Entrained flow gasification of biomass : On atomisation, transport processes and gasification reactions

Sammanfattning: Since the energy consumption in the world is increasing together with an increase of greenhouse gas emission it is of importance to find alternatives to fossil fuels. Biomass is one of the alternative energy sources which is both renewable and CO2 neutral. However, due to the large variability of biomass and the influence from different types of contaminations it is important to find processes that can work with a range of biomass and preferably transform the energy in the biomass into higher value energy forms. Gasification is one of the most robust processes that can achieve this by transforming solid biomass (e.g. wood, bark or rice husks) or liquid byproducts from the forest products industry (black liquor) into a uniform synthesis gas that can be further upgraded into electric power or synthetic motor fuels. This thesis is focused on a specific class of gasifiers called entrained flow gasifiers that converts the biomass to syngas in a reactor where the fuel is entrained in a gas flow.In entrained flow gasification, small fuel particles are gasified together with an oxidizing medium like air or pure oxygen. The fuel particles are either formed a priori by milling of solid biomass or in situ by atomisation of a liquid fuel. In both cases the fuel is thoroughly mixed with a carrier gas and distributed into a hot gasification reactor. One type of entrained flow gasification that is of high relevance for countries with a significant pulp and paper industry is black liquor gasification. Black liquor is a by-product from pulping that is available in large quantities in chemical pulp mills. The energy in the black liquor is normally utilized for steam production in the mill but this steam can easily be produced from low grade biomass, thereby freeing the black liquor for other purposes. The most interesting process is when black liquor is gasified with pure oxygen at high temperature. This process creates a clean synthetic gas with very low concentration of tars that is suitable for catalytic conversion into transportation fuels, e.g. dimethyl ether or methanol.One of the key parts in a black liquor gasifier is the burner nozzle that is used to produce a spray of fine black liquor droplets inside the gasifier. This is a difficult task since black liquor has a very high viscosity. The black liquor atomisation process has therefore been studied with high speed photography to be able to visualize the process and thereby making it possible to optimize the burner nozzle so that it produces a spray with near uniform particle size and appropriate distribution in space. The results show that the fuel particles formed from the considered nozzle consisted of non-spherical and stretched ligaments that in some cases were further broken down into more spherical droplets. The experiments with black liquor were difficult and hazardous since the black liquor is caustic and hot since it needs to be preheated to around 120 °C before it can be atomised. It is therefore of interest to find non-hazardous substitute liquids that will have the same behaviour as black liquor in a nozzle. In a comparison between black liquor at 120 °C and a syrup/water mixture with equal viscosity and surface tension at room temperature it was found that the syrup/water mixture behaved nearly identical to the black liquor in a real burner nozzle.Connected to the atomisation studies, measurements of gas composition in a 3MW black liquor gasifier were made for different black liquor preheat temperatures. The results showed that preheating of black liquor had a significant influence on the syngas composition and the conclusion when this was combined with the results from spray visualization was that the main reason for the observed differences is the smaller droplet size that is achieved with higher preheating temperatures.In a large syngas plant where the goal is to catalytically convert the syngas into motor fuels or chemicals in a catalytic process, the raw syngas from the gasifier must be cleaned and conditioned in several steps. In all contemporary downstream processes the gas must be much colder than when it leaves the gasifier. Hence, gas cooling is an important unit operation in the syngas process. In order to optimize the overall efficiency of the syngas plant it is very important to recover the latent heat in the syngas at the highest possible temperature. One way to do this is to use a counter current condenser that cools the syngas and condenses most of the steam that is mixed with the syngas while at the same time steam that can be used by other processes is produced. The sizing of counter current condensers is therefore of high importance and one part of the thesis work was to develop a computational model that can be used for optimization of these units. In order to validate the code, measurements were carried out in the counter-current condenser in the 3 MW black liquor gasification pilot plant that was mentioned above. The predictions from the model were found to be in very good agreement with the temperature measurements from the pilot plant for the cases that were investigated.Another type of entrained flow gasification process is air-blown cyclone gasification where biomass powder is gasified in a cyclone shaped reactor. This gasification process can be used in combination with a gas engine to produce both heat and power that can be used in district heating applications or as prime mover and heat source in industrial processes where low grade biomass is available at low cost. This type of gasifier has the possibility to operate with ash rich fuels since it operates below the ash melting temperature and the majority of the ash is separated in the bottom of the cyclone.One of the objectives in this thesis was to evaluate the fuel flexibility of the cyclone gasifier by experiments with different fuels in a 500 kW pilot gasifier. From the gasification test it was found that torrefied spruce, peat, rice husk, bark and stemwood powder can be used as fuel to produce a syngas that can be used as fuel in a gas engine.To be able to understand the cyclone gasification process and be able to optimize different sizes of cyclone gasifiers a computational fluid dynamics model of the process has been developed and compared against experimental measurements, both in a 500 kW plant and a 4 MW plant. The results show that the model predicts the main gas species in the product gas and the amount of unconverted fuel reasonably well. It also predicts the effect of increased gasifier size and fuel power well. Therefore the model could be used as a tool for designing cyclone gasifiers in arbitrary sizes and to optimise operating parameters in existing gasifiers.