Detecting, Modelling and Measuring Disturbances in Fixed-bed Combustion
Doctoral thesis, 2010
Combustion of fixed fuel beds in grate furnaces is one of the most common techniques within production of heat and power from solid fuels. The grate furnace used to be a working horse for combustion of hard coal, while today the incitement of using renewable fuel sources has turned biomass and municipal waste into dominating fuel types. These fuels are more conveniently managed by the robust grate furnace than by, for example, fluidised bed or suspension boilers. However, the introduction of the, in many cases, complicated and heterogeneous biomass and waste fuels have, while at the same time subjected to increasingly stringent demands on efficiency and emission of harmful substances, given rise to diffuse challenges to the operation and design of grate furnaces. The influence of the heterogeneous fuels and the low air flow rate required for low nitric oxide emissions are, to a large extent, unclear. The outcome is that different furnaces not only rely on diverging strategies of design and operation – they also show a variety of disturbance characteristics.
To make way for more efficient use of modern biomass fuels in grate furnaces, by improving the understanding of the combustion situation and disturbance characteristics, the following steps were made in this study: (1) a set of grate furnaces were investigated separating hands-on problems from underlying research oriented questions, (2) three methods for detecting and measuring disturbances in grate furnaces were developed, and (3) the fundamentals behind disturbances in the part of a fuel bed of grate furnace dominated by char conversion were addressed by mathematical modelling. The outcome of the inventory of furnaces is that fuel-bed channelling and grate material deterioration are common – the first causing increased emissions of unburned carbon compounds and nitric oxides, while the latter giving rise to high material costs and operation failures. In some furnaces, the disturbances could be detected by logical reasoning and visual observations while abated by hands-on adjustments to the fuel handling, operation conditions or grate design. The continued study adds two instruments to further improve this category: (1) a method of video recording inside furnaces to detect, quantify and fend off disturbing channelling, and (2) a new method of on-line monitoring of the fuel moisture content to detect disturbing changes in fuel moisture content during operation. Furthermore, a third developed method of analysing the residence time field of the gas flow in a furnace by mathematical modelling provides an opportunity to optimise the furnace chamber for both steady conditions and transient disturbances. The inventory of furnaces also identified some fundamental question marks concerning the conversion of the fuel bed – how channels in the fuel bed arise and how these disturbances may be avoided, also during low combustion air flow, without causing grate-material deterioration. Computational Fluid Dynamics (CFD) modelling of two-dimensional conversion of a fixed char bed was developed and used for investigating these matters, in which it was confirmed that a provoked change of bed porosity may cause process disturbing channelling. An obvious solution to avoid bed-channelling is, therefore, to create an as homogeneous fuel bed as possible. However, the structure of the fuel bed may only to a certain extent be affected; the low cost of the inferior quality fuel is, to a large extent, attributable to the fact that it is heterogeneous and, furthermore, advanced fuel pre-treatments drastically reduce the economical outcome. Thus, the most powerful parameter to avoid disturbances in a heterogeneous fuel bed is to manipulate the distribution of air by means of grate design. Here, a considerable flow resistance across the grate – an effect which may be achieved by reducing the amount and/or size of the air holes in the grate rods – was found to dampen channelling. The CFD modelling, however, indicates that the placing or shaping of these holes, under certain circumstances during the char conversion, may cause thermal and/or carburizing material deterioration – one of the most common disturbances in the inventory of furnaces. Consequently, recommendations for placing of holes along the lower part of a combustion grate, counter-acting the risk of grate material deterioration, could be given from the modelling outcome. These recommendations, which also include recirculation of flue-gases into the combustion air, may in a straight forward manner be applied to the design of future grate furnaces to improve the combustion of heterogeneous fuel beds.