1. Ganesh Pidathala Sekar Sebastian A. Avella E.
1
POLITECNICO DI MILANO
School of Industrial Engineering - Campus Piacenza
Master of Science Energy Engineering for an
Environmentally Sustainable World
Prof. EnricoTronconi
Fundamentals of chemical processes for energy and the environment
February 2014
2. WHAT IS SYNGAS?
The term Syngas is derived from (Synthesis Gas) is used for describing a
mixture containing H2 and CO and very often amounts of CO2 and CH4.
Syngas is the direct end-product of the gasification process. Though it can be
used as a standalone fuel, the energy density of Syngas is only about 50
percent that of natural gas and is therefore mostly suited for use in producing
transportation fuels and other chemical products.
As its unabbreviated name implies, Synthesis gas is mainly used as an
intermediary building block for the final production (synthesis) of various fuels
such as synthetic natural gas, methanol and synthetic petroleum fuel (dimethyl
ether – synthesized gasoline and diesel fuel).
In a purified state, the hydrogen component of Syngas can also be used to
directly power hydrogen fuel cells for electricity generation and fuel cell electric
vehicle (FCEV) propulsion.
2
3. APPLICATIONS
In addition to using Syngas to directly manufature products,
through various chemical processes and absorption methods,
each individual component of Syngas can be isolated and/or
purified for other uses or disposal:
Hydrogen – electricity generation and transportation fuels
Nitrogen – fertilizers, pressurizing agents
Ammonia— fertilizers
Carbon monoxide – chemical industry feedstock and fuels
Carbon dioxide – injected into sequestration wells
Steam – turbine drivers for electricity generation
3
4. CHEMICAL COMPOSITION
Substance Composition (%)
H2 20-40
CO 35-40
CO2 25-35
CH4 0-15
N2 2-5
The composition of syngas is highly dependent upon the inputs.
A number of the components of syngas cause challenges which must be
addressed at the outset, including tars, hydrogen levels and moisture.
Hydrogen gas is much quicker to burn than methane, which is the normal
energy source for gas engines.
Range of composition of Syngas
4
5. Fischer-Tropsch example
The basic reaction for formation is:
For example, when octane, a component of gasoline, is formed,
Equation becomes
Similarly, for the formation of olefins,
For ethylene formation, Equation becomes
The other type of main reaction that occurs in this process is the water-
gasshift reaction
5
6. INDUSTRIAL PRODUCTION
OF SYNGAS: PROCESS
TECHNOLOGY
Steam reforming Autothermal reforming or oxidative
steam reforming
Non catalytic Partial oxidation Compound Reforming Method
Catalytic Partial Oxidation
6
7. STEAM REFORMING (SR)
Cm Hn + m H2 O = m CO+ (m + n 2) H 2
Jianjun Zhu Catalytic. SR Process. Partial Oxidation of Methane to Synthesis Gas
7
8. AUTOTHERMAL REFORMING
(ATR)
When the ATR uses carbon dioxide the H2: CO ratio produced is 1:1.
2 CH4 + O2 + CO2 → 3 H2 + 3 CO + H2O
When the ATR uses steam the H2: CO ratio produced is 2.5:1.
4 CH4 + O2 + 2 H2O → 10 H2 + 4 CO
8
9. Other Processes
Compound reforming Method
o The compound reforming method combines the steam-reforming reactor with the
automatic-thermal reactor.
o A major feature of this method is that the early-stage steam reforming reaction
and the late-stage automatic-thermal reforming reaction take place in separate
devices.
o The advantage, as a result, is that low-pressure gas at the outlet of steam
reforming reaction can be transformed into high-pressure gas by means of
automatic-thermal reforming.
o In addition, costs are reduced because a compressor is not required.
o The disadvantages, on the other hand, are that there are two reactors and
construction costs are high
9
11. PARTIAL OXIDATION OF
HYDROCARBONS
All hydrocarbons are possible as feedstocks. Since no water is added, the H2/CO
ratio is lower than compared with SR or ATR.
Partial oxidation of natural gas is used in small plants
Reaction T between 1200– 1600 C and P up to 150 bar.
The concentrations of different compounds in the product mixture are determined
by several equilibria, which are quickly tuned in at these high temperatures.
Partial oxidation can be performed with or without a catalyst.
With catalyst, reaction T can be lower, reactions still reaching equilibrium, since
catalyst lowers the activation energies.
11
12. Important considerations
Development over the years has been to minimize the use of steam in
reforming due to the following disadvantages:
Endothermic reactions
The product gas has a H2/CO ratio of 3
Steam corrosion problems
Costs in handling excess H2O
The trend is to move from steam reforming to “wet” oxidation
(autothermal reforming) to “dry” oxidation. The dry oxidation is the
partial oxidation ( of methane for example).
12
13. NON-CATALYTIC PARTIAL
OXIDATION OF
HYDROCARBONS
PROCESS DESCIPTION-REACTIONS
Producing syngas from heavy hydrocarbons, including deasphalter, pitch and
petroleum coke.
Feed is pre-heated and then mixed with O2 within a burner; after ignition,
reactions occur inside a high T combustion chamber producing an effluent that
contains various amounts of soot, depending on feedstock composition.
Reactor exit gas T are comprised between 1200-1600°C. Syngas has to be
cooled and cleaned within a “washing” section for impurities.
ISSUES-PARALLEL REACTIONS
The high temperature (1100-1400°C) heat recovery in POx is not very efficient.
Oxygen
Feed
Syngas
13
14. NON-CATALYTIC PARTIAL
OXIDATION OF
HYDROCARBONSADVANTAGES
Possibility of utilizing a “low
value” feedstock.
The reaction is exothermic
(energy consumption is less)
Environmentally friendly in
terms of exhaust gases: little
NOx production
DISADVANTAGES
The oxidation step is highly
exothermic reaction, reducing
the energy content of the fuel
Cost of reaction materials are
high
Soot can easily emerge in the
non-catalytic POX process
14
15. CATALYTIC PARTIAL
OXIDATION OF
HYDROCARBONSPROCESS DESCIPTION-REACTIONS
The catalytic partial oxidation process has captured wide attention as a
noteworthy method that can drastically reduce production costs in the
future.
ISSUES-PARALLEL REACTIONS
Curtailment of heat generation and removal of reaction heat
Control of conversion rate through equilibrium management
15
19. Catalysts used for Partial oxidation of
methane
The first row of transition metals (Ni, Co) and precious
metals (Ru, Rh, Pd, Pt, and Ir) have been reported as
active catalysts for CPOM .
The relative rates of carbon deposition were in the order
Ni>Pd>Rh>Ru>Pt, Ir
Carbon deposition is a major issue on nickel catalysts,
which may reduce the number of active sites. Heavy
carbon formation may even increase the pressure on
the feed side or foul downstream process equipment.
19
20. CATALYTIC PARTIAL OXIDATION
OF HYDROCARBONS
ADVANTAGES
High reaction rates
Allows high gas velocities while
still acheiving thermodynamic
equilibrium
Prevents high temperatures
and elimínate termal runaway.
Small size reactor
ISSUES SURROUNDING CPOX
Improvement of heat
resistance of catalyst
Improvement of Coking
resistance of the catalyst
Reduction of costs
20
22. ADVANCES AND NEW
DEVELOPMENTS
Distinguishing characteristics of the catalytic partial oxidation
method
Reaction route of catalytic partial oxidation
Trends in research on catalytic partial oxidation: Direct route
catalyst
Process of direct route catalytic partial oxidation
Two-stage route catalyst
22
23. COMMERCIAL
SPECIFICATIONS
The table presents an evaluation of the economy of five types of
synthetic gas production equipment
SMR POX ATR CR CPOX
Natural gas
consumption
volume
(GJ/t-MeOH)
32 31.6 30.6 30 (29-30)
Oxygen
consumption
Volume
(m^3/t-MeOH)
530 460 280 270-300
CO2 emissions
Volume
(10^3 t/y)
380 375 355 290 250-270
Relative costs 100 95 85-95 80-85 70-80
23
24. Synopsis
The distinguishing characteristics of the catalytic partial oxidation
method are that energy consumption is slight, that the reaction speed is
rapid and that no soot or other unnecessary byproducts are produced.
The issues surrounding this method are improvement of heat resistance
and coking resistance of the catalyst and reduction of costs.
Carbide catalysts and metal oxide catalysts of high oxygen ion mobility
(permeation) are being developed in order to improve the process.
The economy of the syngas production process by the CPOX method,
assuming the process has been completed, is forecast to yield a 30%
reduction in costs at most as compared to the steam reforming method.
Research is expected to continue from the standpoint of practical
application
24
25. REFERENCES
Ke Liu. Chunshan Song. Velu Subramani. Hydrogen and Syngas Production and Purification Technologies
Jianjun Zhu. Catalytic Partial Oxidation of Methane to Synthesis Gas over ZrO2-based Defective Oxides
C. W. MONTGOMERY, E..B. WEINBERGER, AND D. S. HOFFMAN. Thermodynamics and stoichiometry of synthesis gas
production
Carlo Rudy Harold de Smet. Partial Oxidation of Methane to Synthesis Gas: Reaction Kinetics and Reactor Modelling
Gaetano Iaquaniello, Elena Antonetti, Barbara Cucchiella, Emma Palo, Annarita Salladini, Alessandra Guarinoni, Andrea Lainati
and Luca Basini. Natural Gas Catalytic Partial Oxidation: A Way to Syngas and Bulk Chemicals Production
Michael J. Gallagher, Jr.. Partial Oxidation and Autothermal Reforming of Heavy Hydrocarbon Fuels with Non-Equilibrium Gliding
Arc Plasma for Fuel Cell Applications
Saleh A. Al-Sayari. Recent Developments in the Partial Oxidation of Methane to Syngas
Yoon Cheol Yang, Mun Sup Lim, Young Nam Chun. The syngas production by partial oxidation using a homogeneous charge
compression ignition engine
R. Lanza a,b, P. Canu b, S.G. Ja¨ra˚ s .Partial oxidation of methane over Pt–Ru bimetallic catalyst for syngas production
Wenjuan Shan a,*, Matthieu Fleys a, Francois Lapicque b, Dariusz Swierczynski c, Alain Kiennemann c, Yves Simon a, Paul-
Marie Marquaire a. Syngas production from partial oxidation of methane over Ce1XNiXOY catalysts prepared by complexation–
combustion method
Cosme Huertas, Maria Luisa. Tecnologias de produccion de hidrogeno a partir del reformado de queroseno para las aplicaciones
aeronauticas
Luca Basini Topsoe Catalysis Forum, Aug ., 2006 – Aug. H2 generation using SCT CPO SCT-
25
Editor's Notes
Intro Sebastian
GANESH
Syngas is usually produced through the process of gasification of the feedstock at very high temperatures, normally between 800 and 1500°C, depending on feedstock and process. These high temperatures can be obtained from an external heat source or by a process of partial oxidation where a part of the feedstock is reacted with oxygen to release heat.
Ganesh
GANESH
The following table provides a typical range for the composition of syngases. This will be dependent upon the specific chemical composition of the feedstock to the gasifier
For example
Under normal circumstances, faster combustion in the engine cylinders would lead to the potential of pre-ignition, knocking and engine backfiring. In order to counter this challenge the engine has a number of technical modifications and the output of the engine is reduced to between 50-70% of its typically natural gas output. (I.e. a 1,063kW engine running on natural gas is comparable to a maximum 730kW engine on synthetic gas).
SEB
The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen into liquidhydrocarbons. It was first developed by Franz Fischer and Hans Tropsch at the "Kaiser-Wilhelm-Institut für Kohlenforschung" in Mülheim an der Ruhr, Germany in 1925. The process, a key component of gas to liquids technology, produces a synthetic lubrication oil and synthetic fuel, typically from coal, natural gas, or biomass.[1] The Fischer–Tropsch process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons.
Synthesis gas contains a mixture of carbon monoxide and hydrogen and can be obtained from the combustion of coal or natural gas. This gas can be used to produce synthetic crude by the Fischer-Tropsch reaction. Describe two industrial reactors used to convert synthesis gas to a mixture of hydrocarbons by the Fischer-Tropsch process.
The Fischer-Tropsch reaction converts synthesis gas into a mixture of alkanes and alkenes over a solid catalyst Usually containing iron. The basic reaction for paraffin formation is as follows
At the end of the slide …In addition to the simultaneous formation of paraffins and olefins, side reactions also take place to produce small quantities of acids and nonacids (e.g., ethanol).
SEB
With gaseous and liquid hydrocarbons and alcohols as well as carbohydrate feedstock, there are many process options for syngas and hydrogen production. They are steam reforming, partial oxidation, and autothermal reforming or oxidative steam reforming. With solid feedstock such as coal, petroleum coke, or biomass, there are various gasifi cation processes that involve endothermic steam gasifi cation and exothermic oxidation reaction to provide the heat in situ to sustain the reaction process.
The following equations represent the possible reactions in different processing steps involving four representative fuels: natural gas (CH 4 ) and liquefi ed propane gas (LPG) for stationary applications, liquid hydrocarbon fuels (C m H n ) and methano (MeOH) and other alcohols for mobile applications, and coal gasifi cation for large - scale industrial applications for syngas and hydrogen production. Most reactions
(Eqs. 1.1 – 1.14 and 1.19 – 1.21 ) require (or can be promoted by) specifi c catalysts and process conditions. Some reactions (Eqs. 1.15 – 1.18 and 1.22 ) are undesirable but may occur under certain conditions.
Reforming or gasifi cation produces syngas whose H 2 /CO ratio depends on the feedstock and process conditions such as feed steam/carbon ratio and reaction temperature and pressure. Water - gas shift reaction can further increase the H 2 /CO ratio of syngas produced from coal to the desired range for conversion to liquid fuels.
This reaction is also an important step for hydrogen production in commercial hydrogen plants, ammonia plants, and methanol plants that use natural gas or coal as feedstock.
SEB
SR is the most used technology for producing synthesis gas. This technology reacts light desulphurised hydrocarbons (S content ca. 50 ppb) with steam; for instance SR of Methane is represented with:
CH4 H2O CO 3H2 H 298K 206 kJ / mo l (1)
When utilised for H2 production the SR step is followed by a Water Gas Shift (WGS) step for CO conversion:
CO H2O CO2 H2 H 298K 41 kJ / mol (2)
Subsequently H2 is purified with a Pressure Swing Adsorption (PSA) step
CH4 + H2 O = CO+ 3H2
Cm Hn + m H2 O = m CO+ (m + n 2) H 2
CH3 OH + H2 O = CO2 + 3H2
Steam reforming has been used for many decades for synthesis gas production since first
developed in 1926 18 and over the years much progress has been made in reforming
technology.
Steam reforming of methane is highly endothermic and is carried out in fired tubes at
temperature above 900oC. Current industrial catalysts are usually based on nickel. The typical
scheme is illustrated in Fig. 3.
Steam reforming of methane suffers from high endothermicity (intensively energy
consuming), catalyst-coking propensity and requirement for operation at high temperatures
(>900oC). Supported Ni catalyst is very active for steam reforming of methane to synthesis
gas and has been used for many decades. However, Ni catalyst is also very active for
decomposition of CH4 to carbon and hydrogen. At high reaction temperature large amounts of
fibrous carbon can be formed, which influences activity and stability of catalyst, and can even
damage the reactor19. To prevent carbon deposition, a high H2O/CH4 ratio (2.0~6.0) is
required. Excess of water accelerates water-gas shift reaction (CO+ H 2O CO2 +H2),
resulting in a high H2/CO ratio (>3.0), which is not suitable for downstream processes, e.g.
methanol synthesis.
Based on the conventional steam reforming technology, autothermal reforming (ATR) was
first developed in the late 1970s with the aim of carrying out reforming in a single reactor18.
In the first part of the autothermal reforming reactor, homogeneous oxidation of methane is
carried out with O2 in a flame. The final steam reforming and equilibration take place in the
catalyst bed in the second part of the reactor. For normal operation, the autothermal reforming
operates at high temperatures around 2000ÅãC in the combustion zone and l000-1200ÅãC in the
catalytic zone. The product gas composition can be adjusted by varying the H2O/CH4 in the
feed.
GANESH
In the automatic-thermal reforming method, partial oxidation and steam reforming take place with one reactor. Auto thermal reforming (ATR) uses oxygen and carbon dioxide or steam in a reaction with methane to form syngas. The reaction takes place in a single chamber where the methane is partially oxidized. The reaction is exothermic due to the oxidation.
The outlet temperature of the syngas is between 950-1100 C and outlet pressure can be as high as 100 bar.
The main difference between SMR and ATR is that SMR uses no oxygen. The advantage of ATR is that the H2: CO can be varied, this is particularly useful for producing certain second generation biofuels, such as DME which requires a 1:1 H2: CO ratio.
GANESH
GANESH
Partial oxidation is the processes in which the feed gas reacts with insufficient supply of air exothermally to form synthesis gas containing carbon monoxide and hydrogen. The feed gas may be methane and other hydrocarbons.
CH4 + ½O2 → CO + 2H2 (+heat) (Exothermic) ∆Hr (@298K) = -35.7 KJ/mol
This process can be applied to both heavy hydrocarbons ex. (Fuel oil, coal, petroleum coke, asphalt) and light hydrocarbons ex.( refinery gases, LPG, natural gas, naptha etc.)
Any hydrocarbon feedstock that can be compressed or pumped may be used in this technology.
A distinction is made between thermal partial oxidation (TPOX) and catalytic partial oxidation (CPOX).
thermal partial oxidation depends on the air-fuel ratio, proceed at temperatures of 1200°C and above.
The choice of reforming technique depends on the sulfur content of the fuel being used. CPOX can be employed if the sulfur content is below 50 ppm. A higher sulfur content can poison the catalyst, so the TPOX procedure is used for such fuels. However, recent research shows that CPOX is possible with sulfur contents up to 400ppm.
SEB
Partial oxidation (POX) is often used for gasification of heavy oil, but all hydrocarbons are possible as feedstocks. Since no water is added, the H2/CO ratio is lower than compared with steam reforming or autothermal reforming. Partial oxidation of natural gas is used in small plants and in regions where natural gas is cheap. Reaction temperatures are 1350 – 1600 C and pressures up to 150 bar. The concentrations of different compounds in the product mixture are determined by several equilibria, which are quickly tuned in at these high temperatures. Partial oxidation can be performed with or without a catalyst. If a catalyst is used, the reaction temperature can be lower, the reactions still reaching equilibrium, since the catalyst lowers the activation energies. A lower temperature, at this high temperatures, would not significantly change the equilibrium composition [Ullman’s, 1985].
Bharadwaj and Schmidt [1995] claimed that the development over the years has been to minimize the use of steam in reforming due to the following disadvantages:
Endothermic reactions
The product gas has a H2/CO ratio of 3
Steam corrosion problems
Costs in handling excess H2O
The trend is to move from steam reforming to “wet” oxidation (autothermal reforming) to “dry” oxidation. The dry oxidation is the partial oxidation of methane (6). This reaction directly gives the desired ratio H2/CO = 2 for Fischer-Tropsch or methanol synthesis, at temperatures > 900 ºC, as shown in Figure 1 (b) above. However, the selectivites are also affected by the H2O and CO2 formed in the complete oxidation reactions (7, 10) [Bharadwaj and Schmidt, 1995].
Since the partial oxidation reaction is slightly exothermic, the partial oxidation reactor would be much more energy efficient than the energy intensive steam reformer. Since the reaction is fast, the reactor size could be reduced significantly compared to the SR reactor [Bharadwaj and Schmidt, 1995].
The non-catalytic partial oxidation (POX) of methane is already commercialized, for instance in the Fischer-Tropsch plant in Malaysia, Bintulu. However, there are several problems with POX that can be avoided by catalytic partial oxidation (CPOX). In CPOX, only heterogeneous reactions are taking place, lower temperatures are used and no soot or unwanted by-products are formed. No burner is used. It is mainly the absence of homogeneous reactions that prevent the formation of unwanted oxidation products or flames which can lead to soot formation. The newly developed highly selective partial oxidation catalysts can overcome the carbon problem without any steam input. It appears that high selectivity to CO and H2, as compared to CO2 and H2O, can be achieved at short contact times, i.e. at high GHSVs. Results showing high methane conversion at short contact times have also been reported. Bharadwaj and Schmidt [1995] concluded that for partial oxidation of methane, Rh seems to be the catalyst of choice to achieve selectivities to H2 and CO above 95 %, and methane conversions above 90 %. The major engineering challenge is to ensure safe operation with the premixed CH4/O2 mixtures. The reaction mechanism seemed to be direct oxidation via CH4 pyrolysis at contact times shorter than 0.1 s. At longer residence times, reforming reactions with CH4 and H2O or CO2 and shift reactions also may take place.
However, the direct catalytic partial oxidation of methane has not yet been commercialized because it is difficult to study since it involves premixing of CH4/O2 mixtures which can be flammable or explosive [Bharadwaj and Schmidt, 1995].
Due to the high temperatures needed in order to reach high conversions and high selectivities to H2 and CO, the CPOX-reactor needs extremely tolerant materials, which are expensive. Current research at NTNU/SINTEF aims at developing catalysts that can give satisfactory high reaction rates and high selectivities to H2 and CO at lower temperatures (650 ºC). This would be possible if the catalyst could ensure that only the partial oxidation, i.e. reaction (6), occurs, and hence that no water is formed. Reaction (6) is exothermic and hence favored at low temperatures, and if no water is ever present in the reactor, a syngas with a H2/CO ratio of 2.0 could theoretically be formed at low temperatures in relatively cheap steel reactors [Bjørgum, 2006].
SEB
Check this part for the “Example”
This reaction directly gives the desired ratio H2/CO = 2 for Fischer-Tropsch or methanol synthesis, at temperatures > 900 ºC, as shown in Figure 1 (b) above. However, the selectivites are also affected by the H2O and CO2 formed in the complete oxidation reactions (7, 10) [Bharadwaj and Schmidt, 1995].
Since the partial oxidation reaction is slightly exothermic, the partial oxidation reactor would be much more energy efficient than the energy intensive steam reformer. Since the reaction is fast, the reactor size could be reduced significantly compared to the SR reactor [Bharadwaj and Schmidt, 1995].
At the end make the introduction for POX and CPOX
SEB
Non Catalytic POX
is non catalytic partial oxidation process. TPOX (thermal partial oxidation) reactions, which are dependent on the air-fuel ratio, proceed at temperatures of 1200°C and above.
The chemistry of the POx technology is based on the partial combustion of fuels that in case of CH4 is represented with equation (6)
CH4 ó O2 CO 2H2 (6)
However this process is mainly utilised for producing syngas from heavy hydrocarbons, including deasphalter pitch and petroleum coke. These are pre-heated and then mixed with Oxygen within a burner; after ignition, reactions occur inside a high temperature combustion chamber producing an effluent that contains various amounts of soot, depending on feedstock composition. Reactor exit gas temperatures are typically comprised between 1200-1400°C. The obtained syngas has to be cooled and cleaned within a “washing” section for removing the impurities. The high temperature (1400-1100°C) heat recovery in POx is not very efficient and indeed the POx advantage over SR is in the possibility of utilising a “low value” feedstock, even containing sulphur and other compounds that would poison the SR catalysts. Currently the main utilisations of POx are: (i) in H2 production for refinery applications, (ii) synthesis gas production from coal and (iii) in electric energy production from petroleum coke and deasphalter bottoms, through large Integrated Gas Turbine Combined Cycles (IGCC).
GANESH
The chemistry of the POx technology is based on the partial combustion of fuels that in case of CH4 is represented:
CH4 + ½O2 → CO + 2H2
However this process is mainly utilised for producing syngas from heavy hydrocarbons, including deasphalter, pitch and petroleum coke. These are pre-heated and then mixed with Oxygen within a burner; after ignition, reactions occur inside a high temperature combustion chamber producing an effluent that contains various amounts of soot, depending on feedstock composition. Reactor exit gas temperatures are typically comprised between 1200-1600°C. The obtained syngas has to be cooled and cleaned within a “washing” section for removing the impurities. The high temperature (1400-1100°C) heat recovery in POx is not very efficient and indeed the POx advantage over SR is in the possibility of utilising a “low value” feedstock, even containing sulphur and other compounds that would poison the SR catalysts. Currently the main utilisations of POx are: (i) in H2 production for refinery applications, (ii) synthesis gas production from coal and (iii) in electric energy production from petroleum coke.
GANESH
GANESH
Direct route catalyst
Pt, Rh, Ru, Ir, Ni/Al2O3, or (rare earths) 2Ru2O7
the methane conversion rate exceeds 90% with any of the catalysts, and the H2
selectivity rate is reported at 94 to 99%.
SEB
Reaction 3 indicates that synthesis gas can be obtained through a direct route. According to this reaction, production of syngas is theoretically possible at all temperatures once CH4 and O2 have been activated.
But this is an oversimplified approach because other reactions such as reactions 1 and 2, and other oxidation reactions of CH4, H2 and CO, decomposition of CH4 and CO may occur simultaneously.
It is clear that all these reactions play important roles in the partial oxidation of methane, and therefore the product composition at the reactor exit is governed by or limited by the thermodynamic equilibrium of all possible species involved in the process.
The equilibrium composition appears to be essential when discussing experimental results, and detailed thermodynamic analysis of the CH4/O2 (and/or H2O) mixtures has been reported [9, 11,12]. Fig. (1) shows the effect of temperature and pressure on a stoichiometric mixture of methane and oxygen (CH4:O2 = 2:1) [9].
At increasing pressures, higher temperatures are required to obtain high conversion, and high selectivity to H2 and CO. Thermodynamic calculations also revealed that feeding CH4/O2 mixtures at a ratio of 0.5 yields complete combustion products (CO2 and H2O), but CH4/O2 ratio above this limit produce both H2 and CO as major products.
////////////////////////
Although many studies have been conducted with the aim to elucidate the mechanism of CPOM reaction, the mechanism by which CO and H2 are formed is not yet completely clarified. Two reaction mechanisms have been proposed: one is the “direct mechanism ” in which CH4 and O2 react on the adsorbed state on the catalyst surface to yield CO and H2 ; the
second one is the so-called “combustion-reforming mechanism ”. In this latter mechanism, CH4 and O2 first form H2 O and CO2 (Eq. 4), and then dry (Eq. 5) and steam
reforming (Eq. 1) reactions produce CO and H2 . The reaction occurring according to the direct mechanism is shown in equation 3, however the reactions involved in the
combustion-reforming mechanism are more complex assummarized in equations 1, 2, 4 and 5:
In addition to these reactions, other side reactions eventually occur. These include partial oxidations (CH4 + 3/2 O2 CO + 2 H2 O and CH4 + O2 CO2 + 2 H2 ) and the
formation of solid carbon by the Boudouard reaction (2 CO C + CO2 ). Precise knowledge of the mechanism of CPO reaction is of vital importance because of the different
thermal effects, as summarized above for the direct mechanism (Eq. 1) and combustion-reforming mechanism (Eqs. 2-5), which indeed affect both the design and heat
management of industrial units
SEB
The kinetic model used in this work, which will be referred to as the oxygen-assisted
CPO mechanism, is shown in Table 2, together with the corresponding rate equations.
The mechanism consists of six reaction steps;
Dissociative adsorption of oxygen and methane [1, 2]
Surface reactions towards CO and CO2, [3, 4]
Desorption and adsorption of CO are taken into account. [5, 6]
All reaction steps, with the exception of CO adsorption/desorption, are considered to be irreversible.
The mechanism consists of six reaction steps; oxygen and methane adsorption, surface reactions towards CO and CO2, desorption and adsorption of CO are taken into account. All
reaction steps, with the exception of CO adsorption/desorption, are considered to be irreversible. This is justified since both the methane and oxygen conversions are considerably
lower than 100%. Irreversible methane and oxygen adsorption may lead to a structurally unstable kinetic model, as a result of the occurrence of a transcritical bifurcation point
(Nibbelke et al., 1998). Reactor simulations with the kinetic parameters obtained after regression show, however, that these structural instabilities do not occur. This is a result of
the low surface coverages of both adsorbed carbon and oxygen species at the conditions investigated.
Oxygen adsorption. The first step in the reaction mechanism concerns the activation of oxygen through dissociative adsorption. The adsorption rate is assumed to be first order in the
fraction of vacant surface sites 2* (Williams et al., 1992), which implies that the ratedetermining step in the adsorption involves the interaction of molecular oxygen with a single
catalytic site. Oxygen adsorption is considered to be competitive, in contrast to the mechanism proposed by Hickman and Schmidt (1993). In the latter 19-step mechanism,
oxygen is assumed to adsorb on specific catalytic sites, i.e. non-competitively with other species, in order to be able to simulate the experimentally observed O2 adsorption-limited CO
production rate. Methane adsorption. Dissociative adsorption of methane, resulting in the formation of a
surface carbon species and gaseous water, is taken into account in step 2. Methane adsorption is considered to be oxygen-assisted, directly resulting in adsorbed hydroxyl species that
recombine instantaneously to gaseous water. Oxygen-assisted methane adsorption is included in the kinetic model to be consistent with the observed decrease of the CO selectivity at increasing space-times. This will be illustrated later, by examining analytical expressions for
the intrinsic selectivity to CO. Also, oxygen-assisted methane adsorption is in line with the observation that H2O is the only hydrogen-containing product at the conditions investigated.
Reaction 2 obviously is not an elementary step, but proceeds through a number of intermediates such as adsorbed CHx (x=1,3) fragments and adsorbed OH species. A possible
reaction sequence, accounting for the global methane adsorption step is shown in Table 3. Methane adsorption on the Pt surface is considered to be reversible and in quasi-equilibrium.
The abstraction of the first hydrogen atom by adsorbed oxygen species is considered as the rate-determining step in methane decomposition. The subsequent abstraction of hydrogen
atoms from adsorbed CHx fragments, and the recombination of hydroxyl species to water are potentially very fast. As a result, no CHx species appear as surface intermediates in the
kinetic model. The global rate coefficient for dissociative adsorption of methane, k2, is the product of the equilibrium coefficient for molecular adsorption of methane and the rate
coefficient of the rate determining step 2bI, see Table 3. Carbon monoxide and carbon dioxide production. Steps 3 to 6 concern the reaction
paths towards carbon monoxide and carbon dioxide. Reaction 3 describes the formation of adsorbed CO species, which is generally considered to be very fast (Hickman and Schmidt,
1993). Adsorbed CO species are converted to CO2 in step 4, which desorbs instantaneously. CO2 adsorption is not taken into account in the kinetic model, since the heat of adsorption of
CO2 on Pt is very low (Shustorovich, 1990). Finally, CO desorption and adsorption are described in the kinetic model by steps 5 and 6.
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Trends in the development of the catalytic partial oxidation method
Distinguishing characteristics of the catalytic partial oxidation method
The reaction is exothermic, and energy consumption is lower than in the endothermic reaction of steam reforming.
CH4+1/202 → CO+2H2 ΔH = -35.7kJ/mol
Since the reaction speed is rapid, the reactor is extremely small, and in comparison to the non-catalytic partial oxidation process, there is no generation of soot or other unnecessary byproducts. And because H2/CO = 2 synthetic gas is obtained by adding a small amount water, the method is ideal for producing synthetic gas for FT synthesis or methanol production. In addition, equipment costs are reduced by about 30% from conventional methods.
Reaction route of catalytic partial oxidation
As shown in Figure 5, the catalytic partial oxidation reaction route consists of a direct route and a two-stage route. In the direct route, carbon gas and hydrogen are produced directly from methane. In the two-stage route, on the other hand, complete combustion of methane takes place at first, then synthetic gas is produced by subsequent steam reforming, carbon gas reforming, or by the advance of a shift reaction.
Trends in research on catalytic partial oxidation
Direct route catalyst
In the direct route, 100% thermodynamic conversion is possible, but the important point is that this conversion must be stopped at the stage of catalytic partial oxidation byproduct (H2/CO) prior to complete combustion.
• Research with fixed bed reactor
When sundry catalysts, including Pt, Rh, Ru, Ir, Ni/Al2O3, or (rare earths) 2Ru2O7, are used, the methane conversion rate exceeds 90% with any of the catalysts, and the H2 selectivity rate is reported at 94 to 99%.
• Monolith type reactor
When using a monolith catalyst coated with platinum or rhodium, a reaction between air and methane is examined under normal pressure within a residence time range of 10-4 to 10-2 seconds, using a automatic-thermal reactor. As catalyst, rhodium is superior to platinum, and when oxygen is used, at a methane conversion rate of 90% or more, the selectivity of CO is reported at 95% or more, and of H2 at 90% or more.
• Fluid bed reactor
Research with a fluid bed was conducted using a bead-shaped catalyst of 100 μm with Rh, Pt or Ni held in α alumina. With Rh and Ni catalyst, air or oxygen was used, and in the case of reaction under normal pressure, at a methane conversion rate of 90% more, the selectivity rates of CO and H2 are reported at 95% or more.
Process of direct route catalytic partial oxidation
Among the issues involved in the process of catalytic partial oxidation are removal of heat generated by reaction, curtailment of heat generation, avoidance of selective gas phase reaction, and control of conversion rate through equilibrium management. In response to these issues, diverse modes of research are being conducted on process-related factors such as reaction format, reaction conditions and selection of raw materials.
• Curtailment of heat generation
Foremost among the themes mentioned above, given the limit on heat resistance of
catalyst, are curtailment of heat generation and removal of reaction heat. The monolithic type reactor and fluid bed reactor are outstanding countermeasures in this regard.
• Disruption of equilibrium management
What makes it difficult to regulate such things as conversion rates by means of equilibrium management is the production of synthetic gas by the direct route. Research reported thus far has covered cases in which the monolith reactor was used over a very short catalytic time period, or special cases in which adsorption oxygen was used.
Two-stage route catalyst
No research has been directed at the two-stage route, but catalysts are being developed, using nickel catalyst, for instance. In general, rhodium and ruthenium are outstanding catalysts in terms of catalytic activity and coking resistance.
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Economy of catalytic partial oxidation
Not many studies have been completed on the economy of the catalytic partial oxidation method. This may be due to the fact that the technology is still under development and it is difficult to get a picture of it in its fully completed form.
Nevertheless, in the prefaces of various research reports or elsewhere, it is always pointed out that the catalytic partial oxidation method is superior to conventional methods in terms of economy and that great expectations have been pinned on its development.
Processes evaluated for economy
Steam reforming method (SMR)
(Non catalytic) partial oxidation method (POX)
Automatic-thermal reforming method (ATR)
Compound reforming method (CR)
Catalytic partial oxidation method (CPOX)
The evaluation was based on a comparison of economy in the production of 2,500 t/d of methanol. Comparisons were made with other devices, taking the steam reforming method (SMR) as 100. Superiority in economy was found to be in the following sequence: Steam reforming method (SMR), (Non catalytic) partial oxidation method (POX), Automatic-thermal reforming method (ATR),
Compound reforming method (CR), and Catalytic partial oxidation method (CPOX).
It was reported that with the catalytic partial oxidation method, costs were 30% lower than with the steam reforming method.
The same trend was exhibited in the reduction of carbon dioxide gas emissions, the major factor in the phenomenon of global warming. It was reported that emissions from the catalytic partial oxidation method were 35% lower than from the steam reforming meth
The CPO technology has, in recent years, started to migrate from research laboratories into commercial application, exemplifi ed by the growing number of patents, where it is seen
as an alternative to steam reformers for hydrogen production. The reason for this is the effectivenessof catalysts, which would give high selectivity to hydrogen and yield themselves to
a compact design, which allows for rapid response and relatively low capital cost. This has been achieved through fundamental knowledge of the process through years of research. The commercial application relying on CPO and ATR have thus far focused primarily on production of hydrogen for fuel cells, where the compact design makes CPO an obvious choice since
many of these systems are designed to be portable. However, the future will certainly expand the areas where this technology will fi nd its application.
