97/01785 Efficiency of combustion of a specially formulated coal-water froth

Abstract

This work is part of a EU-funded project devoted to the technological development of a specially formulated coal froth and to its use as a fuel for fluidized bed combustion (FBC). In particular, this work is centered on the comparison of combustion performance of this froth from the results obtained with different fluidized combustors; in addition, a critical discussion of factors affecting coal dispersion into the bed and carbon availability for combustion is provided. An experimental program was conceived on the basis of a common fuel, i.e. a low-sulfur, South African bituminous coal. This coal was used as a powder with a maximum size of about 1 mm in the formulation of the froth, i.e. a pressurized coal-water-air mixture (PC-WAM). Further, it was used as a 1--3 mm dry charge for some reference tests. Three different bubbling bed units were used: a pilot-scale, 1,200 mm ID, pressurized combustor at ENEL/CRT (Livorno, I); a pilot-scale, 600 mm ID, atmospheric combustor at CERCHAR (Mazingarbe, F); and a pre-pilot-scale, 140 mm ID, atmospheric facility at CNR/IRC (Napoli, I). The first outcome of the experimental program is that PCWAM can be prepared on site and fed to combustors successfully. In absence of the recyclemore » of fines, combustion efficiency attained values close to 90%. These figures could be improved by properly enhancing post-combustion in the freeboard. PC-WAM injection into the bed produced the same carbon phases that were a characteristic of coal-water slurry combustion: aggregates of relatively large size, tiny carbon deposits on bed particles and char fines. The selectivity toward aggregates was low, whereas formation rates of the other two phases were large and comparable between themselves. The combustion process of fixed carbon, that derived from drying and devolatilization of PC-WAM, was mainly sustained by fines caught by bed particles or flying through the dense phase.« less