Rikard Gebart

THE INFLUENCE OF AIR DISTRIBUTION RATE ON PARTICLE EMISSIONS IN FIXED BED COMBUSTION OF BIOMASS

ABSTRACT Combustion of biomass under fixed-bed conditions will generate both coarse and fine particles that have a negative effect on technical performance or pose health hazards. It is therefore important to reduce the emissions of these particles that are already in the combustion process. The aim of this study was to experimentally investigate how different air supply strategies affect the particle emission in fixed-bed combustion of biomass. The air was supplied either through the grate, through a secondary air register, or equally divided between the two. The results showed that the air supply affects the emissions of both coarse and especially fine fly ash particles. The emissions of fine particles decrease when the air supply through the grate decreases, probably due to lower oxygen concentration in the fuel bed and thereby lower temperature in the burning char particles, which results in less vaporisation of ash elements. Hence, changing or optimizing the air supply strategy appears to be an attractive way to reduce the particle emissions already in the combustion process.

Estimation of numerical accuracy for the flow field in a draft tube

The potential for overall efficiency improvements of modern hydro power turbines is a few percent. A significant part of the losses occurs in the draft tube. To improve the efficiency by analysing the flow in the draft tube, it is therefore necessary to do this accurately, i.e. one must know how large the iterative and the grid errors are. This was done by comparing three different methods to estimate errors. Four grids (122,976 to 4,592 cells) and two numerical schemes (hybrid differencing and CCCT) were used in the comparison. To assess the iterative error, the convergence history and the final value of the residuals were used. The grid error estimates were based on Richardson extrapolation and least square curve fitting. Using these methods we could, apart from estimate the error, also calculate the apparent order of the numerical schemes. The effects of using double or single precision and changing the under relaxation factors were also investigated. To check the grid error the pressure recovery factor was used. The iterative error based on the pressure recovery factor was very small for all grids (of the order 10–4 percent for the CCCT scheme and 10–10percent for the hybrid scheme). The grid error was about 10 percent for the finest grid and the apparent order of the numerical schemes were 1.6 for CCCT (formally second order) and 1.4 for hybrid differencing (formally first order). The conclusion is that there are several methods available that can be used in practical simulations to estimate numerical errors and that in this particular case, the errors were too large. The methods for estimating the errors also allowed us to compute the necessary grid size for a target value of the grid error. For a target value of 1 percent, the necessary grid size for this case was computed to 2 million cells.

THE INFLUENCE OF FUEL TYPE ON PARTICLE EMISSIONS IN COMBUSTION OF BIOMASS PELLETS

ABSTRACT Three different biomass fuels (bark pellets, wood pellets and granulates made from hydrolysis residues) were burned under identical conditions to determine the effect of biomass type on the amount and composition of the combustion-generated particles under fixed-bed conditions. Significant differences in emissions of dust, submicron particles, and the shape of the particle number and mass size distributions were found between the different biomass fuels. For the particles that were dominated by ash elements, the particle emissions were correlated to the ash concentration in the unburned fuel. However, if the combustion condition allowed for organic particles, the “sooting” tendency of the fuel was found to become more important than the amount of ash in the fuel. Furthermore, the fuel type affects the particle emissions more than changes in reactor operating parameters.

Online Characterization of Syngas Particulates Using Aerosol Mass Spectrometry in Entrained-Flow Biomass Gasification

Entrained flow gasification is a promising technique where biomass is converted to a synthesis gas (syngas) under fuel-rich conditions. In contrast to combustion, where the fuel is converted to heat, CO2, and H2O, the syngas from gasification is rich in energetic gases such as CO and H2. These compounds (CO and H2) represent the building blocks for further catalytic synthesis to chemicals or biofuels. Impurities in the syngas, such as particulates, need to be reduced to different levels depending on the syngas application. The objective of this work was to evaluate the amount of particulates; the particle size distribution and the particle composition from entrained flow gasification of pine stem wood at different operating conditions of the gasifier. For this purpose, online time resolved measurements were performed with a soot particle aerosol mass spectrometer (SP-AMS) and a scanning mobility particle sizer (SMPS). The main advantage of SP-AMS compared to other techniques is that the particle composition (soot, PAH, organics, and ash forming elements) can be obtained with high time resolution and thus studied as a direct effect of the gasifier-operating conditions. The results suggest that syngas particulates were essentially composed of soot at these tested process temperatures in the reactor (1200–1400°C). Furthermore, the AMS analysis showed a clear correlation between the amounts of polycyclic aromatic hydrocarbons (PAH) and soot in the raw syngas. Minimization of soot and PAH yields from entrained flow gasification of wood proved to be possible by further increasing the O2 addition. Copyright 2014 American Association for Aerosol Research