On the effects of firing semi-anthracite and bituminous coal under oxy-fuel firing conditions

Traditional wisdom has lead to the design of a boiler being dictated by its fuel. Typically, lignite requires a large boiler to accommodate the moisture content and ash behaviour and anthracite needs a design with a long residence time to allow for complete combustion. Thus the result is that different boiler designs are required for different fuel types. This work demonstrates that it is possible to fire under oxy-fuel firing conditions different fuels in potentially a single combustion environment. In the present work a short series of scoping tests firing Russian semi-anthracite under air and oxy-fuel firing conditions on the RWEnpower Combustion Test Facility (CTF) have been performed and result compared to firing a South African bituminous coal. An IFRF swirl burner was used. The purpose behind this work was to determine whether oxy-fuel firing offered the potential for firing a wider range of coal qualities on a swirl stabilised burner than is conventional showing that stable combustion can be achieved with semi-anthracite as with bituminous coal. In this short communication, it is shown that this is possible. Flame photographs of the Russian semi-anthracite coal fired on air and under oxy-fuel firing conditions at recycle ratios of 75%, 72% and 68% were taken. The air firing condition produced a non-luminous flame in the near burner region. For oxy-fuel firing at 75% recycle ratio, the flame is also non-luminous and more so that the air firing case. When the recycle ratio is reduced from 75% to 68% the flame becomes increasingly luminous and at 68% an intense flame was observed well anchored into the burner quarl. Radiative heat flux measurements were taken with the Russian semi-anthracite coal at 68% recycle ratio and compared to the South African bituminous coal at 68% recycle ratio and on air. In general the peak in radiative heat flux for the Russian semi-anthracite at 68% recycle ratio compared to the South African bituminous coal on air is slightly higher reflecting the effect of oxygen enrichment and the higher calorific value of the semi-anthracite. It can also be observed that the location of the peak in radiative heat flux with Russian semi-anthracite coal at 68% recycle is displaced downstream. In the near burner region, the radiation intensity is lower for the Russian semi-anthracite at 68% recycle ratio compare to South African bituminous coal at 68% recycle ratio and on air reflecting the lower (but not insignificant) intensity of combustion in this region for the Russian semi-anthracite coal.