COMBUSTION OF HYDROCARBON FUELS WITHIN POROUS INERT MEDIA

There has been a recent surge of interest in the combustion of hydroca rbon fuels within porous inert media. The interest has been directed b y the needs of industry to develop high performance radiant heaters wh ile complying with increasingly stringent emissions regulations. This paper reviews the processes associated with non-catalytic combustion w ithin porous media, and describes related experimental and modeling re search. Experimental measurements in porous media are difficult becaus e of the physical limitations caused for both optical and mechanical p robes by the presence of a solid matrix. Modeling is challenging becau se of the limited knowledge of the fundamentals of the thermal, radiat ive, and fluid mechanical processes within porous media, and how these participate in the combustion process. Much of the recent interest in the field has followed the commercial availability of reticulated cer amic foams. This paper describes the structural properties of these ma terials, and their heat transfer properties including convective, cond uction, and radiative behavior. The fluid mechanics of porous material s, relating to pressure drop and turbulence flow characteristics are p resented. Flames stabilized within the matrix of a porous media have h igher burning speeds and leaner flammability limits than open flames. This is due to the internal feedback of heat from the burned gases to the unburned gases through radiation and conduction through the porous medium. This ability to operate at very lean equivalence ratios also contributes to their characteristically low NO emissions. In this pape r, we present experimental measurements which show the effects of the porous matrix on reaction rates, flammability limits, and flame stabil ization. Exhaust emissions and radiant output from porous media burner s are presented for both single-stage and multi-stage burners. The use of liquid fuels in a non-premixed mode of combustion is also discusse d. Modeling the combustion process within porous media is quite comple x because it requires coupled solution of the energy transfer and chem ical kinetics occurring locally in the medium. We present approaches o f varying sophistication which predict flame speeds, temperature and c oncentration profiles,and radiative efficiency. Copyright (C) 1996 Els evier Science Ltd.