Soot is an undesired by-product of entrained flow biomass gasification since it has a detrimental effect on operation of the gasifier, e.g. clogging of flow passages and system components and reduction of efficiency. This study investigated how active flow manipulation by adding synthetic jet (i.e. oscillating flow through orifice) in feeding line affects dispersion of fuel particles and soot formation. Pine sawdust was gasified at the conditions similar to pulverized burner flame, where a flat flame of methane-air sub-stoichiometric mixture supported ignition of fuel particles. A synthetic jet flow was supplied by an actuator assembly and was directed perpendicular to a vertical tube leading to the center of the flat flame burner through which pine sawdust with a size range of 63–112 μm were fed into a reactor. Quartz filter sampling and the laser extinction methods were employed to measure total soot yield and soot volume fraction, respectively. The synthetic jet actuator modulated the dispersion of the pine sawdust and broke up particle aggregates in both hot and cold gas flows through generation of large scale vortex structures in the flow. The soot yield significantly reduced from 1.52 wt.% to 0.3 wt.% when synthetic jet actuator was applied. The results indicated that the current method suppressed inception of young soot particles. The method has high potential because soot can be reduced without changing major operation parameters.

Pulverized fuel (PF) burners play a key role for the performance of PF fired gasification and combustion plants, by minimizing pollutant emission, fuel consumption and hence fuel costs. However, fuel diversity in power generation plants imposes limitations on the performance of existing PF burners, especially when burning solid fuel particles with poor flowability like biomass sawdust. In the present study, a vertically downward laminar flow was laden with biomass particles at different particle mass loading ratios, ranging from 0.47 to 2.67. The particle laden flow was forced by a synthetic jet actuator over a range of forcing amplitudes, 0.35–1.1 kPa. Pulverized pine particles with a sieve size range of 63–112 μm were used as biomass feedstock. Two-phase particle image velocimetry was applied to measure the velocity of the particles and air flow at the same time. The results showed that the synthetic jet had a large influence on the flow fields of both air and powdered pine particles, via a convective effect induced by vortex rings that propagate in the flow direction. The particle velocity, particle dispersion and hence inter-particle distance increased with increasing forcing amplitude. Moreover, particles accumulated within a specific region of the flow, based on their size. The effect on particle dispersion was more pronounced in the forced flows with low mass loading ratios

Short particle residence time in entrained flow gasifiers demands the use of pulverized fuel particles to promote mass and heat transfer, resulting high fuel conversion rate. The pulverized biomass particles have a wide range of aspect ratios which can exhibit different dispersion behavior than that of spherical particles in hot product gas flows. This results in spatial and temporal variations in temperature distribution, the composition and the concentration of syngas and soot yield. One way to control the particle dispersion is to impart a swirling motion to the carrier gas phase. This paper investigates the dispersion behavior of biomass fuel particles in swirling flows. A two-phase particle image velocimetry technique was applied to simultaneously measure particle and gas phase velocities in turbulent isothermal flows. Post-processed PIV images showed that a poly-dispersed behavior of biomass particles with a range of particle size of 112-160 μm imposed a significant impact on the air flow pattern, causing air flow decelerated in a region of high particle concentration. Moreover, the velocity field, obtained from individually tracked biomass particles showed that the swirling motion of the carrier air flow gives arise a rapid spreading of the particles

Soot creates technical challenges in entrained flow biomass gasification processes, e.g. clogging of flow passages, fouling on system components and reduced efficiency of gasification. This paper demonstrates a novel soot reduction method in a laboratory-scale entrained flow reactor by forced dispersion of biomass particles. Gasification of small biomass particles was done in a flat flame burner where a steady stream of biomass was sent. The flat flame burner was operated with a premixed sub-stoichiometric methane–air flame to simulate the conditions in an entrained flow gasifier. The dispersion of biomass particles was enhanced by varying the flow velocity ratio between particle carrier gas and the premixed flame. Primary soot particles evolved with the distance from the burner exit and the soot volume fraction was found to have a peak at a certain location. Enhanced particle separation diminished the peaks in the soot volume fraction by 35–56% depending on the particle feeding rates. The soot volume fraction was found to decrease towards an asymptotic value with increasing inter-particle distance.

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.

Swirling flows have been widely used for many years in engineering applications such as; chemical and mechanical mixing devices, separation units, spray drying technologies, turbo machinery and combustion systems. In practical combustion applications, swirl motion has been adopted to the incoming reactant flows in order to enhance the mixing of fuel and oxidizer and to improve flame stabilization and establishment, especially in regions of relatively low velocities, by recirculating hot product gas to the incoming reactants. At critical operating conditions the recirculation zone exhibit high sensitivity to flow disturbances leading to hydrodynamic instabilities. During combustion, these instabilities can interact with flame structures by modulating the rate of heat release, equivalence ratio, flame surface etc. As a result of these interactions, combustion instabilities can form in the systems. At initial state, combustion instabilities can stay unnoticed due to their relatively small amplitudes, but the amplitudes of the instabilities can increase when they couple with acoustical characteristics of any particular system elements. As a result, combustion systems can suffer from high amplitude noise, vibrations, flame flashback, local flame quenching, and even severe damages in system structures. This thesis provides insights into the interaction of acoustic waves with a swirl stabilized wood powder flame and its effects on flame structures. A high speed photography technique has been applied to wood powder flame under external forcing of the secondary air flow pattern to record spontaneous emission of radiant energy from the flame. Simultaneously, dynamic pressure signals were acquired with a data acquisition board in order to relate pressure data with radiant energy which has been assumed to be representative of heat release. In order to investigate the influence of the interactions on combustion, the resulting data were complemented with gas sampling measurements. From digital still images taken without external forcing, the wood powder flame was observed to expand to occupy the entire combustion chamber. In addition, the flame shape and size appears to be unchanged under a wide range of forcing frequencies, with one exception at a particular low frequency for which a resonant behaviour was observed. The critical frequency was 17 Hz independent of amplitude of the forcing frequency and at this forcing frequency dramatic changes in flame size and shape was observed. Instantaneous and phase averaged images have revealed the presence of large scale vortical structures that closely interacted with the flame surface. A fast Fourier transform of the point wise optical signal also shows that the flame is susceptible to instabilities at the acoustical forcing of 17 Hz. The existence of thermo-acoustically induced combustion instability has been investigated by a Rayleigh criterion which states that the amplitude of a sound wave will be amplified when heat is added less than 90 degrees out of phase with its pressure. In this study, the heat release extracted from high speed images recorded at 17 Hz is approximately 40 degrees out of phase with the pressure data which confirms the thermo-acoustic nature of the instability. Finally, from gas sampling measurements it was concluded that the acoustic oscillations at 17 Hz have increased the NOx emission level to around twice the level without forcing.