WO1994017901A1

WO1994017901A1 - Method for removal of polluting gases from air

Info

Publication number: WO1994017901A1
Application number: PCT/NO1994/000037
Authority: WO
Inventors: Jürgen BITTNER; Ekkehard Schade; Markus Oldani
Original assignee: Abb Miljö Norsk Viftefabrikk As
Priority date: 1993-02-13
Filing date: 1994-02-11
Publication date: 1994-08-18
Also published as: AU6117194A

Abstract

Method for adsorption and cleaning of nitrogen oxides and volatile organic compounds from exhaust contaminated air at environmental temperature, thereby allowing the exhaust contaminated air to pass through a layer (1) of activated carbon as an adsorbant.

Description

Method for removal of polluting gases from air

The present invention is related to a method for removal of polluting gases from air, according to the preamble of the claims.

The method especially is concerned with air which has been polluted by the release of exhaust gases from combustion engines from cars and other vehicles. The polluting components in the exhaust gases primarily comprise carbon monoxide (CO) , nitrogen oxides (NOx), uncombusted volatile organic compounds (VOC) and traces of sulphur dioxide (S0₂). Critical levels of air pollution from exhaust gases may be reached in road tunnels, parking houses and garages and other enclosed areas which are exposed to heavy traffic.

The air in road tunnels, garages and the like is used for breathing. Certain air quality standards therefore must be maintained. The concentration of polluting gases must not exceed levels that are defined as hazardous for the human health.

If the concentration of polluting gases in these areas shall be kept below hazardous levels by dilution alone, quite substantial ventilation air volumes may be required. This is especially the case in long and/or heavily trafficked road -tunnels. A typical figure for road tunnels is a fresh air demand of 100 m³ per second and per km of unbroken length of heavily trafficked tunnel. It is easily understood that the installation and also the operation cost (energy cost) for the ventilation system in a long road tunnel may be substantial. The concentration of the various polluting gas components in exhaust gases will vary with the type of vehicle (gasoline or diesel motor, with or without exhaust catalyzer). The legally acceptable concentrations of the various gases dc vary in relation to the toxicity of each component. Normally, by increasing exhaust pollution, the component N0₂ will be the first to reach health hazardous levels of concentration in the air, followed by NO. These components hence will be decisive for the demand of ventilation of an exhaust pollute area, and these components will also be critical as pollutants in the discharge air from the area.

A method that can purify exhaust polluted air especially by reducing the contents of N0₂ resp. NO in the air, can offer substantial advantages especially for the conditions in long road tunnels. If the air is purified in the tunnel, it can absorb a higher content of exhaust gases, and the requirement for ventilation air can be reduced correspondingly. Eventually a new component of the exhaust gases will reach critical concentration level and limit the reduction of ventilation air volume.

Traditional methods for reduction of NO and N0₂ from flue gases, such as selective catalytic reduction and selective non-catalytic reaction are not applicable for exhaust polluted air. The air is cold, and the concentrations of the pollutants to be removed are of a different magnitude compared to fcr instance industrial flue gases. These methods are not designed for and will not work under these conditions.

Activated carbon is well known as adsorbent for gases such as N0₂ and NO. The gases are bound to the large surface of the activated carbon. It is also known that the surface of the carbon over time will be saturated and assimilation of additional gas is terminated. Adsorbtion as well as saturation are selective processes. Activated carbon may for example adsorb N0₂, NO as well as VOC, however, not CO. It may as well continue to adsorb N0₂ and VOC even when the activated carbon is saturated with NO. thereby allowing NO to penetrate.

With the method according to the present invention gases a high degree of purification of cold air polluted wit automobile exhaust gases may be achieved, avoiding the disadvantages of known methods. This is achieved with the method defined by the features defined in the claims.

The inventors succeeded in achieving very good selective cleaning effects for N0₂, NO as well as VOC in air polluted from exhaust in a road tunnel, by filtrating air through a layer of activated carbon. The inventors have made test with different types of activated carbon. These differ as to specific surface, porosity, particle size and content of ashes and trace metals.

In a preferred embodiment of the present invention, a thickness of the layer of activated coal used in the filter vary from 20 to 60 cm, the air having a velocity through the filter gross area of between 0,2 and 0,6 m/sec.

Good selective purifying effects may principally be s achieved with most types of activated carbon, provided the thickness of the carbon layer and the velocity of the air through the layer, are adjusted to the specific carbon quality. Surprisingly it is found a large difference between the carbon qualitites as to the adsorbation capacity, e.g. the amount of o polluted gas which can be adsorbed before the carbon is saturated and must be regenerated before further gas can be adsorbed.

It has further been found that most of the activated carbon types may be regenerated by desorbtion of the adsorbed gases when the coal is exposed for a high temperature (thermic s regeneration). Some, but not all types of activated carbon which were tested by the inventors, recover complete adsorbation capacity by such a regeneration. The regeneration may be accomplished by blowing hot air through the carbon, the temperature thereby is however strongly limited as the 0 reactivated carbon may oxidize (and glow) already at a temperature below 150°C in air. By such a low regeneration temperature the adsorbed gases are desorbed very slowly and very seldom completely. In the preferred method of regeneration an inert gas is used as a medium and the process is carried out in 5 the temperature range of from 150 to 400°C. The adsorbed gases thereby are desorbed quickly and at high concentrations.

Activated carbon which has been in operation as adsorbent for exhaust gases in air for some time, also will contain a substantial portion of adsorbed water. This water 0 vaporizes and is diverted during the regeneration process. The amount of vapor is by far much larger than the amount of the adsorbed gases which are desorbed during the regeneration. The inventor have succeeded in recirculating the evaporated steam, using this as an inert gas during the regeneration process. This 5 gives a substantial saving in relation to using a separate inert gas from a pressurized container, especially if the activated carbon should be regenerated in the purification plant as such.

Regeneration of the activated carbon with desorbtion of water vapor and adsorbed gases is a strongly energy consuming process. Substantial amounts of heat must be supplied to increase and maintain the temperature in the process. As propellant gas can be used a separate inert gas (such as N₂) or a water vapor. In the preferred embodiment of the present invention, s recirculated vapor is used.

The desorbed gases from the regeneration of activated carbon for purifying of exhaust gases comprise CO, NOx (NO+N0₂) and VOC in relatively high concentrations, as well as traces of S0₂ and H₂S. These gases should not be allowed to escape to the 0 atmosphere and cannot be let out in a road tunnel during the regeneration process. The present invention also comprises a system for reacting the desorbed gases from the regeneration process into not dangerous components, or to remove them.

The method for reaction or removal of the desorbed s gases from the regeneration will be described in connection with Figure 1, whereby a modification is described in connection with Figure 2.

The purification plant comprises a layer 1 of activated carbon of a suitable type and the air to be purified is forced o to pass through this layer by means of a ventilator 2. The layer 1 can be arranged in different ways. In this case is disclosed a layer arranged vertically between perforated plates 3 through which air can penetrate but not the carbon and which are arranged in a chamber 4. During normal operation valves 5 are open and 5 valves 6 are closed. During regeneration valves 5 are closed and valves 6 are opened, the ventilator 7 is initiated and ie heating battery 8 is activated. Preheated air now is penetrating through the layer 1.

The air direction during regeneration may be opposite 0 of the air direction during purifying, but may also be the same. The layer 1 is heated and water vapor and adsorbed gases thereby are emitted, especially CO, NOx and VOC. The surplus of air, vapor and gas in the system, flows out of the outlet pipe 9 and may, if desired, be guided through a cooler 10 where part of the 5 vapor is condenced. This reduces the gas volume through the rest of the system. The gas surplus flows thereafter through a heating element 11 and a catalyst 12 which may be a normal 3-way catalytic converter for Otto-engine exhaust. At correct temperature the following reactions will occur in such a converter :

1 ) CO + VOC + 0₂ → C0₂ + H₂0

2 ) CO + VOC + NOx → C0₂ + H₂0 + N₂

5

Initially the reaction 1 will dominate in the converter, however, air, e.g. free oxygen, rapidly is vented out of the. system. Before the main amount of emitted gas from the layer 1 has reached the catalyst 12, the substantial part of the o air will have been vented out. There will then be very little free 0₂ in the system and the reaction 2, which is a normal reaction in an automobile converter, then will dominate. In this reaction the emitted NOx is reduced to the non-hazardous N₂. This reduction will be nearly quantitative as there is a surplus of s the reducing gases CO and VOC present in relation to NOx. After the catalyst 12 a controlled amount of air is added, whereafter the gas is guided through a further catalytic converter 13 which is of the oxidizing catalyst type as used for diesel exhaust. In this converter the reaction follows equation 1 by means of the o oxygen content in the added air. By surveillance of the air surplus, the oxidation of CO and VOC will be strongly effective. Hereby the most important gases emitted from the regeneration process, are amended to non-hazardous reaction products N₂, C0₂ and H₂0 which without problems may escape to the tunnel. A small 5 ventilator 14 sucks the gas through the system and blows it out, into the tunnel.

Traces of adsorbed H₂S and S0₂ from the layer 1 also will be emitted during regeneration. S0₂ will be reduced to strong smelling H₂S in the catalyst 12 (which is a problem with 0 the automobile converters), however, will oxideze to S0₂ in the converter 13, together with the emitted H₂S from the layer 1. The amount of S0₂, however, will be so small that it may escape to the tunnel during regeneration without problems.

It also may be a problem to ensure complete reactions 5 l and 2 during the starting and termination of the regeneration process, which means that a small amount of not reacted CO, VOC and NOx may escape. It may be desirable also to prevent S0₂ in small amounts to escape during regeneration. To prevent these emissions, a small filter with activated carbon (not disclosed in Figure 1 ) may be used in the system af er the catalyst 13 or the ventilator 14. This filter may separate S0₂ and possible rest gases present and it must be replaced after use.

Another method to avoid discharge of desorbed gases to the tunnel is to adsorbe these gases in a new filter with activated carbon. The arrangement is disclosed in Figure 2. The regeneration process is unchanged in relation to the description above, however, are the catalytic coneverters replaced with a simple carbon filter 11 after the cooler. This filter may have an amount of carbon of only some percentages of the amount of the layer 1. When the filter 11 can absorbe substantially all the gases which are desorbed from the layer 1 during the regeneration, it is because of the fact that the concentration of such gases can be 1000 times as high or more in the vaste gas from the regeneration process, as seen in relation to the concentrations in the tunnel air to be purified through the layer 1. The carbon filter 11 therefore may adsorbe substantially larger amounts of these gases per weight unit with adsorbent, seen in relation to the layer 1. The carbon filter 11 must be replaced after the regeneration. This filter constitutes a minor carbon mass in relation to the layer 1, contains however, by replacement the gases adsorbed by the layer 1 in a concentrated adsorbed forn. This small amount which is easy to handle, very simple may be transported out of the tunnel and to a centralized plant where such used carbon may be regenerated at high temperature in a regeneration plant of a known type.

Claims

Patent Claims

1. Method for adsorbtion and cleaning of nitrogen oxides and volatile organic compounds from exhaust contaminated air at environmental temperature, CHARACTERIZED IN allowing the exhaust contaminated air to pass through a layer ( 1 ) of activated carbon as an adsorbant.

2. Method according to claim 1, CHARACTERIZED IN using a layer (1) of between 20 and 60 cm thickness.

3. Method according to claim 1, CHARACTERIZED IN allowing the air to pass through the layer ( 1 ) at a gross velocity of between 0,2 and 0,6 m/sec.

4. Method according to claim 1-3, CHARACTERIZED IN regenerating the adsorbant in situ by allowing a heated gas to pass through the layer (1) at a temperature of between 150 and 400°C.

5. Method according to claim 4, CHARACTERIZED IN using an inert gas as the heated gas, or a gas being inert in relation to the substances involved.

6. Method according to claim 4-5, CHARACTERIZED IN using water vapor as inert gas during regeneration.

7. Method according to claim 4-5, CHARACTERIZED IN using the desorbed gas from the adsorbant as the heated gas during regeneration.

8. Method according to claim 4-7, CHARACTERIZED IN cooling the desorbed gas from the regeneration before further treatment, thereby allowing water vapour to condense.

9. Method according to claim 7-8, CHARACTERIZED IN allowing the gas from the regeneration first to pass through a 3 way catalytic converter to reduce N0_X to N₂ in a reaction with CO and VOC, and secondly through an oxidicing converter to reduce CO and VOC to C0₂ and H₂0 by supply of oxygen from the air.

10. Method according to claim 7-8, CHARACTERIZED IN the desorbed gas from the regeneration being readsorbed i activated carbon.