GB2418424A

GB2418424A - Producing hydrogen using the Castner reaction

Info

Publication number: GB2418424A
Application number: GB0421518A
Authority: GB
Inventors: Daniel Stewart Robertson
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-09-28
Filing date: 2004-09-28
Publication date: 2006-03-29
Also published as: GB0421518D0

Abstract

Hydrogen for use as a fuel is produced firstly using the Castner process in which sodium hydroxide is reacted with carbon to produce hydrogen and sodium as the main products with sodium carbonate as a by-product and thereafter the liquid sodium is reacted with a counter-current of steam to produce more hydrogen for fuel use and sodium hydroxide as a by-product. The sodium carbonate is then reacted with oxygen followed by water to produce sodium hydroxide and hydrogen peroxide although some carbon dioxide is produced by decomposition of the carbonate. The hydrogen can be burned with oxygen to produce high pressure steam for electrical power generation. The carbon can be coal, coke or from biological sources.

Description

24 1 8424

Introduction.

Two principle reasons are presently advanced to advocate the development of sources of hydrogen gas for use as a fuel in electrical power generation and transport. The first of these is the predicted effects of an increase in the concentration of carbon dioxide in the atmosphere and the second is the recognition that easily accessible sources of oil for use as a fuel and chemical feedstock are becoming rare. There remain however extensive stocks of coal and it is also known that methane hydrate exists in considerable amounts at depths below some parts of the oceans. Resort to these sources is not favourable unless there exists a means of capture and disposal of the carbon dioxide generated from the use of these fuels. As a result it is presently proposed to extend the industrial production of hydrogen for use as a transport fuel initially. Hydrogen is presently produced industrially by electrolysis of water. '['his requires that all the fossil fuel now used in transport be used for electrical power generation. Given the efficiencies involved this policy may require an increased use of fossil fuel.

Under these circumstances it follows that this proposal could in fact lead to an increase in the amount of carbon dioxide released to the atmosphere from electrical power generation. This operation is presently the largest single source of carbon dioxide. Capture and disposal of carbon dioxide is possible and a method of achieving this end has been described. 'lithe chemicals used are not in any way detrimental to the environment (Reference 1). However the use of hydrogen for both electrical power generation and transport is a more effective solution. The work below describes the use of fossil fuels to generate hydrogen for both electrical power generation and transport which results in a reduced use of fossil fuel. The carbon dioxide formed is in a condition and a quantity suitable for removal by the method in Reference 1.

The Chemical Reactions Involved.

The detail of the chemical reactions involved is shown in Table 1. The first of the reactions involved is generally known as the Castner reaction. In this reaction a mixture of sodium hydroxide and carbon is heated to 1000 Celsius. The products are hydrogen gas, sodium metal and sodium carbonate. The hydrogen is either fed to storage tanks or used directly in reaction with oxygen or air to give heat required to produce high pressure steam for electrical power generation.

I'he sodium metal formed (melting point 97.8 Celsius, boiling point 883 Celsius) accumulates on the surface of the molten sodium carbonate (melting point 851 Celsius). The sodium metal is decanted from the molten sodium carbonate as it forms. The metal is then passed into a spray tower and the descending globulised metal is reacted with a rising stream of steam. This generates further hydrogen gas and sodium hydroxide. On completion of the hydroxide-carbon reaction the sodium carbonate is converted to sodium peroxide by introducing an air or oxygen gas stream over the reaction mass at a temperature of 1000 Celsius. The carbon dioxide produced by the reaction is captured and disposed by the method given in Reference l. On complete conversion the sodium peroxide is treated with water giving sodium hydroxide and hydrogen peroxide. Sodium hydroxide from both sources is recycled.

Electrical Power Generation.

The theoretical heat output of a 500 MW electric power generating unit is 5160 Giga Joules per hour (5.16 xlO 12 J him) using powdered coal at the rate of tonnes per hour (Reference 1). The overall efficiency of the unit is 32%. A power station operates two independent closed circuit heat transfer systems. The first is the high pressure high temperature steam circuit used to generate electrical power. The second is the cooling circuit used to cool and condense the the high pressure steam from the from the generator circuit. The heat available from the the steam turbine circuit is 2300 Giga Joules per hour (2.3 x lo 12 J hr1).

On this basis the burning of hydrogen, produced as described, will produce at least the equivalent heat used in the generator circuit. The relevant data and calculations are given in Table 2. The primary hydrogen production is used for power generation and the secondary hydrogen production is used for heating the reaction mixture.

Plant Operation The plant operates a cycle consisting of the following time periods: 1. The loading period.

2 The reaction period.

3. The unloading period.

It is seen from the Table 2 that one cycle reaction mixture consists of 640 tonnes metric of sodium hydroxide and 64 tonnes metric of carbon giving a total charge weight of 704 tonnes metric. The reaction mixture heated in a heavily lagged iron or other suitable metal container fitted with a sealable lid.. The method of heating is either resistance or induction electrical heating using part of the power generated by the power station. 'l'he gas atmosphere within the reactors during this part of the cycle is carbon dioxide derived from reaction mixture. The density of' sodium chloride is 2.13 gms.ml1. The volume of the above charge is 330 x 106 mls equivalent to 330 x 103 litres. Reactors with a maximum volume of 1()3 litres ( 1.0 cubic metre) are considered of sufficient size to allow efficient heating of the reaction mixture. One plant is formed from 330 of such reactors positioned as a rectangle of 33 by 10. Allowing a separation distance of 1 metre all around each reactor the floor area of a plant building is of the order 70 metres by 25 metros. The preformed mixture of carbon and sodium hydroxide is loaded by an overhead hopper loading system. A loading and sealing time of 1 hour for all 330 reactors is considered possible. The reaction period is divided into the heating period and the reaction period. The time of the first is set at 1.0 hour.

The reactions are considered to] 00% efficient. This means that heating continues until the reactions are complete. A time of l.0 hour is set for each ot the hydroxide-carbon reaction and the conversion reaction. The unloading period consists of the cooling time and the product dissolving time. A period of one hour is set for each of these operations. The total cycle time is 6.0 hours. On this basis a reactor hall is loaded and unloaded 4 times in any 24 hour period and produces 64 tonnes metric of hydrogen gas per day. For continuous operation of a 500 MW generating unit the hydrogen production rate required is 16 tonnes hydrogen per hour equivalent to 384 tonnes metric per day. This output requires 6 reactor halls. A further 16 tonnes metric of hydrogen per hour is required to reach the reaction temperature. The hydrogen generating plant will consist of 12 reactor halls operating as described above. The source of the carbon is coal, coke produced from coal or other fossil fuel (oil, biomass, waste). The carbon dioxide generated from the conversion of sodium carbonate not combined with other gases as in the case in conventional power generating units and can be removed by the use of the process described in Reference].

References 1. D.S. Robertson. Old is New. The Chemical Engineer. 12th October. 1995.

pages 40 to 43.

2. G.W.C. Kaye, T.H. Laby. Tables of Physical Constants. Longmans, Green Co. Ltd., 1958.

3 Audus H (1993) Greenhouse gas releases from fossilfuel power stations.

IEAGHG/SRI, IEA Greenhouse Gas R&D Programme, Cheltenham, UK, 178pp 5.

Table l.

The Chemical Reactions Involved.

2112 + 02 = 2H20 Molecular weights2(2) 32 2(18) =4 =36 Heats of Formation 2( -68.32) (KCals) = -136.64 le Castner Reaction. (Primary hydrogen production) 6NaOH + 2C = 2Na + 3H2 + 2Na2C03 Molecular weights 6(40)2 (12) 2(23) 3(2) 2(105.9) (gms) = 240 24 46 6 211.8 lleats of Formation 6 (-101. 99) 2(-270.3) (KCals) = -611.94 = -540.6 The Sodium Rcaction (Secondary hydrogen production) 2Na + 21120 = 2NaOH + H2 Molecular Weights 2(23) 2(18) 2(40) 2 (gms) = 46 Heats of Formation 2(-68.32) 2(-101.99) (KCals) = 136.64 = 203.98 Reagent recovery 2 Na2C03 + 02 = 2Na202 + 2C02 Molecular Weights 2(105.99) 32 2(77.98) 2(44) (gms) =211.8 = 155.96 = 88 Heats of Formation 2(-270.3) 2(120.6) 2(-94.05) (KCals) = -540.6 = -241.2 = - 188.1 2Na202 + 4H20 = 4NaOH + 2H202 Molecular Weights 3(77.98) 6(18) 6(40) 3(34) (gms) =233.94 =108 = 240 = 102 Heats of Formation 3(-120.6) 6(-68.32) 6(-] 01.99) 3(-31.83) (KCals) =-361.8 = -409.92 = - 611.94 = -95.49

Table 2.

Electrical power Generation The steam turbine circuit of a 500 MW electrical power generation unit requires 230() GigaJoules of heat per hour and burns 200 tonnes of coal per hour.

230() x 109J hr1 = 23()0 x 109 x 0.239 Cals hr1 ((1 Cal = 4.184J) = 5.49 x 108 KCals hr Hydrogen requirement 4 gms hydrogen gives 136.64 KCal ( fable 1) 5.49 x 1 o8 KCals requires l 6 x 1 o6 gms hydrogen.

= ] 6 tonnes hydrogen Hydrogen Production. The Castner Reaction Carbon requirement 6 gms hydrogen requires 24 gms carbon 16 x 106 gms hydrogen require (16 x 106 x 24)/6 = 64 x 106gms carbon = 64 tonnes carbon Sodium Hydroxide requirement 6 ems hydrogen require 240 gms NaOH 16 x 106 gms hydrogen require (16 x 106 x 24())/6 = 64() x 106 gms NaOIl = 640 tonnes NaOH Heat requirements. - Castner Reaction.

704 tonnes of NaOH plus carbon require to be heated to 1000 Celsius.

Specific Heat of NaOII = 0.88 J gm1 Cl(taken as equal to NaC1 Reference 2) Heat required to reach reaction temperature =1.47 x 108 KCal.

Assuming effective insulation and automatic temperature control only a fraction of the above will heat will be required to maintain reaction temperature for 1 hour.

Sodium Reaction 240 gms NaOH produce 46 gms Na metal.

604 tonnes NaOH produce 122.7 tonnes Na metal 46 gms Na metal produce 2 gms H2 122.7 tonnes Na metal give 5.3 tonnes 1-12 4 gms H2 produce 136.6 KCal.

5.3 tonnes 112 produce 1.8 x 1()8 KCals to

Claims

Claims.

1. The industrial scale production of hydrogen gas for use as a fuel in electrical power generating and transport is feasible by the method described.

2. Assuming an overall plant conversion efficiency of 60%, equal to that predicted for present day natural gas fired 500 MW power plants (Reference 3), the quantity of hydrogen produced by the conditions.

described is sufficient for a 300 MW power plant minimum.

2. The use of fossil fuel is no greater and may be reduced Each plant uses 64 tonnes metric of carbon based fuel 4 times per day giving a total of 3072 tonnes metric per day for the whole plant compared with a loading 4800 tonnes metric per day for a 500 MW plant or 2880 tonnes metric per day for a 300 MW power plant 3. The carbon dioxide produced is reduced. Each reactor hall produces 119 tonnes metric of carbon dioxide per reaction equal to 476 tonnes metric per day. Volume of carbon dioxide produced per day 240,160 m3 per day compared with 576,000 m3 per day from a conventional 500 MW power plant or 345600 m3 per day from a 300 MW power plant 4. The capture and disposal of the carbon dioxide using the method given in Reference 1 is facilitated since the carbon dioxide produced is not mixed with other gases and is at a high concentration.

Amendments to the claims have been filed as follows Claims.

1. The industrial scale production of hydrogen gas for use as a fuel in electrical power generating and transport is feasible by the method described.

2. Assuming an overall plant conversion efficiency of 60%, equal to that predicted for present day natural gas fired 500 MW power plants (Reference 4), the quantity of hydrogen produced by the conditions described is sufficient for a 300 MW power plant minimum.

2. The use of fossil fuel is no greater and may be reduced Each plant uses 64 tonnes metric of carbon based fuel 4 times per day giving a total of 3072 tonnes metric per day for the whole plant compared with a loading 4800 tonnes metric per day for a 500 MW plant or 2880 tonnes metric per day for a 300 MW power plant 3. The carbon dioxide produced is reduced. Each reactor hall produces 119 tonnes metric of carbon dioxide per reaction equal to 476 tonnes metric per day. Volume of carbon dioxide produced per day 240,160 m3 per day compared with 576,000 m3 per day from a conventional 500 MW power plant or 345600 m3 per day from a 300 MW power plant 4. The capture and disposal of the carbon dioxide when thermal decomposition of the carbonate is used in conjunction with the method given in Reference 1 is facilitated since the carbon dioxide produced is not mixed with other gases and is at a high concentration S. The improvement of the sodium carbonate-carbon reaction by the use of cobalt and nickel metal powders or other catalyst.

6. The use of the hydrogen peroxide produced by the reactions described as an alternative to air or oxygen in the burning of hydrogen