WO1991012875A1

WO1991012875A1 - Disposal of flue gases

Info

Publication number: WO1991012875A1
Application number: PCT/GB1991/000325
Authority: WO
Inventors: Miroslav Radojevic
Original assignee: Miroslav Radojevic
Priority date: 1990-03-02
Filing date: 1991-03-04
Publication date: 1991-09-05
Also published as: AU7311191A; GB9004743D0

Abstract

A method and apparatus for the disposal of flue gases is disclosed wherein the flue gases are transported to a discharge point below the surface of a body of salt water and discharged into the salt water of a depth sufficient to ensure that the gases are substantially dissolved before reaching the surface of the body.

Description

Disposal of Flue Gases

This invention relates to the disposal of waste gases, in particular flue gases, and more particularly to the disposal of flue gases by the use of salt water.

It is well known that flue gases produced by fossil fuel combustion, for example in fossil fuel fired electricity generating plants or other industrial boilers and by steel making plants, refineries,

incineration plants, chemical plants and such like contain substances considered to be harmful or

potentially harmful to the environment, including sulphur dioxide (SO₂), carbon dioxide (CO₂), hydrogen sulphide (H₂S), hydrogen chloride (HCl), hydrogen fluoride (HF), oxides of nitrogen (NOx), nitric acid vapour (HNO₃) and trace metals such as transition metals. Alkali and alkaline earth metals may also be present. Recently, measures have increasingly been required to remove at least some of these substances from the flue gases before discharge of the gases to the atmosphere.

Particular importance has been attached to the removal of SO₂, which is associated with the problem of "acid rain" and also to the removal of CO₂, which is associated with potential global warming.

The removal of SO₂ is conventionally effected by means of a lime and/or limestone scrubber through which the flue gas is passed in conjunction with a freshwater - lime and/or limestone slurry. The SO₂ in the flue gas is thereby oxidised to sulphate (SO₄ ^2-). However high costs are involved in the necessary provision of the impinger, scrubber, bubbler devices and their housings, of chimney stacks for the disposal of non- absorbed gases and in pumping the required solutions to and from the scrubber. Moreover, significant problems arise with this process in the transportation of the large quantities of lime and/or limestone which are required, in the preparation of scrubbing solutions and in the transport and disposal of the spent slurry which is formed in considerable bulk. Problems due to plugging of the scrubber are also common.

Furthermore, the use of limestone in the oxidation of SO₂ results in the formation of CO₂. Thus the removal of one pollutant gas from the flue gases merely results in the formation of another pollutant gas.

It is also known that the requirement of lime and/or limestone in scrubbers for the removal of SO₂ can be significantly reduced or eliminated by the use of sea water instead of fresh water since the oxidation of SO₂ in sea water occurs considerably faster than in fresh water. This has been attributed to the catalysis of the SO₂ oxidation by chloride ion present in the sea water.

Both the above types of scrubber are, also, limited in their use by the short residence time of the pollutant gas in the scrubber. This problem is not confined to scrubbers for the removal of SO₂ but also affects scrubbers and such like techniques for the removal of other gaseous components from flue gases and the like. There is therefore a need for excellent gas/liquid contact by means of a high liquid:gas ratio, which may not easily be achieved within the other operating parameters of a scrubber.

The present invention seeks to provide a means of disposal of flue and like gases which avoids the use of lime and/or limestone, which obviates the requirement to provide scrubbing towers, chimney stacks and such like and which results in a minimum of environmental damage.

According to a first aspect of the present

invention there is provided a method of disposing of flue gases comprising conveying said gases to a discharge point beneath the surface of a body of salt water and discharging said gases, which flue gases are substantially dissolved in the body of salt water before reaching the surface of the body.

According to a second aspect of the present invention there is provided an apparatus for the disposal of flue gases characterised in that the apparatus comprises a pipe having a first end and at least one second end, wherein the first end is

connected to a source of the flue gases and at least a portion of the pipe including the at least one second end is disposed beneath a body of salt water, the said portion comprising means for discharging the flue gases and which portion is located at a depth below the surface of the body of salt water which is sufficient to ensure that the flue gases are substantially

dissolved in the salt water before reaching the surface of the body.

In a first embodiment of the invention the salt water is sea water.

In a second embodiment of the invention the flue gases are discharged as small bubbles. Bubbles having a diameter of the order of 0.05 to 10cm are desirable, although diameters in the range of 0.1 to 1cm are particularly preferred.

In a third embodiment of the invention the flue gases are passed through a dust extraction means before discharge.

In a fourth embodiment of the invention air is mixed with the flue gases before discharge.

In the method of the present invention, the flue gases are desirably discharged into the salt water as small bubbles, depending on the operational parameters of the method, such as the required rate of gas

throughput. The gas/liquid contact is thereby

maximised, and together with the very large volumetric ratio of liquid to gas, efficient transfer of the flue gas components from the gaseous to the aqueous phase is achieved. However, high efficiencies are also achieved with larger bubbles.

The method of the present invention is suitable for the disposal of many soluble components found in flue gases including SO₂, CO₂, H₂S HCl, HF, NOx and HNO₃. Greater than 99% removal of these components can be achieved.

The following scheme for the dissolution of SO₂ and CO₂ has been proposed:

XO₂ + H₂O XO₂.H₂O (1)

K _1x

XO₂ .H₂O

HXO₃- + H⁺ (2)

K_2x

HXO₃- _XO₃ ^2- + H⁺ (3)

where X represents S(IV) or C(IV)

H_x is the Henry's Law Constant

K_1x is the first dissociation constant

and K_2x is the second dissociation constant.

Dissolved S(IV) species are rapidly oxidised by a chloride ion catalysed reaction to sulphate (SO₄ ^2-) whilst dissolved CO₂ will be mainly present as HCO₃-.

The dissolved species resulting from the various flue gas components are rapidly diluted with the surrounding salt water by natural mixing processes such as currents, tides, waves and the like. Turbulent mixing due to the momentum of discharge of the flue gases also assists in the dilution of the dissolved species.

The efficiency of the process of the invention in the dissolution of SO₂ and CO₂ may be calculated as follows:- The constants H_x, K_1x and K_2x of equations (1) to (3) above may be defined in terms of activities and partial pressures of the various components. Thus,

H_x = (4)

K_1x = (5)

K_2x = (6)

A pseudo - Henry's Law coefficient,

can be defined in terms of all the dissolved species

= H_x (7)

As seawater has a pH in the region of 8 and as pH = -log₁₀ (a_H+), a_H+= 1 x 10^-8 molℓ^-1

Values at 5°C of H_x, K_1x and K_2x and of derived

from these values by equation (7) are shown in table 1 for S(IV) and C(IV). H_x, K_1x and K_2x are independent of pH, but increases with increasing pH for these

species, illustrating the increasing solubility of these species at higher pH. The value quoted for

is for seawater at pH8.

Consider a unit volume immediately outside the discharge point of the flue gas outlet. The volume consists of liquid seawater and bubbles of flue gas. Equilibrium between the gas and liquid is assumed, which is reasonable given the relatively small size of the bubbles and the long residence time of the bubbles in the sea water. A flue gas component has a partial pressure p_1x inside the discharge pipe and a partial pressure p_2x inside a gas bubble after equilibrium with the sea water. A partial pressure P_3x is defined as the partial pressure which would result if the

equilibrium dissolved species were transferred to the gas phase in the bubbles. Thus,

P_1x = p_2x + p_3x (8)

The total activity of the dissolved species at equilibrium is then given by ax(IV)aq. = (9)

where R is the gas constant

(R=0.082 ℓtm mol^-1K^-1)

T is the absolute temperature (K) L is the volumetric ratio of liquid to gas in the unit volume (m³ liquid/m³ gas) p_3x may be estimated from p_3x = L.R.T. a_x(IV)aq (10)

Thus from equations (9) and (10)

p_3x = (11)

The efficiency of the absorption E, is defined as

E(%) = x 100 (12)

since p_3x represents the amount of gas removed into the liquid phase. Thus E(%) = x 100 (13)

E(%) = x 100 (14) :

The efficiency of absorption of SO₂ and CO₂ at various volumetric ratios and at 5°C (278K) is shown in table 2.

The value of L will increase with increasing distance from the discharge point. Values of L greater than 1000 can be achieved by means of the method of the present invention and accordingly both SO₂ and CO₂ can be dissolved with an efficiency of 100%. In comparison, L values of 0.1 or less are representative of flue gas disposal devices including conventional scrubbing towers.

Even bubbles of flue gas having a diameter of for example 2.5 cm will rise through the salt water at a maximum velocity of 0.38ms^-1. Hence when, for example, the outlet of the discharge pipe is 50m below the surface of the salt water, the flue gas bubbles have a potential residence time of at least 132s, and where the depth of the outlet is 200m, the potential

residence time is at least 526s. In contrast, the time available for liquid/gas contact in a conventional scrubbing tower is of the order of 10s or less.

Table 3 shows the average concentration of various ions in sea water:-

From Table 3 it is clear that SO₄ ^2-, HCO₃- and Cl'are present in sea water in high concentration and in view of this and also the high natural pH and excellent buffer capacity of sea water it can be seen that the discharge of SO₂, CO₂, H₂S, HCl HF, NOx, HNO₃ and other acidic components will have minimal environmental effects. Furthermore, any alkali or alkaline earth metals present in the flue gases will have no

significant effect on the sea water, since these metals are already abundant in sea water.

If desired and depending on local coastal conditions, the flue gases may be passed through dust extraction means before discharge to sea. Any suitable dust extraction means may be used, for example

electrostatic precipitators, cyclones, baghouses including fabric filters, gravity settling chambers and inertial separators.

At or in the outlet of the discharge pipe, any suitable means may be employed for the production of small bubbles. Suitable means include propellers, impellers, spargers, nozzles or such like, a perforated pipe having holes of, for example, 3 to 15mm in

diameter, a pipe comprising a porous material or a cover of porous material attached to the end of the pipe. It is not necessary for the discharge of the flue gases to be confined to the end of the discharge pipe. The gases can desirably be discharged from substantially the whole length of that part of the discharge pipe which is located at a suitable depth.

The flue gases are transported to the point of discharge by means of an underwater pipe. The length, diameter and composition of the underwater pipe and the depth of the point of discharge may be chosen as desired depending on the prevailing local coastal conditions and on the source and nature of the flue gases, and on the proximity of the source to the salt water.

It is preferable, however, that the discharge point is not very close to the sea bed so that the sea bed is not disturbed by the flue gases. The outlet pipe may, for example include an underwater chimney to ensure that the initial discharge of the flue gases is in an entirely upward direction thus preventing any disturbance of the sea bed.

Where necessary, means may be provided to propel the flue gases along the discharge pipe and to overcome the static pressure of the water at the discharge point. If desired, air may be introduced into the flue gases to increase the level of oxygen and thus to promote the oxidation of SO₂. Where the flue gases are produced at elevated temperature, introduction of air will also serve to cool the gases before discharge. Suitable means for propelling the flue gases include induced draught fans, blowers, compressors and air pumps.

Where a dust extraction means is not included, and depending on the nature of the dust, trace transition metals such as Fe, Mn, Zn and Cu may be present in the flue gases at the point of discharge. Such metals are known not to be highly soluble in aqueous solutions of high pH. Thus these metals will not be highly soluble in sea water (pH8) and will simply be discharged and eventually deposited on the sea bed or dissolved.

It is noted that the present invention is not confined to the use of sea water but is applicable to many bodies of salt water, for example a salt lake.

The following examples illustrate the invention. Example 1

A small industrial coal fired boiler located on the coast is equipped with an electrostatic

precipitator to remove dust particles. The flue gas exiting the electrostatic precipitator has the

following characteristics:

Flow rate = 425m³ min^-1

Temperature = 150°C

SO₂ = 1000 ppmv

CO₂ = 12% by volume

O₂ = 6% by volume

N₂ = 81% by volume

The flue gas is discharged to sea at a depth of 50m in a nearby bay by means of a pipe 63.5cm in diameter, constructed from a corrosion resistant material (for example polyvinyl steel). On discharge, the flue gas is passed through a device capable of producing small bubbles of flue gas in the sea. The CO₂ and SO₂ are completely absorbed in the sea water. Example 2

A contact sulphuric acid manufacturing plant is located on the shore of a saltwater lake and is equipped with an electrostatic precipitator to remove dust particles from the flue gases. Gas leaving the precipitator has following characteristics:

Gas flow rate = 283m³min^-1

Temperature = 30ºC

SO₂ = 0.35% by volume

CO₂ = 6% by volume

O₂ = 9% by volume

N₂ = 81% by volume

The saltwater composition is:

pH = 10

HCO₃- = 0.04 mol 1^-1

CO₃ ^2- = 1.5 mol 1^-1

SO₄ ^2- = 0.40 mol 1^-1

Cl- = 1.3 mol 1^-1

Na⁺ = 5.0 mol 1^-1

Temperature = 10°C

The flue gas is piped by means of a corrosion resistant pipe (for example polyvinyl steel) 50.8cm in diameter and 300m long and is discharged at a depth of 20m to produce small bubbles. Complete absorption of SO₂ and CO₂ takes place.

Example 3

An HCl manufacturing plant has the following flue gas characteristics:

Flow rate = 6m³ min^-1

Temperature = 15ºC

HCl = 0.01% by volume

The plant is on the coast and the flue gas is piped by means of a corrosion resistant pipe (polyvinyl steel) 10cm in diameter and 200m long to a depth of 15m underwater and discharged in the form of fine bubbles.

HCl gas dissolves according to Henry's law

equilibrium.

HCl _gas HCl _aq ; H = 20 mol 1^-1 atm ^-1

HCl

_aq H⁺+ Cl-; K₁> 1 mol 1^-1

Complete absorption of HCl takes place to give soluble Cl- in seawater.

Claims

1. A method of disposing of flue gases

characterised in that the method comprises conveying the said gases to a discharge point beneath the surface of a body of salt water and discharging the said gases into the body of salt water wherein the said gases are substantially dissolved in the body of salt water before reaching the surface of the body.

2. A method according to claim 1 wherein the salt water is sea water.

3. A method according to claim 1 wherein the gases are discharged as small bubbles.

4. A method according to claim 3 wherein the diameter of the bubbles is of the order of 0.05 to 10cm.

5. A method according to claim 4 where the diameter of the bubbles is of the order of 0.1 to 1cm.

6. A method according to claim 1 wherein the gases are passed through a dust extraction means before discharge.

7. A method according to claim 1 wherein the gases comprise at least one member selected from the group comprising SO₂, CO₂, H₂S, HCl, HF, NO, N₂O, and HNO₃.

8. A method according to claim 1 wherein air is mixed with the gases before discharge.

9. A method according to claim 1 wherein the gases are cooled before discharge.

10. An apparatus for the disposal of flue gases characterised in that the apparatus comprises a pipe having a first end and at least one second end, wherein the first end is connected to a source of the flue gases and a portion of the pipe including at least the second end(s) is disposed beneath a body of salt water, the said portion comprising means for discharging the flue gases and which portion is located at a depth below the surface of the body of salt water which is sufficient to ensure that the flue gases are

substantially dissolved in the salt water before reaching the surface of the body.

11. An apparatus according to claim 10 including means for propelling the flue gases along the said pipe.