Hydrogen Generator

The present invention relates to apparatus for generating hydrogen from a hydrocarbon feed and a process for the production of hydrogen from a hydrocarbon feed, and in particular to such apparatus and process for use in producing a supply of hydrogen for a fuel cell.

Fuel cells offer the potential to generate electricity directly from fuel gases with hitherto impossible quietness and efficiency. For simplicity of construction it is desirable to fuel cell manufacturers to develop systems that operate at low temperatures, this being particularly important for portable and low power systems. A number of low temperature fuel cell systems are produced commercially and these include proton exchange membrane fuel cells, alkaline fuel cells and phosphoric acid fuel cells. Each of these systems is preferably fuelled using pure hydrogen and is adversely affected if the hydrogen contains oxides of carbon.

The proton exchange membrane fuel cell utilises a proton exchange membrane closely coupled to a catalyst that is able to dissociate hydrogen fuel into protons and electrons. This type of fuel cell represents one of the simplest fuel cell systems to manufacture and operate but is beset by the problems associated with delivering the hydrogen fuel. Hydrogen is a gas that is not amenable to liquefaction other than under extreme cryogenic conditions. This

factor makes delivery of hydrogen as a fuel problematic, as the energy density of compressed gas, stored in heavy metal or composite cylinders, is untenably low. There is clear commercial need for a system to allow hydrogen to be generated from commercially available fuels such as propane, butane, petroleum spirit and paraffin using a simple, low cost apparatus. Moreover, it is highly desirable that the system should deliver hydrogen substantially free of contamination by carbon monoxide or carbon dioxide both of which gases have a deleterious effect on fuel cell operation.

Throughout this application the process of thermo lytic cracking of a hydrocarbon feedstock into carbon and hydrogen is referred to as pyrolysis. The device for achieving such reaction is referred to as a pyrolyser. For the avoidance of doubt pyrolysis may be taken, within the context of this specification, to be synonymous with 'thermolytic cracking', 'thermolysis' and 'cracking'.

The problem of liberating hydrogen from a hydrocarbon fuel has been the subject of considerable research. The approach usually adopted is the use of a reformer system wherein the hydrocarbon is partially oxidised, to yield a mixture of carbon monoxide and hydrogen. In such systems the fuel gas is mixed with an oxidant, typically air or steam, and is passed over a heated catalyst. The object of this process is to render the fuel into a form wherein the carbon content is converted to carbon monoxide and the hydrogen content

of the fuel is liberated as either hydrogen or water vapour. The reformate, thus produced, is cooled and then passed through a separate catalytic reactor to perform a so-called water gas shift reaction. In this process the carbon monoxide is reacted with water vapour to produce carbon dioxide and hydrogen. The latter reaction is significantly exothermic and is adversely driven to reactants by high temperature. It is therefore common for the shift reactor to have multiple stages with cooling between each stage. This can make the shift reactor expensive to produce and control. Once the carbon monoxide has been safely converted to carbon dioxide it is necessary to remove the carbon dioxide via some purification system. Typically oxidative and steam reformers employ a pressure swing apparatus (PSA) to effect such purification. A chemical representation of the processes is included below.

C _n H _2n+2 + Y ₂ O ₂ ^{Catalyst/Tenip} ) nCO + (n + l) H ₂

Partial oxidation reformer reaction

C _n H _2n+2 + nH ₂ 0 ^Cata ' ^yst/TemP > nCO + (2n + l) H ₂

Steam reformer reaction

CO + H ₂ O ^Catø ' ^yst/Tenip > CO ₂ + H ₂ Shift reaction

An example of a steam reforming (SR) apparatus is to be found in US Pat. No. 5,932,181 to Kim et al. The apparatus is designed to produce a continuous supply of high purity hydrogen from natural gas and comprises a desulphurisation unit to removed the sulphur bearing odorant from the gas, a steam reformer to liberate the hydrogen from the incoming gas feed, a water gas shift reactor to oxidise carbon monoxide to carbon dioxide and a PSA to effect final purification.

An example of a catalytic partial oxidation reactor (CPOX) is to be found in US patent No.6,221,280 to Anumakonda et al. In this system a heavy hydrocarbon, having in excess of 6 carbon atoms, is volatilised and mixed with air before contacting a rhodium/alumina catalyst maintained at 1050 ⁰ C.

The hydrocarbon is partially oxidised to a mixture of hydrogen and carbon monoxide whilst any sulphur containing materials in the fuel are converted to hydrogen sulphide. As previously the product gas must be purified to remove all traces of sulphur and carbon monoxide before it is suitable for use in a low temperature fuel cell.

An example of a fuel processor that combines both steam reforming and partial oxidation is described in US Patent No 6,555,259 to Carpenter et al. In this system a fuel is mixed with air and steam before contacting a rhodium/zirconia catalyst. The advantage of such a system is that the SR reaction is endothermic whilst the POX reaction is exothermic. Control of the

relative intensity of each reaction allows the processor to operate autothermally (self control of temperature). This class of fuel processor has become widely known as an autothermal processor (AP).

It can be appreciated from the above that the principle difficulty with existing fuel processors for the production of hydrogen for fuel cells is that all produce a mixture of various gases some of which are unsuitable for introduction to a fuel cell system. Particularly problematic are carbon oxides which are damaging to the performance and endurance of the cells. Carbon dioxide is a significant problem within alkaline fuel cells, wherein it reacts with the electrolyte to create a precipitate. In other cell systems carbon dioxide is less damaging, but can indirectly cause catalyst poisoning due to carbon monoxide, which can be produced via a reverse shift reaction occurring within the cell. Carbon monoxide is a much more significant threat to fuel cell operability, as it is a prolific poison for the catalysts used to dissociate hydrogen within the cell. It is therefore necessary to ensure that the output from the reformer system is below 50ppm carbon monoxide and is preferably below 5ppm carbon monoxide.

Fuel processor systems capable of meeting this purity target are currently i) bulky due to the need for a multi-stage water gas shift reactor and

pressure swing apparatus, ii) expensive due to the use of platinum group metal catalysts,

iii) unreliable due to the tendency of the fuel cell catalyst to perform a reverse shift reaction in the presence of trace amounts of carbon dioxide.

In addition, the water gas shift reaction has an equilibrium condition favouring low temperature, that is to say the lower the temperature the lower the concentration of carbon monoxide in the gas produced. The low temperature required to achieve satisfactory gas quality means that the rate of the shift reaction is slow and the reactor must be large to allow the gas sufficient catalyst contact time. The optimisation between reactor size/weight, operational temperature and gas quality is complex, requiring complex and expensive control facilities.

As a result of these issues it is fair to say that the production of hydrogen for fuel cells has become one of the most significant impediments to the uptake of this technology.

An alternative route to the generation of hydrogen from hydrocarbon fuels is via a pyrolysis reaction in a pyrolyser. A pyrolyser is an entropy driven device that breaks down the fuel at high temperature into its elemental constituents, in the case of a hydrocarbon fuel, hydrogen and carbon. A pyrolyser can typically be used to generate hydrogen from any hydrocarbon fuel that when reduced to its elemental composition will generate more molecules of

hydrogen than there were molecules of the fuel. Examples of such fuels are methane, ethane, propane, butane, petroleum spirit and paraffin.

The pyrolysis reaction is generally endothermic (requires external energy) but is driven at high temperatures by the increased entropy of the gases produced.

In general, this gives pyrolysers a unique advantage over oxidative reformers in that the entropy change is maximum when the fuel is of high molecular weight and saturated. This means that the system is extremely beneficial when used with high carbon alkanes typical of those found in paraffin, petroleum, butane and propane. For comparative purposes the entropy change during the pyrolysis of methane is +82 J.moF'.K ^"1 , whilst for butane the equivalent value

The use of a simple pyrolyser for hydrogen production has been described in US Patent Nos. 6,653,005 and 6,670,058 to Muradov. In 6,653,005 a system is described wherein the hydrocarbon gas is decomposed over a carbon or metal catalyst to yield a deposited carbon material and a hydrogen rich exhaust gas. The problem with the system described is that excessive maintenance would be required to remove the build up of carbon and maintain the pyrolyser in an operational mode. In US Patent No 6,670,058 a system is described wherein the pyrolysis of a hydrocarbon fuel is conducted in the presence of a carbon material, preferably activated carbon. The process is

used for the continuous production of hydrogen and carbon, the latter being separated, activated and partially recycled to the reaction chamber.

International Patent WOO 1/20703 to Manikowski and Noland describes a pyrolyser system having two pyrolysis chambers. One chamber is used for active pyrolysis whilst the other is oxidised using air to yield two output streams, one of which is hydrogen rich and is used to power a fuel cell and the other of which is carbon monoxide rich and is used to fuel a heat engine and generator. After some period of operation the gas feeds to the two pyrolysers are reversed. Such a system may be described as a reciprocating pyrolyser. A reciprocating pyrolyser is a system which has a plurality of pyrolysis chambers one of which is used for pyrolysis whilst deposited carbon is burned in the other. In such systems it can be appreciated that the pyrolysis reaction is endothermic, requiring external heating, whilst the combustion of the carbon is exothermic, generating heat.

One of the problems with pyrolyser systems disclosed in the literature is that a separate fuel stream is required to heat the pyrolyser to a temperature in the range 600-1000°C for pyrolysis to occur. This use of the fuel for heating creates considerable inefficiency in the pyrolyser, which is undesirable.

This invention seeks to address the problem of maintaining the temperature of a pyrolyser without the use of a separate fuel stream other than for start-up and shut-down.

According to a first aspect of the invention there is provided apparatus for generating hydrogen from a hydrocarbon feed, the apparatus comprising a plurality of zones, each zone being adapted for pyrolysis of the feed, each zone being further adapted for oxidation of deposited carbon, and each zone comprising means for utilising deposited carbon from pyrolysis in a first zone to partially or completely sustain pyrolysis in a second adjacent zone.

According to a second aspect of the invention there is provided a process for the production of hydrogen from a hydrocarbon feed, comprising the step of utilising deposited carbon from pyrolysis in a first zone to partially or completely sustain pyrolysis in a second adjacent zone.

According to a third aspect of the invention there is provided a method of operating a fuel cell, including the step of producing hydrogen utilising apparatus or a process as set out above.

The basis of this invention is to operate a reciprocating pyrolyser, such that heat from the combustion of the carbon is transferred to the endothermic pyrolysis reaction, thereby minimising the need for external heating. It is

preferred in the development of such a system to control the deposition of the carbon during pyrolysis so that carbon is deposited in close proximity and preferably in contact with the heat transfer medium. One example of a method of achieving this is by manufacturing the heat transfer medium from a stainless steel, most preferably grade 304 stainless steel. Stainless steel has the curious property, under the conditions of pyrolysis, that it will catalyse the growth of a nanofibrous carbon deposit in close proximity to the steel surface. The fibres are believed to be produced by the dissolution and re-precipitation of carbon on steel crystallites that are ejected from the surface by the growth of a carbon deposit at the grain boundary.

The invention will further be described, by reference to the following examples and drawings, in which;

Figure l is a schematic, longitudinal sectional view of a first embodiment of apparatus according to the invention;

Figure 2 is a schematic, three-quarter perspective view of a second embodiment of apparatus according to the invention exploded to show its internal configuration;

Figure 3 is a schematic, three-quarter perspective view of the apparatus of Figure 2 assembled;

Figures 4a and 4b are respectively, schematic, longitudinal and transverse sectional views of apparatus according to the invention for demonstrating the process of the invention;

Figure 5 is a thermal log for the apparatus of Figures 4a and 4b during start up and normal operation; and

Figures 6 and 7 are plots of gas analysis data for the same run illustrated in Figure 5.

Example 1

In the simplest embodiment of this invention a reciprocating pyrolyser is constructed having pyrolysis chambers comprising the inside and outside of a stainless steel tube as shown in Figure 1 which define zones in which pyrolysis and oxidation can take place, hi this embodiment two pyrolysis chambers are defined, respectively by the inside of the stainless steel tube 3 and the exterior annular gap between the stainless steel tube 3 and the outer containment 2. The outer containment 2 is constructed of a material that is not conducive to the catalysis of nano fibre growth. In the device of this invention the outer containment is made from silica, however, most non-ferrous metals capable of withstanding IQOO ⁰ C in the presence of carbon and air would be

suitable. The outer containment 2 is wound with an electrical heater 1 which is used to heat the stainless steel tube 3 during start-up.

The operation of the preferred embodiment is thus:- On start-up power is supplied to the electrical heater 1 until the temperature of the stainless steel tube 3 reaches a temperature in the range 600-1000°C and most preferably 850°C.

A hydrocarbon fuel gas, most preferably propane or butane, is passed into the inlet 6 and is pyrolysed on the inside surface of the stainless steel tube 3. A nano fibrous deposit of carbon is formed in intimate contact with the tube wall. A hydrogen rich gas stream exits the system via tube 7 and after cooling can be used to fuel a fuel cell.

After some period of operation the tube 3 becomes choked with deposited carbon and the flow becomes restricted. At this point the flow of hydrocarbon into 6 is switched off and the flow replaced by air. Hydrocarbon is now flowed into the outer containment via the inlet tube 4 and pyrolysis occurs on the outer wall of the tube. A hydrogen rich gas now issues from the outlet tube 5. The air flowing through the inner stainless steel tube 3 causes partial combustion of the carbon deposit into carbon monoxide and/or carbon dioxide with the generation of heat. This heat flows through the wall of the stainless steel tube heating the tube surface for carbon deposition. The availability of

this heat source allows the power supplied to the electrical winding 1 to be switched-off or considerably reduced. The carbon monoxide produced from the oxidation of the carbon deposit may be further oxidised into carbon monoxide to create more heat. Additionally or alternatively, the carbon monoxide issuing from outlet 7 may be transferred into a burner for combustion into carbon monoxide

After some further period of operation the carbon deposit inside the tube 3 has burned away and the flows in the system are again switched over. Air now flows through the outer containment and causes the partial combustion of deposited carbon as described above. Heat flows through the tube wall and carbon is deposited within the tube.

The pyrolyser is cycled in this way to provide a continuous flow of a hydrogen rich gas.

Example 2

Figures 2 and 3 illustrate a second embodiment of apparatus 1 according to the invention. The following description of operation of this embodiment is specific to the use of propane as a fuel. This is not intended to limit the invention to the use of propane, as the skilled person will appreciate and the system could equally well be fuelled by any hydrocarbon fuel.

The pyrolyser of this embodiment is shown in an exploded view in Figure 2 and as assembled in Figure 3. The apparatus is assembled around a central tube 8 which contains a catalysed ceramic porous burner suitable for the combustion of propane and carbon monoxide. Fuel is delivered to the porous burner via the fuel pipe, 9, whilst air is supplied from a fan attached to plate 10. Also contained within the core tube 8 is a small electric start-up heater to heat the porous burner to a suitable strike-off temperature during start-up.

About the core tube, 8, a construct is made up from two dividers, 11, two end plates, 12, sixteen porous elements, 13, and two outer shells, 14. The construct is such that two semi-annular chambers are created each one having an inlet pipe, 15, and an outlet pipe, 16. The semi-annular chambers will be referred to as semi-annular chambers A and B for the remainder of this description. Gas passing through the semi-annular chambers is forced to pass through the evenly spaced porous elements, 13, which have been produced from an expanded metal such as Expamet 926S.

The operation of the pyrolyser is as follows:-

On start-up current is applied to the internal heater until the temperature at the lower face of the porous burner reaches 400°C. Gas is then admitted via the fuel tube 9 and air via a fan attached to plate 10. Combustion of the fuel gas leads to rapid heating of the porous burner to a temperature upwards of 85O ⁰ C.

The hot gases from the porous burner heat the core tube 8 to a temperature such that pyrolysis can occur on the outer surface of the tube.

Temperature sensors attached to the outside of the inner tube, within one of the semi-annular chambers detect when the wall temperature above the burner reaches 850°C and signals that the pyrolysis gas flow should be started. Propane is supplied to semi-annular chamber A from the upper inlet pipe. Propane experiences an increasing temperature gradient on passage through the chamber. The metal of construction of the chamber is a 300 series stainless steel and most preferably grade 304 stainless steel. This metal has a composition that catalyses the growth of carbon nanoflbres from any carbonaceous gas at temperatures above 650 ⁰ C. The passage of the propane over the hot stainless steel core tube and over the radiatively heated Expamet landings results in rapid pyrolysis and the growth of a thick mat of carbon nanofibres. The outfall gas leaving the reactor is approximately 85% hydrogen with a balance of methane.

Flow of propane into the reactor is monitored and declines as the passage of gas through the semi-annular chamber is restricted by the growth of carbon nanofibres. At some pre-defined flow-rate an electronic sensor signals the control unit for the pyrolyser to switch the inlet to semi-annular chamber A over to air and to pass propane through semi-annular chamber B. At the same

time the outfalls from the system are also switched over via a high temperature change-over valve of a proprietary design.

Air passing through semi-annular chamber A causes the combustion of the deposited carbon and the generation of carbon monoxide. Little or no carbon dioxide is produced as the carbon is in excess. Air is passed through semi- annular chamber B until oxygen is detected at the chamber outfall via a lambda sensor of the type used in automotive vehicle exhaust systems. As soon as oxygen break-through is detected the air flow is shut-off and semi- annular chamber A becomes dormant. The flow of air into semi-annular chamber A is moderated such that the rate of combustion of carbon is slightly faster than the rate of deposition of carbon in semi-annular chamber B.

As mentioned above, an important aspect of the operation of the pyrolyser is the utilisation of deposited carbon from pyrolysis, here by heat transfer by conduction via the separator plates between semi-annular chamber A and semi-annular chamber B and vice versa. The carbon monoxide produced by oxidation of the deposited carbon may be further oxidised within the pyrolyser chamber into carbon dioxide to produce more heat. In addition or alternatively, the combustion of carbon monoxide produced during the oxidation cycle, within the porous burner provides an additional energy source further assisting in providing for the apparatus to be self sustaining once started.

Example 3

An apparatus for demonstrating the process of this invention is shown schematically in Figure 4. The apparatus comprises a silica tube 20, sealed at each end with a silicone rubber stopper, 27. Passing through the silica tube, and supported in holes through the silicone stoppers so as to be centrally located in the silica tube is a stainless steel (grade 304) tube of 20mm nominal diameter with a 16mm bore. The silica tube is wound with a resistive wire, 28, such that the passage of electricity through the wire results in the heating of the assembly to temperatures up to 1000°C. The apparatus is externally insulated with ceramic fibre insulation such that there is at least 38mm of insulation over the entire heated zone.

Internal to the stainless steel tube is a thermocouple, 29, to monitor temperature. The reactor system is set up with suitable valve means such that by means of a single switch-over operation the reactor will operate in one of two modes: in mode one, propane is supplied into the bore of the stainless steel tube whilst air is supplied to the silica tube; in mode two air is supplied to the bore of the stainless steel tube and propane to the silica tube.

The propane flow rate is set in the range 20-200ml/min and the air flow rate is set in the range 200-2000ml/min. Typical running conditions for this reactor were a propane flow rate of 50ml/min and an air flow rate of 500ml/min, in

all experiments ratiometric control of the inlet gases at 10:1 airrpropane was maintained, giving a 33% excess stoichiometry in relation to the air required for complete gasification of deposited carbon.

Normal operation of the reactor is as follows.

Electricity is supplied to the heating element and the reactor heated to 850°C. Once the internal thermocouple reads at least 750°C the gas flow is started with air supplied to the inside of the stainless steel tube, 22, and propane to the outside, 23. Propane is pyrolysed on the outer surface of the tube and a hydrogen rich gas issues from the gas outlet 25. After 10 minutes the inlet valves are cycled such that the gas flows are switched. Propane now enters through 22 and the hydrogen rich gas escapes from 24. Air entering at 23 is used to combust deposited carbon and is extracted from 25. The process continues with the inlet and outlet valve cycling every 10 minutes. Figure 5 shows the thermal log for the reactor during start-up and normal operation. It can be seen that there are two significant thermal peaks associated with the combustion of deposited carbon in each cycle. The larger peak relates to the combustion of carbon inside the stainless steel tube in close proximity to the thermocouple. The smaller peak relates to the combustion of carbon on the outside of the stainless steel tube and is demonstrative of the heat transfer that at the core of this process.

Gas analysis for the same ran data as is shown in Figure 5 is shown in Figures 6 and 7. It is clear that the pyrolysis of propane on the internal surface of the tube is marginally more effective than on the outside. After 8 cycles the exit gas from the internal surface is 93% hydrogen whilst the gas generated on the external surface is only 81% hydrogen. The difference in performance presumably reflects the 1.95:1 variance in space velocity between the inside and outside of the stainless steel tube in this particular reactor. Figure 6 shows carbon monoxide concentration in the hydrogen rich exit gas stream, it can be seen that carbon monoxide peaks at approximately lvol % when the gas change-over is effected and then declines rapidly to 0.2 vol%. There is also some evidence that carbon dioxide may be produced in this period. The high concentration of carbon monoxide at change-over results from air entering the system and from the gradual reduction of metal oxides that form within the reactor. When the volume of the total system is large this volume of carbon monoxide is unimportant. However, in larger systems an improvement in performance can be garnered (1) by rejecting the initial production of hydrogen rich gas and incorporating a ca 1 minute gas buffer prior to the gas entering the fuel cell, carbon monoxide was removed from the hydrogen rich gas stream by a proprietary process and the resultant gas passed to a fuel cell; and/or (2) carbon monoxide is further oxidised in the pyrolysis chamber into carbon dioxide; and/or (3) carbon monoxide is processed in a burner. The second and third option are advantageous as they provide an additional energy source further assisting in providing for the apparatus to be self sustaining.