Henrik Thunman

Combustion of wood particles—a particle model for eulerian calculations

A simplified model for the combustion of solid fuel particles is derived, relevant for particle sizes and shapes used in fluidized and fixed-bed combustors and gasifiers. The model operates with a small number of variables and treats the most essential features of the conversion of solid fuel particles, such as temperature gradients inside the particle, the release of volatiles, shrinkage, and swelling. Typical shapes (spheres, finite cylinders, and parallelepipeds) are also considered. The model treats the particle in one dimension, and to describe the conversion inside a fuel particle the model only needs the transfer of heat and mass to an element of its external surface. When modeling a large combustion system, it is a great advantage that the conversion is related to the external surface, because the model does not have to be limited to just a single particle. In fact, it can handle the conversion of a solid phase in a computational cell, where the conversion is related to surface area per unit volume, instead of the surface area of a single particle. The model divides the particle into four layers: moist (virgin) wood, dry wood, char residue, and ash. The development of these layers is computed as function of time. The model shows satisfactory agreement with measurements performed on more than 60 samples of particles of different sizes, wood species, and moisture contents. Comparison with the experiments shows that the simplifications made do not significantly influence the overall accuracy of the model. The model also demonstrates the great influence of shrinkage on the times of devolatilization and char combustion.

Ignition and propagation of a reaction front in cross-current bed combustion of wet biofuels

Grate firing is the most common way to burn bio-fuels in small-scale units. Different combustion modes are achieved depending on how fuel and primary air are introduced. In continuous systems fuel and air are usually fed in cross-current and counter-current flow. Here, combustion of wet biofuels is studied in a 31 MW reciprocating grate furnace (a cross-current how combustor), and additional experiments have been made in batch-fired pot furnaces. The fuel was forest waste with moisture content of approximately 50%. The combustion in a cross-current flow furnace is generally assumed to start by ignition on the surface of the bed, followed by a reaction front propagating from the surface down to the grate. Measurements and visual observations presented in this paper show, however, that in the case of wet fuels the ignition takes place close to the grate, followed by a reaction front propagating from the grate up to the surface of the bed. Hence, the progress of combustion in the bed is opposite to the expected one.

Co-current and counter-current fixed bed combustion of biofuel - a comparison

A generalised model is developed for combustion of solid fuel in a fixed bed on a grate. The model can be applied to co-current and counter-current combustion and treats a bed consisting of thermally large fuel particles of optional shape (spheres, finite cylinders and parallelepipeds) of any solid fuel. The result of model calculations agrees well with the measurements available in the literature, and the validity of the model is also shown to be satisfactory for the investigation of the differences between co- and counter-current combustion in a fuel bed, simulated by a fuel batch. The results show how the different phases, drying, devolatilisation and char combustion interact during conversion.

Thermal conductivity of wood—models for different stages of combustion

Abstract The effective thermal conductivity is one of the most important parameters for modelling of thermo-chemical conversion of wood. It changes both with temperature and with conversion of the wood. There have been suggestions on modelling of this problem, together with measurements, in earlier works, especially for wet and dry wood, but for char the knowledge is poor. Here, two principle models of effective thermal conductivity on the basis of the pore structure in wood are validated by a comparison with direct numerical simulation of the fibre structure. The validation leads to a more general model, both for conductivity in the perpendicular and parallel directions relative to the fibres in the wood. In addition, the model expresses the effective thermal conductivity of char, since the wood material maintains its fibre structure during conversion. The effective thermal conductivity is estimated from given values of temperature, density and moisture content of the wood. It can also be applied to pellets and chipboards.

First Experiences with the New Chalmers Gasifier

During summer 2007 a 2–6 MWth indirect gasification section was integrated into the loop of the existing 82➀2 MWth circulating fluidized bed boiler at Chalmers University. With help of a particle distributor the gasification unit is connected to the loop after the cyclone. Hot bed material entrained from the boiler is so transferred to the gasifier providing the heat for the production of a nearly nitrogen free product gas. Non-gasified char is returned together with the bed material into the boiler and converted. Biomass can be fed into both sections; the boiler and the gasifier. The gasification is separated from the boiler via two loop seals and a particle distributer, directing particles either back to the boiler or into the gasification section. For that reason the CFB boiler can be operated even after the retrofit independently, just like before, or in combined combustion/gasification mode. This possibility keeps the risk for a retrofit low. As, furthermore, the investment costs for the integration are considerably lower than standalone gasification units of that size, the retrofit is an easy way to extend the potential of a CFB Boiler towards bi- and tri-generation (heat, power, fuel) and enter new markets.

Reactor residence time analysis with CFD

The residence time of the fluid in a reactor can be analysed with at least three different computational methods: (a) Eulerian simulation of the residence time measurements; (b) solution of the Eulerian transport equation for residence time; (c) Lagrangian particle tracking. Methods (a) and (c) are compared with analytical and experimental data from a pilot lagoon for validation, and the superiority of the Eulerian approach is demonstrated. Method (b), which has been validated in earlier studies, is applied to study the flow in the secondary combustion chamber of a biomass grate furnace. An inefficiently exploited zone of the furnace is identified, and a change in operating conditions, aimed at improving the reactor utilisation, is discussed.

Modeling of the combustion front in a countercurrent fuel converter

A generalized model is developed for combustion of solid fuel in a fixed bed on a grate. The model treats a bed consisting of thermally large fuel particles of optional shapes (spheres, finite cylinders, and parallelepipeds). In the special case of countercurrent combustion, the model shows how the fuel undergoes drying, devolatilization, and char combustion simultaneously in a conversion front that progresses with constant rate from the surface of the bed to the grate. The progress of the temperature front down through the bed is caused by the combustion of char and some volatiles on, or close to, the surface of the fuel particles before the remaining gas between the particles is ignited. The results show the movement of a reaction wave through the bed. The width of this combustion front is aaout 0.05m in agreement with measurements in similar cases. The processes of drying, devolatilization, and char combustion within the front emphasize the importance of a rather detailed description of the conversion of the thermally large fuel particles used here, and prove that a continuous treatment (such as when the bed is represented by a porous medium) does not give a correct picture of the conversion front for most of the particle sizes used in fixed-bed combustion of biofuel. Propagation rate, temperature profile, and width of the conversion front obtained by the model agree well with experimental data.

Terahertz Spectroscopy for Real-Time Monitoring of Water Vapor and CO Levels in the Producer Gas From an Industrial Biomass Gasifier

In this paper, we present a study of THz transmission spectroscopy as a novel tool for the monitoring of the steam and CO contents of the raw gas from industrial biomass gasifiers. A THz gas spectrometer with a frequency range of 300-500 GHz was designed and constructed. Proof-of-principle testing was performed at laboratory conditions using mixtures of different gases at high temperatures (600-700 K). The results demonstrate the feasibility of applying strong rotational water vapor lines at 448 and 383 GHz, so as to obtain reliable online measurements of water vapor with an absolute precision of about 0.2 vol.% for the current device. CO lines were identified at 461 and 346 GHz, facilitating measurement of this gas. The gas spectrometer was integrated into an industrial gasifier and boiler, and its performance was tested in terms of online measurements of steam and CO in the hot raw gas and flue gases under real-life conditions. Considering the error intervals, the results are in complete agreement with data acquired by solving loose mass balances around the system. The onsite experiments demonstrate that THz gas spectroscopy is a promising tool for fast, robust, and reliable monitoring in industrial applications.

Behaviour Of Biomass Particles In A Large Scale (2–4MWth) Bubbling Bed Reactor

Biomass is regarded as an interesting fuel for energy-related processes owing to its renewable nature. However, the high volatile content of biomass adds a number of difficulties to the fuel conversion and process operation. In the context of fluidized bed reactors, several authors have observed that devolatilizing fuel particles tend to float on the surface of a gas-fluidized bed of finer solids. This behaviour, known as segregation, leads to undesired effects such as poor contact between volatiles and bed material. Previous investigations on segregation of gas-emitting particles in fluidized beds are conducted in small units and they are often operated at rather low gas velocities, typically between the minimum fluidization velocity (umf) and 2·umf. Therefore, it is not known to what extent such results are of relevance for industrial scale units and for higher fluidization velocities that are commonly used in large bubbling beds. In this work the behaviour of biomass particles in a large scale bubbling bed reactor is investigated. Tests were conducted at a wide range of fluidization velocities with three different bed materials of varying particle size and density. The fuel was wood pellets and the fluidization medium was steam, which makes the findings relevant for indirect gasification, chemical looping combustion (CLC) and bubbling bed combustion applications. The experiments were recorded by means of a digital video camera and the digital images were subsequently analysed qualitatively. The results show high level of segregation at fluidization velocity up to 3.5umf. Beyond this point fuel mixing was significantly enhanced by increasing fluidization velocities. At the highest fluidization velocity tested (i.e. >8umf), a maximum degree of mixing was achieved.