Fluidized Bed Combustion Of Biomass Research Articles

Power-to-Green Methanol via CO2 Hydrogenation—A Concept Study including Oxyfuel Fluidized Bed Combustion of Biomass

A revolution of the global energy industry is without an alternative to solving the climate crisis. However, renewable energy sources typically show significant seasonal and daily fluctuations. This paper provides a system concept model of a decentralized power-to-green methanol plant consisting of a biomass heating plant with a thermal input of 20 MWth. (oxyfuel or air mode), a CO2 processing unit (DeOxo reactor or MEA absorption), an alkaline electrolyzer, a methanol synthesis unit, an air separation unit and a wind park. Applying oxyfuel combustion has the potential to directly utilize O2 generated by the electrolyzer, which was analyzed by varying critical model parameters. A major objective was to determine whether applying oxyfuel combustion has a positive impact on the plant’s power-to-liquid (PtL) efficiency rate. For cases utilizing more than 70% of CO2 generated by the combustion, the oxyfuel’s O2 demand is fully covered by the electrolyzer, making oxyfuel a viable option for large scale applications. Conventional air combustion is recommended for small wind parks and scenarios using surplus electricity. Maximum PtL efficiencies of ηPtL,Oxy = 51.91% and ηPtL,Air = 54.21% can be realized. Additionally, a case study for one year of operation has been conducted yielding an annual output of about 17,000 t/a methanol and 100 GWhth./a thermal energy for an input of 50,500 t/a woodchips and a wind park size of 36 MWp.

Multifunctional Additives for NOX Abatement in Fluidized Bed Biomass Combustion

Multifunctional Additives for NO_X Abatement in Fluidized Bed Biomass Combustion

Modern State and Topical Issues of Studying Solid Fuel Particle Primary Fragmentation Processes as Applied to Biomass Combustion and Gasification in Fluidized and Dense Bed (Review)

It is shown that it is important to take into account the variation of particle sizes due to their fragmentation in fluidized bed biomass combustion. The present state of investigations into the fragmentation process is analyzed. It is shown that primary fragmentation, a process involving cracking and disintegration of initial fuel particles into two or more parts due to thermal stresses and growth of pressure in the particles during their rapid heating at the drying and devolatization stages is the most essential issue. Factors causing the cracking of fuel particles and the nature of this process are considered. The particle fragmentation quantitative characteristics and criteria are analyzed. It is shown that the particle critical diameter is the simplest criterion for estimating the susceptibility of different fuels to fragmentation. The main factors influencing the occurrence of primary fragmentation, namely, particle size, heating rate, bed temperature, and fuel characteristics, are considered. The list of fuel’s main characteristics affecting its primary fragmentation includes the volatiles content, porosity, moisture and ash content, susceptibility of particles to swelling or shrinking, and the organic part composition. Matters concerned with predicting primary fragmentation of fuels are considered. Information about the interrelation between the main characteristics of fuels, their susceptibility to primary fragmentation, and its nature is presented. In view of biomass properties and its combustion conditions, both of the primary fragmentation mechanisms, namely due to devolatization induced stresses and thermal stresses, are supposed to take place. It can be expected that the domination of one or another mechanism will depend on the combination of particle size and heating temperature. The lines of and methods for studying the nature of biomass particles primary fragmentation and its quantitative characteristics under different conditions are outlined. The data obtained as a result of such fundamental investigations will form the basis for elaborating methods for designing furnace devices and gasifiers operating on biomass taking into account the effect of particle fragmentation on the combustion, gasification, carryover, and heating surface contamination processes.

Agglomeration mechanism in biomass fluidized bed combustion – Reaction between potassium carbonate and silica sand

Agglomeration is one of the operational problems in fluidized bed combustion of biomass, which is caused by interaction between bed materials (e.g. silica sand) and the biomass ash with a high content of potassium species. However, the contribution of different potassium species to agglomeration is not fully understood yet. In the present work, the reaction between K2CO3 and silica sand has been studied extensively by thermogravimetric analysis. The reacted samples were analyzed by SEM-EDX to reveal the reaction mechanism. The results indicated that the reaction occurs in a solid-solid phase already at temperatures around 700°C. The reaction rate increases with increasing temperature, but decreases with an increase of CO2 partial pressure. Using smaller particle size and well mixed solid reactants results in an increased reaction rate. It is observed that the reaction initiates in the contact area between K2CO3 and silica sand, forming a thin product layer. The layer acted as a reactive media further reacting with K2CO3 and silica sand. The results provide a basis for understanding of potassium induced agglomeration process in fluidized bed biomass combustion.

Optimization of a 50 MW bubbling fluidized bed biomass combustion chamber by means of computational particle fluid dynamics

An efficient utilization of biomass fuels in power plants is often limited by the melting behavior of the biomass ash, which causes unplanned shutdowns of the plants. If the melting temperature of the ash is locally exceeded, deposits can form on the walls of the combustion chamber. In this paper, a bubbling fluidized bed combustion chamber with 50 MW biomass input is investigated that severely suffers deposit build-up in the freeboard during operation. The deposit layers affect the operation negatively in two ways: they act as an additional heat resistance in regions of heat extraction, and they can come off the wall and fall into the bed and negatively influence the fluidization behavior. To detect zones where ash melting can occur, the temperature distribution in the combustion chamber is calculated numerically using the commercial CPFD (computational particle fluid dynamics) code, Barracuda Version 15. Regions where the ash melting temperature is exceeded are compared with the fouling observed on the walls in the freeboard. The numerically predicted regions agree well with the observed location of the deposits on the walls. Next, the model is used to find an optimized operating point with fewer regions in which the ash melting temperature is exceeded. Therefore, three cases with different distributions of the inlet gas streams are simulated. The simulations show if the air inlet streams are moved from the freeboard to the necking area above the bed a more even temperature distribution is obtained over the combustion chamber. Hence, the areas where the ash melting temperatures are exceeded are reduced significantly and the formation of deposits in the optimized operational mode is much less likely.

Investigations on the Mechanisms of Ash-Induced Agglomeration in Fluidized-Bed Combustion of Biomass

Interactions between fuel ash and bed material, which lead to agglomeration, are impeding a widespread use of alternative biomass fuels for energy conversion in fluidized-bed boilers. Reactions of certain ash components (alkaline metals in the case of biomass) with silicon of the quartz sand, commonly used as bed material, have been identified as main drivers for this phenomenon. Still, there is no consensus on the detailed mechanisms causing the attachment of ash on bed particles and the consequent growth of agglomerates. Only through a better understanding of these processes will it be possible to cast precise predictions and implement effective countermeasures for agglomeration issues. This paper reviews the current state of knowledge and delivers new insights into the mechanisms of coating-induced agglomeration. In particular, the influence of heterogeneous reactions on coating formation and the metamorphism of the coatings during agglomeration are investigated. The results are derived from experiment...

Experimental determination of agglomeration tendency in fluidized bed combustion of biomass by measuring slip resistance

A method by measuring the slip resistance between particles was used to determine the agglomeration tendency in fluidized bed combustion of biomass. Solid particles were taken from different stages of biomass combustion in a fluidized bed and loaded into a bench-scale test apparatus with two concentric cylinders. A precision variable frequency motor and a torque sensor were employed to measure the torque driven by the inner cylinder at a constant speed, which is directly related to the slip resistance of the solid particles. The measurement results showed significant difference in the slip resistance of the bed solids taken from different stages of biomass combustion at different operating temperatures. A strong correlation was found between the onset of agglomeration and increase in the slip resistance, especially near the onset of agglomeration, due to the build-up of molten biomass ashes, presence of liquid bridges between particles and formation of incipient agglomerates. With further validation, the method developed in the present study can potentially be used to quantify agglomeration tendency in a fluidized bed reactor and characterize the dynamic process of agglomeration.

The Phase Transformations during Fluidized-Bed Combustion of Biomass

FactSage6.1 was used to study the phase transformation at high temperature when biomass combustion in a fluidized bed reactor. The results show that eutectic was formed during the reaction process, the eutectics are formed mainly by the reaction between the silica in bed particles and the alkali species in biomass ash. The solid phase transformed to melt layer on the surface of sands particle mainly contains potassium, some calcium and magnesium, and also a few phosphorus and chlorine are found in the melt layer. The result utilizing FactSage equilibrium modeling shown that the distribution ratio of potassium in the gas phase increased with the increase of temperature, moreover, the melt of bed material surface increased when defluidized occurred.

Low-cost Camera System for Online Estimation of Grain Size in Fluidized Bed

AbstractAt present, the production of energy has to be capable of responding to fast and unexpected changes, which may originate from changes in the fuel quality or in the energy demands, for example. New fuels and their co-combustion, added to the growing demands for process efficiency and the efforts to reduce harmful emissions have generated an increasing need for monitoring the processes, and new kinds of measurements and tools are needed to meet those challenges. Monitoring the condition of the bed is increasingly important in fluidized bed combustion of biomass, and it is usually performed manually by sieving the bottom ash. This is expensive and laborious, however, and should be either automated or alternative methods for estimating the bed quality should be developed if possible. In this paper we present an online inspection system for estimating the coarseness of the bed material using digital images taken from the bottom ash.

Detecting and Counteracting Agglomeration in Fluidized Bed Biomass Combustion

Agglomeration in fluidized bed combustion can be a big operational problem, leading to unwanted defluidization and shutdown of the installation. Therefore, the onset of such events has to be reliab...

A Fluidized Bed Biomass Combustion Model with Discretized Population Balance. 1. Sensitivity Analysis

A model of a 1 MWth atmospheric bubbling fluidized combustor burning waste wood fuel (feeding) is presented. The model incorporates both the solid and gas phases. The bed is assumed to consist of two phases, of which the emulsion phase takes both gas and solids into account, while the bubble phase consists only of gas. A wide size distribution of biomass feed, representative of the actual boiler feed, has been assumed. The model calculates the gas composition, velocities, and other important hydrodynamic parameters in both the emulsion and bubble phase. A particle size distribution model is included to calculate elutriation losses of fine char particles. This approach is novel in the sense that a population balance for fine particle class is derived, using the well-known mass balance principles, and a coupled discretized population balance equation, valid for the whole particle size range, is presented. The model takes into account devolatilization, fragmentation, and attrition of the solid phase along with gaseous profiles. It includes nine components for which differential equations have been derived and solved to calculate species concentration at any point along the bed height. In total, 20 particle size classes have been considered, 10 of which are considered as feed and the rest as fine classes. The model aims to assess the effect of different parameters on boiler performance and gaseous emissions. A sensitivity analysis of the gaseous emission profiles with respect to different variables and parameters defining different submodels has been carried out. A homogeneous NOx model has been included with individual kinetic parameters for relevant species. It has been found that gas hydrodynamics play a significant role, and the system can be optimized using these parameters.

Capture of Potassium and Cadmium by Kaolin in Oxidizing and Reducing Atmospheres

Nutrient elements including alkali metals in biomass ash need to be returned to the forest by ash recycling. It is, however, difficult to recycle the total biomass ash due to the high concentrations of heavy metals in some ash fractions. During fluidized bed combustion of biomass, alkali metals tend to attach on bed particles or deposit on heat transfer surfaces, causing bed agglomeration, slagging, and fouling problems, while heavy metals are found enriched in the fly ash. Therefore, it is important to control both alkali and heavy metals during combustion of biomass. In this study, the competition between potassium and cadmium adsorption on kaolin was investigated experimentally in a fixed bed reactor equipped with an alkali detector. The experiments were made at 850 °C under both oxidizing and reducing conditions, which are relevant to fluidized bed combustion of biomass. The oxidizing gas was air, while the reducing gas was composed of 80% N2, 10% CO, and 10% CO2. Fine aerosols of droplets containing the metals in water were produced from their aqueous solutions by means of an atomizer and subsequently vaporized in the reactor. Atomic absorption spectrometry (AAS) and X-ray powder diffraction (XRD) techniques were used for quantitative and qualitative analyses of the products. The results indicate that potassium is captured by kaolin in both oxidizing and reducing atmospheres. Kaolin captures cadmium in air, but not under reducing conditions. In addition, reducing conditions appear to promote potassium adsorption on kaolin. The copresence of potassium and cadmium compounds in the gas phase promotes the capture of both metals by kaolin. This promotion was more pronounced for potassium than for cadmium, indicating a preference of kaolin for potassium capture over cadmium capture. In air, the total amount of potassium captured by kaolin increased when cadmium was added, but the water-soluble fraction decreased slightly. Moreover, kaolin captures KCl slightly more effectively than KOH in air, but less effectively than KOH under reducing conditions.

The relevance of attrition to the fate of ashes during fluidized-bed combustion of a biomass

The fate of ashes during the fluidized-bed combustion of a biomass fuel (Robinia pseudoacacia) has been investigated both experimentally and theoretically. Fluidized-bed combustion experiments with Robinia have been carried out with a bed of pure quartz at temperatures ranging from 700°C to 850°C at different oxygen concentrations in order to investigate the tendency of the biomass ashes to deposit on the bed particles and to give rise to bed agglomeration. SEM/EDX analysis of the silica bed particles after the tests was carried out to determine the formation of alkali-rich layers on inert bed particles and possibly of silicate melts. Results indicated that large quantities of biomass ashes are retained on the bed particles under all operating conditions, but only at the higher temperatures could molten surface layers and agglomerated bed particles be noticed. Experimental results have been interpreted on the basis of a single-particle combustion model applied to both fine and coarse char particles' burnout in a fluidized bed. Calculations show that extremely high temperatures are rapidly reached by fine particles even at very low oxygen concentrations. These temperatures are well beyond typical potassium silicate melting temperatures. On the other hand, coarse particles burn at temperatures only slightly above bed temperature. Experimental and theoretical results indicate that the following mechanism is relevant to the fate of ashes during fluidized-bed combustion of biomass: ash is mostly detached from the coarse char as attrited fines whose temperature is raised significantly by carbon afterburning. Fines can further adhere onto inert bed particles, with formation of alkali-rich surface layers. If the bed temperature is higher than the alkalisilicate eutectic, a melt forms that enhances bed particle stickiness and may ultimately lead to bed agglomeration.