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Blue water gas (sometimes referred to as blue gas) is produced in a similar manner to produce gas but allows the production of a higher heat content gas by intermittent blasting the incandescent bed with air and steam such that the overall heat balance is maintained. The products of the air blast contain the nitrogen which reduces heat content of the product gas.

The process follows the similar principles to the production of producer gas with the exception that the problem of nitrogen dilution is overcome. The feedstock bed is simultaneously blasted with air followed by steam – the air reaction is exothermic causing the bed to increase in temperature, and this is balanced by the endothermic reaction of the steam. The typical composition of the product gas is on the order of carbon dioxide 5% v/v, carbon monoxide 41% v/v, hydrogen 49%, v/v, methane 1% v/v, and nitrogen 4% v/v. If oxygen is used instead of air, the process can be continuous and coke is preferred to coal because coal can continue to devolatilize in the blow period thereby reducing process efficiency – there are also issues related to process efficiency that arise from the use of caking coal as the feedstock.

The blue water gas may be enriched by adding a carburetor in which heavy (high density, high-boiling) fuel oil is sprayed into a brick lined chamber during the blow period of the blue water gas plant with air. During the make period, the air is turned off and the oil is cracked into smaller hydrocarbon derivatives in the now-heated chamber. This produces a mixture of hydrocarbon derivatives (predominantly methane) which increases the heat content and enriches the gas product.

See also: Blue Gas.