Pilot-scale bubbling fluidized bed gasification

Gasification system

The pilot scale atmospheric BFB gasification system at University of Zaragoza, Spain consisted of a feeding system, fluidized bed reactor, gas cleaning, tar sampling, ash disposal and gas measuring equipment including with a number of pressure and temperature sensors. The detailed description of the individual segment of the gasification system can be found elsewhere in paper II. Here, only their key features are given. The entire gasification unit is illustrated by Figure 3.8. Additionally, some of the important sections of the gasification system are shown by Figure 3.9.

The pilot scale gasifier operated with air as gasifying agent and processed biomass with a throughput capacity of approximately 10 kg/h (GPT, 2015). The reactor was made of AISI 310 refractory stainless steel with a diameter and height of 0.36 m and 3.5 m respectively, stepped vertically in two sections called bed and freeboard.

Along the height of the reactor seven temperature sensors (T1-T7) were spaced

equally apart (140 mm) around the bed (T2-T6) and one each located at the

bottom (T1) and at the freeboard respectively (see figure 1, paper II).

Additionally, two pressure sensors were positioned around the bed, spaced 0.115 m apart along the reactor height. To minimize heat loss and to reduce operational risk the entire reactor was covered from top to bottom with glass wool insulation blanket of 0.3 m thickness on the reactor’s outer wall.

Air utilized for gasification was supplied to the bottom of the reactor through air nozzles and regulated by a mass flow controller operating between the range of

0.5 Nm3/h and 100 Nm3/h. Biomass and bed-material (silica-sand, avg. size: 367

m) was continuously fed to the reactor via two augers attached to the

corresponding hoppers of 40 kg each (one each for biomass and bed-material respectively).

Figure 3.8: Pilot scale gasification system at University of Zaragoza, Spain

Augers were independently driven by variable speed control motors and calibrated for a desired feeding rate before the start of a particular experiment.

The resulting raw gas from the gasification was conditioned and cleaned by a series of devices (such as cyclone, wet scrubber and char bin) located at various gas streams and solid recovery points respectively (refer figure 1, paper II). The cyclone removed the particles from the producer gas, wet scrubbers stripped condensable tars and cooled the gases while ash and char bin collected the solid residues left after the gasification tests. The solid and liquid residues obtained after each gasification operation were measured to account mass balance.

Figure 3.9: Close view of some important sections in gasification system (c) control panel for displaying operational parameters e) -GC standalone PC

storing gas data; the rest of the figures are marked)

Test protocol

Test protocol of the current gasification facility followed several operational phases identified as pre-heating, combustion and gasification. During pre-

Air preheater Gas sampling line Temperature & pressure sensors

heating, hot air from an air-preheater (Figure 3.9a) was delivered to the reactor to increase bed temperature around 450 – 500 C which was achieved approximately 3 h after start-up. As soon as the bed temperature reached to ca. 500 C, a small amount of biomass was combusted, contributing to further increase temperature to a desired ca. 800 C in about 20 to 30 min. The high temperature at this point set the gasifier in an auto-thermal regime at which gasification was initiated. Gasification was usually achieved by adjusting air flow rate (4.16 to 4.99 Nm3/h), but keeping fuel flow rate constant at 4.7 kg/h. Present gasification tests were primarily investigated for two different stints characterized with two different values of equivalence ratio, namely 0.25 and 0.30. Each stint lasted for about 3 h during which stable data in terms of gas quality were collected and analyzed. After the end of an experimental run, the solid and liquid residues were recovered and accounted for mass balance. Further, to determine the proportional distribution of char and ash, the collected solid residues were muffled as according to the standard CEN/TS 15403:2006 (CEN/TS15403, 2006) and by-differenced. The expressions used to calculate various gasification parameters such as ER, gas LHV, gas yield, CGE and CCE are given in paper II (eq. 1 through 4).

Analytical measurement

Part of the gas from the main gas stream was diverted through a sampling line (Figure 3.9b) containing options for tar measurement and gas compositional analysis. By such analyses, performance of gasification was monitored continuously. The gas passing through the main exit was mixed with propane and flared outside the gasification unit (Figure 3.9g).

Tar measurement

The tar sampling section was located close to the gas exit in the gas sampling line, which served both as tar recovery and sampling gas cleaning. The entry of the gas sampling line was kept heated by an electric heating element (ca. 400 C) to avoid tar condensation before the tar train. In the tar train, the sampling gas was first cooled through two cold condensers (~ 0.75 L each, filled with ice) from where tar was recovered and measured (Figure 3.10). Afterwards, the gas

was further cleaned by a cotton filter and then directed to the gas analysis system through a flow meter monitoring the quantity of the gas sampled. Fractions of the tar recovered from the condensers were analyzed for composition by using a GC- MS (Gas chromatography mass spectrometry) (Agilient 7890 A, USA).

Gas analysis

Gas quality, in terms of composition, was analyzed every two minutes by means of a micro-gas chromatograph (Agilient 3000A GC, Model G2801A, USA) located downstream of the gas sampling line. The GC consisted of two modules (Plot U and Molsieve 5A) which were calibrated for measuring different gas species prior experiments. Plot U determined gas components CO₂, C₂H₄, C₂H₆, C₂H₂ and H₂S while Molsieve analyzed gas species in terms of H₂, N₂, CH₄, CO and O₂ respectively. GC data were recorded in a standalone PC (Figure 3.9e) for data analysis and interpretation.

Figure 3.10: Real-time tar sampling