Analysis of deposits from combustion chamber of boiler for dendromass

Abstract

This article records the degradation of the refractory alumina-silicate lining in a boiler furnace combusting dendromass after ten years of use. The deterioration of refractory alumina material was evaluated from phase and chemical analyses of accretions. The fusion temperatures of accretion were measured with a high-temperature microscope and the results were compared with the solidus (point of first liquid formation) temperature predicted by FactSage 7.3 thermodynamic simulation software. Content of SiO₂ (50–65%) in accretions confirms that fine silicate particles of ash adhere to the lining. These silicates react with subliming alkalis from the dendromass and form aggressive eutectic melts on surface of lining at operating temperatures. Increase in operating temperature, inhomogeneity (porosity) and alkali content in accretions are the main factors influencing accretion viscosity, melt convection and lining corrosion. For this reason accretions on the vertical walls lower down are much thicker than on the walls in the upper part and the arch. The Al₂O₃ concentration is higher in the arch accretions; there the refractory material corrodes intensively. The fusion temperatures of the glassy accretions (measured at the furnace atmosphere: accretion boundary) are around 1150 °C. However, the calculated temperatures of slag formation stated by FactSage are about 150 °C lower, and these correspond to the operating temperature in the upper section of the combusting chamber. The fusion temperatures increase with the Al₂O₃ content in accretions closer to the lining. The obtained results will be applied to define the requirements for the development of boiler furnace refractories.

Introduction

Biomass/dendromass ranks among the fuels in the category of renewable energy and nowadays it often replaces the fossil fuels used in the heating stations preparing heat and hot water for households. Modern boilers using dendromass provide higher efficiency at lower cost. The development of biomass combustion technology is intensive; it is focusing on the enhancement of combustion effectiveness and reduction of emissions, so that energetic biomass utilization should be in compliance with EU norms [1], [2], [3].

The refractory materials for boiler linings were not specifically developed for biomass combustion. The same refractory materials (concrete and fire-bricks) are mostly used just as they were applied in older boiler and furnaces [4], [5], [6].

The expansion of biomass use in energy and metallurgy forced to focus on their combustion efficiency, ecology and economy. Some researchers worked on the corrosion resistance verification of various types of refractory materials in simulated atmosphere at high temperatures [4], [6], [7], [8], [9], [10], [11], [12]. The studies reveals that the interaction of gas with the refractory materials depends on the composition of combustion gas, its flow rate, and operating temperature in boiler, as well as the chemical and phase composition and density of the refractory material. For biomass combustion, the linings should be thermally stable, abrasion and wetting resistant and dense (e.g. refractories with high content of Al₂O₃ and SiC) [9].

There is a lot of research on biomass related to slagging and fouling of chambers in boilers and furnaces [13], [14], [15], [16], [17], [18]. The generated combustion gas is different from that produced in the case of coal combustion, and so the corrosion effect of the gas atmosphere on the lining in boilers is different as well.

The gas, ashes and slag produced from biomass combustion contain in addition to carbon dioxide also more water vapour and alkali. The heating capacities of different kinds of biomass are significantly different, as well as the ratio of combustible components to non-combustible residues (ash matter) and the chemical composition of the gases and ashes formed. The various amounts of ash (from 0.1% (soft wood) to 15% (bark)) created from different types of dendromass fuel and the proportion of major contents (SiO₂ and CaO) in ashes can vary from single figures to high percentages (3–70%) [3], [19], [20], [21], [22]. An important role is played by humidity of the biomass, vapour and volatile compounds (mainly H₂O, carbon oxides, sulphur, phosphorus, chlorine, alkaline and heavy metal compounds), which react together and condense in different parts of the combustion chamber and cooler zones in the heat-exchanger.

Only part of the ash remains in the chamber bed. Very fine particles are carried away with hot aggressive combustion gas from the combustion chamber bed to the top and into further parts of the boiler. In the furnace atmosphere the vapours of water, compounds of carbon, sulphur, phosphorus and chlorine react with alkalis (Na, K) and metal alkaline earths (Ca, Mg), which are aggressive to silicon. Together they create the various multi-component compound particles with low eutectic temperature. At the beginning, the aggressive gases and then the low-fusible melts attack the refractory materials. Sticky particles are caught on the hot surface of the refractory lining and gradually cover the walls and arch with glassy deposits. Later on the thick viscose glassy deposits start to flow due to their weight [7], [13], [14].

In an attempt to monitor these processes, previous researchers measured the fusibility temperature of ashes and slags. Slagging, fouling and sintering indicators were evaluated similarly as in the case of coals [16], [17], [23], [24], [25], [26].

The main difference between coal and biomass ash is that the coal ash contains higher amounts of SiO₂ and Al₂O₃ but lower amounts of K₂O and Na₂O. The alkalis have decisive role on fusibility of the ashes. Despite these differences, it is possible to use the same methodology for the fusibility characterization of ashes [23]. In addition to the measurement of the fusibility temperature [23], [25], [27] the indexes for evaluation of ashes fusibility derived from their chemical composition were calculated [17], [23]. In the study [23], which focuses on characterization of the biomass, the ashes are divided into classes in relation to the base/acid index: if B/A $0.2$ it indicates a lower deposition tendency of slagging; 0.2–1 a medium tendency and $1$ a high tendency. In the study [17], which deals with coal combustion, the intervals of slagging formation are <0.11; 0.1–0.14 and >0.14 respectively. The prediction of slag formation start is calculated on the basis of thermodynamic principles and chemical analysis [16]. In the study [13] it was experimentally shown that 15–20% of liquid phase (temperature at T₁₅) makes the particles sticky, and if the deposit material contains more than 70% liquid (T₇₀) the deposit flows down from the vertical walls.

The combustion processes in boilers are highly complex. The refractory linings are loaded thermally, mechanically and chemically; simultaneously and repeatedly. The higher the temperature, the more intensive are the thermodynamic interactions between alkali compounds and refractory materials and the low melting-point compounds formed on the hot surface of the lining. Problems can occur during the combustion processes if the type of fuel-biomass is changed. Increasing temperature together with changes in oxide-reduction conditions and abrasive effects of solid particles in the aggregate shorten the life of the lining material [7], [13], [14], [18], [28]. For these reasons materials resistant to thermal shock and CO such as moulted refractory alumina-silicate and spinel concrete, which were developed for metallurgical and energy plants using coal combustion or natural gas, are often applied in the boilers [4], [5], [29], [30].

The study investigates the extent of wear on the alumina linings in a dendromass-fuel boiler after 10 years of operation. The inside of the boiler was not monitored during the operation. The formation of accretions on the refractory lining was evaluated in relation to the composition of the ashes of woodchips combusted at low temperature in laboratory and slags from the grate of boiler. The results of the chemical and thermal analyses of ashes and slags were the basis for modelling the combustion gas composition in combustion chamber and the conditions of accretion formations. The determination of the softening and melting points of the accretions provides information about their formation and the temperature distribution in the chamber. The progress of the accretion melting process and corrosion of the lining and the temperature in the boiler needs to be known in order to establish the economical combustion process and the identification of the proper refractory materials.