| US20150336051A1 | 2015-11-26 | |||
| US20110200505A1 | 2011-08-18 | |||
| US20130149225A1 | 2013-06-13 | |||
| US20110041481A1 | 2011-02-24 |
| Claims 1. A method for the removal of nitrogen oxides from flue gas from combustion facilities, comprising the steps of passing the flue gas through a catalyst for selective reduction of nitrogen oxides in presence of ammonia add¬ ed to the flue gas either as such or in form of a pre¬ cursor thereof; at a flue gas temperature below 250°C injecting an ef¬ fluent gas containing nitrogen dioxide into the flue gas upstream the catalyst for selective reduction of nitro¬ gen oxides; providing the effluent gas containing nitrogen dioxide by steps of catalytically oxidizing ammonia or a precursor thereof with an oxygen containing atmosphere to an effluent gas containing nitrogen monoxide and oxygen in presence of an oxidation catalyst; cooling the effluent gas to ambient temperature and oxi¬ dizing the nitrogen monoxide in the cooled effluent gas to the nitrogen dioxide containing effluent gas. 2. The method of claim 1, wherein the oxygen con- taining atmosphere comprises flue gas. 3. The method of claim 1, wherein the oxygen atmos¬ phere is ambient air. 4. The method according to any one of claims 1 to 3, wherein the nitrogen dioxide containing effluent gas is injected into the flue gas in an amount resulting in 45 to 55% by volume of the nitrogen oxides is nitrogen di¬ oxide at inlet to the catalyst for selective reduction of nitrogen oxides. 5. The method according to any one of claims 1 to 4, wherein the oxidation of the nitrogen monoxide in the cooled effluent gas to the nitrogen dioxide containing effluent gas is performed in presence of an oxidation catalyst . 6. System for use in the method according to any one of claims 1 to 5, comprising within a flue gas duct a catalyst for selective reduction of nitrogen oxides; upstream the catalyst for selective reduction of nitro¬ gen oxides, injection means for injection of ammonia or a urea solution into the flue gas duct; upstream the catalyst for selective reduction of nitro¬ gen oxides, injection means for injection of nitrogen dioxide containing effluent gas; and outside the flue gas duct, an ammonia oxidation catalyst; and means for cooling and oxidizing nitrogen monoxide containing effluent gas withdrawn from the ammonia oxida¬ tion catalyst to the nitrogen dioxide containing efflu¬ ent gas connected at its outlet end to the injection means for injection of the nitrogen dioxide containing effluent gas. 7. The system of claim 6, wherein the means for cooling and oxidizing the nitrogen monoxide containing effluent gas is in form of a heat exchanger. 8. The system of claim 6, wherein the means for cooling and oxidizing the nitrogen monoxide containing effluent gas is in form of a spirally wound tube. 9. The system of any one of claims 6 to 8, wherein the means for the cooling and oxidizing nitrogen monoxide containing effluent gas is provided with an oxida¬ tion catalyst. |
The present invention relates to a method and system for reducing emission of nitrogen oxides (NOx) from off-gasses. In particular, the method and system of the invention provides an improved reduction of NOx during cold start of the engine . Flue gases from different combustion facilities, e.g. boil ¬ ers in solid or liquid fired power plants gas, oil-fired generators or cement kilns, biofuel fuelled combustion plants and waste incineration plants contain a number of environmentally problematic or even poisonous compounds. These comprise NOx.
Catalytic cleaning of flue gas reduces the amount of NOx and is therefore beneficial for the environment in general. In most areas, legislation requires reduction of NOx in the flue gas.
In Selective Catalytic Reduction (SCR) of NOx, nitrous ox ¬ ide compounds are selectively reduced to harmless nitrogen and water by reaction with a reduction agent, e.g. ammonia, over a catalyst.
In power plants typically a heat recovery unit steam generator unit (HRSG) is installed that recovers heat from a hot flue gas stream.
The HRSG unit comprises a superheater, an evaporator and an economizer . In the superheater and evaporator, the heat in the flue gas is used to superheat steam and to preheat feed water before it is pumped to the boiler, which increases the boiler ef ¬ ficiency of the power plant. The flue gas temperature is thereby typically cooled to approximately 150°C.
The low temperature of the flue gas downstream the evapora ¬ tor encounters a problem in the NOx removal by means of SCR.
The problem with the known SCR catalysts is the relatively low efficiency at flue gas temperatures below 250°C.
This usually means that the evaporator section of the HRSG will have to be split and the SCR placed in between the two sections .
The SCR catalyst is typically installed downstream the su ¬ perheater and upstream the economizer.
It is known that the SCR reaction can be considerably ac ¬ celerated, and the low temperature activity can be signifi ¬ cantly raised at equimolar amounts of NO and O 2 in the flue gas by the so-called "fast" SCR reaction:
2NH 3 + NO + N0 2 → 2N 2 + 3H 2 0.
This invention is based on forming NO 2 externally to the flue gas duct and injecting the prepared NO 2 into the flue gas in an amount that promotes the fast SCR reaction. NO 2 can be formed from N¾ by oxidation of the N¾ to NO over a precious metal containing catalyst in a first step and sub ¬ sequently oxidation of NO to O 2 in a second step.
Thus, the invention provides in a first aspect a method for the removal of nitrogen oxides from flue gas from combus ¬ tion facilities, comprising the steps of
passing the flue gas through a catalyst for selective re ¬ duction of nitrogen oxides in presence of ammonia added to the flue gas either as such or in form of a precursor thereof;
at a flue gas temperature of below 250°C further injecting an effluent gas containing nitrogen dioxide into the flue gas upstream the catalyst for selective reduction of nitro ¬ gen oxides;
providing the effluent gas containing nitrogen dioxide by steps of
catalytically oxidizing ammonia or a precursor thereof with an oxygen containing atmosphere to an effluent gas compris ¬ ing nitrogen monoxide and oxygen in presence of an oxida- tion catalyst;
cooling the effluent gas to ambient temperature and oxidiz ¬ ing the nitrogen monoxide in the cooled effluent gas to ni ¬ trogen dioxide. The problem with the known methods and systems is the rela ¬ tively low efficiency of the SCR catalyst at flue gas tem ¬ peratures below 250°C, as mentioned hereinbefore. This problem is solved by the invention with injection of O 2 into the flue gas at the low temperatures to promote the "fast" SCR reaction. This reaction is responsible for the promotion of low temperature SCR. At flue gas temperatures above 250°C, SCR catalysts have sufficient efficiency and injection of O 2 into the flue gas can be disrupted when the gas temperature reaches
250°C.
A particular advantage of the invention is that tail end SCR installations (i.e. downstream the ecomonizer) are pos ¬ sible without the need of re-heat the gas to higher temper ¬ atures needed for reducing the catalyst volume.
Thus, a flue gas reheater unit, which is typically arranged upstream the SCR catalyst in the known flue gas cleaning systems can be omitted or much reduced in duty. Ammonia oxidation to NO externally to the flue gas duct, is usually performed in a reactor with a noble metal catalyst, typically platinum or an alloy of platinum with other precious metals as minor components at reaction temperatures of between 250 and 800°C in presence of oxygen containing atmosphere.
To provide the required reaction temperature, the oxidation reactor can be heated by e.g. electrical heating or induc ¬ tion heating.
In an embodiment, the oxygen containing atmosphere includes hot recirculated gas which provides then additionally part of the oxidation reactor heating duty. NO formed from N¾ by oxidation of the N¾ in contact with a precious metal containing catalyst in a first step, is subsequently oxidized to NO 2 in the NO containing effluent gas from the first step by cooling the gas to ambient tem ¬ perature to push the equilibrium reaction 2 ΝΟ+θ 2 ^2 NO 2 to ¬ wards formation of O 2 in the above reaction scheme. The term "ambient temperature" as used herein, shall mean any temperature prevailing in the surroundings of a combus ¬ tion facility employing the method and system of the inven ¬ tion. Typically, the ambient temperature will be between -20°C and 40°C.
Cooling and oxidation of the NO containing effluent gas can be performed in an aging reactor sized so that the resi ¬ dence time of the gas is about 1 minute or longer. In an embodiment the oxidation reaction is performed in presence of a catalyst promoting the oxidation of NO to NO 2 · Those catalysts are known in the art and include Pt on T1O 2 , Pt on S1O 2 and activated carbon or Pt and/or Pd on alumina .
As mentioned hereinbefore the desired fast SCR reaction re ¬ quires equal amounts of NO and NO 2 . Consequently, the amount of NO 2 injected into the flue gas at cold start con ¬ ditions with a temperature below 250°C is controlled to re- suit in 45 to 55% by volume of the nitrogen oxides content in the flue gas is NO 2 at inlet to the SCR catalyst unit.
In a further aspect, the invention provides a system for use in the method according to the invention.
The system comprises within a flue gas duct a catalyst for selective reduction of nitrogen oxides; upstream the catalyst for selective reduction of nitrogen oxides, injection means for injection of ammonia or a urea solution into the flue gas duct;
upstream the catalyst for selective reduction of nitrogen oxides, injection means for injection of nitrogen dioxide containing effluent gas; and
outside the flue gas duct,
an ammonia oxidation catalyst; and
means for cooling and oxidizing nitrogen monoxide from the ammonia oxidation catalyst to nitrogen dioxide containing effluent gas connected at its outlet end to the injection means for injection of nitrogen dioxide containing effluent gas . As mentioned above, the oxidation reaction of NO to NO 2 needs a residence time of the NO containing gas of at least 1 minute. Typically, 1-2 minutes.
This can be achieved in a heat exchanger either gas cooled or water cooled or alternatively when shaping the cooling and oxidizing means as a spirally wound tube with a length resulting in the desired residence time of the gas passing through the tube . In another embodiment, the means for cooling and oxidizing nitrogen monoxide containing effluent gas is provided with an oxidation catalyst is provided with an oxidation catalyst promoting the oxidation of NO to NO 2 .