Entrained flow gasification of biomass: soot formation and flame stability
Sammanfattning: Entrained flow gasification (EFG) is a well-proven, commercially available technology for large scale coal gasification processes, with a production of a high quality syngas (a mixture of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4) and other compounds). For biomass, the process is still under development and there are several hurdles that must be cleared before it can become commercial. In entrained flow gasification, solid fuel particles are milled to a size of a couple of hundred micrometers to ensure good heat and mass transfer with the surrounding hot gases, priorto be fed in a co-flow of oxidizer stream that can be either air or pure oxygen. The milled biomass particles have cohesive behavior and poor flowability, leading to serious challenges associated with consistent particle feeding and effective mixing. The pulverized fuel injector is a vital part of the gasification/combustion system and a well optimized fuel injector can help to promote the process efficiency by enhancing mixing, minimizing pollutant emission and fuel consumption. Biomass differs from coal not only in chemical composition (in terms of carbon, oxygen, volatile etc. contents) butalso in aerodynamic properties depending upon some factors, e.g. shape sphericity, aspect ratio, particle size, bulk density and particle cohesion force etc. One of the key challenges to implement biomass in entrained flow gasification is to ensure a good mixing of biomass particles with the oxidizer stream. A common concept is to impart swirling motion into the oxidizer stream, forming a recirculated hot gas flow that can participate in the gasification. The dispersion behavior of biomass particles in turbulentisothermal swirling flows has therefore been studied by using a two-phase particle image velocimetry technique. This technique provides simultaneous measurements of continuous (air) and disperse phase (pulverized pine particles) velocities. The results show that the addition of pulverized pine particles (with a size range of 112-160 μm) into turbulent air flow significantly affect the dispersion rate and velocity fields of thesuspending air flow in the burner near field, inducing a “blockage effect” where the air velocity is reduced along the jet core corresponding to a region of high particle concentration. It was also found that imparting swirling motion to the co-annular jet flow increased the particle dispersion due to strong centrifugal effects induced by the swirling motion. The entrained flow gasifier is operated at high temperatures to maintain high conversionand high cold gas efficiency, resulting in low tar yields, high oxygen demand and a viscous slag flow. High operating temperatures also favors soot formation that can be detrimental to the operation of the gasifier, e.g. clogging of flow passages, fouling on system components and reduced efficiency of gasification. A novel soot reduction method on the basis of forced dispersion of fuel particles has therefore been applied toa laboratory scaled entrained flow reactor. Pulverized pine particles with a size of 63- 112 μm were gasified in a sub-stoichiometric methane-air flame stabilized on a flat burner. Soot formation was measured along the reactor height in terms of volume fraction by a two-color laser extinction method. The results show that particle dispersion and inter-particle distance were enhanced by varying the flow velocity ratio between the particle carrier gas and the premixed flame. The soot volume fraction was found todecrease towards an asymptotic value with increasing inter-particle distance.There are other techniques to control particle dispersion and promote mixing, e.g. acoustic forcing or a synthetic jet flow. Both techniques induce a periodic motion to the gas phase flow that influences the motion of solid fuel particles. A synthetic jet actuator was used in both isothermal and reactive flows in a laboratory scale entrained flow reactor. It was found that the synthetic jet actuator formed local flows of dilute and dense gas particle suspensions via a convection effect induced by large scale flow structures. It was also shown that the synthetic jet actuator provided controlled particle dispersionin isothermal flows with respect to forcing amplitudes. The resulting flow field imposed significant effects on the amount of soot formed during gasification of pulverized pine particles.Acoustic forcing was applied to a 150 kW wood powder burner to excite one of thenatural system instabilities during combustion of wood powder particles. The effect of the instabilities on the flame shape and NOx formation were investigated at differentair/fuel ratios. The powder flame gave a quick response to external flow perturbations at 17 Hz showing irregular wobbling and increased NOx emission in the presence of acoustic excitation. Based on the experiences gained from the experiments, dispersion characteristics of particle-laden flow are of utmost importance to reliably predict and optimize pollutant emission. Controlled particle dispersion can be simply achieved by external forcing ofthe gas flow by a synthetic jet actuator without any need of a source of external fluid or time-consuming, expensive burner modifications.
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