2. IN A FIXED-BED, REGENERATIVE, PYROLYTIC, HYDROCARBON GAS CONVERSION PROCESS WHERE, IN A PROCESSING ZONE, A COOL GASIFORM HYDROCARBON REACTANT AT A TEMPERATURE SUBSTANTIALLY BELOW REACTION TEMPERATURE IS INITIALLY CONTACTED FOR ABOUT 0.001-0.1 SECOND IN A REACTION ZONE COMPRISING A HOT, AGGREGATE MASS OF GRANULAR, REFRACTORY, HEAT-TRANSFER PARTICLES HEATED TO A REACTION TEMPERATURE OF ABOUT 1000*-3500*F, TO INDUCE C--C SCISSION AND C--H SCISSION IN SAID HYDROCARBON REACTANT TO PRODUCE A REACTION EFFLUENT CONTAINING LOW MOLECULAR WEIGHT UNSATURATED HYDROCARBONS WHICH IS IMMEDIATELY THEREAFTER CONTACTED IN A QUENCHING ZONE COMPRISING A FIRST, COOL, AGGREGATE MASS OF GRANULAR, REFRACTORY, HEAT-TRANSFER PARTICLES MAINTAINED AT A TEMPERATURE SUBSTANTIALLY BELOW SAID REACTION TEMPERATURE SUPERPOSED ON SAID REACTION ZONE IN RELATION TO THE FLOW OF REACTANTS THROUGH SAID REACTION ZONE WHEREBY THE TEMPERATURE OF SAID REACTION EFFLUENT IS RAPIDLY REDUCED BELOW THE TEMPERATURE AT WHICH UNDESIRABLE SIDEREACTIONS OCCUR, THE INITIAL EXTENT OF SAID REACTION ZONE AND SAID QUENCHING ZONE BEING SUFFICIENT TO EFFECTIVELY CARRY OUT SAID GAS CONVERSION PROCESS, PASSING SAID REACTANT THROUGH SAID HOT, AGGREGATE ZONE UNTIL THE INTERFACE BETWEEN SAID REACTION ZONE AND QUENCHING ZONE IS POSITIONED SO AS TO AFFORD A QUENCHING ZONE OF MINIMUM EFFICIENCY AND THE TEMPERATURE OF SAID REACTION ZONE HAS DECREASED TO A MINIMUM CONVERSION LEVEL, THEREBY PRODUCING A COOL ZONE COMPRISING A SECOND, COOL, AGGREGATE MASS OF GRANULAR, REFRACTORY HEAT-TRANSFER PARTICLES ADADJACENT SAID REACTION ZONE IN SAID PROCESSING ZONE, SAID PROCESSING ZONE COMPRISING A UNITARY, AGGREGATE MASS OF SAID HEAT-TRANSFER PARTICLES, THE PROCESS FOR REHEATING AND REPOSITIONING SAID REACTION ZONE IN SAID PROCESSING ZONE WHICH COMPRISES TERMINATING THE FLOW OF SAID REACTANT WHEN THE TEMPERATURE OF SAID REACTION ZONE IS REDUCED TO ABOUT MINIMUM CONVERSION LEVEL, PASSING A HEAT-TRANSFER GAS DOWNWARDLY THROUGH SAID PROCESSING ZONE INTO A HEAT-RESERVOIR VESSEL CONTAINING A REFRACTORY MASS OF A HEAT-TRANSFER MATERIAL, SAID REFRACTORY MASS OF A HEATTRANSFER MATERIAL AT THE TIME OF INITIAL CONTACT WITH SAID HEAT-TRANSFER GAS BEING SUBSTANTIALLY COOLER THAN SAID REACTION ZONE, CONTINUING THE FLOW OF SAID INERT GAS UNTIL THE COOL ZONE ADJACENT SAID REACTION ZONE AND A PORTION OF SAID REACTION ZONE HAS BEEN DISPLACED FROM SAID PROCESSING ZONE AND AT LEAST A PORTION OF THE REFRACTORY MASS IN SAID HEAT-RESERVOIR VESSEL HAS BEEN HEATED TO AN ELEVATED TEMPERATURE, SUBSTANTIALLY THE SAME AS THE TEMPERATURE OF THE REACTION ZONE, TERMINATING THE FLOW OF SAID INERT GAS, PASSING A COOL COMPONENT OF A COMBUSTIBLE ADMIXTURE THROUGH THE HEAT-TRANSFER ZONE, IN DIRECT HEAT EXCHANGE WITH THE HEATED, REFRACTORY MASS THEREIN, ADMIXING THE THUS-HEATED COMPONENT OF THE COMBUSTIBLE MIXTURE WITH ANOTHER COMPONENT TO PROVIDE A COMBUSTIBLE MIXTURE, IGNITING SAID MIXTURE IN A COMBUSTION ZONE TO PRODUCE HEATED FLUE GASES AT ABOUT SAID ELEVATED REACTION TEMPERATURE, PASSING THE HEATED FLUE GASES RESULTING FROM THE COMBUSTION OF SAID MIXTURE THROUGH SAID PROCESSING ZONE UNTIL THE INTERFACE BETWEEN SAID QUENCHING ZONE AND SAID REACTION ZONE IS SUBSTANTIALLY IN THE INITIAL POSITION TERMINATING THE FLOW OF SAID FLUE GASES, AND REINTRODUCING SAID HYDROCARBON REACTANT INTO SAID REACTION ZONE.