Possibilities of removing H 2 S from gas from gasification of biomass Ing. Pavel Machač, CSc, Dr. Ing. Vladislav Krystl, Ing. Sergej Skoblja, Ing. Petr Chalupa Institute of Chemical Technology Prague Technická 5, 166 28 Praha 6 Dejvice, Czech Republic, Department of Gas, Coke and Air protection. Pavel.Machac@vscht.cz Department of Inorganic Technology Introduction The first research of removing H 2 S from energetical gas at high temperature by means of limestone or dolomite application was in the U.S.A in the early the eighties years of last century. This method should be used instead of classical procedure which use absorption as e single step. Typical concentration level of sulfane in raw pressure generator gas is about 0,6 0,8 % vol. Typical facilities for O 2 steam gasification at pressure about 25 Bar is LURGI generator.
Introduction Applications of gas from gasification process as a fuel for FUEL CELL is researched at present. Raw gas contains tar, fly-ash, H 2 S, HCl, HF and other. In case of high temperature (800 C) FUEL CELL is calculated upon removing H 2 S to value 1 ppm (from 100 200 ppm), for HCl and HF to value 10 ppm. Using of limestone and dolomite was tested at high temperature. Chemism of system H 2 S CaO Bacic reaction is: CaO + H 2 S CaS + H 2 O (CaO + MgO) + H 2 S (CaS + MgO) + H 2 O (CaCO 3 + MgO) + H 2 S (CaS + MgO) + H 2 O + CO 2 CaO and (CaCO 3 + MgO) is calcinated limestone or dolomite. CaS is product and can be regenerated: 2CaS + 3O 2 = 2CaO + 2SO 2 CaS + CO 2 + H 2 O (g) = CaCO 3 + H 2 S (t) (t,p)
Chemical composition of limestones Limestone Comp. % Čertovy Vrchlabí Čížkovice Krušné Orlické Hejnáschody hory hory Hydčice SiO 2 0,24 1,92 14,03 3,23 2,02 6,36 Al 2 O 3 0,07 0,36 5,69 0,16 0,36 1,22 Fe 2 O 3 0,13 0,16 1,51 0,85 0,22 0,32 CaO 55,19 52,90 43,67 26,92 37,72 45,23 MgO 0,45 1,14 0,62 20,11 13,64 5,22 Na 2 O 0,04 0,01 0,02 - - 0,29 K 2 O 0,04 0,13 0,63 0,13 0.07 0,45 CO 2 43,82 42,70 32,27 44,76 45,57 39,89 H 2 O 0,50 0,90 2,33 0,70 0,35 0,50 Thermodynamic of reaction CaO or (CaCO 3 ) + H 2 S Eguilibrium of this chemisorption is positive from catching of H 2 S point of view at wide temperature range 600 to 1400 C. Eguilibrium is described by following equation: [ H log [ H 2 O] 1 = 3519,2 0,268 S] T 2 where T is absolute teperature [K] [H 2 O],[H 2 S] molar concentration Eguilibrium of system (CaCO 3 + MgO) H 2 S is positive at temperature over 800 C. From previous relation and from eguilibrium of calcinate reaction (CaCO 3 = CaO + CO 2 ) was deriveted equation: P 5280,5 log([ H 2O][ CO2 ]) = 7,253 where P is 101,3[ H 2S] T pressure [kpa]
da dn dt E K = z exp( Kinetics of CaO + H 2 S reaction For: c is H 2 S concentration of gaseous phase i x is molar quantity of reactive CaO t is time there is relation: where K is speed constant 1,2,3 RT ) dx = dt Kxc Mechanism of adsorption was studied and time decreasing of no-converted CaO N and quantity of reaction capable CaO A and quantity of blocked CaO B and quantity of product G were measured. dn dt da dt Kinetics CaO + H 2 S reaction Reaction constant K has form : E 1,2,3 R E K = z exp( 1,2,3 RT where z is exponential factor, dn is activation energy of adsorption, desorption and chemical dt reaction, is gass constant )
Konversion - time dependence of CaO <=> CaS for limestone Čertovy schody Konversion - time dependence for varied size of particles (t = 850 C)
Apparatus for mmeasurement kinetics of systém CaO + H 2 S Matematical models of process Validity of a few models for reaction heterogeneous systém solid matter gas was verificated: Model according to Borgwardt r 0 = K s S g C SO2 η where K s is speed constant where S g is specific surface of calcinate (CaO) where C SO2 is concentration of SO 2 and η is factor of efficiency β Is the so name sensitivity on sulfatation r = r 0 exp (-βx) x is conversion Empirical formula from ÚTZCHT x = 1-exp[ -(t/t 0 ) n x is conversion of CaO ] t is time t 0,n are empirical parameters depend on SO 2 concentration and size of particles
Matematical models of process C t Model according to Pigford De = R 2 τ R R 1 1 2 1 C Sv r ϑ DsC R ε rϑ ϑ R C 1 D e τ ε S v r D s thickness of reaction product radius of spherical particle concentration SO 2 at radius R equivalent diffusivity tortuousity porosity is specific surface of unit volume of particle is radius inside grain equvalent diffusivity of solid reaction components Comparison of models for limestone Čertovy schody at 800 C For evalution of reaction CaO + SO 2 models was find out that a very good results gives empirical formula and model by Borgwardt.
Conclusion Knowledges obtained by means of research of process power gas dry desulphurisation can be apply for the use of gasification gas from biomass as a feed gas in the FUEL CELL.