WO2012005644A1

WO2012005644A1 - Method for providing a homogeneous mixture of liquid fuels and oxidants for use in a catalytic reactor

Info

Publication number: WO2012005644A1
Application number: PCT/SE2010/051220
Authority: WO
Inventors: Bård LINDSTRÖM; Daniel HAGSTRÖM
Original assignee: Reformtech Sweden Ab
Priority date: 2010-07-09
Filing date: 2010-11-08
Publication date: 2012-01-12

Abstract

The method according to the invention in its broadest aspect is a method for generating a homogeneous mixture of oxidant(s) and vaporized fuel for a catalytic process in a reactor having a reaction space. In particular it comprises a method for generating a homogeneous mixture of oxidant(s) and vaporized liquid fuel for a catalytic process in a reactor having a reaction space (8; 14), and a catalyst (6), comprising the steps of providing a fuel having a boiling point range; providing one or more oxidants and introducing said oxidants in said reaction space (8; 14); heating the oxidant(s) to a temperature above the boiling point range of the fuel, and introducing the fuel into the oxidant(s) in said reaction space; wherein the reaction space is restricted in size such that the dwelling time of the fuel and oxidant mixture is below the explosion limit of the fuel, is outside the cool flame regime, and is not below 25 ms.

Description

Method for providing a homogeneous mixture of liquid fuels and oxidants for use in a catalytic reactor

The present invention, relates to a method for providing a homogeneous mixture of liquid fuels and oxidants for use in a catalytic reactor.

Background

Conventional processes for upgrading or for converting a fuel, such as flame combustion, are related with high emissions, high reaction temperatures and complex system designs. The complex designs also entails use of expensive materials rendering the material costs of the equipment used in these processes unduly high.

By replacing a flame combustion system with a catalytic combustion system it is possible to eliminate the emissions of nitrous oxides and particulates as well as to reduce cost and complexity of a system through the use of lower operation temperatures and reduction of the number of parts in the system.

Catalytic reactor systems are also easily modified for use in alternative applications. A catalytic combustor can for example be modified to a catalytic hydrogen generator (reformer), by simply changing the catalyst in the reactor and adjusting the operating conditions, thereby enabling the development of a single technology platform for many applications.

Industrial catalytic processes are today mainly focused on using gaseous fuels

(Methane, LPG) or single component fuels (methanol, ethanol) as these are easily gasified and can be treated as a gas in the mixing chamber. Gaseous fuels are commonly employed as they can readily be mixed with oxidants at low

temperatures and present a low risk for damaging the catalyst through droplet contamination or soot formation on the catalyst surface.

The ability to create a homogeneous of a liquid fuel(s) and oxidant(s) is a critical requirement for any catalytic reactor as the failure to accomplish such a mixture will lead to poor conversion of the fuel in the catalyst as well as hotspots that can lead to thermal degradation of the system and shorten the life time of the products. The fact that the today available fuels, such as diesel, cannot simply be heated into a vapour makes the task of designing a catalytic reactor for such a fuel much more complex than for a gaseous fuel that can simply be mixed a low temperatures in a static mixing equipment.

There are several methods that have been employed in catalytic reactors to realize a mixture between a liquid fuel(s) and oxidant(s) . The related systems all have a common method of operation:

1. The fuel is atomized at elevated pressure

2. The fuel is introduced into a high temperature oxidation gas (or gasses),

which results in complete or partial evaporation of the fuel 3. The mixture is then delayed so that complete mixing can be achieved before the fuel -oxidant mixture is introduced to the catalyst

In US-4,302, 177 there is described how a pre-heated fuel is mixed with, and evaporated in a hot oxidant gas stream to produce a mixture of evaporated fuel and oxidant. The concept of direct fuel evaporation in a hot oxidant and allowing the fuel to be slowly mixed with gas is a proven method when using steam as the oxidant, in for example a hydrogen reforming system.

However when using a logistical fuel such as diesel or gasoline in reforming (or any other oxidation process), air is required as part of the process mixture, and while the introduction of fuel into hot air in the reactor facilitates the vaporization of the fuel , it also initiates chemical reactions that lead to autoignition of the fuel-air mixture.. Autoignition of the fuel causes hot flame combustion that leads to high emissions as well as the risk for thermal degradation of the catalyst and reactor housing.

Kohne et al, proposed using the cool flame as method for preparing mixtures for catalytic reactors, USP 6,793,693. The concept of using the cool flames to prepare fuel oxidant mixtures have however been shown to have several drawbacks:

1. The cool flame reactions are precursors to complete combustion and the long delay times, from 25 up to the preferred 500 ms increase the risk of autoignition of the fuel oxidant mixture through the formation of the OH radical at 700 K

2. In low oxygen containing systems (such as reformers) the cool flame

consumes only the alkanes, leaving the more complex aromatics that require oxygen for conversion in a oxygen depleted environment that cannot be converted in the catalyst, resulting in a high fuel slip from the reactor - that can damage e.g. a fuel cell utilizing the fuel, and poison the environment

3. The flame velocity of the cool flame, due to the prolonged dwelling time, often exceeds the gas velocity at low loads, increasing the risk for flashback explosions that can damage the catalyst and the reactor housing

4. The radical precursors in the cool flame have been found to initiate fuel polymerization at the walls and at the fuel nozzle

Summary of the Invention

The object of the present invention is to overcome the drawbacks connected with prior art systems and methods.

Thus, the inventors have devised a method of operating a catalytic reactor as defined in claim 1.

The method according to the invention in its broadest aspect is a method for generating a homogeneous mixture of oxidant(s) and vaporized fuel for a catalytic process in a reactor having a reaction space. In partiular it comprises a method for generating a homogeneous mixture of oxidant(s) and vaporized liquid fuel for a catalytic process in a reactor having a reaction space (8; 14), and a catalyst (6), comprising the steps of providing a fuel having a boiling point range; providing one or more oxidants and introducing said oxidants in said reaction space (8; 14);

heating the oxidant(s) to a temperature above the boiling point range of the fuel, and introducing the fuel into the oxidant(s) in said reaction space; wherein the reaction space is restricted in size such that the dwelling time of the fuel and oxidant mixture is below the explosion limit of the fuel, is outside the cool flame regime, and is not below 25 ms.

Brief Description of the Drawings

Figure 1 is a graph of ignition delay times for Diesel fuel;

Figure 2 is a graph showing cool flame and explosive combustion regions observed for diesel fuel;

Figure 3a shows Diesel Droplet life time for Catalytic Burner;

Figure 3b shows Diesel Droplet lifetime for Reformer;

Figure 4 illustrates identification of optimal conditions for a 3 kWth Catalytic Diesel Burner;

Figure 5 illustrates an embodiment of a reactor;

Figure 6 illustrates an embodiment of a combustion reactor and its size;

Figure 7 shows quenching distances for fuels, adapted from Law (2007);

Figure 8 illustrates a conventional method for pre-heating a catalytic reactor; and Figure 9 shows a reactor concept for starting a catalytic reactor.

Detailed Description

For the purpose of this application, the following terms will have the indicated meanings.

The "dwelling time" (t) is the time constant describing the time spent of the fuel(s) and oxidant(s) in the mixing chamber before contact is made with the catalysts, and is the ratio of the volume of the reactor (Vr) and the volumetric flow of the oxidant and fuel mixture (v), t=Vr/v. Expressed in a simpler manner: the dwelling time is the actual time that the gas is present in the reactor.

The "delay time" is the maximum time that a gas can stay in the reactor without exploding.

"Logistic fuel" means a fuel that is commercially available to consumers. The use of the conventional fuels available today, such as diesel, gasoline, E85, or biodiesel (FAME) has been limited in catalytic systems, as these are complex multi component mixtures hydrocarbons, that cannot be gasified through boiling as the wide boiling point range of the fuel components (Table 1) results in fuel distillation when heated on a hot surface, which in turn leads to soot formation through pyrolysis and low fuel utilization in the catalytic reactor.

Table 1: Conventional fuel Characteristics, Bosch (2007)

The ignition of hydrocarbon fuel(s) is initiated through the reaction of free radicals, by a series of chemical processes that propagate the system to ignite, Chung Law (2006). The pathway, radicals and time constants required to reach complete combustion through autoignition are dependent on the hydrocarbon fuel, pressure, composition and temperature of the oxidant mixture.

Ignition delay times were studied extensively for NASA by Spadaccini (1980) and Lefebvre (1986) and they were able to determine the relationship between the temperature, pressure and fuel concentrations on the time delay for autoignition for logistical fuels.

wherein

τ = = delay time

A = Arrehnius constant

P - pressure

E = activation energy

R = general gas constant

T = temperature

When a fuel is injected into an air containing oxidant mixture at temperatures > 1000 K, the reactions are rapid and ignition delay times < 20 ms are observed as seen in Figure 1. In lower temperature regions, the radical reactions are slower and although reactions do take place, the initial temperature increase is low in these regions. Injection of a fuel in a medium temperature region (550<T<880 K), under prolonged delay times > 45 ms and less than the delay time for autoignition at these conditions, can cause the air to react with the fuel to form a pale blue flame, associated with the emission of peroxides and formaldehyde. This phenomenon is referred to as "cool flames" and can appear in the upstream region of a premixed flame or as a precursor to an explosion, Lewis and Von Elbe (1987). The reaction rates of cool flames are significantly lower than those for high temperature oxidation and consumes only 5- 10 % of the hydrocarbons in the fuel. The temperature typically only rises with 100-200 K in the cool flame and the reaction rate decreases with increasing temperature, providing a balance to the increasing temperature.

FIG 2, shows the different combustion regions, that were observed for mixtures of air and diesel fuel, for reactions with dwelling times of 25 ms or greater.

On the basis of these known conditions and behaviours, the present inventors have developed a new method for providing a homogeneous mixture of liquid fuels and oxidants for use in a catalytic reactor, wherein the risk for explosion or uncontrolled behaviour is eliminated or at least substantially reduced.

Injecting a liquid fuel into a hot gas, will result in a temperature drop, as the fuel evaporation requires energy, additionally the temperature of the oxidation gas must be above the boiling temperature of the liquid fuel injected into the oxidant gas. The effect of heat loss due to evaporation is low for diesel and gasoline fuels, at around 15 K for a reformer under standard conditions and 10 K for a catalytic burner operating at lambda 3.

The time required to completely evaporate a fuel is as stated earlier a critical factor, that must be considered and the droplet life time must be considered as the liquid droplet could have serious detrimental effects on the catalyst and on the ability to create a homogeneous mixture of fuel and oxidant.

Spray penetration is strongly dependent upon the density of the atmosphere. As the density increases the penetration reduces and the spray angle increases due to turbulence and aerodynamic drag forces at the liquid surface. Increased

temperature of the atmosphere also reduces penetration as it increases the evaporation rate from the liquid surface.

It is possible to evaporate a complex fuel by injecting small droplets of liquid fuel into the hot gas stream. As the liquid evaporates, heat is absorbed from the gas. During the time it takes for the droplets to evaporate, they will be swept along with the gas stream. It is important to know how far the droplets will travel, so that the liquid will not affect any catalysts or sensors downstream of the injection point.

A precise calculation would be very difficult to do. Presented here is a rough estimate of the distance travelled by the droplets. The formula should still be quite useful in giving order-of-magnitude estimates, and to see the effect of different factors such as droplet size. Lefebvre (1986) gives the following formula for predicting droplet lifetimes:

2

8(* /<¾)ln(l + B)(l + 0.22Re_D^)

t_e = droplet lifetime

p_w = density of fuel

D = diffusion constant

kg = thermal conductivity of gas

C_p = heat capacity of gas

B = Biot number

Reoo = Reynolds number

Correlating, the theoretical data of Lefebvre with test results from a 1 kW catalytic diesel reactor (FIG 3a and FIG 3b) provides a method for determining, droplet life times and distance travelled by droplets in various oxidant fuel mixtures.

By employing the results and methods from Figures 1-3, the present inventors have been able to determine the optimal regions for mixing hydrocarbon fuels for catalytic reactors, and by doing so a novel method for designing and developing catalytic reactors has been developed.

FIG 5, schematically illustrates a reactor in which the novel method is employed in the practical case for a 3kW thermal reformer.

The optimal operating conditions are specific for the conditions of a specified reactor and conditions, and the principles according to the invention provide a powerful tool for designing catalytic reactors to their optimal reaction conditions.

The objective, of creating a homogeneous mixture of oxidant(s) and vaporized fuel(s) that is stable and does not partake in undesirable radical reactions that lead to an explosive mixture and byproducts, is achieved by the method according to the invention for liquid fuels of mixed hydrocarbons (such as diesel, gasoline, JET-A and other fuels) that have a boiling point range of from not less then 20°C, suitably not less than 50°C or not less than 100°C or not less than 150, and up to not higher than 500°C, suitably not higher than 375°C or not higher than 210°C at 1 ban

Thus, the method according to the invention in its broadest aspect is a method for generating a homogeneous mixture of oxidant(s) and vaporized fuel for a catalytic process in a reactor having a reaction space. In partiular it comprises a method for generating a homogeneous mixture of oxidant(s) and vaporized liquid fuel for a catalytic process in a reactor having a reaction space (8; 14), and a catalyst (6), comprising the steps of providing a fuel having a boiling point range; providing one or more oxidants and introducing said oxidants in said reaction space (8; 14);

The global parameters that govern the control of the reaction are i) the temperature of the oxidant and the fuel, ii) the pressure of the reactants, iii) the geometry of the reactor and iv) the heat transfer to and from the mixing reactor.

The dwelling time of the reactants should be higher than 30 ms under normal conditions, but could be increased if heat transport from the mixing reactor is improved to lower the temperature of the reactants to slow down the initiation of the radical reactions.

The dwelling time (t) is the time constant describing the time spent by the fuel(s) and oxidant(s) in the mixing chamber before contact is made with the catalysts, and is the ratio of the volume of the reactor (Vr) and the volumetric flow of the oxidant and fuel mixture (v), i.e. t = Vr/v.

The method according to the invention, utilizes "safe" operating parameters for which it is possible to mix and evaporate a liquid fuel in a hot oxidant without starting radical reactions that form precursors to a fuel explosion as well as to ensure complete evaporation of the fuel.

In contrast to conventional methods, where the fuel is either, boiled to a vapour and then mixed at lower temperatures or injected into a hot gas simply to evaporate the fuel, this method comprises setting appropriate operating conditions that are ideal for a certain fuel under specific operating conditions. The method also considers the density and heat capacity of both the fuel and the oxidants and thereby enables an optimized solution for a specific application.

For example when operating with an catalytic reformer the delay time required for fuel evaporation is approximately 10% lower than for a catalytic combustor, due to the higher heat capacity of the steam in the process - which enables through the method according to the invention the construction of a smaller and optimized reactor for the reforming system, allowing for a more cost efficient design in the production of the catalytic reactor.

Additionally whereas a method such as cool flame evaporation focuses on the reaction stabilization of the fuel and oxidant to prevent a runaway reaction, the method according to this invention eliminates this problem by operating outside of the risk zones and thereby it is possible to create a more stable solution that has low risk of overheating or damaging the catalyst that is used in the process. Experiments, performed on non logistical fuels such as ethanol, methanol and DME have shown that method according to the invention is highly suitable for predicting optimal operation for these fuels and that the prevention of radical reactions create stabilization in the mixture zone, allowing for the design of robust catalytic reactors based upon both logistical as well as environmentally friendly fuels.

Figure 5, shows a reactor design that is particularly suited for operating a catalytic process according to the invention. In the reactor system described in FIG 5, fuel is provided through fuel line 5, into the reactor via an atomizing nozzle 4, where it is mixed with the oxidant(s) in the mixing chamber 8. The oxidant(s) are provided to the reactor via inlet 1, in an outer reactor chamber 2, which is used to pre-heat the oxidants before introducing the oxidants in the reactor via orifices 3 at the bottom plate of the reactor.

The oxidant/ fuel mixture is then passed on to the catalyst 6, where the catalytic process takes place and then the gaseous products are transferred from the system at the catalyst exit 7. The use of the reactor housing as a means for providing heat to a reactor and thereby stabilizing the reactor is a proven method used in the processing industry and is well illustrated in USP 3,955,941.

In order to illustrate the effectiveness of the method according to the invention, an illustrative example is given. For this example a 5 kWth diesel fuelled catalytic burner operating at 1 bar is considered. For such a system, the mass fuel flow rate of fuel (Gf) is determined from the energy content of the fuel (Table 1) and the air flow rate is determined from the air-fuel ratio (λ) of 3.2, which was determined through experiments to give the best performance for the catalytic burner.

The flow rate of the oxidant for this system case example is 5.34 g/s (0.184 mol/s) and the fuel flow is 0.1 16 g/s (~6.1 10-4 mol/ s), and the temperature of the oxidant/ fuel mixture in the mixture zone 8, is under these conditions 700 °C as a result of the thermal radiation from the catalyst in which the average temperature under the operating conditions is 920 °C.

Under the operating conditions, described above the average volumetric flow of the fuel and oxidants, as measured in the reactor is - 1.5 10-2 m³/s. The reactor diameter is set, according to the available catalyst dimensions, and in this case a 3.16 inch (80.3 mm) catalyst was chosen and the reactor diameter was therefore set to 82 mm (0.082 m) and thus the oxidant fuel mixture had an average velocity of 2.62 m/s in the mixing zone of the reactor 8.

Under these conditions, it can be seen from the data in Figure 4 that for a temperature of 700 °C (973 K) the required delay time for complete droplet evaporation is 22 ms (0.022 s) yielding a minimum distance of 57 mm in length of the mixing zone in order to achieve complete fuel evaporation.

From the data in Figure 4, applied to a 6 kWth burner the maximum it can be derived that the dwelling time allowed to prevent the formation of undesirable pre- reactions is 35 ms, setting a restriction of maximum length of 91 mm for the entire mixing zone. In order to improve the mixing in such a reactor and to minimize the length, a static mixer 9, can be introduced before the catalyst, as illustrated in Figure 6. The total distance of the mixing zone in this case is 60 + 5 + 20 mm (85 mm) providing a mixing reactor within the restrictions necessary for complete fuel evaporation and the prevention of undesirable pre-reactions that would lead to explosion.

It has been observed that a flame in a combustible mixture will be extinguished if it is forced to propagate through a constriction or near a wall. The walls are evidently able to exert some repressive influence on the flame. The quenching effect of the walls on the flame propagation is termed "wall quenching". The minimum size of an opening through which a flame will travel is called the "quench distance"

In an added safety feature, the size of the orifices of the static mixer 9, should be set to be smaller than the quenching distance of the fuel used, Figure 7, so that in the unlikely event of a flame formation it would be quenched before contact is made with the catalyst, in the case for diesel fuel the quenching distance is <2 mm.

The size of the catalyst channels should also be selected so that an eventual flame would be quenched at the entrance of the catalyst, as specified in FIG 7 for designing robustness in a catalytic reactor.

In order to start the system described in FIG 5, heat must be provided to the oxidants to provide the heat necessary for initial fuel vaporization. In FIG 8 there is shown a conventional method for heating the oxidants by burning fuel as an initial heat provided.

Thus, the incoming oxidants are heated in a heat exchanger 10, before being transported to the reactor through inlet 1. The heat is provided by an external burner 1 1, which is operated with a conventional flame during start-up.

To resolve the size issue as well as to protect the system from being polluted with carbon particles from the conventional combustion, a novel method for pre-heating the system was designed, as illustrated in FIG 9.

In the reactor set-up shown in Figure 9, the catalytic reactor 14, is housed inside an outer reactor 15, in which a secondary fuel nozzle 13 and a spark ignition device 12 is mounted. During start-up the secondary nozzle 13, is used for introducing fuel into the outer chamber and to provide a flame for heating the catalytic reactor 14 and the products from this combustion is transported to the catalytic reactor through air inlet 1 , to provide direct heat for the catalyst. The configuration described in FIG 9 allows the outer wall of the catalytic reactor 14 to act as a heat exchanger as well as to provide a soot protection for the catalyst and the mixing zone 8. When sufficient heat has been provided the fuel supply to the secondary nozzle 13 is shut off and normal supply through nozzle 4 is started and the catalytic reactor is able to operate under normal conditions.

The method according to the invention is suitable for any catalytic reactor in which air used as an oxidant, either alone or in combination with other oxidants such as steam or carbon monoxide. The catalytic reactors are suitable for a wide range of applications including but not limited to hydrogen production for fuel cells, hydrogen production for NOx reduction, hydrogen production for combustion engines and catalytic burners for both mobile and stationary applications.

Claims

CLAIMS:

1. A method for generating a homogeneous mixture of oxidant(s) and vaporized liquid fuel for a catalytic process in a reactor having a reaction space (8; 14), and a catalyst (6) , comprising the steps of:

providing a fuel having a boiling point range;

providing one or more oxidants and introducing said oxidants in said reaction space (8; 14);

heating the oxidant(s) to a temperature above the boiling point range of the fuel, and

introducing the fuel into the oxidant(s) in said reaction space;

wherein

the reaction space is restricted in size such that the dwelling time of the fuel and oxidant mixture

i) is below the explosion limit of the fuel,

ii) is outside the cool flame regime, and

iii) is not below 25 ms

2. The method as claimed in claim 1, wherein the fuel is a logistical fuel, suitably selected from diesel, gasoline, E85, biodiesel (FAME).

3. The method as claimed in claim 2, wherein the fuel is a multi component fuel and has a boiling point in the range of not less then 20°C, suitably not less than 50°C or not less than 100°C or not less than 150, and up to not higher than 500°C, suitably not higher than 375°C or not higher than 220°C at 1 bar.

4. The method as claimed in any preceding claim, further comprising providing an outer reactor chamber (2; 15) enclosing the reaction space (8; 14), passing oxidant through said outer reactor chamber (2; 15) so as to preheat it, before introducing it into the reaction chamber (8; 14).

5. The method as claimed in any preceding claim, further comprising introducing fuel into the outer chamber (8; 15) during start-up so as to provide a flame for heating the reactor chamber (2; 14).

6. The method as claimed in any preceding claim, wherein the reactor further comprisies a static mixer (9) placed in the mixing zone between said inlet (1) and orifices (3) and said catalyst (6) , wherein the size of the orifices in the mixing device are smaller than the quenching distance for the fuel that is used for preventing accidental flames from coming into contact with the catalyst.

7. The method as claimed in any precding claim, wherein the catalyst (6) is provided with channels which are dimensioned to quench any accidental flames that could form in the mixture zone during transient conditions.

8. The method as claimed in any of claims 4-8, wherein the outer reactor chamber (2) has a secondary fuel nozzle (13) for providing a flame for heating the catalytic reactor (8; 14).