FROM

FLUE-GAS USING A CERAMIC FILTER WITH AN SCR CATALYST

The present invention relates to a method and system for reducing emission particulate matter and nitrogen oxides

(NOx) from off-and flue gas. In particular, the method and system of the invention provides an improved reduction of NOx at low gas temperatures. 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 particulate matter and NOx.

Use of particulate filters and catalytic cleaning of flue gas reduces the amount of particulate matter and NOx and is therefore beneficial for the environment in general. In most areas, legislation requires reduction of NOx in the flue gas .

Ceramic filters e.g. in form of filter candles are used in many industries for removal of particulate matter from pro- cess gases. They are one of the most efficient types of dust collectors available and can achieve collection effi ^¬ ciencies of more than 99% for particulates. The filters can be made from various ceramic materials comprising ceramic fibres made of alkali and alkaline earth silicates, or alu- mino silicates. Ceramic filters based on ceramic fibers impregnated with active catalysts for the removal of NOX, N¾, dioxins, CO and different VOCs together with dust are disclosed in WO 2016150464.

In particular, vanadium oxide-based catalysts are commonly used catalysts for NO _x reduction by selective reduction of NO _x with N¾ in stationary and automotive applications. These catalysts are active both in the removal of hydrocar ^¬ bons (VOC) and of NOx by combined oxidation and the SCR re ^¬ action with NH3.

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.

A problem with the known SCR active catalyst is the rela- tively low efficiency of the catalysts at gas temperatures below 250°C. A low flue gas temperature in particular at start up and shut down of combustion facilities creates a problem in the NOx removal by means of SCR. This problem is solved by the invention through injection of NO ₂ into the flue gas at the temperatures below 250°C to promote the "fast" SCR reaction.

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 NO ₂ in the flue gas by the "fast" SCR reaction: 2NH ₃ + NO + N0 ₂ → 2N ₂ + 3H ₂ 0.

Thus, this invention is based on forming O ₂ externally to the flue gas duct and injecting the prepared O ₂ into the flue gas in an amount that promotes the "fast" SCR reac ^¬ tion. O ₂ can be formed from N¾ by oxidation of the N¾ to NO over a precious metal containing catalyst in a first step and subsequently oxidation of NO to NO ₂ in a second step.

The invention provides in a first aspect a method for the removal of nitrogen oxides from flue gas from combustion facilities, comprising the steps of

passing the flue gas through one or more ceramic filter catalysed with a catalyst for selective reduction of nitro ^¬ gen oxides in presence of ammonia added to the flue gas ei ^¬ ther as such or in form of a precursor thereof;

at a temperature below 250°C, injecting an effluent gas containing nitrogen dioxide into the flue gas upstream the one or more ceramic filters;

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 oxida ^¬ tion catalyst;

cooling the effluent gas to ambient temperature and oxidiz ^¬ ing the nitrogen monoxide in the cooled effluent gas to the nitrogen dioxide containing effluent gas. Preferably, the ceramic filter (s) is shaped in form of can ^¬ dle filter ( s ) .

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.

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 flue gas which provides then additionally part of the oxidation reactor heating duty.

Preferably, the oxygen containing atmosphere is ambient air.

NO formed from N¾ in a first step by oxidation of the N¾ in contact with a precious metal containing catalyst 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 NO ₂ . 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. Typical ^¬ ly between 1 and 2 minutes.

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 a temperature below 250°C is controlled to result 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 second aspect, the invention provides a system for use in the method according to the invention.

The system comprises within a flue gas duct a filter house with one or more ceramic filters catalysed with a catalyst for selective reduction of nitrogen oxides; upstream the one or more ceramic filters or the filter house injection means for injection of ammonia or a urea solution into the flue gas duct;

upstream the one or more ceramic filters or the filter house, injection means for injection of nitrogen dioxide containing effluent gas; and

outside the flue gas duct,

an ammonia oxidation catalyst unit; and

means for cooling and oxidizing nitrogen monoxide contain- ing effluent gas withdrawn from the ammonia oxidation cata ^¬ lyst to the nitrogen dioxide containing effluent gas con ^¬ nected at its outlet end to the injection means for injec ^¬ tion of the 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 promoting the oxidation of NO to NO ₂ .

In all the embodiments of the system according to the in- vent, the one or more ceramic filters are in form of ceram ^¬ ic candle filters.