The bench-scale gasification set-up is composed of a stainless-steel cylindrical bubbling fluidized bed gasifier (internal diameter 100 mm and height 850 mm) externally heated by means of a 6 kW electric furnace. Steam, air, enriched air and/or N2 can be used as gasification agents; they are fed from the bottom of the gasifier by percolation through a porous ceramic plate. The height of the bed in the reactor is approximately 200 mm. The feedstock can be pre-dried Rhenish lignite, pre-treated WTA lignite, and/or SRF (solid recovered fuel); it is fed continuously in the bed of the reactor by means of a screw feeder and a feeding probe. The feeding probe can be purged with a small N2 flow in order to help the fall of the feedstock and to avoid the material from clogging the tube. A porous ceramic candle is installed in the upper part of the freeboard inside the reactor; the aim of the candle is to filter the solid particulate from the product gas, that is forced to pass through the filter in order to exit the gasifier. In order to investigate the effect of increase of temperature in the freeboard, it is possible to operate small air injections in the freeboard through a steel tube of a 6 mm diameter.
Downstream the gasification reactor, the product gas flows through a series of heat exchangers in order to condense and separate the unreacted water; the flow rate and composition of the dry product gas stream are measured by means of mass flow meters and online gas analysers (IR, UV and TCD), for the volume composition measurement of H2, CO, CO2, CH4, NH3, and H2S, fed at cold gas rates of 1.5 L min-1. A slipstream of the product gas was used for tar sampling according to the technical specification CEN/TS 15439. The basic setup is shown in figure 1.
Temperatures are measured by means of three K-type thermocouples, one in the reactor bed (T1), one in the freeboard (T2) and another at the head of the candle, just at the outlet of the cavity of the filter (T3). Pressure drops through the candle (∆P1) and through the reactor (∆P2) are measured by means of pressure probes located at different points in the reactor. Pressure fluctuation signals inside the gasifier can be also acquired, with a vertical probe in the freeboard, connected to a pressure transducer (P/E); each signal is then amplified, digitally converted and stored in a PC. The frequency of data acquisition was 100 Hz, much higher than those typically observed in the gas-fluidised bed under study (less than 10 Hz); the duration of each acquisition is of 2-3 min, to ensure their repeatability and significance. Signals are processed and analysed to obtain their power spectral density functions (PSDF) by fast Fourier transform; in this way the frequency of the erupting bubbles can be estimated and the fluidization quality of the bed can be evaluated.