Truls Liliedahl

Experimental determination of bed agglomeration tendencies of some common agricultural residues in fluidized bed combustion and gasification

Abstract Ever increasing energy demand and the polluting nature of existing fossil fuel energy sources demonstrate the need for other non-polluting and renewable sources of energy. The agricultural residues available in abundance in many countries can be used for power generation. The fluidized bed technology seems to be suitable for converting a wide range of agricultural residues into energy, due to its inherent advantages of fuel flexibility, low operating temperature and isothermal operating condition. The major ash-related problem encountered in fluidized beds is bed agglomeration which, in the worst case, may result in total defluidization and unscheduled downtime. The initial agglomeration temperature for some common tropical agricultural residues were experimentally determined by using a newly developed method based on the controlled fluidized bed agglomeration test. The agricultural residues chosen for the study were rice husk, bagasse, cane trash and olive flesh. The results showed that the initial agglomeration temperatures were less than the initial deformation temperature predicted by the ASTM standard ash fusion tests for all fuels considered. The initial agglomeration temperatures of rice husk and bagasse were more than 1000°C. The agglomeration of cane trash and olive flesh was encountered at relatively low temperatures and their initial agglomeration temperatures in gasification were lower than those in combustion with both bed materials. The use of lime as bed material instead of quartz improved the agglomeration temperature of cane trash and olive flesh in combustion and decreased the same in gasification. The results indicate that rice husk and bagasse can be used in the fluidized bed for energy generation since their agglomeration temperatures are sufficiently high.

Pyrolysis of large wood particles : a study of shrinkage importance in simulations

Shrinkage models have been developed and included in a model for the pyrolysis of large wood particles. Shrinkage is modelled in three different ways: uniform shrinkage, shrinking shell and shrinking cylinders. These models and a reference model without shrinkage are compared with experimental data for mass loss versus time during pyrolysis of birch cylinders at different temperatures. In the experiments a wood particle was introduced into a pyrolysis furnace held at constant temperature. The particle mass and volume were recorded using a balance and a video camera. Uniform shrinkage slows down the pyrolysis whereas shrinking shell and cylinder models enhance the pyrolysis rate. The effect was sufficiently small to be neglected given the uncertainty about some wood physical properties.

Modelling of char-gas reaction kinetics

Abstract A semi-empirical gasification kinetic model is developed and the most common rate models are reviewed. Comparisons are made with experimental data for lignite char and published data on chars of coal, peat and biomass. In the gasification experiments, finely ground lignite char samples of 0.5–1 g were gasified in a thermobalance at atmospheric and elevated pressures, at temperatures between 750 and 850°C, using a number of CO-CO 2 -H 2 O-Ar mixtures.

Heat transfer controlled pyrolysis kinetics of a biomass slab, rod or sphere

Abstract A theory for deriving the pyrolysis rate of a single infinite slab, infinite cylinder or sphere in a constant temperature furnace is suggested. In analogy with the shrinking-core model a pyrolysis propagation front velocity is defined. The velocity is thereafter used in a compartment-model approach for deriving a set of ordinary differential equations for solving the burn-off over time. A comparison with experimental and published data is also made.

A thermodynamic study of dolomite as hydrogen sulphide adsorbent when pyrolysing or partially gasifying coal

Abstract Hydrogen sulphide absorption on dolomite when pyrolysing or partially gasifying coal is discussed with respect to thermodynamics. It is assumed that the absorption equilibrium over half-calcined dolomite sets the hydrogen sulphide concentration and that the gas phase composition is determined by the equilibria of the water-gas shift and the Boudouard reactions. When partially gasifying coal, temperature, pressure, air potential, moisture in and ultimate composition of the coal are the only parameters to vary in practice. Given the assumptions, theoretical minimum equilibrium hydrogen sulphide concentrations are derived as functions of these parameters. In the computations, the carbon content in the gas phase is accounted for indirectly, as it cannot be directly controlled. Results indicate that theoretical minimum hydrogen sulphide (i.e. sulphur) concentrations are influenced mainly by temperature, pressure and moisture content in the coal. An increase in temperature will enhance the absorption potential, whilst a pressure increase has the opposite effect. The theoretical minimum hydrogen sulphide concentrations were found to be 10–20 mg S MJ −1 , depending on the parameter values chosen. The reason for the existence of a minimum is that above temperatures of around 800 °C, an increase in temperature has to be coupled to an increase in total pressure to prevent the decomposition of the half-calcined dolomite.

Modelling and Simulation of Biomass Conversion Processes

By utilizing biomass gasification, the energy content of the biomass can be utilized to produce gas to be used for cogeneration of heat and power as well as other energy carriers such as fuels for vehicles. The concept is suitable for application to existing CHP plants as well as for utilizing spent liqour in small scale pulp and paper mills. The introduction would enable flexible energy utilization, use of problematic fuels as well as protects the environment by e.g. avoiding the release of toxic substances. In this paper, the possibilities to develop this concept is discussed. In this paper we compare different gasification processes with respect to what gas quality we get, and how the gasification can be modelled using different modelling approaches, and how these can be combined. Results from simulations are compared to experimental results from pilot plant operations in different scales and with different processes like CFB and BFB Technologies, athmospheric and pressurized, and using steam, air and oxygen as oxidizing media.