GB2291868A - Treating exhaust gases with a suspension of dolomite - Google Patents

Abstract

A process is described which allows the removal of up to 100% of the carbon dioxide and acid gases, such as oxides of nitrogen, from the exhaust gases produced by fossil fuelled engines. The exhaust gases are contacted with a suspension of dolomite in aqueous sodium chloride solution (eg sea water) under pressure the products of the reaction being NaHCO3 precipitate and MgCl2.

Description

The Removal of Carbon Dioxide and Other Atmosphere Contaminants from the Exhausts of Fossil Fuelled Engines.

Introduction.

The main components of the exhaust fumes of fossil fuelled engines are nitrogen, oxygen, water, carbon dioxide, carbon monoxide, oxides of nitrogen, oxides of sulphur, unburnt hydrocarbons and elemental carbon particles. With the exception of nitrogen, oxygen and water, all the other components of the exhaust are atmosphere contaminating materials and gases. Of the contaminating gases carbon dioxide is in the preponderance. The process described below is capable of removing carbon dioxide gas, some carbon monoxide gas, oxides of nitrogen, oxides of sulphur, unburnt hydrocarbons and elemental carbon particles from the exhausts of fossil fuelled engines.

The Amount of Carbon Dioxide Produced.

The amount of carbon dioxide produced by typical road vehicles is required in order to assess the effectiveness of any absorber system. This being so, two cases will be examined, the light motor car used for personal transport and the heavy commercial transport vehicle of the articulated type, consisting of a tractor unit and a load carrying trailer. It is assumed for the purposes of the calculation that the reactions are complete. In practice this is not the case.

Petrol Fuelled Passenger Motor Car.

Loaded vehicle weight (5 passengers) = 1402 kilograms Average kilometres covered annually = 32,180 Fuel use - litres per 100 kilometres = 8.3 Litres per year = 2671 Fuel tank capacity = 68.3 litres Composition of petrol taken as dodecane (C12H26) Density = 0.7487 grams per millilitre Molecular weight = 170.34 Weight burned per kilometre = 62.1 gms 2C12H26 +3702= 24C 2 +26H20 34.068 + 118.4 105.6 + 46.8 Oxygen used per kilometre = 215.9 grams, volume = 151.1 litres.

Carbon dioxide produced per kilometre = 192.5 grams, volume = 97.4 litres.

Water produced per kilometre = 85.3 grams, considered as a liquid.

Air used per kilometre = 938.6 grams = 728.7 litres.

Nitrogen in air used per kilometre =722.7 grams, volume = 577.7 litres.

Total gas volume after combustion, per kilometre = 675.1 litres.

Volume percentage of carbon dioxide after combustion = 14.4.

Carbon monoxide produced per kilometre = 5.0 grams, volume = 6.25 litres Volume percentage of carbon monoxide after combustion = 0.93 Heavy Commercial Transport.

loaded vehicle weight = 38,000 kilograms.

Average kilometres annually = 160,900 Fuel use, litres per 100 kilometres (fully laden) = 55.7 Litres per year = 89,621 Fuel tank capacity = 182 litres Composition of Diesel oil, taken as pentadecane (C15H32) Density = 0.7685 grams per millilitre Molecular weight = 212.42.

Weight burned per kilometre = 427.9 gram C15H32 + 2302 = 15C02 + 16H20 21.242 73.6 66.0 28.8 Oxygen used per kilometre =1481 grams, volume =1483.8 litres.

Carbon dioxide produced per kilometre =1328.3 grams, volume = 617.8 litres.

Water produced per kilometre = 579.7grams, considered as a liquid.

Air used per kilometre = 6451.3 grams = 5008.9 litres Nitrogen in air used = 4967.5 grams, volume = 3970.9 litres.

Total gas volume after combustion, per mile = 4588.7 litres.

Volume percentage of carbon dioxide after combustion = 13.5 From the above it is seen that one motor car adds 6.2 tonnes of carbon dioxide to the atmosphere every year and the heavy commercial transport vehicle adds 210 tonnes in the same period.

The Process for the removal of Carbon Dioxide.

As far as is known it is not possible to effect the removal of the carbon dioxide and some of the other atmosphere contaminating gases and materials generated by fossil fuelled engines by modification of the engine in any way.

Therefore to remove the atmosphere contaminating gases produced by such engines it is only possible to proceed by treating the gases by means of equipment added to the vehicle.

The chemicals to be used in the process for the removal of these atmosphere contaminating gases are sodium chloride (common salt) and a natural mineral called dolomite, which is a mixed chemical compound of magnesium carbonate and calcium carbonate. Common salt is mined in Cheshire and large deposits of dolomite are found in Yorkshire. The usual minor constituents of dolomite are the oxides of iron, aluminium, titanium and manganese, silica, (either free or combined as clay and felspar), sulphur as iron sulphide (pyrites) or calcium sulphate (gypsum), sodium and potassium carbonates and organic matter.

Dolomite is virtually insoluble in water. However when carbon dioxide gas is dissolved in water carrying a suspension of a fine powder of dolomite the magnesium carbonate part of the dolomite dissolves preferentially. Only about five percent of the calcium carbonate in the dolomite dissolves under these conditions. The amount of magnesium carbonate that can be dissolved in a fixed volume of water is dependent on the pressure of carbon dioxide above the liquid.

For example, if there is no carbon dioxide gas in the atmosphere above the liquid, then less than one gram of magnesium carbonate will dissolve in a litre of water.

However if there is a pressure of pure carbon dioxide of 15 pounds per inch (1 atmosphere pressure) above the water the amount of dissolved magnesium carbonate is 25 grams per litre and at a pressure of 150 pounds per square inch (10 atmospheres pressure) the amount dissolved is 60 grams per litre (Graph 1).

It would be possible by passing the engine exhaust gases into a reservoir containing a suspension of a fine powder of dolomite in water and allowing the pressure of carbon dioxide to rise to remove carbon dioxide gas from these gases.

However as soon as the solution prepared in this manner is exposed to normal atmosphere the carbon dioxide is released and the magnesium carbonate reappears as a solid. This is because there is only a low partial pressure of carbon dioxide in normal atmosphere. Thus although the carbon dioxide may have been prevented from reaching the atmosphere while being trapped by chemical reaction with the suspension, the process of discharge of the solution causes release of the gas to the atmosphere. To prevent this situation requires the modification of the reaction.

The modified reaction involves passing carbon dioxide into a suspension of dolomite in sodium chloride (common salt) solution in water under pressure. The most convenient source of water for dissolving the sodium chloride is filtered sea water. These conditions result in the conversion of the dissolved magnesium carbonate to soluble magnesium chloride. The carbon dioxide undergoes a chemical reaction which produces sodium bicarbonate. This compound is not soluble in magnesium chloride/sodium chloride solution (Graph 2) and it is deposited from the solution as a solid.

The reaction of magnesium carbonate in the dolomite with sodium chloride and carbon dioxide in water under pressure plus the relevant heats of formation in Kcal per mole is as follows: MgC03 + 2NaCl = Na2CO3 + MgCl2 Exothermic 0.2 Kcal NaCO? + CO7 + H7O = 2NaHCO3 Exothermic 20.3 Kcal MgC03 + 2NaCl + CO2 +H2O = 2NaHCO3 + MgC12 The reaction is exothermic, that is, it produces heat, giving 20.2 Kcal of heat per mole of magnesium carbonate converted. The fact that the reaction is exothermic means that the temperature of the reaction cannot be allowed to rise significantly. The estimated maximum reaction temperature is 38e C.

In order to estimate the amounts of chemicals required for use of the reaction as an absorption process for the atmosphere contaminating products in the exhaust of fossil fuelled engines the operating pressure has first to be decided.

The selected pressure is 150 p.s.i. total pressure or four times the normal tyre pressure of motor cars. From the estimated percentage of carbon dioxide in the volume of gases produced this pressure means that the partial pressure of carbon dioxide at 150 p.s.i. is 21.6 p.s.i. (1.46 atmospheres). At this pressure the solubility of magnesium carbonate is 28 grams per litre.

The above reaction is dependent on the presence of excess sodium chloride to reduce the solubility of the sodium bicarbonate formed and to cause it to precipitate. Therefore the reaction must be performed in concentrated solutions.

Such solutions also reduce the likelihood of alternative insoluble magnesium compounds, which do not remove carbon dioxide, precipitating in preference to the desired sodium bicarbonate. One litre of a saturated solution of sodium chloride contains 357 grams of this compound. Using Graph 2, which shows the reduction of the solubility of sodium bicarbonate by various concentrations of sodium chloride, the amount of sodium bicarbonate formed and precipitated from 357 grams of sodium chloride is 436 grams as opposed to 513 grams if the reaction proceeded according to the above equation. The amount of carbon dioxide absorbed is 114 grams using 219 grams of magnesium carbonate, or 497 grams of dolomite, and producing 247 grams of magnesium chloride. All of the other atmosphere contaminating gases will be removed by the solution.Nitrogen and sulphur oxides dissolve in water to give acids which immediately react with the magnesium carbonate and calcium carbonate producing soluble non-volatile salts and carbon dioxide. Carbon monoxide is soluble in alkaline bicarbonate solution under a pressure of 56 p.s.i. (4 atmospheres) of this gas, forming sodium formate.

Since there is no data available for the solubility of carbon monoxide in alkaline solution as the pressure is increased from one atmosphere it is assumed that that amount dissolved will proportional to the partial pressure of this gas in the exhaust gases. This has the value of 0.1 atmosphere at a total pressure of 10 atmospheres. On this basis the reduction amounts to a minimum of 1.0% It is apparent from the weights of required chemicals and the amounts of carbon dioxide produced per year by either the ordinary motor car or the heavy commercial transport that for such vehicles to carry sufficient chemicals in order that the absorber is only changed once per year is not practical.

The principle involved in the use of this process is that the driver of the vehicle involved changes the absorber solution each time he or she refills the fuel tank of the vehicle.

Application to the Motor Car.

There are two positions on present motor cars that an absorber tank can be fitted: (i) Underneath the vehicle between the wheels.

(ii) At the rear of the vehicle in front of the present luggage space (the boot).

Most of the present designs of motor car world wide can be fitted with a tank beneath the chassis of the vehicle with dimensions 1.5 metres by 1.2 metres by 0.15 metres (5 feet by 4 feet by 6 inches) since the distance between the front and rear wheels is of the order of 1.8 metres. The fitting of such a tank would be easier in front wheel drive vehicles bearing in mind that the present silencer unit would no longer be required. The added weight in this position would lower the centre of gravity of the motor car thereby increasing the stability. The design of the absorber system is shown in diagrams 1 and 2. The diagram 1 shows the main components of the system, the condenser units and the absorber units.

These components cannot be made of iron metal in any form since this metal is rapidly corroded by carbon dioxide solutions. The use of aluminium metal is also precluded because this metal is reacts chemically with sodium alkali solutions, such as sodium bicarbonate. However there are no high temperatures involved in the absorber reactions and plastic can be used for the construction of the units shown. The units are required to withstand pressures of up to 150 p.s.i.

and are therefore enclosed in a protective case of steel or aluminium metal.

Most fossil fuelled engines cannot tolerate any pressure in the exhaust exit system and the exhaust gases are drawn into the absorber system by a compression pump which is driven from the engine or electrical system of the vehicle. The gases leaving the engine exhaust manifold are hot and carry water, unburnt hydrocarbons and carbon particles. To remove these materials and cool the gases below 38 C, before being fed into the absorber units, the gases are passed through a series of the condenser units cooled by the air flow of the moving vehicle. The gases then pass on to the absorber units, which are also cooled in the same manner, where they are bubbled through the absorber solution. This procedure keeps the solution well mixed. The precipitating sodium bicarbonate deposits in the base of the absorber units.

The volume of an absorber system which can be reasonably fitted under the vehicle between the wheels is 143 litres. As shown in diagram 1, free space is required above the liquid in both the condenser and absorber units. This is estimated at 25% of the above volume, giving a total required volume of 175 litres.

All the units have the dimensions of 10 centimetres by 10 centimetres by 15 centimetres deep. Each has a volume of 1.5 litres and 117 absorber and 3 condenser units are contained in the absorber system, arranged in a rectangle of 10 by 12 units with dimensions of 1 metre by 1.2 metres and 0.15 metres deep. The materials produced in the absorber and condenser unit are withdrawn for the bases of the units. All the bases of each type of unit are connected together as shown in diagram 1. To obtain and control the operating pressure of 150 p.s.i. the outlet of the system is fitted with a pressure control valve.

The exhaust gases are drawn into the absorber and compressed to the required value. The rate that gas is produced is dependent on the on the speed that vehicle is being driven which decides the fuel consumption. In practice the speed and therefore the fuel consumption is seldom constant and so a mean speed of 80 kilometres per hour has been selected. To pressurise the absorber to the operating pressure 1,750 litres of gas is required and this will be delivered in two minutes at the fuel consumption of the selected speed. The gas flow rate through the absorber unit is 900 litres a minute and the gases are passed through the condenser and absorber units in parallel giving a flow rate through the condenser units of 300 litres per minute and through the absorber units of 7.8 litres per minute.The overall depth of the unit, including pipes is estimated as 25 centimetres (10 inches) maximum. The pipe for the extraction of used absorber solution and the refilling with fresh absorber solution is taken to a point near the fuel tank filler. Using a suitably designed vacuum and pump system, available at the fuelling station, the driver of the vehicle could perform this operation in under 2 minutes. Emptying of the condenser units will be performed when the vehicle if serviced. For this absorber: Dolomite required = 72 kilograms.

Sodium chloride required = 51 kilograms Sea water required = 143 kilograms Sodium Bicarbonate produced = 62 kilograms Magnesium Chloride produced = 37 kilograms Increase in vehicle weight = 26% Increase in fuel consumption = 9% Reduction of carbon dioxide = 10% Reduction of acid gases = 100% Reduction of carbon monoxide = 1.0% The second space available for an absorber system is in front of the present rear luggage space. By extending the length of a motor car by 50 centimetres would allow the accommodation of an absorber system 1 metre wide by 0.7 metre deep and 0.5 metres long with a total volume of 375 litres. From the figures above it can be seen that the conditions will be as follows: Dolomite required = 144 kilograms.

Sodium chloride required = 102 kilograms Sea water required = 286 kilograms Sodium Bicarbonate produced = 124 kilograms Magnesium Chloride produced = 74 kilograms Increase in vehicle weight = 52% Increase in fuel consumption = 16% Reduction of carbon dioxide = 20ago Reduction of acid gases = 100% Reduction of carbon monoxide = 2.0to Application to Heavy Commercial Transport.

The absorber system for this vehicle is composed of condenser and absorber units the same design as shown in diagram 1 and is conveniently composed of 120 absorber units and 8 condenser units each being 0.3 metres (1 foot) long, wide and deep and having a volume of 27 litres each. This gives a total volume of 3240 litres. The individual units are arranged as eight units wide and fifteen units long giving an overall length of 4.5 metres by 2.1 metres by 0.3 metres (15 feet by 7 feet by 1 foot). This unit is fitted under the trailer of the articulated heavy commercial transport. The weights of chemicals involved are: Dolomite required = 1609 kilograms Sodium chloride required = 1157 kilograms Water required = 3240 kilograms Sodium bicarbonate produced = 1412 kilograms Magnesium chloride produced = 709 kilograms.

Increase in vehicle weight = 16% Increase in fuel consumption = 6% Reduction in carbon dioxide = 85% Reduction in acid gases = 100% Reduction in carbon monoxide = 1.0% The heat produced by the reactions of the above weights of chemicals, in the case of both vehicles, will raise the temperature of the solution by less that 1 C in either case. The increase in fuel consumption caused by the increase in weight is given in graph 3.

Safety of the System for the Users and the Environment.

Any biological life forms in the sea water will not interfere with the process and may in fact survive the conditions in the absorber solution since both sodium bicarbonate and magnesium chloride are components of sea water. All of the chemicals involved in the above process have no dangerous properties whatsoever. Sodium bicarbonate for example is the chemical from which washing soda is obtained. On this basis they can be disposed of into the normal sewage system, directly into the ocean or collected at the filling station for recovery of the chemicals. In the event of a road accident involving vehicles fitted with the above system and rupture of the absorber tank occurring, the chemicals will not present any serious danger. In fact since carbon dioxide and the solution involved suppress fire, their presence may help to reduce the danger from this source.The presence of salt solutions mean that the liquids in the absorber are unlikely to freeze, reducing the danger of the absorber tank rupturing in cold weather.

Useful Chemicals Produced.

The solid residue from the chemical reactions in the absorber containsvarying amounts of useful chemicals dependent on the source and impurity content of the original dolomite, sodium chloride and sea water used.

1. Sodium bicarbonate, used in the glass industry and the detergent industry.

2. Calcium carbonate, used in the cement industry.

3. Iron oxide, used in the steel industry.

4. Aluminium oxide, used in the aluminium metal and ceramic industries 5. Titanium oxide, used in the paint industry.

6. Manganese oxide, used in the steel industry.

7. Sodium nitrate, used as a fertiliser.

& Sodium sulphite and sulphate, used in the textile industry.

9. Potassium salts.

Claims (1)

1. Claims.

   (i) A process whereby part or all of the carbon dioxide from the exhausts of fossil fuelled engines is captured and prevented from entering the normal atmosphere of the Earth.

   (ii) A process whereby other noxious gases produced by fossil fuelled engines, such as nitrogen oxides and sulphur oxides are captured and prevented from entering the atmosphere of the Earth.

   (iii) A process whereby unused fossil fuel and solid products are captured and prevented from entering the atmosphere of the Earth (iv) A process for the applications in claims (i), (ii), where the captured noxious gases give rise to products which are not environmentally damaging and readily disposable.

   (v) A process for the applications in claims (i), (ii) whereby useful chemicals are produced and can be collected and used industrially.

   (vi) A process for the applications in claims (i), (ii), (iii) and (iv) where the chemicals involved have no dangerous characteristics, (vii) A process for the applications in claims (i), (ii), (iii) and (iv) whereby the required chemicals are added to and removed from the absorber system by a method which is of equal simplicity to the addition of fuel to the fuel tank of the vehicle.