2 Experimental setup

2.1 Description of facilities and typical experimental conditions

The analysis performed in this book was attained from two different-sized bubbling fluidized bed biomass gasifiers: a 75 kWth gasifier located at the University of Aveiro [1] and a 250 kWth gasifier located at the Polytechnic Institute of Portalegre [2].

The installation from the University of Aveiro (Figure 2.1) integrates a pilot-scale bubbling fluidized bed reactor (75 kWth) with a reaction chamber of 2.3 m height and 0.25 m internal diameter. About 17 kg of quartz sand, with particle size range between 355 and 1000 mm, gives the bed a static height of 0.23 m. Dry atmospheric air enters the distributor plate from the bottom of the bed. The distributor is composed of 19 injectors, each with 3 holes (1.25 mm diameter) placed perpendicularly to the direction of the gas flow in the reactor, thus providing a uniform distribution of the primary air to the bottom bed of the reactor. A preheating system heats this air stream before its injection into the gasifier vessel. Then, a screw feeder placed 0.30 m above the distributor plate discharged the biomass into the reactor.

Figure 2.1: Schematic layout of the experimental gasification facility with a pilot-scale BFB reactor. Dashed line, electric circuit; continuous line, pneumatic circuit; A, primary air heating system; B, sand bed; C, bed solid level control; D, bed solid discharge; E, bed solid discharge silo; F, propane burner for preheating; G, port for visual inspection of bed surface; H, air flow meter (primary air); I, control and command unit UCC2; J, biomass feeder; M, raw gas sampling probe; N, water-cooled probe for gas sampling, pressure and temperature monitoring; O, gas exhaust; P, gas condensation unit with impingers for condensable gases (water and tars) removal; Q, gas sampling pump; R, gas condensation unit for moisture and other condensable gases removal; S, filter for particle matter/aerosol removal; T, gas flow meter; U, dry gas meter; V, computer for data acquisition from SICK analyzer; X, computer for data acquisition; Y, security exhaust pipe; Z, raw gas burner; UCD0, UCD1, electropneumatic command and gas distribution units; UCE1, electronic command unit; O2, online gas analyzer for O2; SICK, online gas analyzer for CO2, CO, CH4, and C2H4; micro-GC fusion, gas chromatograph with TCD [3].

The reactor's start-up until an operating bed temperature of around 773.1 K was done by a propane burner and preheating of the primary air. After reaching a bed temperature of around 773.1 K, the biomass feeding was started, and the gas burner and primary air preheating system were switched off. Afterward, the biomass combustion allowed the delivery of necessary heat to achieve the desired operating bed temperature. The equivalence ratio was controlled at the wanted level by adjusting the biomass feeding rate while keeping the primary air gas flow rate constant. Then, the direct gasifier was operated under autothermal and steady-state conditions without any external auxiliary heating systems. Thus, the necessary heat for the gasification process is delivered from the partial combustion of the biomass fuel in the reactor.

The fluidized bed was operated at atmospheric pressure and in a bubbling regime, with a superficial gas velocity of around 0.28-0.30 m/s (depending on the operating conditions, namely the bed temperature) and with average bed temperatures in the range of 973.15-1143.15 K. The bed temperature was maintained at the desired level by regulating the insertion of a set of eight water-cooled probes located at the bed level. In addition, nine water-cooled sampling probes placed at different heights along the reactor (two immersed in the bed and seven along the reactor freeboard) monitor the temperature and pressure inside the reactor.

Furthermore, the gasifier produced raw gas with combustible properties that supported its continuous combustion when mixed with air in an atmospheric gas burner located downstream of the gasifier; all the produced gas in the gasifier was burned continuously in the burner before being released to the atmosphere.

The operating conditions of the reactor were characterized, namely the fuel feed rate, air feed rate, equivalence ratio, temperature and pressure along the reactor, and gas composition at the exit.

Gas samples were collected and passed through a range of detectors, including paramagnetic (O2), nondispersive infrared (CO2, CO, N2O, and SO2), chemiluminescence (NO), and flame ionization (hydrocarbons). Further details concerning the experimental facility can be found in [4, 5].

The Polytechnic Institute of Portalegre unit consists of an upflow fluidized bed gasifier operating at up to 1123.15 K, under a total pressure below 1 bar, and at a maximum pellet feeding rate of 70 kg/h. Figure 2.2 displays a diagram of the biomass gasification plant used in the experiments.

Figure 2.2: Biomass gasification semi-industrial plant at Polytechnic Institute of Portalegre, Portugal. The main components of the unit are described [2].

The main components of the unit are as follows: (a) a biomass feed system with two inline storage tanks that allow discharging the biomass into the reactor using an Archimedes' screw at variable and controllable speeds - the two storage tanks act as buffers to avoid air entering through the feeding system; (b) a tubular fluidized bed reactor, 0.5 m in diameter and 2.5 m in height, internally coated with ceramic refractory materials; biomass enters the reactor at a height of 0.4 m from the base, and preheated air enters the reactor from the base through a set of diffusers, with a flow of about 70 m3/h (the schematic of the fluidized reactor is depicted in Figure 2.3); (c) a gas-cooling system consists of two heat exchangers: the first exchanger cools the syngas to about 570 K using a cocurrent air flow that enters the unit, and the second heat exchanger cools the syngas to 420 K by a forced flow of air coming from the exterior; (d) a cellulosic bag filter that allows the removal of carbon black and ash particles produced during the gasification process; filter cleaning is done with pressurized syngas injection; black carbon is collected at the bottom of the bag filter and stored in a proper tank; (e) a condenser where liquid condensates are removed by cooling the syngas to room temperature on a third tube heat exchanger. Gasification runs were performed using coffee husks, forest, and vine-pruning residues at 1063.15 and 1088.15 K.

Figure 2.3: Schematic of the mesh and the corresponding bubbling fluidized bed gasifier [2].

Syngas analysis was performed in a Varian 450-GC gas chromatograph with two thermal conductivity detectors that allow the detection of H2, CO, CO2, CH4, O2, N2, C2H6, and C2H4 (equipped, respectively, with CP81069, CP81071, CP81072, CP81073, and CP81025 Varian GC columns), using helium as carrier gas. Additional details concerning the experimental facility can be found elsewhere [6, 7].

2.2 Biomass characterization

Biomass is the only natural energy source with a lot of carbon, which can be used as a substitute for fossil fuels [8]. Part of the process of turning biomass into energy is to figure out its composition. This way, thermal analytical methods have been used extensively and fundamentally in the past few years. The promising energy production through thermochemical conversion technologies, which include pyrolysis, gasification, and combustion, depends on the thermal analysis in an actual qualification of the process [9]. Even though the elemental composition and heating value are essential for using biomass, thermal decomposition is needed to accurately figure out the properties and economic value of different samples and mixtures of biomass. The ultimate analysis or elemental analysis needs special equipment. However, the proximate analysis produces data with standard equipment like a furnace [10].

Before the actual gasification process, most biomass analyses were carried out in the Laboratory of Chemistry of the High School of Technology and Management located in Portalegre, Portugal, since biomass characteristics can provide valuable information on how the gasification process will occur. This analysis also provides crucial data to treat the implemented numerical model. The instruments used in the performed analysis are thermal gravimetric analysis (data for proximal analysis), elemental analysis (determination of biomass composition concerning the percentage of C, H, N, and O), humidity (sample moisture content assessment), and calorific value (appraisal of energy contained in biomass). As for the characterization and analysis of the Portuguese municipal solid waste (MSW) residues, these were carried out using data from the Oporto metropolitan area obtained from LIPOR, the entity responsible for the management, treatment, and recovery of MSW produced in the city. From the pretreatment of MSW conducted by LIPOR, usually via shredding and dehydration, a refuse-derived fuel containing only cellulosic and plastics is obtained...