1141

40. Small Scale Catalytic Syngas Production Small Scalewith Plasma C Adam A. Gentile, Leslie Bromberg, Michael Carpenter

40.1 Plasma ...... 1142 The production of syngas has been desired for 40.1.1 Introduction ...... 1142 nearly a century as a starting point towards an 40.1.2 Types of Plasmas ...... 1144 effort to achieve the synthesis of higher-energy liquid fuels. Methods that are utilized for obtain- 40.2 Partial Oxidation Reformation ing syngas have undergone profound technological Using Cold Plasma...... 1147 transformations over the years. The formation of 40.2.1 Methane Reformation ...... 1147 plasma is one such concept capable of energizing 40.2.2 Plasma Catalysis ...... 1151 molecular chains through dielectric manipula- 40.2.3 Reactor Apparatus ...... 1153 tion in order to excite electrons and form radicals. 40.3 Cold-Plasma-Assisted Many types of plasma configurations exist under Experimentation...... 1156 a range of variable parameters such as pressure, 40.3.1 Steady State Operation ...... 1156 temperature, current, power intensity, and phys- 40.3.2 Transients: Startup/Shutdown...... 1157 ical structure. Nonthermal plasma in conjunction 40.3.3 Conversion and Efficiency...... 1157 with catalysis is a relatively new concept that takes 40.4 Analysis and Discussion...... 1159 a more subtle approach that eliminates the inten- 40.4.1 Process Challenges...... 1159 sive energy requirement while maintaining high 40.4.2 Design Improvements ...... 1159 conversion efficiency. The technology and process 40.4.3 Higher Hydrocarbon Reformation ...... 1160 presented in this book chapter will encompass a hybrid plasma/catalysis (PRISM) system having 40.5 Synergistic Benefits of Plasma ...... 1160 40.5.1 Chemical Processes 1160 the ability to ionize and reform any vapor phase ...... 40.5.2 Commercial Implementation ...... 1160 specieswithafocusonshorttolongchainhydro- carbons. The apparatus promotes reactions that 40.6 Conclusion...... 1161 would otherwise be disadvantageous from either References...... 1162 a high activation energy standpoint or from the yield of unfavorable side products. Catalytic partial oxidation (CPOX) using a low-energy, nonthermal plasma is capable of nearly perfect conversion of a carbonaceous starting material into clean, ho- mogeneous, hydrogen-rich syngas.

This chapter documents work done on small- to mid- There exist several applications of stationary and scale plasma enhanced catalysis for platform-flexible onboard hydrogen production: fuel reformers in an effort to improve conventional (i. e.,  Supplementation and augmentation of hydrogen internal combustion) vehicle operation as well as the into internal combustion engines (ICEs) for im- commercial standard reformation efficiency. Although proved operation and pollutant elimination industry has been focusing on methane, the system per-  Diesel PRISM fuel reformation for after-treatment PartD|40 forms as a cost-effective method in the production of application hydrogen-rich, process-ready syngas from any carbona-  Onboard hydrogen generation for fuel cell power ceous source/feedstock. production  Clean-burn electrical generation (turbines, gen sets)  Liquid drop-in fuel production.

© Springer International Publishing AG 2017 C.S. Hsu, P.R. Robinson (Eds.), Springer Handbook of Petroleum Technology, DOI 10.1007/978-3-319-49347-3_40 ,and 4 H 2 ,C 2 H 2 )inaplasma- 4 40.1b using par- Visual of excited then quickly recombine ) / ]) an electrical field supplied a 2 ( 1 H C of the gas molecule into plasma H Simple plasma discharge apparatus ]. Strongly bonded molecules like ) 2 C b ion motion that makes( up plasma. (after [40. Fig. 40.1  40.5, which leads to the conclusion 4 < x Œ 40.4, we expound on a discussion of the H 1 breakdown ]. This can be achieved and maintained as Gas 3 C ! Electrode [40. 4 6 Generation of Plasma Field H H 1 2 C into intermediate compounds such as C long as the methaneThe is intermediate exposed to moleculesand the are electrical will a field. reform short-livedwhile yielding back species some amount into of molecular methane hydrogen. if not converted Multiple methods exist to produce a viablethe plasma most field, practical being two electrodesproximity in close supplied enough withelectricity, a or plasma, voltage through creating theelectrodes. an air This gap arc can between of the allel be conductive seen plates. incurrent The Fig. between power the conductive supply electrodesgas drives ionizing the the flowing throughrole it. for the Pressure generation plays of the an field. important At pressures below induced state have. been documented to disassociate C may also be achieveda when distinct temperatures rise thresholddifficulty beyond level than applying of extreme the amountsization can of molecule. be heat, reached ion- With inside by less an adequatea voltage dielectric source. Thisconsisting approach of causes charged species (electrons,and ions, neutral radicals) species (atoms, molecules,ticles). and excited The par- plasmastate thus of becomes a mattercal conductivity highly characterized [40. energetic by an elevated electri- hydrocarbons such as methane (CH tive. In Sect. data including processment, challenges, and areas future development. of Someeffects of improve- of the synergistic plasma overbe typical gas-phase found processes in can Sect. of how andenvironment why is plasma-assisted a POXogy. in viable a and catalytic cost-effective technol- 40.2. Plasma rf – + V b) 40.1. The primary reaction of in- ]. Typically generated from a gaseous – 1 + + details the experimental process from start + – – – 40.3 + – + The chapter is organized as follows: First, an intro- Breakdown and Ionization – a) duction of plasma and itsare different defined forms and in attributes Sect. terest is partialthe oxidation micro- (POX) and and macrokinetics ofterms this, the of theory, PRISM along will process be with in described in detail in Sect. There are many different pathstion that lead plasma, to which the is produc- state universally of matter known and as alsoin the the most fourth the abundant state present universe.methods Some of generating generally plasma knownplasma include jet the devices or plasmatron, and torch,or spark, corona gliding discharge, arc, electrical andplasma glow is discharge. For regardedparticles our as in purposes, random a motion collection thatcally neutral is, [40. of on average, freely electri- charged 40.1 Plasma 40.1.1 Introduction phase, application through ana electrical temporary field conditionof induces bulk of medium a temperature.complish Additional this plasma ways include, state to butfrequency (RF) ac- regardless are treatment not or limited athe microwave gas to, treatment in a of a strong radio to electromagnetic augment field. a When utilized reaction,cial plasma factors adds to substantial the benefi- molecularthe interactions reaction and at kinetics hand. of The standard gas molecule iswhen stable at or in near an a environment stateings. of equilibrium As with its surround- intensivemolecules and begin to extensive losement their properties rapidly increases stability. change, and Electron ionization move- iselectrons achieved when gain enoughfrom energy their respective to orbitals removemal unbound position from themselves within their or nor- around the molecule. Ionization Petrochemicals Section to finish with a broad overview of operational perspec-

Part D

PartD|40.1 1142 Small Scale Catalytic Syngas Production with Plasma 40.1 Plasma 1143 atmospheric and above vacuum, the electrical energy lifetimes (nanoseconds) [40.1]. Electrical field energy required to create a plasma field arc or glow discharge is directly proportional to electron energy responsible is reduced to a minimum. The augmentation of the di- for greater molecular decomposition from the electron- electric strength of vacuum by matter and pressure is impact dissociation and ionization reactions. As an the primary function for sustaining the plasma field. example, bonding energy of most hydrocarbons is be- Another factor in field generation refers to the ge- tween 36 eV. This range expresses CHandCC ometry of the electrodes. Industrially, these can vary bond strength varying from molecule to molecule of from sharp singular points to infinitely long plates to any hydrocarbon in general. CH bonds are stronger annular arrangements physically similar in appearance and closer to 6 eV in strength than that of CC bonds, to concentric pipe heat exchangers. In the case of pro- which reach the lower interval of 3 eV in strength. An cessing gas, the annular apparatus is highly efficient electron-volt (eV) is energetically equivalent to about and isolates the system from the surrounding environ- 1:61019 J (Joules). The following electron-impact re- ment. It is difficult to classify and analyze the plasma actions of methane show the threshold energy barrier generated from an external electromagnetic field be- necessary to achieve specific levels of plasma excita- cause of the inconsistent nature of the ionized particles. tion [40.3]: The generated plasma is on a very short time and    Vibrational excitation space scale (10 710 9 s) as the excitation weakens, dissipating quickly upon departure from the applied  . /  electrical source. If not utilized properly, the plasma CH4 C e ! CH4 24 C e will return to a stable gaseous ground state. Threshold energy D 0:162 eV   CH4 C e ! CH4.13/ C e Stability and Equilibrium Threshold energy D 0:361 eV Plasma requires constant treatment via an external en- ergy source in order to maintain its phase stability, Dissociation making it a short-lived species at ambient conditions.  In a continuous process, it’s wise to consider the place-   ment of the plasma generating field in order to optimize CH4 C e ! CH3 C H C e overall system efficiency. Ionization and dissociation Threshold energy D 9eV   are a requirement for the existence of plasma, but these CH4 C e ! CH2 C H2 C e induced conditions also cause a fluctuation in equilib- Threshold energy D 10 eV rium. Plasma density, also defined as electron or ion CH C e ! CH C H C H C e density (ni), is the number of free electrons per unit vol- 4 2 ume. Ion density can be used to determine the degree of Threshold energy D 11 eV fractional ionization (xiz)ofplasmarepresentedbythe equation  Ionization

ni  C  x D CH4 C e ! CH4 C 2e iz n C n g i Threshold energy D 12:6eV  C  where ng is neutral gas density. If the degree of ion- CH4 C e ! CH3 C H C 2e ization is  1 then the plasma can be considered fully Threshold energy D 14:3eV ionized and if xiz 1, the plasma is weakly ionized. No matter how minute the presence of ionization in Methane is a highly stable compound and, as a re- the gas phase, plasma characteristics still show to some sult, requires more energy to break the threshold barrier degree [40.3]. Threshold levels for ionization exist dur- than most other hydrocarbons. This data can be used ing excitation of a gas-phase substance and depending as an initial guide for determining the necessary en- on the amount of energy applied, different charged ergy requirement for any intended plasma application. and neutral molecules will be produced that are of Threshold levels affecting plasma stability lead to one PartD|40.1 overall equal constituents to the original gas molecule. possible reason as to why the plasma state is almost These excited forms consist of rotational excitation, never in chemical or physical equilibrium with its sur- vibrational excitation, disassociation, and full plasma roundings. Since the discharges are electrically driven ionization. Rotational and vibrational excited states are and weakly ionized, the applied power preferentially generally considered impractical for most processes heats the mobile electrons while the heavy ions ex- due to their low threshold energy (< 2 eV) and short change energy through collisions with the background 40.3a) and helicon 10 kW). This form of plasma 1 eV) at ambient conditions. The  40.3b) are produced by an axial vary- Glow Discharge Electron Cyclotron Resonance (ECR) and Helicon Discharge Cascaded Arc discharge (Fig. ing DC magneticcoils. The field coils surround a sustained cylindrical chamber inmicrowaves by which are electromagnetic injected throughpropagating a a dielectric rotating window polarizedthe source wave. plasma This where it generates cessing can chamber. be directed Electron into cyclotron theis resonance pro- (ECR) operated withcon a discharge, strong which magnetic isHelicon much field discharge weaker uses versus an by heli- with RF-driven comparison. the antenna weaker magnetic coupled field tocon create wave a at resonant the heli- plasmathe source particle-wave where it interaction is isenergy believed that transfer the into cause the ofinterest plasma. for the these Conventional plasma wave areas devices include of semiconductor manufacturing, electric propulsionvanced devices, medical or treatment. in ad- Glow discharge is produced by acal low field frequency induced electri- by DCinteracting or RF electrodes. in the Once openionization area adequate occurs between releasing voltage a is vibrantionized light met, gas. based on The the systemconducting is transparent tube. usually Known for enclosed its in usageorescent in lamps a flu- and non- neon lights astelevision well devices, as glow plasma-screen discharge plasmaplications also in has analytical ap- chemistrymass via spectrometry spectroscopy and and in material surfacea treatment method via known as sputtering. field is stabilized by cascaded platesage with a potential floating volt- andOperated is under typically a DC structured supply,in the cylindrically. the power lower requirement kW is range (1 and device is usedtreatment as in well. material deposition and surface Electron cyclotron resonance (Fig. stilling an electromagnetic fieldcurrent that (DC) negates the requirement direct power necessary in CCP. For to analytical methods reach such asetry, spectrom- efficient ICP provides a reliablerange detection limit of over a sample wide elements.also be The directed into RF the plasma powerthrough gap the source itself electrodes as for can opposed both to ICP and CCP. High-density, low-temperature cascaded arcin is a plasma relatively homogeneouscreates thermal equilibrium. an This exceptionsification to since the hot-thermal thethermally plasma equal gas, clas- ( ions, and electrons are all ]. ). e 1 )is T T 40.2). The 1atm) ), the phase i < T

e Plasma ). This is known as T P T < D i T D e T ]. 40.3a–d) are produced by high fre- 2 ) of the plasma is at least equal to the T ) representing the overall temperature ( i T ]. At or below ambient pressure, the ion tem- 1 40.3c,d) placed outside of the reactive zone in- Conventional plasma is synthetic and every syn- Low Field Pressure (0 atm ambient system process temperature. thesized type carries differentbenefits micro- for and process macroscopic use. Plasmapressure in is response also to the categorized method by structure of of ionization and the the electrical field. Forpressure instance, environments in plasma very low mayered always nonthermal. The be following consid- is aplasmas list with of descriptions conventional grouped by pressure. typically much less than the electron temperature ( hot plasma and the requirement differselement from where, element for to example, a temperaturemust of be 20 reached 000 K for completelium thermal while ionization around of 4000 K he- is needed for cesium [40. When not in thermal equilibrium ( state is considered coldall plasma temperature meaning ( that the over- Capacitive coupled plasma (CCP) and inductive coupled plasma (ICP) (Fig. quency RF electric fieldsare where the positive dominant ion factor leading collisions toThe electron bombarding dissociation. ions arenitude directly of affected frequency, typically by at the the MHzof mag- level, causing capable an electroning or the Townsend electrons avalanche cascad- intoused a in plasma material state. processing CCP toties is enhance like surface generally that proper- ofdeposition. integrated A circuitry substrate via is etching presentinteracting electrodes or upon that one film are of aided bymaterial the maintaining a two excess positive negative sheath charges and direct- ing them appropriately. ICP(Fig. utilizes a coiled electrode 40.1.2 Types of Plasmas As stated before,plasma, there both natural exist and many conventionalfocus (Fig. different of types this of plasma chapter used is in on awhile, one reformation however, to form process; note it of other isplasma types. conventional still is For worth- naturally instance, most occurring inand the interstellar universe matter. from An stars exampleknown of as thermal hot plasma, plasma, is also theof Sun’s lightning. surface or A a neon bolt mal light plasma can or cold be plasma.on considered This temperature, nonther- classification electronic depends density,of and the level medium of [40. energy gas [40. perature ( Petrochemicals An immense amount ofspecies energy and is required charged for speciesrium neutral with to each be other in ( thermal equilib-

Part D

PartD|40.1 1144 Small Scale Catalytic Syngas Production with Plasma 40.1 Plasma 1145

Fig. 40.2 Synthetic and universal –3 log n(cm ) plasma on energy (logarithmic 25 Solid Si at temperature voltage equivalent) room temperature versus density plot. De D Debye length – the distance scale over which charge densities can exist 4 7nλ3 <1 3 De spontaneously or more simply, how far the electrostatic effects travel. As Laser 20 an example, low-voltage and undriven plasma Focus plasmas are generally a few Debye lengths wide (middle portion of figure) High Shock (after [40.1]) tubes pressure Theta arcs pinches Fusion 15 reactor

Low pressure Alkali Fusion metal experiments plasmas 10 Glow discharges Flames

λ >1cm Earths´s De ionos- phere 5 Solar corona

Interplanetary Solar wind 0 –2 –1 0 1 2 3 4 5 log Te (V)

High Field Pressure (PPlasma > 1atm) components. The high power requirement pitfall limits Plasma Torch and Thermal Plasmatron widespread use of the thermal plasmatron in industry, Plasmatron is a high-powered DC thermal plasma gen- but the plasma torch finds a niche in plasma cutting, erated by electric resistive heating of a turbulent flow of pollution control and hydrocarbon reformation. gas molecules. The plasmatron provides highly control- lable electrical heating, but requires resilient electrodes Corona Discharge to withstand corrosion from the strong arcs and extreme This form of nonthermal plasma is generated through heat. A relatively large amount of energy is required to finely pointed electrode tips at a voltage high enough harness a homogeneous field in thermal equilibrium due to initiate a conductive field, but not so high to cause PartD|40.1 in part to the elastic collisions of the propagating ener- an electrical arc discharge to the grounded path. Al- gized electrons. Utilizing an annular configuration, the though the corona charge can be positive or negative, input gas is injected tangentially to produce a vortex the positive corona is undesirable due to the extremely that extends the plasma arc field inside of the ionization high energy requirement to excite positive ions. This is zone. Water is used as a heat sink to cool and control not the case for a negative corona, in which electrons electrode temperature preventing thermal damage to the are much lighter and more easily ionized in a non- ]) 1 Fig. 40.3a–d Example sources of low-pressure, high-density plasma in basic configurations (after [40. RF lasers. 2 40.4. DBD is another form of nonther- Helicon Inductive Gliding arc power source. The materialrier used for ranges the dielectric fromnonconductive bar- in polymers nature. Planar to ormay axial glass be configurations utilized depending or onbe flow ceramics, applications seen and all in can Fig. mal plasma that canand be low generated temperature that at is ambientrium. not pressure in Microdischarges total occur particle at equilib- and propagate the like dielectric a surfaces glowhomogeneity, discharge but while within maintaining ament. Originally more designed pressurized for ozone environ- find generation, DBDs use intion of catalysis materials, emissions enhancement, control, excimer lamps, surfacesilent-discharge and CO modifica- Gliding arcs arepoint arc of discharges shortest distance that between initiate parallel, but from diverg- the RF Antenna b) d) 10 kV)  RF Multipoles bias RF ganics (VOCs) removal, ECR Microwaves Helical resonator Dielectric Barrier Discharge (DBD) c) a) thermal low-pressure setting. The Townsend avalanche causes the highly energizedmove and radially in separated all electrons directions to creatingAs a the plasma cloud. electronsthe move voltage further point, and theyno further lose longer away energy able and from to eventuallyproach force are the ionization as grounded they electrode.electrons, begin The to maintains field, ap- ionization or forage cloud as of is long supplied.ozone as Corona production, a volatile volt- dischargeair or is clean-up, and a wateralytical method techniques. sanitization as for well as for an- An insulating dielectricincluding barrier the is spacetrodes that used gap, uses low-RF to the ora separate, microwave frequency field high from producing voltage elec- alternating current (AC) (1 Petrochemicals

Part D

PartD|40.1 1146 Small Scale Catalytic Syngas Production with Plasma 40.2 Partial Oxidation Reformation Using Cold Plasma 1147

a) High voltage b) Dielectric barrier Discharge gap electrode High voltage Discharge gap Dielectric barrier High AC voltage Generator electrode

Ground electrode

Ground electrode

Fig. 40.4a,b Configurations of different DBD plasma devices (apparatus similar to other plasma devices minus the dielectric barrier) (after [40.4]) ing electrodes. The arcs travel along the length of the reaches extreme temperatures (T > 10 000 K) energiz- electrodes until complete breakdown of the arc occurs ing all the chemically active species (CAS) present near the endpoints of the divergent electrodes [40.5]. in the medium. The power requirement varies from As the gliding arc dissipates, the encompassing tur- 110 kW based on the parameter conditions. Gliding bulent gas maintains a glow discharge plasma state. arc can be considered a hybrid form of thermal and This continuous cycle is determined by the system nonthermal plasma reaching only temporary equilib- parameters, which include characteristics of the elec- rium with traits similar to the plasmatron and glow trical power source (AC or DC), range of current and discharge. Some of the many applications of gliding voltage capacity, gas type, gas velocity, electrode ge- arc plasma include metallurgy, chemical reformation, ometry, and field pressure. Associated with a high level reformation of flue gas, dissociation of CO2, and de- of current ( 50 A=arc discharge), this thermal plasma contamination.

40.2 Partial Oxidation Reformation Using Cold Plasma

40.2.1 Methane Reformation  Steam methane reformation (SMR)

Industrial practices often vary in regards to reformation CH4 C H2O  3H2 C CO ı techniques as the end goal is unique to each process. Hr 298 K D 206 kJ=mol Hydrocarbon reformation can be utilized as a supple- ment to larger-scale processes by making use of utility  Water gas shift (WGS) streams and side products that would otherwise be a detriment to overall system efficiency. For instance, H2O C CO  H2 C CO2 ı when a surplus amount of steam is generated after pri- Hr 298 K D41 kJ=mol mary and secondary cooling, it can be in the engineer’s PartD|40.2 interest to make use of it. If a steady supply of natu-  Methanol synthesis ral gas is also available, then steam methane reforming (SMR) could be a source of additional revenue for 2H2 C CO  CH3OH ı a wide range of production applications, the most com- Hr 298 K D91 kJ=mol mon and basic of these being the synthesis of hydrogen or methanol. Below are the ideal reactions that accom- Being diverse in its usage and easily storable, pany the SMR process: methanol typically equates into a positive revenue mol mol = = mol = mol = mol 36 kJ mol = 891 kJ = 2 heat mol 206 kJ D = D mol C 247 kJ CO 42 kJ O 84 kJ CO = 2 heat heat 2 O D C D D 2 C 298 K C C 298 K H 2H CO ı 2 O 2CO D 172 kJ 2 r ı 2 O r 2 C C C O H 122 kJ C 298 K 298 K 298 K 2H 2 H 4 4 4 ı 2 ı ı CO 2H H  r r r D H D 298 K  2  H H ı H 2 2H CH CH CH r    C 2    2 2 2 H CO O C 298 K 2H 298 K     O O ı  ı r à 2 2O r 2 2 2 C C à à  1 2 H H C 2 2 1 1 C C 2 3H 2H 4H CO    4   H  C C C  C C C C C 2 4 4 4 CH 2 CO H CO 2CO CO CH CH CO 2CO CH Thermal decomposition Dry methane reformation CO reduction (undesired) Boudouard reaction (undesired) Methanation (undesired) Methane partial oxidation Combustion reactions Other Possible Side Reactions        ation. The methane POX reactiontions and of other macroreac- concern that are(which associated include with methane the POX below. The SMR following reactions can reactions bepetitive considered above) as com- well are as reversible listed in nature: ]. Volatility, 2 g), and gaso- g compared to = l for methanol at = = ften, realities of opera- ogen. Some presented solu- lversus16MJ = g), methanol (20 kJ = g), hydrogen is characterized by a low = mol): 11 kJ 8kJ after-treatment (thus removal of the three-way : x Hydrogen is unfortunately a difficult molecule Reformation with the goal of hydrogen production The integral step of methane reformation is the 016 g= : line (42 catalyst). Dilution also allowsfrom for improved either efficiency, reducedlower pumping in-cylinder temperature, thus losses reducing heat at trans- fer light to the load cylinder or rich walls. gas Running with further hotextends improves hydrogen- the the dilution limit. flame Fastercontributor flame speed to speed as is increased another engine wella efficiency as more by isochoric enabling combustion zone. to store fora extended high periods mass of heating time. value, Aside 120 kJ from methane (50 kJ source. It is possible tocess, deploy but SMR would as a requirea primary large-capacity high pro- equipment rate and ofcost of production generating to steam compensateefficiency. given The for SMR’s low SMR the conversion process high currently is used well at the documented commercial and method level, of but is methane only reformationchapter one and is on the partial oxidation focus reformation of techniques. this has seen real promiseplications for like small-scale fuel and cell on-handinternal technology ap- or combustion enhancement engines of of (ICEs). hydrogen The include fast advantages (which flame enables speed high-pressure and operation), highther allowing octane high ei- compressionwith ratios associated or downsizing. strong Thegas use turbocharging of addition hydrogen-rich tohighly spark-ignited diluted (SI) operation. enginesdilute-operation It allows burn is limits for possible wherereduced to nitrogen to near oxides extend nonexistent levels, are eliminating the thefor need NO Petrochemicals volumetric heating(2 value due to its low density flammability, and smallstorage molecule difficulties size of hydr alltions add to to these the containment, issues which are have some cryogenic validity but anddue are not high-pressure to ideal furtherplosion increase hazards in thatapplications. are efficiency deemed Methanol losses high has and riska been storage ex- for medium used for mobile on utilities likeically occasion fuel designed cells to as that operate are on typ- a source of hydrogen. production of synthesis gas ordrogen syngas, and a carbon mixture monoxide. of O hy- tion interfere, producing a syngas withas adulterants such carbon dioxide, methane,gen. water, Generally, oxygen these and sideand nitro- constituents are limit unfavorable overall efficiencyrequiring minimization in when possible downstream throughout oper- processing standard temperature and pressure [40.

Part D

PartD|40.2 1148 Small Scale Catalytic Syngas Production with Plasma 40.2 Partial Oxidation Reformation Using Cold Plasma 1149

Partial oxidation is an exothermic reaction capable 1 correlates to stoichiometric POX while a ratio of of producing an ideal syngas product of a 2 W 1 ratio of 0.25 gives way to stoichiometric and complete com- hydrogen to carbon monoxide. The reaction is able to bustion. Oxygen, within these stoichiometric bounds, reach temperatures in excess of 1300 ıC that need to is depleted quickly after injection due to its fast reac- be levied and maintained through use of a catalyst in tion rate and will not be present in the product stream. order to avoid nitrogen-based reactions. Nitrogen ox- Oxygen can only be present postreactor if the system ides (NOx) are considered an emission that have, by is not functioning properly or if lean operation is being orders of magnitude, a much greater environmental im- conducted. Any ratio < 0:25 is considered lean (or fuel pact than unburned hydrocarbons and carbon oxides. deficient) combustion and a ratio > 1:0 inactivates the Not only does NOx cause the formation of ground-level reaction by extending the gas mixture past its flamma- ozone, but it also has negative effects upon the human bility and reactive limits. respiratory system. The rate of formation of NOx in- In order to lower reaction temperature, eliminate creases exponentially as temperatures rise in a lean and NOx formation, and increase methane conversion, a cat- oxygen-rich flame zone during partial or total oxidation alyst is typically present inside the reactor. Currently, with air. Nitrogen, if not removed before reformation, catalytic reformation is commonplace in the industry must remain inert throughout the process. This factor and as so, it is the most economically well-developed is taken into consideration along with reactant conver- technique for hydrogen production. Typical POX cata- sion and hydrogen yield, all of which are influenced by lysts include Pt, Rh, Ni, Co, Ir, Pd, or Ru metals that thermal conditions. are supported on metal oxides, which include Al2O3, CeO2, MgO, and TiO2 in different proportions, mix- Thermal Equilibrium tures, and geometric configurations [40.3, 6]. Though Not including plasma interaction, the factors that af- difficult to fully model theoretically, the experimental fect syngas production are reactor size, space velocity, reaction pathways using catalysis are consistent and flow dynamics, catalyst type, reactor and catalyst ge- repeatable. Initially, exothermic reactions of POX and ometry, feed material and feed ratio, system pressure, combustion are activated together to provide heat to the and thermal equilibrium. Exothermic processes are con- reactor. This spike in temperature allows the intermedi- trolled not only for safety reasons, but also for system ate products of H2O, CO, and CO2 to then immediately efficiency and minimization of side products. Thermal react via the SMR and WGS reactions with unconverted equilibrium helps justify why the SMR reaction is inef- hydrocarbon as well as the hydrogen product. Due to ficient and energy intensive while also explaining POX the continuous nature of the plug flow reactor and high efficiency and NOx formation. gas space velocity, the residence time of the reactive The tubular reactor is commonly employed in in- species is short (< 1:0 s). The short residence time in dustry and most often utilized for homogeneous gas- relation to the competing exothermic and endothermic phase reactions. It is assumed that the reactants are reactions forces the process to reach completion that is consumed over the length of the reactor while experi- not necessarily preferential to the best possible results. encing some concentration variance both radially and Thermal equilibrium tends to develop in localized in the direction of flow. This forces the rate of reac- spots of the POX reactor and is contributed mostly from tion to adjust accordingly as reactants are consumed and gas flow convection, downstream or upstream radiation, products are formed that become intermediate reactants catalytic reaction, and heat conduction of the reactor themselves. In a POX reactor, the plug flow apparatus and catalyst wall [40.7]. Even through well-controlled is selected to maximize production rate. The reactor operation, the active species are always in a constant can be used with, or without, a form of packed cata- state of reactive flux. The bed temperatures along the lyst; when under a turbulent flow regime, equilibrium length of the reactor help justify these claims, which is maintained in a quasisteady state of operation creat- have been seen thorough experimentation and are well ing a relatively stable, predictable thermal profile. As documented in literature. the process approaches completion under this configu- Maximization of hydrogen yield and selectivity can ration, different zones of the reactor experience a rise be achieved through manipulation of reactor tempera- and fall in temperature due to the multitude of occur- ture. Figure 40.5 shows this relationship and provides PartD|40.2 ring reactions. This effect is more apparent in a shorter an example of thermal equilibrium influencing the amount of reactor length using a packed-bed catalyst product composition [40.7]. It is easy to see the asymp- design and even more so if the reactor diameter is re- totic increase in conversion and the perfect selectivity duced. The thermal profile and reactions that take place that is achieved as temperatures rises in the reactor. are directly proportional to the ratio of fuel and oxidizer. The syngas yield of purely two hydrogen molecules More specifically, a carbon-to-oxygen atomic ratio of to each CO molecule from a 1.0 ratio of C/O would the- 0  0 ı r 2

H H ı  r i 2 n H H 2  C Á H Á i 2 m 2 m n O  0 2 C h H 0 C < mol), the process can Á Á = 2 ı C > r 2 m 2 O m O CO ı 2 H r laced within the reactive n  0kJ à CO h H H  / n n C  C   m 2 C 2 ı 2  r 2 4 C O H CO O O n CO Á 2 n 2  n Á 2 n . H 2 n  n     C C C C m m m m ]. The heterogeneous catalytic environment H H H H 6 n n n n C C C C , a process recycle stream, or simply the exhaust The above global reactions give a straightforward Combustion Autothermal reformation (ATR) Partial oxidation Steam reformation 2 that the total netproximately enthalpy zero change ( to the system is ap- overview and arereformation the process. Reaction initial kineticslized can basis to then help for be more uti- modeling thoroughlyproperties map for any the determination intrinsic of system syngassition product compo- and yield. Temperature,play pressure, a and role phaseway all for in reactions to the follow. The homogenousreactions mechanism gas phase of determining POX the domation path- of not a inherently high-quality promote syngas;resolved this the with problem for- is the partly priate insertion supported of catalystzone a [40. p durable and appro- enhances the normal molecular pathwayon via active the sites catalyst thatfor reduce stoichiometric the POX energy and barrier lowerbustion the required heat amount necessary of for com- study, reaction the CPOX stability. of In methane on alyst a prior has rhodium-coated cata- been indirectly modeledsteam based reformation, on water gas combustion, shift, andormation dry methane (DMR) ref- kinetics. Theselisted equations, below, follow which the are Arrhenius model and have been     from an internal combustion engine: then be considered autothermal. In order tosome compensate, form of directCO injection is required using water, 1.00 0.90 0.80 0.70 0.60 0.50 1400 85) where : conversion 0 4 and CO selec-  CH 2 is necessary for ]. 8 2 Temperature (K) 1300 ]) d possible system defi- cannot initiate without the 1200 conversion with H 4 2 1100 H CO Experimental POX using Rh-coated foam 1000 Chemical Kinetics 1.00 0.95 0.85 Selectivity 0.90 The reformation process can bethalpy summarized changes by exhibited the throughout en- thereaction system. either Each imparts orapproach removes completion. heat The in POXatively an reaction effort itself slow to has kinetics rel- and assistance of energy from an externalnal source source or an such inter- asPOX combustion. is Overall, still exothermic the even process withciated the of multitude competitive of endothermic asso- reactions. In the event tivity versus temperature (after7 [40. further addition ofyield. oxygen Peak begins ratio is to unique topendency lower each on process hydrogen with reactor high structure de- assistance. and The plasma stability or ofgreatly catalysis feed to ratio thermal alsoical equilibrium contributes standpoint, and, reactorindication from temperature of provides a syngas a quality mechan- an quick ciencies. If the reactionactivation is controlled then solely product bywill selectivity thermal is limit maximum predictable,becomes product but a potential. dominant When presenceequilibrium plasma is in overcome and the the achievement system, ofgreater an efficiency thermal even is then possible [40. methanol production as well, and though mostlyas viewed a negative aspectcontrolled to the to process, assist combustionis can in introduced be reformation. to As thecrease until feed, more a hydrogen oxygen peak yield is reached tends (C/O to ratio in- Fig. 40.5 oretically be perfect formethanol downstream production, applications but like during combustion POX is making unavoidable thisble. idealization It should next be to noted, impossi- that some CO Petrochemicals monolith catalyst. CH

Part D

PartD|40.2 1150 Small Scale Catalytic Syngas Production with Plasma 40.2 Partial Oxidation Reformation Using Cold Plasma 1151 determined using empirical methods [40.9, 10]. further intent to isolate the product hydrogen. This  à happens through the most economical form of sepa- 80 : : ration possible and can be accomplished using one of R D 2:119 1010 exp p0 2 p1 3 Combustion CH4 O2 the several following techniques: pressure-swing ad- ÂRT à sorption (PSA) to separate hydrogen, PSA to separate  47:3 R D 1:0 10 9T5 exp the CO followed by condensation of the remainder of SMR RT 2  Á water, and membrane or sieve separation of the hydro-  p p  K1 p3 p gen [40.12]. With regards to the overall POX process, CH4 H2O eq,SMR H2 CO  à WGS is unavoidable and can be seen as neither bene- 7 4 32 ficial nor detrimental to downstream application. This RDMR D 7:0 10 T exp  RT Á is especially true when engine combustion or electrical  generation directly follows syngas production, as CO is  p p  K 1 p2 p2 CH4 CO2 eq,DMR H2 CO also a suitable, burnable energy source.

For the WGS reaction (r D steam/CO ratio) 40.2.2 Plasma Catalysis  Á 1 RWGS D k pCOpH2O  Keq pCO2 pH2 Low-input energy capable of producing highly active species has become the focal point of potential for de- k D 1:78 1022.1 C 0:0097r  1:1364r2/T8 HTSR  à velopment of more efficient plasma reactors. In plasma 70 catalysis, there exist a number of different configura-  exp RT tions that rely upon location of the plasma field and  : catalyst zone. If the placement of the catalyst zone is k D 1:74 1017.1 C 0:154r C 0:008r2/T 8 5 LTSR  à prior to or much further downstream of the plasma field, 35  exp then the process is a two-stage system (Fig. 40.7). Hot RT thermal plasma imparts extreme amounts of heat en- ergy that cannot be used in conjunction with catalysis Overall thermodynamic equilibrium constant (Keq) due to the structural limitations of catalytic material.  à The coating and the substrate are unable to withstand ı ı P S H N v 00v 0 the high temperatures of the plasma field resulting in K D exp  .P / jD1 j j : eq R RT atm immediate thermal degradation as well as sintering of the coating material. In the immediate area surrounding The equilibrium constant (Keq) takes into account the the thermal plasma field, the cost of processing equip- reversibility of each of the process reactions. This ment and piping material would also rise in an effort must be addressed due to the variability of conditions to resist the high temperatures using higher durability during operation. As an example, the WGS reaction equipment. This leads to applications in which thermal approaches completion at two different temperature plasma has very limited appeal because of the ineffi- ranges. The high-temperature shift (HTSR) and low- ciency from the high energy demand and the inability to temperature shift (LTSR) are performed in the ranges integrate into established chemical processes. For ther- of 300500 and 200300 ıC respectively. Lower tem- mal plasma to have a role in catalysis, ample distance peratures support better CO conversion, but the kinetics between the catalyst and plasma field is required, elim- favor an increased reaction rate at higher tempera- inating most of the potential synergy of the combined tures. The WGS reaction is always present, shifting the systems. product composition towards thermodynamic equilib- When the catalyst zone is located inside the plasma rium between CO, CO2,H2, and water. The subsequent field or in close proximity postplasma generation, the production of hydrogen-rich syngas with minimal CO process is known as a single-stage system (Fig. 40.6). content, in particular to polymer electrolyte membrane In a single-stage plasma catalysis system, the effects of (PEM) fuel cell application, is also based on WGS ki- plasma and catalysis that contribute to the process be- netics [40.11]. Industrial-scale applications looking to come dependent on each other. This differs greatly from PartD|40.2 maximize hydrogen production tend to operate both conventional catalysis due to the introduction of highly HTSR and LTSR in separate reactors in series down- active species and the creation of a voltage potential and stream of the reformation reactor. The use of both current flow by result of plasma discharge across the shift reactors as ancillary units downstream from the catalytic surface. The active species alter the status of reformer allows for near-total conversion of CO into the gas-phase reactions and associated reaction kinetics, CO2 and provides additional hydrogen from steam with which no longer ensues from the ground state. Elec-  R R 40.8). ]) and overhead 13 ) a ( Schematic drawing of (after [40. ) b ( Diffusion view Fig. 40.7a,b rotating arc reactor usedstudy in conducted a by prior Daeet Hoon al. Lee Side view stands for the reactant in a vibrational state  is the threshold energy for the formation of R T E Physical adsorption and chemisorption are the ]). The thermal energy required to initiate the catalytic and 3 vibrational excitation it hasenergy is been reduced noted in gas-phase that reactions. activation methods where gas-phase moleculescles or attach ionized to parti- Physical the adsorption is slightly surface exothermic and is ofattributed mainly the to the catalyst weak (Fig. Van der Waals attractive forces R Reaction Desorption b) a E +++ ++++++ ++++++ Adsorption High voltage High + ++ 25 mm T E + + Diffusion ++ is the activation barrier of chemisorption for * a R R ++ ++ E Illustration of vibrationally excited species from a plasma-induced state accelerating a catalytic reaction. 90 mm90 Boundary layer Catalyst surface Gas-phase a) represents the gas-phase molecule at ground state (uncharged) and process is now much less than the standard catalytic reaction and even further so than the reaction without catalyst trical field enhancement dependsangle, on and curvature, the dielectric contact constant ofHigher the packing electric pellets. fieldpromoting the leads electron-impact reactions to responsible for higherhydrocarbon reforming. electron The density decompositioncarbons of is hydro- attributed todissociation ionization of and electron-impact the excited molecules. Simply through through a plasma-induced electron-impact reaction (after [40. Fig. 40.6 Petrochemicals (charged). Part D

1152 PartD|40.2 Small Scale Catalytic Syngas Production with Plasma 40.2 Partial Oxidation Reformation Using Cold Plasma 1153

Fig. 40.8 Re- Heat transport in solid wall action effects Diffusion flowing through a catalytic Transport of momentum, Gas-phase monolith, which energy, reactions Adsorption, include transport species surface reactions, processes and Thermal desorption a microscopic radiation view at the nanoscale level (after [40.11], reproduced with permission of The Royal Soci- ety of Chemistry)

between the interactive surfaces. As the temperature generating field within the plasma head. The plasma profile surrounding the catalyst rises, the adsorption of head is a precision-designed, stainless steel structure particles steadily declines until critical temperatures are consisting of a tapered rod that acts as the anode, which obtained in which physical adsorption no longer oc- is housed inside a cylindrical wall of like material (the curs. At this point, physical adsorption gives way to the cathode). The point of largest diameter along the length sole influence of chemisorption. In the rate-influencing of the anode rod is where the plasma field initiates and step of chemisorption, valence forces create highly re- can also be considered the choke point of the flowing el- active instances via bond stretching of the exposed and ement. The axial design makes use of the annular gap to absorbed chemical species on the surface of the cata- hold steady the electrical field. This closely resembles lyst. Chemisorption produces heat comparable to the that of a dielectric barrier discharge (DBD) apparatus, exotherm of the chemical reactions being performed. but without direct internal insulation while incorporat- Atom and metastable compound formation along with ing more of a coronal discharge approach to the plasma electron multiplication cascade into the catalyst bed type utilized. In place of a fine-tipped point that is typ- manipulating the normal conditions in which the cat- ically seen in a coronal design, the annular region is alyst performs. These effects are apparent and usually used to grant a larger volume coverage that increases happen under very short electrical contact times. The the duration of electrical exposure to the gas molecules heightened interaction between the surface and adsor- ensuring sufficient ionization. The plasma chamber of bate then create new bonds, forming molecules that are the PRISM mimics the flow patterns of the schematic specific to the catalyst and operating conditions. shown in Fig. 40.9 for the rotating arc reactor, but dif- fers in dimensionality and in orientation. The radial 40.2.3 Reactor Apparatus injection points allow for a toroidal arc that further increases residence time within the plasma head appa- In a lab setting, a reactor that imparts all of the above- ratus. As concentrated charges dissipate from the added listed chemical and physical effects has been con- rotational turbulence, both metal erosion and degrada- structed. The proprietary unit, known simply as PRISM tion upon the electrodes are reduced, extending their and shown in Fig. 40.9, has demonstrated successfully overall lifetime. An observation of the anode after long- the ability to reform methane gas into syngas at high term operation reveals minor impressions of spiral-type efficiency and near perfect conversion using very lit- markings across its metal surface indicating the tangen- PartD|40.2 tle energy input. The process incorporates a three-step tial path taken by the flowing gases. approach coinciding with intermediate stages that con- The annular chamber provides uniform exposure tribute to the final composition of the produced syngas. to the incoming gas assisted by the well-mixed effect At ambient or elevated temperatures, a stream of dry of the turbulent flow regime promoting molecular in- air or pure oxygen is introduced to a stream of natural teractions and ion collisions, while the toroidal flow gas and is well mixed prior to exposure of the plasma- lengthens the space and time of the overall exposure. Fuel Fixed bed catalyst Syngas ⎩ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎧ C. This suggests all the participat- ı 0 AC : 1 ˙ PRISM reactor body and internals near T In the subsequent portion of the reactor, the diame- H bond breakage and excited electrons that are ini-   Air Plasma electrode tially released contribute toas the well as reactions to downstream intermediate the hydrocarbon final compounds syngas alsoreadily product. react in The their more remaining hydrogen-depleted configurations.first This step helpsa drive level sufficient the for effective reaction overall system rate operation. of POXter is up increased to to allow expansionlonger of the exposure to gases granting the high surfaceEven area distribution of the from catalyst. turbulentonto flow the active of sites the ofstability the particles catalyst of is the observed temperature through feed profile. composition Given at a aoperation, consistent specific ratio the during specific steady-state zonala temperatures fluctuate at Fig. 40.9 the formation of ozone moleculesto are occur. the Not first only reactions isis ozone also formed, apparent but decomposition drocarbons. in These the feed complementing streamsa stream are preferential well syngas of yielding mixed ratio hy- and priorinto to in introduction the plasmaC zone. Hydrogen atoms released from ing reactions are heldreactor constant along in the their lengthoxygen corresponding of the atoms zones. As are stated the prior, first particles to be consumed ), each in diameter. 40.2.1 00 5 is an example schematic kV compact transformer monitored through temper- C 40.9 3 kV) and very low current expend-  in length and roughly 8: 00 Thermal expansion follows quickly after the gas In this chamber, the core of the partial oxidation An external power source is placed above the re- Zonal Reactions in thickness. This refractory participates as an insu- 00 that allows forproperties additional drastically change, reserve and voltage willcally without adjust when manual automati- input control fluid to sustainarc the field. plasma stream is ionized byflow the path plasma continues field.the by From reactor there, entering that the the housesspecifically the main for proprietary portion promoting catalyst of crafted thecomposition is POX a reaction. mostly Catalyst upon nickel-based an coating alumina prepared substrate. Thesecally monolithic, sculpted cylindri- blocks ofa catalyst tightly packed remain fashion; stationaryactor these wall. in extend to the inner re- reaction takes place. This is ature readouts using multiplethe thermocouples drilled center to ofreactor each is catalyst. 22 The external body of the ing no more than 100 Wat of maximum combined processing feed power flow rate.is Voltage to variable the based plasma on head themarily defined fluid by properties, gas which density and aresystem velocity. pri- The operates PRISM with a 12 These factors allhighly energized contribute cold plasma state to necessaryormation for the the stage ref- of formation the process. of the actor providing voltage directlyare to held inside the a electrodesmaintain barrier that the of uniform ceramic/quartzthroughout three-dimensional the in annular order gap. electric The to field powerat supply high operates voltage ( The inner wall oftive the refractory reactor material is of lined high1 by alpha a alumina nonconduc- roughly Petrochemicals of an older generation model of the PRISM reactor. As listed in thethat section of occur methane throughout modeling the reactions process (Sect. lator, but more importantly ensureselectrical containment properties of that the were impartedionized to gas the stream. Figure incoming zone of thea reactor fluctuation and in catalyst primary reactionsditions bed when arise itself favorable con- during experience eachbe reaction. manipulated through This, a however,tially, well-controlled the can plasma system. zone Ini- operates asthe an ignition controlled source combustion for heatingWhen to optimal bed the temperature catalyst isfield bed. achieved, then the takes responsibility plasma forby molecular breakdown ionization and electroncompounds. dissociation Oxygen of ionization into the its atomic exposed state and

Part D

PartD|40.2 1154 Small Scale Catalytic Syngas Production with Plasma 40.2 Partial Oxidation Reformation Using Cold Plasma 1155 and do not appear downstream of the reactor. If the readings of this zone. Temperature measurement for oxygen atoms are not reduced with the carbon source a predictable profile is thus only conducted in the main into CO, then the atoms along with some CO will body of the reactor housing the fixed bed catalyst, be fully oxidized into CO2 before leaving the catalyst which is monitored via eight equally spaced thermocou- zone. The POX reaction not only produces the syngas ples. The thermocouples are placed in predrilled holes mixture, but also releases sufficient enough heat to sta- of appropriate dimensions and sealed tightly at the pro- bilize the reactor and perpetuate the overall reactionary truding ends that are connected to the wiring relay. The process. Due to the fuel-rich environment and high temperature of the catalyst bed is directly linked to the process temperature, the unconverted methane tends to exo- or endothermicity of the reactions, and also to the react with the steam side product via the SMR reac- conditions of the catalyst, plasma head activity, system tion also enhancing the syngas ratio [40.14]. The final pressure, and gas flow rate. As a comparative example stage falls to the WGS reaction and the reverse WGS that provides an accurate visual of catalyst temperature reaction, which augment the CO2 component forming distribution, this noninduced flow in a laminar to tran- either more hydrogen or more CO. The WGS reaction sitional regime produces the temperature profile shown is not optimized in this configuration and purely partic- in Fig. 40.10. ipates as an indirectly controlled and closely monitored The effect of modifying the flowrate through the side reaction. Should the syngas ratio require more CO catalyst suggests that at higher flows, the reaction en- than hydrogen or vice versa, then a downstream WGS compasses a larger volume and initiates further down- reactor can be implemented at a low additional capi- stream within the catalyst itself. The PRISM differs tal cost to the system. The combination of reactions only slightly in flow path due to its transitional-to- resulting in a net exothermic process leaves the prod- turbulent regime reducing the radial variability of key uct stream hot (> 600 ıC) requiring heat exchange, if parameters. desired, to cool the syngas as well as condense any The thermal profile shown in Fig. 40.11 represents residual water. the consistency in occurring POX reactions down the length of the catalyst. In actual operation where flow Thermal Profile rate and ratio are unaltered set points, the bed tem- With the operation of the plasma head being under perature and product composition do not deviate by a cold plasma configuration, there is not a significant more than the error present in the sensors-measurement enough generation of heat to give cause for thermal equipment. The overall temperature drop within the

Catalyst Front heat shield (rhodium coated monolith) Back heat shield (foam) (monolith)

Inlet Outlet

8 T(K) 2slpm 1200 6 4 2 700 0 035 10 15 20 25 0 Axial (mm) Fig. 40.10 Example POX catalyst PartD|40.2 Radial (mm) Radial 8 T(K) temperature profile in laminar 6slpm 1260 6 setting at differing flowrates. The 4 catalyst coating is rhodium and the hydrocarbon is iso-octane 2 600 (after [40.11], reproduced with 0 035 10 15 20 25 0permission of The Royal Society of Axial (mm) Chemistry) 55, : Exper- 57 at the : 85 C/O ratio. : 0 Fig. 40.11 imental operation of a PRISM re- actor displaying average temper- ature readings for each cata- lyst zone over increasing rates of flow. The operational ratio was adjusted only to the automatic feedback control for the temper- ature rise in the system. The initial set point of the C/O atomic ratio was 0 completion of the experiment which increased to 0  23 25 /CO product ratio. It stands to show 2 Air flow rate (scfm) Thermal Equilibrium, it was stated that Thermocouple 2 Thermocouple 3 Thermocouple 4 Thermocouple 5 Thermocouple 6 8H 21 , : 1  40.2.1 19 The PRISM is a relatively simple system requiring that improvements canhighly still effective be process, but madeability are of on not the this technology necessary in for already the field. vi- V configuration (OHV/V8)engine natural and gas, a turbocharged been 100 integrated kWe with a corresponding single PRISM alternatorcontinuous unit slipstream for have engine successful operation. a fuel valve, airtem valve, along power with source, some corresponding and instrumentation.continuous In control operation, the sys- system is stablecontrolled under apparatus a of well- a quick responseloop ratio based feedback on systemflow temperature. controllers Attached that areratio dictate mass control the and feed oxidizerbe input flow allocated based rate. to Adjustments on matic can either change set in point fuel flow. resulting The in feedback an control auto- loop scheme was operated closerratio to at a 0.55 combustion-oriented ofin C/O Sect. species. As expressed previously peak hydrogen yield is achieved at While this holds true,citing results were consistent more results than ofsion adequate and over a 99% methane conver- 17 15 ted at a facility 13 e rise and fall of zonal 11975 een conduc 1 l overhead valve/eight-cylinder : C translating to good thermal stability ı 840 820 800 780 960 940 920 900 880 860 Temperature (°C) Temperature 40.3 Cold-Plasma-Assisted Experimentation 40.3.1 Steady State Operation Many experiments have b in Brooksville, FLand through error. The research PRISM asuration system is well in ready a as for standalonestable, trial config- early-stage continuous production commercialization of for Another usable experimental-phase application clean has syngas. formed been per- for syngas-hydrogen enhancedspark-ignited combustion (SI) in engines.fuel The addition benefits during of combustionscientific are hydrogen well community, known and to thementation the difficulties have been of discussed. its Aa imple- skid that General consists Motors of 8 catalyst bed betweenmore thermocouples than 60 2 and 6 is no Petrochemicals throughout reactor operation. Th temperatures correspond directly to eachdependent other on and the are more prior so zone’s the temperature, active zone andAt temperature even (thermocouple each 2). discrete flowtation set of point, the an PRISM’sserved if temperature accurate no profile manipulation represen- of can flowshould be rate be had noted ob- been that the done. PRISM It under this experimental

Part D

PartD|40.3 1156 Small Scale Catalytic Syngas Production with Plasma 40.3 Cold-Plasma-Assisted Experimentation 1157 is responsive to changes in catalyst bed temperature, proaches a fuel-to-air ratio that is richer in operation. but focused on the main zone of operation, which is Above 900 ıC the methane will flood the system and also the highest temperature zone of the process. This be too rich for any oxidation, thus cooling the catalyst keeps reactor conditions favorable for higher hydro- bed and stabilizing the reactor conditions. At this point, gen production and safe during long term operation. the temperature of the system reaches its peak moment The control mapping has been optimized through many  1000 ıC and will no longer approach this temper- phases of testing on the PRISM unit. Manual control in- ature range again under steady operation. The system put is unnecessary when consistent homogeneous fuels is now self-sustained and very stable. Any adjustments are being used as the system will respond accordingly. to the fuel mapping at this point will only induce mi- This follows suit for the power supply, which operates nor changes in temperature. If flow is increased, then within its voltage limits to always provide the elec- the main reaction moves toward the next catalyst zone trically induced plasma field within the plasma head. followed by the controls. Given added backpressure, The simplicity of the system and stability of the reac- a gas chromatograph (GC) analysis can be processed at tor make it possible for the need of only one operator any point during this time. In order to shut the system overseeing the process. As syngas is being produced, down, simply turn off the power supply and close the it is then guided through heat exchange removing the fuel valve. A slight air peak may cause a minor com- formed water and lowering gas temperature to ambient bustion instance with any remaining fuel in the system, levels. After a final water knockout point, the syngas but quickly subsides as shown by the drop in measured is led to a low-tolerance oxygen sensor confirming the temperature. The reactor retains heat well and may re- absence of oxygen before compression and then to stor- quire roughly 3 h to return to ambient levels. age of the syngas product. Excess syngas is sent to flare In engine operation, many more transient situations during experimentation. must be considered. Startup ensues in the same manner, When added to the 8:1 l engine skid, the PRISM with normal engine startup first followed by PRISM functionality remains the same; filtered air is either ac- startup. Once the temperature change in zone 1 ap- tively fed to the reactor or naturally pulled through proaches zero, syngas is being produced and can now the reactor via the engine requirement. The controls in give way to reduced fuel consumption on the engine’s place respond accordingly to either arrangement. In this main line. Discontinuities from the piston’s compres- case, the gen-set engine is brought to steady operation sion strokes do not affect the stability of the reactor even first, followed by the PRISM reactor, which is fully in- when air is being pulled through. This setup requires troduced to the engine fuel inlet. The natural gas valve minimal manipulation to operate steadily, but additional is then slowly closed until the engine is no longer be- instrumentation would be necessary in a real-life appli- ing fed with fuel other than freshly produced syngas. cation using a motor vehicle where stops are frequent. From this point, system operation will continue until A few ideas are in place to alleviate these potential tran- shutdown. It must be noted that there is a drop in en- sient issues. The most logical approach would be to gine power return when fully supplied with only syngas, implement the lowest possible flow through the reactor which is mainly due to the excess dilution of nitrogen during engine idle. The system, though it retains heat from air being processed twice during the same system well, would fluctuate more than desired since the unit is loop. Syngas can also be introduced hot or cold, elimi- designed for continuous flow and not for on–off appli- nating the need for heat exchange if deemed unrequired. cations. If added to current motor vehicles, the benefit The main focus of this system is to eliminate NOx pro- of the hydrogen-rich syngas can be used in a cursory duction and the need for exhaust gas after-treatment. manner; this makes the PRISM advantageous in high- Using a five-gas analyzer, NOx levels have been rou- way situations. To shut the system down, fully open tinely recorded at less than 10 ppm prior to after- the engine fuel valve while shutting off the reactor fuel treatment while using slipstream syngas operation. valve and power to the unit. After this, total shutdown is as simple as turning the ignition key. 40.3.2 Transients: Startup/Shutdown 40.3.3 Conversion and Efficiency

The startup sequence for operation of the PRISM begins PartD|40.3 with simply turning on the power supply and opening In order to achieve high conversion and efficiency, op- system flow. The plasma field is on at all times and timization on a working system is vital. True optimiza- initiates the process acting as a spark to ignite a very tion requires experimentation, analysis of experiments, lean mixture of air to fuel. These gases are well mixed and adjustment thereafter from theory. This is a repeat- before introduction to the plasma head chamber. As able, stepwise process that demonstrates the versatility temperatures rise, the fuel mapping for the controls ap- of the technology and potential for performance im- GC results of PRISM operation with pure oxygen Fig. 40.12 C. Efficiency is also ı ainable and adjustable. 8.030 800 2  CO T 6.893 4 Valve noiseValve 6.676 CH 85 and : Retention time (min) time Retention 0  A number of reactions occur simultaneously along Product Measurement increased at higheroxidation temperatures. ratios suggest Thetially that assists leaner in a producing partial greater littlethrough amounts of the combustion hydrogen side ini- reactionsimmediately (mainly downstream steam in the reformation) reactor. the length ofproperties the of reactor the while plasmaThe carrying field participating the throughout reactions electrical the areidation, process. combustion, steam partial methane ox- reforming,All and of water these gas shift. reactionsit are reversible difficult in nature, tocome making from surmise so many and variables.the The predict goal outcome is the to tobe influence product accomplished the out- through endsary manipulation user’s parameters. of With needs, the aon neces- and internal feedback catalyst control this temperature loopble and can reactor based reactant conditions ratio, are sta- maint Through experimentation and analysis ofeters these at param- varied setspecific points, product a yield. database Alows is production for established early database problem for also detection and al- error analysis. The product stream isof analyzed gas via into a a small high-precision slipstream SRI 1068C gas chromato- POX by decreasing the C/O ratioity and enhances yield the of selectiv- hydrogen (peaked curve)around approximately C/O CO 4.453 2.993 2 N 2.520 2 2.16 2 2.350 O 2 H H nstills the necessary amount of 0% conversion. In an environment sup- : 0%, while thermal plasma with pure methane : 0% conversion of starting material. : Conversion weighed against the energy input to the Intensity (arb. units) (arb. Intensity is near a 12 provement. For instance, nonthermal plasma ondoes its own not provide enough energyachieve any or significant the selectivity environment of to hydrogenbon and monoxide car- production. Onplasma the other is hand, capable thermal ofgen unassisted, converting but methane the high intofor amount hydro- of thermal energy required plasmamethane generation conversion and coupled low hydrogen selectivity with isan not the economically feasible path low to anycation. commercial appli- There exists athermal plasma, point, which between i nonthermalenergy and required to fullycoupled convert with the other feed processes.exist stream Plasma in does when one not form; simply itjust is as affected any by intensive otherin properties phase its density of or matteris viscosity. would the The constant by difference fluctuation with a oftime plasma change every intervals. property Methane over short conversioning into only a hydrogen pure us- methane feedclose and to nonthermal 6 plasma is Petrochemicals system is at itsin peak conjunction when nonthermal with plasmasystem catalysis. is The provides used an active fieldcal ideal of species environment the for to the react. chemi- Deviation from stoichiometry of portive of POXnonthermal using plasma air systems anda99 are methane, capable thermal of and achieving

Part D

PartD|40.3 1158 Small Scale Catalytic Syngas Production with Plasma 40.4 Analysis and Discussion 1159

Table 40.1 Volume/mole percentage display of GC results Table 40.2 GC-MS results of producer gas composition for air and oxygen directly after PRISM system reformation PRISM syngas product – GC analysis (% by volume) Component Mole percent Weight percent Compound With air With oxygen Oxygen and argon 1:995 2:787 H2 30:7% 58:5% Nitrogen 53:749 65:723 CO 15:9% 32:2% Methane 10:668 7:471 CO2 3:3% 2:9% Hydrogen 17:789 1:565 CH4 1:3% 6:4% Carbon monoxide 11:273 13:783 O2 0:0% 0:0% Carbon dioxide 4:488 8:622 N2 48:8% 0:0% Ethylene ND ND Total 100:0% 100:0% Ethane 0:038 0:049 Acetylene ND ND graph (GC) attached with Haysep D and Moleseive Propylene ND ND 13X columns. The GC uses a thermal conductivity Propane ND ND detector (TCD) with the attachments to accurately mea- Isobutane ND ND sure the molecular content of the syngas that the unit n-Butane ND ND is receiving. An algorithm has been implemented to trans-2-Butane ND ND the GC interface based on calibration gas at various 1-Butane ND ND compositions pertaining to the compounds of interest Isobutylene ND ND cis-2-Butene ND ND being synthesized. Based on the GC configuration, ev- iso-Pentane ND ND ery compound known to be present in the gas product is n-Pentane ND ND accounted for and returned to the user in a percent vol- 1,3-Butadiene and ND ND ume composition. Refer to Table 40.1 and Fig. 40.12 methyl acetylene for actual experimental GC results. The GC is main- C5 Olefins ND ND tained frequently and recalibrated on a three-month Hexanes ND ND cycle. Third-party analysis of a process-derived fuel Heptanes plus ND ND (PDF) from municipal solid waste (MSW) reformed into syngas from producer gas has also been performed percentage of tar reduction in the gas stream. Results resulting in high levels of hydrogen yield and a large from this experiment are shown in Table 40.2.

40.4 Analysis and Discussion

40.4.1 Process Challenges The transient issue is minor and ideas are in place to remedy these negatives, such as timing control to maxi- The areas of concern that were encountered during de- mize the benefits of hydrogen supplementation. sign and operation of the PRISM process are system controls, modeling, catalyst durability, safety precau- 40.4.2 Design Improvements tions, variable feedstock, and overall scale-up. The feed- back control loop is responsive to changes in zonal re- Startup requires short-term ignition in the form of actor temperature where the fuel-to-air ratio set point a modified fuel-to-air ratio. This is viewed more as an adjusts accordingly to balance reaction heat, firstly pre- unavoidable hindrance that has a greater affect on an venting catalyst damage and secondly for safety in elim- operation with a high-frequency modulation between inating any potential runaway reaction. Though stable, transient and steady-state operation. It is safe to as- the system must always be monitored. Plasma-assisted sume that syngas is in full production from cold startup POX with catalysis provides a wide range of variables after approximately one minute; this dead time is less- that are difficult to model at the atomic level. This limits ened if system operation was recently shutdown due PartD|40.4 the depth of process understanding to broader general- to the substantial thermal mass of the system. If pos- izations using separate macrolevel modeling techniques sible, a more durable catalyst that has greater resistance and theories that are well established. The catalyst is to sulfur poisoning and thermal decomposition would susceptible to thermal deactivation, coking, carbon de- be ideal for future experimentation and commercializa- position, chemical poisoning (typically sulfur), and de- tion. This would loosen the restrictions that decrease the activation after heavy usage over long periods of time. lifespan of the catalyst bed. 0, : 0than2 : nol. Because of its low oper- to CO much closer to 1 99% at higher process flows while 2 98% range otherwise. >  and CO molecules as building blocks for the 2 It has been deemed plausible to reform all chains generally in the and types ofGiven hydrocarbons a using vapor the phasematics PRISM of like reactor. feeds benzene such in stoichiometric asoxygen proportions for heptane with POX, or a aro- productdrogen that and consists of carbon mostlyplasma hy- field monoxide imparts enough can energy andalyst be the bed heated achieved. allows cat- for The activationcould of the result POX in reaction that the near-perfect use of conversion methane. Higher comparable hydrocarbon reformation to sults re- in a ratio of H ample, average hydrogen percent bywas volume of 30% syngas withend low-end results being results 32% being by volume.also 28% Some be variance could attributed and to high- thenatural inconsistent gas composition of pipeline. the This rangeis of observed syngas for composition allduring short-term components operation as of well the astinuous month-long product con- experimentation. stream Data havethoroughly examined been to show logged stable productionvolume and of and high quality syngas. Reformation efficiencysus ver- stoichiometrically perfect conversion is80% high range in for the airoxygen operation operation. and in Methane the conversionbeen 90% efficiency recorded range has to for value in them unlessis further true for processing a isis side done. difficult product This such to handle as and dirtyPRISM glycerol, costly is which to able remove to asadditional heating make well. to these primary The processes side or for productsside a valuable product useful such for as metha ating expense, the PRISM can belooking utilized to in any maximize process itssolid hydrogen oxide potential. fuel PEM cells and thatanother run potential beneficiary directly of on this hydrogen technology. are This process hasmarket-worthy products, the allowing flexibility thetheir user to path to create of choose can profitability. be numerous taken Syngas, down any oncethe processing synthesized, H path that would use 40.5.2 Commercial Implementation product. Olefins are the raw material for most plastics which would inhibit downstream processing,could but be again alleviated by acontent. PSA unit The to reformation increase hydrogen cessfully of observed waste at has the facility. also been suc- , approximately. The 00 50 C. Not only is it a loss in hat reduces product out- ı  00 18  00 ). 40.13 2% of the average data acquired over time. For ex- The system’s flow capacity is high for the small The production of hydrogen is possible through ˙ 95% oxygen content from air in a continuous man- footprint of the apparatus itself providing overof 150 scfm high-quality syngasPRISM per unit unit. are Dimensions 15 of each limitation thus farcontrollers, is which reach the maximumbefore allowable capacity the flow reaction of rate is extended beyond theA the catalyst safe mass bed. assumption flow isduce that upwards the of PRISMwith 200 scfm is nitrogen of able displacement syngas. toput t and This pro- system coincides efficiency. Using purecylinders oxygen is from gas not aoperation, negating viable the avenue low cost for benefitsPressure long-term of the system swing process. adsorption (PSA)swing adsorption or are vacuum both viable pressure methods> of producing ner at low operating costs. Thesystems physical tend footprint of to the be large,oxygen but from nitrogen even would a provide dramatic small improve- separationments of in production efficiency and power recovery. In gasification, athrough portion tar of buildup, potential whichchained product consists molecules is of that lost higherhigh are temperatures carbon up condensable, to but 400 only at 40.5 Synergistic Benefits ofPlasma 40.5.1 Chemical Processes Longevity tests showingducted regularly system and for durability months atoperation, a are time. process Under con- efficiency stable time does and, not in diminish fact,der results over a tend previous to control improveproduct schema, yields slightly. a Un- and fluctuation conversion in isof syngas observed in the range 40.4.3 Higher Hydrocarbon Reformation Petrochemicals production, if not properly removed before downstream processing, the tars will obstruct processeventual flow shutdown. and cause These concernsby can a be plasma-induced dealtsupportive with field of partial in oxidation of a thesyngas hydrocarbons catalytic (Fig. into environment many processes, as it isvarious such methods an abundant of element. obtaining The tensive hydrogen are and energy also in- another typically compound involve that the couldIn hold destruction processes value that of onhydrogen-containing create its compounds, excess there own. side is hydrocarbons little or to no

Part D

PartD|40.5 1160 Small Scale Catalytic Syngas Production with Plasma 40.6 Conclusion 1161

new types of exhaust treatment, electric and fuel-cell powered vehicles, and alternative fuels. One outlet for plasma reforming technology, which also presents a solution to the negatives of internal combustion in transportation, is for onboard vehicular implementation. In a fully integrated system, the gen- eration of hydrogen as syngas into an ICE allows for lower combustion temperatures, better flame speed and propagation, increased sparkplug life, and decreased NOx emissions during engine operation. Onboard pro- duction of hydrogen may be attractive for reduction of cold-start emissions and minimizing engine cranking during cold start. This is beneficial for catalyst regen- eration and posttreatment as well. The robust, compact design of the PRISM unit can be used with a variety of fuels to provide a rapid response without the ne- cessity of reformed gas storage. Complete integration of the vehicle operating system with the PRISM re- actor would call for an intuitive approach to achieve Fig. 40.13 Operation and view of the ionized plasma field a seamless, well-controlled operation. This would fac- exposed to the air (precatalysis) tor in startup, shutdown, ratio control, valve timing, and additional temperature sensors, but would otherwise not and can be produced in a high-pressure, temperature- require further manipulation of either new or old stan- controlled environment when provided with the proper dard vehicle engine designs. catalyst. Methanol, ammonia, and diesel fuel are just Upon total oxidation or combustion of fuel with some of the many other products that can be made from or without syngas, large quantities of water and CO2 feedstock derived of syngas. are produced in the exhaust. The hydrogen-rich gas Decreasing emissions from motor vehicles and in- may be useful during cold start, both minimizing the creasing engine efficiency is a necessary step toward cranking, as well as potentially decreasing the hy- improving air quality and decreasing greenhouse gases. drocarbon emissions due to cold-start ignition. In an Internal combustion engines for transportation consti- exhaust gas recycle (EGR) process the dry reforming tute as one of the largest consumers of imported oil reaction plays a major role in altering the stoichiome- and are also a major source of ozone-eliminating gases try of the system. The main goal of EGR is to lower such as NOx. A variety of potential improvements are peak combustion temperature in the piston chamber. presently being investigated: lean-burn engines, engines This provides a smoother cycle of operation reducing designed with increased compression ratios, improved NOx and ensuring maximum conversion while boosting catalyst formulations, use of close-coupled catalysts, thermal-to-mechanical efficiency.

40.6 Conclusion

Very few configurations exist, outside highly special- POX has great potential to address many environ- ized areas of use, which allow thermal plasma to be mental concerns while maintaining profitability for the implemented in a profit-worthy system. A low en- end user. In its present design, PRISM is a viable ergy, nonthermal plasma creates a better source for tool for implementation into small- to middle-scale utilization of resources, providing efficient production syngas production plants. Expending roughly 100 W of high quality syngas. Nonthermal plasma also re- of energy in a robust system configuration built for PartD|40.6 quires assistance from catalysis in the form of a tai- extended operation, nonthermal single-stage plasma lored reaction as opposed to simple decomposition. catalysis has proven to be a viable technology for ref- If unassisted, gaseous carbon and hydrogen atoms in ormation. Full integration of the PRISM reactor into a heated environment will preferentially recombine future commercial endeavors is possible with only into methane instead of separating into hydrogen and minor adjustments to the current benchmark opera- carbon monoxide molecules. Plasma-assisted catalytic tion. , 158 , 12808– 35 , 48–82 (2012) , 10967–10976 24 35 , 291–302 (2010) 35 , 13643–13648 (2013) 38 10.103g/9781849734776-00048 , 17–28 (2013) of methane catalyticeffects partial of oxidation varied under reynoldsnumber, number Int. the J. Hydrog.and Damköhler Energ. partial oxidation of methane inInt. plasma J. reforming, Hydrog. Energ. els at high-temperatures, Catalysis duction from methanecatalytic under partial oxidation, the water gasand interaction shift heat recovery, of Int. reaction J. Hydrog. Energ. (2010) oxidation of methane to6 syngas, Open Catalysis J. doi: Interaction of heterogeneouskinetics and with homogeneous massreforming and of heat logistic transfer796–808 fuels, in (2011) Combust. catalytic Flame controlled chemistrymethane, in Int. J. plasma Hydrog. Energ. reforming of 12820 (2010) 40.9 W.H. Chen, T.W. Chiu, C.I. Hung: Hysteresis loops 40.8 S. Jo, D. Lee, Y. Song: Effect gasof temperature on 40.11 O. Deutschmann: Catalytic reforming of logistic fu- 40.10 W.H. Chen, T.W. Chiu, C.I. Hung: Hydrogen pro- 40.14 S.A. Al-Sayari: Recent developments in the partial 40.12 L. Maier, M. Hartmann, S. Tischer, D. Deutschmann: 40.13 D.H. Lee, K.T. Kim, M.S. Cha, Y.H. Song: Plasma- contributors whose works were incorporatedas and an cited influencelike to to the thank Shiva completion Wilson andEnergy of Nat Tech, Mundy LLC this of and Freedom chapter. Mr.efforts I’d and Larry dedication Bell to the for company as theirtheir well tireless as wisdom lending and expertiseopportunity. Freedom to Energy Tech, the LLC technology has thesive and exclu- license the on the PRISMalso Plasma like technology. I to would acknowledge Chetman Staron of and TopLine Automotive Mike Engineering, Wirth- Inc.I would Finally, like to acknowledge the Chemicalical and Engineering Biomed- Department of theFlorida University of for South the thorough educationevery provided student to involved me with and thegoes department. to The the credit well-establishedkind curriculum faculty devoted and to one-of-a- the wellbeingand of to each the individual engineering society as a whole. , 32 cations to , 5012–5019 Principles of 33 (Appl. Catalysis B 85, 1–9 Yu. Chao, M.B. Chang, H. Chen: This chapter was written with (6), 1301–1310 (1994) 66 7(C4), 47–66 (1997) rier discharges, principle and applications, J.IV Phys. Review of plasmaforming catalysis for on hydrogenintegration hydrocarbon and production prospects re- –2008) Interaction, engineering and environment control,Chem. Pure Appl. 2848–2867 (2007) production via catalytic partial oxidation reforming of liquid fuels, Int. J. Hydrog. Energ. (2008) modeling ofrhodium partial in oxidationSymp. a (Int.) of on short Combustion/The methanetute, Combustion contact Naples Insti- on (1998) time pp. 2283–2291 reactor, 27th Plasma Discharges(Wiley-Interscience, Hoboken 2005) and MaterialsR. Processing Mekemeijer, L. Fulcheri,ative G. Petitpas: study A of compar- forming non-thermal technologies, Int. plasma J. assisted Hydrogen re- Energ. 40.4 U. Kogelschatz, B. Eliasson, W. Egli: Dielectric-bar- 40.5 A. Czernichowski: Gliding arc – Appli 40.3 H.M. Lee, S.H. Chen: 40.6 P.K. Cheekatamarla, C.M. Finnerty: Synthesis gas 40.7 O. Deutschmann, L. Schmidt: Two-dimensional 40.1 M.A. Lieberman, A.J. Lichtenberg: the assistance of Dr.Michael Carpenter Leslie of RTI Bromberg International. from Leslie40 has MIT years over and ofof plasma and which reformation wasing experience spent most students, in and writingdocuments conducting for a research, the vast educat- field numberengineering. of Michael of plasma has published science degrees andchemistry in plasma with both physics a and focushas participated on first-hand nanoscience inPRISM at key system NCSU. developments and He successful ofof downstream the our processing fresh syngas product. His insightable has been towards invalu- the growthlike of our to technology. We thank would contributions Leslie to our and research andA Michael kind to thank for this you book to their alland chapter. previous time studies plasma on and technology, reformation especially to the authors and Acknowledgments. References 40.2 J.D. Rollier, A. Darmon, D. Gonzalez-Aquilar, Petrochemicals

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