An object of the present invention is to overcome {TAIL ~ the disadvantages} of the prior art by directly contacting a non condensible gas as a feed gas with heated water, thereby humidifying the feed gas, while utilizing the heat possessed by the formaldehyde product gas as {HEAD ~ a heat source} for heating the water 
An object of the present invention is to overcome { TAIL ~ the disadvantages } of the prior art by directly contacting a non condensible gas as a feed gas with heated water , thereby humidifying the feed gas , while utilizing the heat possessed by the formaldehyde product gas as { HEAD ~ a heat source } for heating the water
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