Investigation of ammonia formation during gasification in an air-blown spouted bed: Reactor design and initial tests

A laboratory scale, pressurized gasifier has been developed to simulate the operating conditions of air-blown spouted bed gasifiers. The new 28 mm i.d. reactor operates with a continuous coal feed of typically 3 g min(-1), at temperatures up to 980 degreesC and pressures up to 20 bar(a). Fluidizing gases can be combinations of air, steam, and nitrogen. The gases and coal enter the gasifier through the inverted conical base of the reactor, as a high velocity jet, which induces the rapid mixing of the fresh coal with the bed char. At the pilot scale this mode of operation had been found to avoid the operating difficulties associated with the agglomeration of sticky coal particles. Gases within the reactor may be sampled from the spout jet (using a specially designed water-cooled probe) and from the exit of the reactor. In the present study, the reactor has been used to study NH3 formation under conditions relevant to those of the pilot-scale gasifier that was used during the process development stage of the Air-Blown Gasification Cycle. The pilot-scale work had found high, variable, and at times difficult-to-predict concentrations of NH3 in the fuel gas. The present stage of the work was aimed to help understand the pilot-scale data by focusing on the effect of operating conditions on NH3 formation reactions. The gasifier has been shown to run with continuous feed for periods of up to 30 min, before sinter builds up on the base and prevents normal operation. Within this time data are collected on the spout and exit gas compositions. In this paper we report preliminary findings, which suggest that the NH3 is produced during pyrolysis and by the effect of steam on the char-N. The latter is the most important reaction and the mechanism is at present unclear. High concentrations of NH3 appear to form through the latter mechanism and to vary with operating conditions, temperature, steam content, presence of sorbent, and coal:air ratio.