of NOx removed is about equal to the fraction of the overall process stream contained in the at least one selected process gas streams to which ozone is added. U.S. Patent No. 8,965,098 focuses on instances where a fixed proportion of the total NOx must be removed, e.g. 50%, or where a series of different fractions of NOx should be removed, e.g. 25%, 50% and 75% for example to meet current and future regulatory needs. Various embodiments are described that cost effectively achieve this, including: placing fixed partitions into existing exhaust gas ducts and or scrubbers and injecting ozone into one or more or the separated streams; injecting ozone into one of more of multiple ducts containing the exhaust gas stream from a combustion or chemical source of NOx contamination; and, injecting ozone into one or more separate zones in a spray drier, or other air pollution control equipment, so that a fixed proportion of the exhaust gas is treated with ozone. These embodiments work well and economically if emission regulations require that a fixed percentage (e.g. 75%) of NOx be removed from a source with constant or varying NOx levels, or that a specified NOx level be maintained in the stack from a source with a roughly constant NOx level (for example an inlet NOx level of about 400 mg/Nm3 be reduced to less than 100 mg/Nm3: in which case 75% of the exhaust gas is treated with ozone and then the oxidized products scrubbed out).

[0012] However, in the case of a source that generates a variable NOx level and where regulations require that a fixed level of NOx be maintained in the stack, then these partial NOx removal ozone oxidation solutions are not ideal. The required fraction of NOx removed, and hence the required proportion of the total process gas that must be treated varies continuously between the minimum fraction required to reduce the lowest NOx input level to the regulatory limit and the maximum fraction required to reduce the highest NOx inlet level to the regulatory level, not in a series of fixed increments, e.g., 25%, 50% and 75%.

[0013] U.S. Patent 8,865,098 teaches a partial solution to this problem in the embodiment described in column 11, line 39 to column 12, line 9 as well as Figure 8. This constitutes the nearest prior art to the current invention. In this embodiment a fan driven by a variable frequency drive is used to divert a variable proportion of the process gas stream to an ozone oxidation duct and scrubber that removes the oxidized NOx and other contaminants present, if any.

[0014] It is not feasible to implement this particular solution in many real world applications because of space limitations, the geometry of the process flow duct (more particularly if the plant has multiple ducts), or because of the impact of the wide range of different fractions of the process flow diverted through the fan on the pressure drop, the ozone mixing efficiency and mixing time and the residence time for the ozone oxidation in the ozone oxidation duct as well as scrubber efficiency.

[0015] The present inventors have discovered a flexible ozone-based oxidation system that can remove and capture a variable partial fraction of the nitrogen oxides contained in an exhaust gas. This variability can be in response to the source of nitrogen oxides contamination that continuously or step-wise varies with fuel or other operational parameters such as furnace load or changes in the required stack nitrogen oxides level.

SUMMARY OF THE INVENTION

[0016] In one embodiment of the invention, there is disclosed a method for the partial removal of contaminants from a process gas stream comprising the steps of: separating a process gas stream into at least two process gas streams by means of a partition, baffle or damper, feeding ozone into contact with at least one of the separated process gas streams to oxidize the contaminants in the gas stream, and feeding at least one of the process gas streams contacted by ozone to a scrubber for removal of the oxidized contaminants from the gas streams.

[0017] This invention provides a cost effective solution for the removal of variable amounts of NOx from a combustion or chemical sourced process stream, utilizing close to the stoichiometric ratio of ozone to NOx removed, for all fractions of NOx that need to be removed, without increase in the levels of N02 emitted in the stack or issues with ozone slip.

[0018] It provides a flexible ozone-based oxidation system that can cost effectively remove and capture a variable partial fraction of the NOx contained in the exhaust gas from a source in response to either a source of NOx contamination that continuously or step-wise varies with fuel or other operational parameters, such as furnace load, or to planned or seasonal changes in the required stack NOx level.

[0019] The present invention can be compared and contrasted with a generalized but similar NOx removal opportunity and comparable results can be obtained versus these earlier ozone oxidation methods for NOx removal and capture. A feed stream containing 100 Kg/hr of NOx is to be treated. As is normal in industrial practice the NOX levels are reported as N02 although typical NOx sources from combustion processes comprise 90-95% NO. In this case the NOx is assumed to be 100% NO for simplicity. The ozone oxidation processes of US Patent numbers 6,162,409, 5,206,002 and 7,303,735 (hereafter referred to as conventional LoTOx), US Patent number 8,965,098 (hereafter referred to as fixed treatment) and the current invention (hereafter referred to as variable treatment) are compared and contrasted. Figure 3 shows the ozone to NOx molar ratios for these different processes as a function of the required NOx removal level, 0 to 100, relative to the stoichiometric ozone to NOx removal ratio at 100 % NOx removal. In real industrial situations, additional margin would be required to account for non-ideal flow distribution, mixing, temperature, pressure, other contaminants, etc.

[0020] As can been clearly seen from Figure 3, conventional LoTOx, utilizes significantly more ozone than does the current invention, variable treatment especially at low and intermediate levels of NOx removal. In addition, the residual NOx emitted from conventional LoTOx is essentially fully converted to N02, whereas in the case of variable treatment it remains as the lower environmental impact NO.

[0021] Fixed treatment is in part able to solve the issues of conventional LoTOx at low and moderate NOx removal levels, but because of the restrictions imposed by the fixed partitions or selection of one or more of multiple exhaust streams for ozone treatment and NOx removal, then this solution has sweet spots of ozone usage at the removal levels corresponding to the fractions of the flow treated in the selected process gas streams, in this case 25%, 50%, 75% (and 100%). In order to prevent the formation of residual N02 it is necessary to fully treat 25%, 50%, 75% or 100% of the flow, even when intermediate levels of NOx removal are required. If 66% removal of NOx is required, then 75% of the NOx should be removed in this example, to ensure that additional residual NOx is not generated, thus requiring more ozone to be used than in the ideal variable treatment case. This leads to the stepwise nature of the curve for fixed treatment in Figure 3.

[0022] The contaminants contained in the process gas stream for treatment are selected from the group consisting of nitrogen oxides, sulfur oxides, acid gases, particulate matter and mercury. The nitrogen oxides treated are selected from the group consisting of nitric oxide and nitrogen dioxide. The process gas stream is an exhaust gas stream from an industrial process, typically from a source selected from the group consisting of fixed sources and mobile sources. The industrial process is selected from the group consisting of industrial boilers, power generation systems, chemical processing, kilns, furnaces and combustion processes.

[0023] The scrubber is typically selected from the group consisting of dry, semi-dry and wet scrubbing equipment. The amount of ozone that is added to the at least one of the two or more separated gas stream is an amount of greater stoichiometry than the amount of nitrogen oxides present therein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 3 is a chart showing the fractional amount of nitrogen oxides removed versus the amount of ozone employed for various processes.

[0025] Figures 4(a) through 4(e) are schematics of embodiments of the present invention showing different mechanisms for dividing the stream to be treated.

[0026] Figures 5.1,5.2,5.3 and 5.4 are schematics of nitrogen oxides absorption systems.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Figure 4 of US Patent No. 8,865,098 depicts a fixed treatment ozone based oxidation solution for the removal of NOx from process streams that utilizes fixed partitions to separate the process stream into two or more parts.

For example if the stream is divided into 4 equal parts, then this provides a good solution for the removal of 25%, 50%, 75% or 100% of the NOX, but less effective solutions for intermediate NOx removal levels.

[0028] 4.1 (a) and 4.1 (b) show how this design may be modified in accordance with the current invention by the addition of variable dampers in one or more of the gas streams, that allow the fraction of gas being treated to be varied between the minimum level required and the maximum level required.

[0029] The dampers may be single vanes that can restrict the flow of gas in one of more of the gas streams, or comprise multiple louvers. Additional baffles, flow straighteners, static mixers, etc, may be required in order to ensure good flow distribution and that the mixing of ozone be optimized in these designs. Computational fluid dynamics code is commonly used to optimize these factors.

[0030] The damper may be automatically controlled to respond to continuous changes in the NOx removal requirement or adjusted manually, for example with a screw gear, to allow adjustments to be made to different fuels burned or different regulatory demands.

[0031] If there is insufficient residence time for the ozone oxidation to be completed in the duct selected for this purpose, then an additional oxidation reactor may be required. Figure 4(c) shows one such embodiment of the current invention. Again dampers may be placed in one or more of both the untreated and treated gas streams in order to control the split of treated and untreated process gas in response to varying NOx removal requirements.

[0032] An alternative embodiment is shown in Figure 4(d) in which a variable speed drive is used with a fan to divert the calculated fraction of process gas that is not to be treated with ozone. The reactor is sized so that the residence time is sufficient for complete reaction of the ozone with NOx for both the minimum and maximum fractions of NOx that need to be removed. The ozone injection grid and flow diversion elements are designed to ensure rapid and complete mixing of ozone into the process gas stream. If necessary multiple bypass streams or multiple reactors can be utilized to keep the flows within optimal conditions.

[0033] Figures 5.1,5.2, 5.3 and 5.4 show examples of various nitrogen oxides absorption schemes showing placement of fans, reactor zones and related process details.

[0034] The ozone can either be injected before or after the fan depending on the oxygen/ozone concentration, on the humidity level and on the construction material of the elements in touch with the process gases. To inject before the fan will allow to the operator to lower the residence time.

[0035] In Figure 5.4, a nitrogen oxides absorption system is shown. The principle is to control the temperature via steam injection or demiwater spray. In both cases the water added to the system is then removed in the next phase where the coalescer collects the excess of water in the system. In some situations the second injection of water may also not be needed, in particular when the humidity into the system is high enough to provide the system with the required amount of liquid to remove the N205.

[0036] If the ozone treated and untreated streams are recombined and then the combined stream is treated with the selected Air Pollution control equipment, for example a wet, dry or semi-dry scrubber, then the selected fraction of NOx is removed together with other contaminants, if any, in this additional device.

[0037] If the ozone-oxidized stream is fed through an air pollution control device prior to mixing with the untreated stream, then the selected fraction of NOx as well as the same fraction of other contaminants, if any, may be removed.

Claims (2)

1. Having thus described the invention, what we claim is:
2. 1. A method for the partial removal of contaminants from a process gas stream comprising the steps of: separating a process gas stream into at least two process gas streams by means of a partition, baffle or damper, feeding ozone into contact with at least one of the separated process gas streams to oxidize the contaminants in the gas stream, and feeding at least one of the process gas streams contacted by ozone to a scrubber for removal of the oxidized contaminants from the gas streams.