Processing of off-gas from waste treatment

Abstract

Waste treatment comprises heating it in a chamber to effect pyrolysis of the waste, introducing oxygen into the chamber to effect combustion of the pyrolyzed waste, and contacting off-gas from the pyrolysis and/or combustion steps with an oxidation catalyst to convert carbon monoxide and hydrocarbons in the off-gas into carbon dioxide and water and with a reduction catalyst to convert nitrous oxides to nitrogen and oxygen. Thus, domestic waste is treated in a batch process using catalytic converters to reduce the level of toxic components before off-gas reaches the atmosphere.

1 . A method for treatment of waste, comprising:
 heating waste in a chamber to effect pyrolysis of the waste;   introducing oxygen into the chamber to effect combustion of the pyrolyzed waste; and   contacting off-gas from the pyrolysis and/or combustion steps with an oxidation catalyst to convert carbon monoxide and hydrocarbons in the off-gas into carbon dioxide and water.   
     
     
         2 . A method according to  claim 1  wherein the oxidation catalyst comprises platinum, palladium or rhodium. 
     
     
         3 . A method according to  claim 1  or  2 , comprising operating the oxidation catalyst at a temperature of from 200° C. to 900° C. 
     
     
         4 . A method according to  claim 3 , comprising operating the oxidation catalyst at a temperature of from 300° C. to 800° C. 
     
     
         5 . A method according to any previous claim, wherein the oxidation catalyst is a diesel particulate filter. 
     
     
         6 . A method according to any of  claims 1 - 4 , wherein the oxidation catalyst is a 2-way catalytic converter. 
     
     
         7 . A method according to any of  claims 1 - 4 , wherein the oxidation catalyst comprises palladium on alumina. 
     
     
         8 . A method according to any previous claim, comprising contacting off-gas with a first oxidation catalyst which is a diesel particulate filter and a second catalyst which is a 2-way catalytic converter. 
     
     
         9 . A method according to any of  claims 1  to  7 , comprising contacting off-gas with a first oxidation catalyst which is a diesel particulate filter and a second catalyst which comprises palladium and alumina. 
     
     
         10 . A method according to any previous claim, further comprising contacting off-gases with a reduction catalyst to convert nitrous oxides into nitrogen and oxygen. 
     
     
         11 . A method according to  claim 10  wherein the reduction catalyst comprises platinum, palladium or rhodium. 
     
     
         12 . A method according to  claim 10  or  11  comprising operating the reduction catalyst at a temperature of 300° C.-700° C. 
     
     
         13 . A method according to any of  claims 10 - 12 , comprising operating the reduction catalyst of a temperature of from 400° C.-600° C. 
     
     
         14 . A method according to any of  claims 10 - 13 , wherein the reduction catalyst comprises rhodium. 
     
     
         15 . A method according to any of  claims 10 - 14 , wherein the reduction catalyst is a 3-way catalytic converter. 
     
     
         16 . A method according to any previous claim, comprising contacting the off-gas with the oxidation catalyst and subsequently contacting the off-gas with a reduction catalyst. 
     
     
         17 . A method according to any previous claim, comprising contacting the off-gas with a wet scrubber to remove inorganic acids and volatile metals. 
     
     
         18 . A method according to any previous claim, comprising filtering off-gas to remove ash prior to contact of off-gas with the oxidation and/or reduction catalysts. 
     
     
         19 . A method according to  claim 18 , comprising filtering off-gas so that ash remains in the chamber. 
     
     
         20 . A method according to any of  claims 1 - 17 , comprising contacting off-gas with a cyclone to remove ash prior to contact of off-gas with the oxidation and/or reduction catalysts. 
     
     
         21 . A method according to any of  claims 1 - 20 , comprising passing exhaust gas from the catalyst through a heat exchanger. 
     
     
         22 . A method according to any of  claims 1 - 21 , comprising maintaining the oxygen content of gas output to the atmosphere within the 3-10% range. 
     
     
         23 . Waste treatment apparatus, comprising:
 a chamber to receive waste;   a heater to heat waste in the chamber;   an outlet for exit of off-gas from the chamber; and   an oxidation catalyst to catalyse oxidation of carbon monoxide and hydrocarbons in the off-gas.   
     
     
         24 . Apparatus according to  claim 23 , adapted to be sealed so that waste can be heated to effect pyrolysis of the waste and comprising an inlet for air to enable combustion of the waste. 
     
     
         25 . Apparatus according to  claim 23  or  24  wherein the oxidation catalyst comprises platinum, palladium or rhodium. 
     
     
         26 . Apparatus according to any of  claims 23 - 25  wherein the oxidation catalyst is a diesel particulate filter. 
     
     
         27 . Apparatus according to any of  claims 23 - 25  wherein the oxidation catalyst is a 2-way catalytic converter. 
     
     
         28 . Apparatus according to any of  claims 23 - 25  wherein the oxidation catalyst comprises palladium and alumina. 
     
     
         29 . Apparatus according to any of  claims 23 - 28 , comprising a first oxidation catalyst which is a diesel particulate filter and a second catalyst which is a 2-way catalytic converter. 
     
     
         30 . Apparatus according to any of  claims 23 - 28 , comprising a first oxidation catalyst which is a diesel particulate filter and a second catalyst which comprises palladium and alumina. 
     
     
         31 . Apparatus according to any of  claims 23 - 30 , further comprising a reduction catalyst to catalyse reduction of nitrous oxides in the off-gas. 
     
     
         32 . Apparatus according to  claim 31  wherein the reduction catalyst comprises rhodium. 
     
     
         33 . Apparatus according to any of  claims 31 - 32  wherein the reduction catalyst is a 3-way catalytic converter. 
     
     
         34 . Apparatus according to any of  claims 23 - 33  comprising, in sequence, an oxidation catalyst for treatment of off-gas exiting the chamber and a reduction catalyst for treatment of off-gas exiting the oxidation catalyst. 
     
     
         35 . Apparatus according to any of  claims 23 - 34  comprising an ash filter between the chamber and the catalyst to prevent ash in the off-gas reaching the catalyst. 
     
     
         36 . Apparatus according to any of  claims 23 - 34 , comprising a cyclone to prevent ash entering the oxidation and/or reduction catalysts. 
     
     
         37 . Apparatus according to any of  claims 23 - 36  further comprising a heat exchanger to process exhaust gases exiting the catalyst and entering the chamber. 
     
     
         38 . Apparatus according to any of  claims 23 - 37  further comprising an oxygen monitor to monitor the oxygen content of the off-gas. 
     
     
         39 . An apparatus according to  claim 38  further comprising an input to combine air with the off-gas prior to contact with the oxidation catalyst. 
     
     
         40 . An apparatus according to  claim 39 , wherein the input of air into the off-gas prior to contact with the oxidation catalyst is controlled by the oxygen monitor such that the oxygen content of the off-gas after contact with the oxidation catalyst is within the range 3-10%. 
     
     
         41 . An apparatus according to any of  claims 23 - 40 , further comprising a thermocouple for monitoring the operating temperature of the catalyst. 
     
     
         42 . An apparatus according to  claim 41 , wherein the thermocouple is connected to the air input such that the input of air can be controlled to modulate the operating temperature of the catalyst. 
     
     
         43 . A method of waste treatment substantially as hereinbefore described with reference to the examples. 
     
     
         44 . Apparatus substantially as hereinbefore described with independent reference to each of  FIGS. 1 ,  2 ,  3 ,  4 ,  5  and  6 .