Thermodynamic Study of Water-Steam Plasma Pyrolysis of Medical Waste for Recovery of CO and H2

Plasma Science & Technology, VoL.7, No.6, Dec. 2005Thermodynamic Study of Water-Steam Plasma Pyrolysis ofMedical Waste for Recovery of CO and H2Huang Jianjun (黄建军)-2, Guo Wenkang (郭文康)1，Xu Ping (须平)1 Institute of Modern Physics Fudan University, Shanghai 200433, China2 Department of Applied Physics, School of Science Shenzhen University, Shenzhen 518060,ChinaAbstract This paper describes the equilibrium compositions of the typical medical waste un-der high temperature pyrolysis by a steam plasma torch using the NASA CEA2 program. Variouscomponents from selected typical medical waste were input to the program along with the treat-ment temperature from 1000 K ~4100 K. The program then performed the Gibbs free energycalculations and searched for the equilibrium composition with minimizing the total system Gibbsfree energy. The calculation results indicate that, the equilibrium composition of a system C-H-Oat C/O = 1 in the temperature range of 1400 K ~ 2000 K has demonstrated that gas compositionare CO and H2 mainly, the other components (CO2, C2H4, C2H2, CH4 etc.) is less than 1%by volume and the degree of raw material transformation is about 100%. Comparison with airplasma, the steam plasma treatment will not produce nitrogen oxides, if the materials are free ofnitrogen element.Keywords: steam plasma, gasification, Gibbs free energyPACS: 52.75H; 82. 20w; 82.30L1 Introductioncardboard and plastics. Medical staff's usually place sy-ringes and blades, known as "“sharps”in plastic or card-There are two very pressing problems faced by to-board containers to prevent injuries.day's society: waste treatment and disposal, and cleanThe medical waste stream has changed significantlysources of energy. The water-steam plasma technologyin the last few years and the plastics content of theprovides a revolutionary way of conversion of organicwaste stream has grown from 10 percent to 30 per-waste, such as medical waste, into hydrogen-rich gas.cent in this decade. Therefore, the heating value ofThis technology is a waste of high heat pyrolysis pro-the medical waste stream is much larger than before.cess powered by a water-steam plasma torch, which en-The components of typical medical waste were shownsures that all complex organic compounds are destroyedin Table 1.and gasified to convert waste to valuable products ofThe ratio between amounts of components of typicalhydrogen-rich synthesis gas, clean sources of energy.medical waste and steam are defined by the followingThe objective of this paper is to conduct a theoreticalreactionsanalysis of typical medical waste processing under theC2H4+ 2H2O→2CO + 4H2，(1)steam plasma pyrolysis. There are two common waysC3H6+3H2O-→3CO+6H2,(2)to express the chemical equilibrium. One is based onequilibrium constants, while the other one is minimiza-CgH1oO3+ H20- + 6CO + 6H2,(3)tion of the free energy. One of the disadvantages ofCH2CHCl+ 2H2O- + 2C0+ 2H2+ HCI，(4)using the former is that it is more dificult to test forCgHs+ 8H2O-→8CO + 12H2.(5)the presence of some condensed species in the reactionproducts than the latter. However, it is anticipated thatsolid carbon may be produced during the fuel reforming3 Minimization of gibbs energyprocess and deactivating the catalytic reactions. There-In this study, the equilbrium simulations involvedfore, a method based on minimization of free energy isthe use of the NASA CEA 2[1,2 program developednormally used in fuel reforming analysis. The conditionfor calculating complex chemical equilibrium composi-of the chemical equilibrium may be stated in terms oftions based on the minimization of system free energy.thermodynamic functions such as the minimization ofThe calculations required the input of thermo -dynamicentropy. Among these, minimization of the Gibbs freedata [3~7] including enthalpy and entropy for each po-energy is more favorable when the reaction temperatureand pressure are specified.中国煤化二上e inthe system. Thepro; data into the Gibbs2 Typical medical wastefreeHCNMHGrding to the followingcompositionG=H- TS,(6)Medical waste is the name given to the rubbish con-taining human and animal tissue, blood, excreta, drugs,where G,H,T and S are Gibbs free energy, enthalpy,swab, syringed, as well as non-toxic items such as paper,temperature and entropy, respectively.Huang jianjun et al. : Thermodynamic Study of Water-Steam Plasma Pyrolysis of Medical Waste for RecoveryTable 1. Typical medical wasteMedical wastewt%22 akjainjT;+E ak;On;+i=1j=1j= NG+1Cellulose18.40Polypropylene11.04NGpolyatPolyethyleneOlnT=akin;)AInn+ (2一TPolystyrene7.36^ j二1^ j=1Organic tissue3.50VC3.68.Glassbh-be+'RT(k=1,.,I),(15)Stainless steel3.68Moisture30.25_H9Total100.002 , aijT2a;△lnT =:山(j= NG + 1..,.NS)，(16)i=For a mixture of NS species the Gibbs free energy1. Nper kilogram of mixture g is given byaijnjTi +in- n)Inn+ .VS=1g= 2 ijnj,(7),Ni njHNOM(2!" RT)OmT=n-Sni+2 RI,2 (7)j=1where nj is kilogram-moles of species j per kilogram ofmixture, the chemical potential per kilogram-mole ofspecies j is defined to be)πi+.ROn;+二1 j+1j=NG+1 .(8)山=(唱))r,P,nt;'E njHj) Alnn +For ideal gas, the chemical potential μj can be writ-j=ten as心n;(H9)2]ho-h .心n;Hqu;μj=凹+ RTIn(禁)+ RTIn((保)(i=1.,m), (9)Re2pi jOInT =RT't2R2T2，(18)where 2, n,po,p, R are the chemical potential in the一aijnjS;standard state, kilogram-moles of mixture per unitE(2R)π+ 2鄂Onj+mass, pressure in the standard state, pressure, universal=1j=1j=NG+1gas constant respectively and solid or liquid the chem-e njSNS njC;ical potential μj becomes(E R )0lnn+(ZTR+j=μ (j= NG+ 1..,NS).ME njHjSSo- sThe condition for chemical equilibrium is the mini-乙R2T)OlnT=0p+n- .mization of free energy. This minimization is usuallysubject to certain constraints, such as the followingmass-balance and conversation of atoms constraints:NS njS;uj(19)Caijnj-b=0， i=,,-,l,(11)R2Twhere Cj, ho, Hg, l, NG, So, Sj, πj are molar heat :capacity' at constant pressure for standard state for(12)species or reactant j, specific enthalpy of total reac-bq=2 aijnj=b;，i=,1,tants, molar standard -state enthalpy for species j, thenumber of chemical elements, the number of gases inwhere aij, by, b; are stoichiometric coefficients, kilogramthe mixture, specific entropy of total reactants, molaratoms of element i per kilogram of species j, assignedstandard state entropy for species j, Lagrangian multi-kilogram-atoms of element i per kilogram of total reac-plier for element i divided by RT respectively.tant, assigned kilogram- atoms of element i per kilogramThe main simulation parameters are system temper-of total oxidant (k= 1) or total fuel (k = 2) respectively.ature ranging from 1000 K to 4100 K. In the calculationDefining a term G as:the speciation less than 10- 5 is neglect.G=g+ >入:(b:-b%),(13)4 Results and discussionGenaral seimlgtinn reslts fr the components of typ-ical中国煤化Iigs.1~5 under difer-where λi is Lagrangian multipliers.entE show that the targetHence the condition for the equilibrium becomes:procTYHCN M H Gig. 4 shows that thetarget products CO and H2 account for almost 82% ofδG=2 (4j+\a;)0n;+2 (b,- 6y)8λ; = 0.(14)the total and the HCl is about 17%, the amount of ad-二1i=1mixtures (CO2, C2H4, C2H2, CH4 etc.) is less than 1%The reduced Gibbs iteration equations are written [叫:by volume within the temperature range of 1400 K~3149Plasma Science & Technology, Vol.7, No.6, Dec. 2005It is essential to point out that actual system perfor-mance may or may not be predicted precisely by thesimulation results due to kinetic and mixing effects.The results of the simulation presented in this paperare close to the Startech corp.'s experimental results.Whatever, the simulation results provide valuable in-formation regarding potential chemical reactions in thesystem. The most challenging task associated with thesimulation is usually the collection of reliable thermo-dynamic data.Fig.1 Equilibrium speciation of the polyethylene reactionwith the steam5 ConclusionsThis study investigated the equilibrium compositionsf typical medical waste components under diferenttemperature using the NASA CEA2 equilibrium calcu-lation program. The simulation results have indicatedthat, the equilibrium composition of system C-H-O atC/O=1 in the temperature range of 1400 K ~ 2000 Khas demonstrated that the actual target products are .CO and H2 only, the amount of admixtures (CO2,C2H4, C2H2, CH4 etc.) is less than 1% by volumeFig.2 Equilibrium speciation of the polypropylene reac-and the degree of raw material transformation is abouttion with the steam100%. With the increasing of the temperature, whenthe temperature is over 2500 K the amount of the H2and CO decrease dramatically and the H increases. So,for increasing of the amount of the H2, CO and decreas-ing the consumption of the electric energy, the temper-ature of the hydrogen-rich synthesis gas should be con-trolled between 1300 K ~ 1500 K. Comparison with airplasma, the water-steam plasma makes none nitrogen多goxides, if the materials to be treated have no nitrogenelement in it. This study provided a revolutionary way,auexnwhich notmly dispose the waste but also convert it toFig.3 Eequilibrium speciation of the cellulose reactionthe clean energy- hydrogen-rich gas under friendly tothe environment.References1 McBride B J, Gordon S. Computer Program for Cal-culation of Complex Chemical Equilibrium Composi-tions and Applications, Cleveland, Ohio, USA, LewisResearch Center, 1994, pts. I? McBride B J, Gordon S. Computer Program for Cal-市525354045Fig.4 Equilibrium speciation of the PVC reaction with thesteam3 Chase M W, Davies C A, Downey, et al. J. Phys.Chem. Ref. Data, 1996, 25(2): 551 ~ 6034 Cox J D. Pure and Applied Chemistry, 1982, 54(6):1239~ 12505 Gurvich L V, Veyts I V, Alcock C B. Thermodynamic40Properties of Individual Substances, Fourth ed, NewYork: Hemisphere Publishing Corp., 1989. Vol.1 pts.图3 Marsh K N, Frenkel A, Gadalla M, et al. Thermody-. namic Tables. Hvdrocarbons. TX, USA, Thermody-中国煤化工: A&M university, ColFig.5 Equilibrium speciation of the polystyrene reactionYHC N M H GM A. ThermodynamieData for Fifty Reference Elements. Cleveland, Ohio,2000 K. Above 2500 K, the amount of CO and H2 willUSA, Lewis Research Center, 1993, (NASA TP-3287)reduce and H will increase. Meanwhile, the amount ofC, CH, C2H2, 0, OH and so on will increase slightly(Manuscript received 20 January 2005)(not plotted in the figures).E-mail address of Huang Jianjun: huangji@szu.edu.cn3150.