production of fuel gas which generates little nitrogen oxides on combustion , has a prolonged combustion period for unit volume , and presents a high combustion temperature . alcohol or other liquid fuel is burnt in a fuel layer 20 in a combustion chamber 18 to generate a primary fuel gas , which is mixed in a gas pipe 14 with pressurized air ejected by an air nozzle 16 , where spiral flow of air is formed around the ejected air . the ratio of the cross section of the primary fuel gas flow to that of air ejected from the air nozzle 16 is first decreased and then increased , so that vortices are formed in the area where the cross section changes to react the primary fuel gas with air and decompose hydrocarbons in the primary fuel gas into carbon and hydrogen with high reactivity , thus producing a fuel gas capable of high temperature combustion with low amount of nitrogen oxides generated .