NOVELTY - Apparatus (I) comprises: a combustion furnace (100) for generating a carbon dioxide rich flue gas, where the combustion furnace has a flue gas outlet (102); and a flue gas treatment apparatus (104) having a flue gas inlet in communication with the flue gas outlet, a treated gas outlet (108) downstream of the flue gas inlet, and a gas flow path between them. The flue gas treatment apparatus comprises: a particulate removal device; a first heater; a first catalytic converter; a second heater; a second catalytic converter; and at least a first fan.    USE - The apparatuses is useful for carbon dioxide enrichment of a greenhouse (all claimed) to enhance photosynthesis and improve the growth of greenhouse plants.    ADVANTAGE - The apparatus eliminates nitrogen oxides, carbon monoxide, toxic contaminants, sulfur dioxides and particulate matter from the flue gas, thus providing contaminants-free carbon dioxide for carbon dioxide enrichment in greenhouse.    DETAILED DESCRIPTION - Apparatus (I) comprises: a combustion furnace (100) for generating a carbon dioxide rich flue gas, where the combustion furnace has a flue gas outlet (102); and a flue gas treatment apparatus (104) having a flue gas inlet in communication with the flue gas outlet, a treated gas outlet (108) downstream of the flue gas inlet, and a gas flow path between them. The flue gas treatment apparatus comprises: a particulate removal device in the gas flow path for eliminating at least some particulate matter from the flue gas, a first heater in the gas flow path downstream of the particulate removal device, where the first heater is for heating the flue gas to a first treatment temperature, a first catalytic converter in the gas flow path downstream of the first heater for eliminating at least some carbon monoxide and sulfur dioxide from the flue gas, a second heater in the gas flow path downstream of the first catalytic converter for heating the flue gas to a second treatment temperature, a second catalytic converter in the gas flow path downstream of the second heater for eliminating at least some nitrogen oxides from the flue gas, and at least a first fan for forcing the flue gas from the flue gas inlet to the treated gas outlet. INDEPENDENT CLAIMS are also included for:    (1) a method (M1) for carbon dioxide enrichment of a greenhouse, comprising (a) combusting a hydrocarbon fuel to generate a carbon dioxide rich flue gas, (b) treating the carbon dioxide rich flue gas to eliminate at least some toxic contaminants and generate a treated carbon dioxide rich gas stream, where the treating comprises (b1) eliminating at least some particulate material from the flue gas, (b2) heating the flue gas to a first treatment temperature, (b3) using the first catalytic converter to eliminate at least some carbon monoxide and sulfur dioxide from the flue gas, (b4) heating the flue gas to a second treatment temperature, (b5) using the catalytic converter to eliminate at least some nitrogen oxides from the flue gas, and (c) feeding the treated carbon dioxide rich gas stream to a greenhouse;    (2) an apparatus (II) for carbon dioxide enrichment comprising the combustion furnace, the flue gas treatment apparatus comprising the particulate removal device, at least a first catalytic converter in the gas flow path downstream of the particulate removal device for eliminating at least some toxic contaminant gases from the flue gas, and the first fan;    (3) the flue gas treatment apparatus for treating the carbon dioxide rich flue gas; and    (4) a method (M2) for carbon dioxide enrichment of a greenhouse, comprising the steps (a) and (b) as above per se, where the treating comprises the step (b1) as above per se and (b2-1) eliminating at least some toxic contaminant gases from the flue gas by passing the flue gas through the first catalytic converter, and the step (c) as above per se.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of the apparatus for carbon dioxide enrichment.    Combustion furnace (100)    Flue gas outlet (102)    Flue gas treatment apparatus (104)    Treated gas outlet (108)    Frame (111)