Fluidized Bed Combustion System Research Articles

Potassium release and mitigation by additives in different biomass combustion systems

Deposition formation on heat-exchanging surfaces and corrosion limit the use and efficiency of solid biomass combustion systems. The main influencing factors on these ash-related problems are the potassium content, the form in which the potassium is bound to the fuel matrix, and the temperature in the combustion system. To avoid these deposition problems, choosing the right fuel for a particular combustion system and vice versa is important.In this work, the temperature in fluidized bed, grate firing, and suspension combustion systems is analyzed by using the example of the power plants Altenstadt and Avedøre, as well as a European municipal solid waste incineration plant. Additionally, the temperature-resolved potassium release of ten different biofuels is analyzed in an electrothermal vaporization unit (ETV) connected to an inductively coupled plasma optical emission spectrometer (ICP-OES). The potential of additives to mitigate the potassium release is analyzed by adding kaolin and coal fly ash to miscanthus, torrefied wood, and beechwood.The results show that hardly any potassium is released into the gas phase at the typical temperatures of fluidized bed systems. Most of the potassium release occurs between 1000 °C and 1400 °C, the typical temperature range of grate combustion systems. This shows that reducing the temperature of the fuel on the grate could drastically reduce the potassium release. However, due to the negative effect on the efficiency, other measures, such as using additives for capturing potassium species, are favored. At the temperatures in suspension-fired combustion systems, almost all potassium is released into the gas phase, and additives cannot mitigate the release at these temperatures. Additives can shift the potassium release of miscanthus and torrefied wood while showing no positive effect when mixed with beechwood.All in all, this work enables a quantitative comparison of various biofuels and a rough evaluation of the suitability of a fuel regarding the deposition formation for different combustion systems.

Quantitative measurement of sodium emissions during oxy-fuel combustion of a beech wood biomass particle using laser-induced breakdown spectroscopy

Oxy-fuel biomass combustion in a circulating fluidized bed is a clean and sustainable biomass utilization technology. Woody forest biomass is considered a promising source of renewable energy because of its widespread availability across the globe. However, effective biomass utilization in thermal power plants is largely limited by severe ash deposition and corrosion of heat transfer surfaces due to alkali metal and chlorine emissions during biomass combustion. In this study, laser-induced breakdown spectroscopy (LIBS) was applied to detect in-situ gas phase sodium (Na) emission during single particle combustion of widely available forest biomass, namely, beech wood. The operating conditions simulate the thermo-chemical conditions relevant to a fluidized bed combustor system, where the operating temperature varies between 1073K–1273K. This work aims to investigate the effect of O2 concentration, CO2 (oxyfuel environment), and particle size on the emission of sodium during the combustion of a single beech wood particle. The unresolved sodium doublet was used as the LIBS signal to quantify sodium emission concentration.Oxygen concentration is the most critical parameter affecting Na emissions, and an increase in O2 concentration led to higher Na emissions. It is observed that there is a significant reduction in the emission of Na in an O2/CO2 environment as compared to an O2/N2 environment. It is hypothesized that this drop in Na emissions in O2/CO2 environment could be due to lower particle temperatures caused by the lower diffusivity of O2 in CO2 as well as the endothermic gasification reaction in high CO2 environments. In addition, it is reported that the dissociation of sodium carboxylates and carbonates in the biomass matrix to release gaseous Na due to thermal decomposition is retarded at higher CO2 concentrations which could explain the drop in Na emissions. The effect of biomass particle size is pronounced mainly at lower concentration of O2 where, larger particles keep Na trapped in its matrix, however, at higher O2 concentrations (30%), the effect of particle size on Na emission is negligible.

Energy efficiency analysis and optimization of a pressurized oxy-fuel circulating fluidized bed combustion system

The commercial application of oxy-fuel combustion technology is limited by energy deduction which is caused by addition of air separation unit (ASU) and carbon dioxide compression and purification unit (CPU). Pressurized circulating fluidized bed (CFB) oxy-fuel combustion technology is one option to improve the net electric generation efficiency. In this research, a series of simulations and optimizations were carried out to analyze the energy efficiency of a pressurized oxy-fuel combustion system. The boiler structure and heat exchange surface arrangement under different pressure were calculated. The cross-sectional area of the furnace and tail flue were both reduced up to 10 % of the original size and the floor space of the boiler was considered to be the lowest at 1.1 MPa. The energy efficiency and exergy analysis were used to explore the influence of pressure on the net efficiency and the optimization potential of system. The net electric generation efficiency reached a maximum value of 24.16 % at 1.1 MPa, which is 5.4 % higher than that at atmospheric pressure. Finally, four parameters, i.e. preheat temperature, oxygen concentration, ASU outlet oxygen purity and CPU compressor outlet pressure, were optimized to improve the system net efficiency to 26.67 %, 29.04 %, 25.87 % and 24.36 %, respectively.

Alternative sorbents for mercury capture in flue gas from lignite combustion

This paper presents an experimental study of two of alternative sorbents for mercury capture from the lignite combustion flue gas. The sorbents are based on calcium hydroxide and aluminosilicate, using different modifications. A commercially available activated carbon (AC) with bromine impregnation was selected as the reference sorbent. This sorbent is a state-of-the-art solution for mercury removal in industrial combustion facilities. However, its application is costly for power plants because of the market price of the AC and because the presence of carbon in the fly ash can complicate its further use, which gives a strong motivation for exploring new alternatives. Experimental investigation was carried out in a 500 kW pilot-scale bubbling fluidized bed combustor using a lignite with naturally high mercury content. A injection system of our own design and construction was used to supply the sorbents to the flue gas downstream of the combustor at a flow rate in the range of 100–500 mg/m3N of the flue gas. Continuous measurement of mercury concentration was used upstream the sorbent injection and downstream the fabric filter, simultaneously. Different sorbent injection rates and flue gas temperatures at the sorbent injection point were examined.The mercury capture ratio was evaluated considering the Hg balance in the combustor. It was shown that the mercury self-capture by the fly ash in the fluidized bed combustion system varies approximately from 30 to 90 %, depending on the SO2 concentration. The Hg capture ratio for calcium hydroxide was in a range of approximately 15–25 %, while for aluminosilicate it was 25–45 %, determined by the actual conditions of the flue gas and the initial Hg concentration. This is in the range of the reference AC, which typically reaches from 25 – 95%. The increased injection rate of the sorbents was reflected in the Hg capture ratio, but its promotion was observed only in the case of the aluminosilicate sorbent. A correlation of the Hg capture ratio with the flue gas temperature at the injection point was also clearly identified only for the aluminosilicate sorbent, when an increase in the temperature led to increasing in the capture ratio.

Effect of back pressure on gas-solid distribution characteristics of cyclone distributor applied in powdered coal-fired circulating fluidized bed combustion system

The cyclone distributor is an essential part of the powdered-coal-fired circulating fluidized bed (PC-CFB) combustion system, and its gas-solid distribution characteristics are significantly affected by the back pressure of the lower outlet. A four-way coupling method using a dense discrete phase model (DDPM) was adopted to simulate the cyclone distributor. The results indicate that the effect of the back pressure on the axial gas velocity is more pronounced compared to that on the tangential gas velocity. With the increase in the back pressure, the particle separation efficiency decreases monotonically, while the rate of change gradually increases. Meanwhile, the gas separation efficiency increases as the back pressure becomes higher. Furthermore, increasing back pressure improves the particle concentration at the outlets of the cyclone distributor. To maintain high particle separation efficiency and ensure the safe operation of the boiler, it is recommended to set the back pressure between 1000 Pa and 2000 Pa.

Evaluation of Multi-Utility Models with Municipal Solid Waste Combustion as the Primary Source under Specific Geographical and Operating Conditions

Developments in waste incineration technology in terms of efficient fuel preparation, combustion, and emissions reduction, as well as the growing needs of the community in terms of electricity, water, and air conditioning loads, are the prime motive for this study. This study presents a novel approach, in which three models of the fluidized bed combustion of municipal waste for simultaneous power generation, freshwater production, and district cooling are analyzed for their energy and exergy performance. The three simultaneously evaluated utility models are different configurations of a fluidized bed combustion system with Rankine cycle power generation, cooling with a vapor absorption refrigeration system, and fresh water production using multiple effect desalination. The output from the turbine, cooling system, and desalination system is determined using the Engineering Equations Solver for different boiler operating pressures. Energy and exergy analysis data for different pressures are used to identify the best configuration. Two variants of the absorption cooling system, namely, single effect and double effect, are considered. The variants of the multiple-effect desalination are the three-stage and five-stage methods. Input parameters used in this study are municipal solid waste generation and composition data collected for an urban community in an arid climate zone with high demand for electric power, cooling, and fresh water. Model 2, which contains two turbines with the reheating and cooling systems connected to a high-pressure turbine and water desalination connected to a low-pressure turbine, gave the best overall performance. Significant savings in terms of the replacement of conventional energy were observed from these waste conversion plants with greater benefits in arid weather conditions. The results obtained by different models under different operating criteria constitute a guideline for municipal planners for the selection of appropriate waste utilization technology, as well as the appropriate operations.

Exergy and Energy Analysis of the Shell-and-Tube Heat Exchanger for a Poultry Litter Co-Combustion Process

Increasing production of poultry litter, and its associated problems, stimulates the need for generating useful energy in an environmentally friendly and efficient energy system, such as the use of shell-and-tube heat exchangers (STHE) in a fluidized-bed combustion (FBC) system. A holistic approach which involves the integration of the First Law of Thermodynamics (FLT) and Second Law of Thermodynamics (SLT) is required for conducting effective assessment of an energy system. In this study, the STHE designed by the CAESECT research group, which was integrated into the lab-scale FBC, was investigated to determine the maximum available work performed by the system and account for the exergy loss due to irreversibility. The effects of varying operating parameters and configuration of the space heaters connected to the STHE for space heating purposes were investigated in order to improve the thermal efficiency of the poultry litter-to-energy conversion process. Exergy and energy analysis performed on the STHE using flue gas and water media showed higher efficiency (75–92%) obtained via energy analysis, but much lower efficiency (12–25%) was obtained when the ambient conditions were factored into the exergy analysis, thus indicating huge exergy loss to the surroundings. From the obtained experimental data coupled with the simulation on parallel arrangement of air heaters, it was observed that exergy loss increased with increasing flue gas flow rate from 46.8–57.6 kg/h and with increasing ambient temperature from 8.8 °C to 25 °C. To lower the cost of STHE during final design, a larger temperature difference between the hot and cold flue gas is needed throughout the exchanger, which further increases the exergetic loss while maintaining an energy balance. In addition, this study also found the optimal conditions to reduce exergy loss and improve energy efficiency of the designed STHE. This study shows the possibility to evaluate energy systems using integration of exergy and energy analysis.

Investigating the Performance of Fluidized Bed Combustion Systems for Biomass Energy Conversion

Investigating the Performance of Fluidized Bed Combustion Systems for Biomass Energy Conversion

Energy Efficiency Analysis of Oxy-Fuel Circulating Fluidized Bed Combustion System with High Oxygen Concentration

Energy Efficiency Analysis of Oxy-Fuel Circulating Fluidized Bed Combustion System with High Oxygen Concentration

Potassium capture by ilmenite ore as the bed material during fluidized bed conversion

Biomass is a complex fuel containing a heterogeneous mixture of organic matter and, to a lesser extent, inorganic matter. Inorganic ash-forming elements can cause serious ash-related problems in fluidized bed combustion systems, such as bed agglomeration, fouling and corrosion of heat transfer equipment. Potassium (K), a biomass ash component, is a major contributor to these phenomena. The aim of this work, in the context of high-potassium biomass fuel combustion, is to estimate the potential for the absorption of potassium by bed materials under different oxygen carrier aided combustion (OCAC) operating conditions, in terms of reaction gas composition, exposure duration, bed temperature, potassium salt type, and concentrations of potassium salt. Experiments were conducted with an electrically heated fluidized bed reactor (FBR). The potassium solution is injected into the dense bed region via a small tube. In the studied temperature range of 700–900 °C, the captured K amount increases significantly at temperatures lower than 850 °C. The gas atmosphere had a less notable effect on K capture, but the capture rate of potassium dropped to half after the K-solution was switched from KOH to K2CO3 in the CO2 gas atmosphere. The percentage of injected potassium that is captured is high (∼40–60 wt%) in the first 5 h, and it remains at a high level of about 35 wt% after 25 h. KTi8O16 is the only crystalline K-compound found in the bed solid samples when ilmenite ore was used as bed material. KAlSi3O8 and K0.43(NH4)0.53Al0.89Si2.11O6 were detected when olivine sand was used as bed material. Thermogravimetric analysis (TGA) indicated no significant effect of the spent ilmenite on its redox reactivity after its reaction with K-solutions in the fluidized bed; however, the O2-carrying capacity of spent ilmenite shows a 7 % reduction after 25 h of exposure to potassium in the fluidized bed.

Coupling of spectral thermal radiation model with a comprehensive system model for co-combustion of biomass in bubbling fluidized bed

A previously developed 1-D comprehensive model for a bubbling fluidized bed combustor (BFBC) co-firing lignite with cotton residue and limestone is coupled with a 3-D spectral and gray radiation models utilizing method of lines (MOL) solution of discrete ordinates method (DOM). The performance of the coupled models is assessed by comparing their predictions of temperature profiles and gaseous emissions (i) with those of original FBC model without coupling and (ii) with the in-situ measurements from Middle East Technical University (METU) 0.3 MWt BFBC. Comparisons between fluidized bed combustion (FBC) system models with and without radiation reveal that the spectral model improves the predictions only slightly. This is considered to be due to the very small temperature difference between the freeboard medium and hot refractory side walls which results in lower rates of heat transfer from the freeboard to the walls. Furthermore, radiative heat transfer in the freeboard is dominated both by combustion gases and particles.

Optimization of oxy-fuel circulating fluidized bed combustion system with high oxygen concentration

High oxygen concentration oxy-fuel circulating fluidized bed (CFB) combustion technology is a high efficiency and low cost oxy-fuel combustion technology, which also has high potential of optimization and efficiency improvement. In this research, energy efficiency is analysed to acquire the effects of flue gas recirculation mode, preheating temperature and oxygen purity on the net efficiency and energy consumption of the 50% oxy-fuel combustion system, so as to obtain the further optimization strategy. The following results are obtained by simulation. The optimal extraction position of recycled flue gas (RFG) is gas preheater outlet, and the net power generation efficiency increases by 0.23% to 25.08%. As the preheating temperature increases from 230 to 400 °C, the net efficiency increases by 1.19% to 26.04%. The net efficiency increases first then decreases as the oxygen purity at ASU outlet increase from 89% to 97.4%, and the optimal value is the 94%. Finally, the optimal system is obtained when RFG is extracted from gas preheater outlet, the preheating temperature is 400 °C and the oxygen purity at ASU outlet is 94%. The net efficiency of the optimization system reaches 27.4%, increasing by 2.55%.

Evaluation of the use of lignite of Turkeys’ with biomass as Agricultural waste as fuel in terms of emissions

Agricultural wastes as Biomass contains low carbon, high hydrogen, high oxygen and a lower amount of sulfur. Coals contain higher amounts of carbon, lower amounts of hydrogen, lower oxygen and higher amounts of sulfur. With the use of lignite and biomass mixture as fuel will provide less CO2 and SO2 emissions and a more economical mixture will be obtained. Considering these emissions, fluidized bed combustion systems are recommended in the literature for the combustion of lignite and biomass. In this study, rice husks, corn cobs, walnut shells, sunflower shells, olive cake and woodchips were used as agricultural waste. 10 different lignite extracted from Turkey were used as fuel. It has been assumed that the combustion process was carried out by taking the biomass rate of 10%, 30% and 50%. When burning of 1 kg of lignite and biomass mixture, the highest CO2 emission occurs from 10% woodchips - 90% Kütahya - Ömerler (washed) mixture as 2.938 kg and the highest SO2 emission obtained from 10% olive cake - 90% Kütahya Seyitömer-Ayvalı lignite mixture as 0.061 kg. The highest H2O emission was obtained by mixing 50% woodchips - 50% Manisa-Kısrakdere lignite as 0.563 kg.

Experimental investigations on the chlorine-induced corrosion of HVOF thermal sprayed Stellite-6 and NiAl coatings with fluidised bed biomass/anthracite combustion systems

Stellite-6 (Co-based) and NiAl coatings (Ni-based) were deposited via HVOF spraying onto 304 stainless steels and tested in a 20 kWth biomass fired bubbling fluidised bed (BFB) combustor for 20 h and an industrial scale anthracite fired CFB boiler for 1630 h. Stellite-6 showed excellent corrosion resistance in both fluidised bed combustion systems because of the formation of the outermost Cr2O3 layer and the spinel CoCr2O4 beneath, whereas NiAl coatings’ anti-corrosion performance was significantly depleted due to the chlorine attack, and the resultant formation of Al2O3 layer at the coating/substrate interface finally led to coating spallation in both systems.

Identification of Circulating Fluidized Bed Boiler Bed Temperature Based on Hyper-Plane-Shaped Fuzzy C-Regression Model

Bed temperature in dense-phase zone is the key parameter of circulating fluidized bed (CFB) boiler for stable combustion and economic operation. It is difficult to establish an accurate bed temperature model as the complexity of circulating fluidized bed combustion system. T-S fuzzy model was widely applied in the system identification for it can approximate complex nonlinear system with high accuracy. Fuzzy c-regression model (FCRM) clustering based on hyper-plane-shaped distance has the advantages in describing T-S fuzzy model, and Gaussian function was adapted in antecedent membership function of T-S fuzzy model. However, Gaussian fuzzy membership function was more suitable for clustering algorithm using point to point distance, such as fuzzy c-means (FCM). In this paper, a hyper-plane-shaped FCRM clustering algorithm for T-S fuzzy model identification algorithm is proposed. The antecedent membership function of proposed identification algorithm is defined by a hyper-plane-shaped membership function and an improved fuzzy partition method is applied. To illustrate the efficiency of the proposed identification algorithm, the algorithm is applied in four nonlinear systems which shows higher identification accuracy and simplified identification process. At last, the algorithm is used in a circulating fluidized bed boiler bed temperature identification process, and gets better identification result.

Fluidized bed combustion of poultry litter at farm-scale: Environmental impacts using a life cycle approach

Combustion can concentrate phosphorus (P) from poultry litter into an ash product that is easier to transport for land application. This was the first life cycle assessment (LCA) study to investigate the efficacy and sustainability of a fluidized bed combustion (FBC) system using poultry litter to heat poultry houses and produce electricity, in the United States, to replace liquid propane gas (LPG) and natural gas (NG) use. The ‘Baseline scenario’ used results from a 16-month FBC monitoring study, and an ‘Improved scenario’ based on an increased biomass feed rate (0.246 tons/h), increased run-time (6720 h/y), and net positive electricity production. In the Baseline scenario, climate change potential was 32% and 44% lower than use of LPG and NG, respectively, for poultry house heating, but the low electricity production from the FBC system resulted in net electricity import and lower sustainability compared to LPG use in 12 of the 18 impact categories. The Improved scenario had 48–98% less environmental impacts than the Baseline scenario in the 18 categories. The sensitivity analysis showed that a 10% change in the electricity input for FBC operation resulted in the highest average change (4.8%) in the 18 impact categories, indicating the necessity of a net positive electrical energy output from the FBC unit for increased sustainability. The results also highlighted the impact of the replacement heating fuel type, as replacing NG through poultry litter combustion had a greater impact than replacement of LPG.

Modeling and dynamic characteristics of the dense phase region of a biomass-fired circulating fluidized bed combustion system using Modelica

Using Modelica language, a mathematical model combining static and dynamic contributions of the dense phase region of a 130 t/h biomass circulating fluidized bed boiler combustion system was established on MWorks simulation platform. The mathematical model adopted modular packaging to increase the universality of the model, and it used an implicit, high-order, and multi-step Dassl integration algorithm to conduct the simulation. Under the design condition parameters, the relative error between the bed temperature of the dense phase region obtained by the simulation model and the actual temperature was less than 3.8%, which indicated that the static characteristics of the established simulation model were accurate. The effects of biomass feed and primary air volume step changes on the bed temperature, oxygen content in the flue gas, height of the dense phase region, and the bed pressure difference in the dense phase region were investigated. Both the biomass feeding and the primary wind step of 10% reduced the temperature, and it was obvious that the primary wind had a greater impact on the bed temperature. Meanwhile, the primary wind had a greater impact on the bed pressure difference than the biomass feeding.

Distribution of PAHs in coal ashes from the thermal power plant and fluidized bed combustion system; estimation of environmental risk of ash disposal

The comparison of fly ash generated from lignite combustion in a thermal power plant Kolubara A (Veliki Crljeni) and bottom and fly ash from coal waste combustion in a semi-industrial fluidized bed boiler (Vinča) was performed as the function of particle size. The average total concentrations of the 16 EPA priority PAHs in ash fractions are 0.49 mg kg−1 of ash (thermal power plant) and 17.48 mg kg−1 of ash (fluidized bed boiler). The sum of 3- and 4-ring PAHs accounts for more than 93% of overall PAHs concentration, and the most abundant among them is fluoranthene.The portions of PAHs groups defined based on their physico-chemical properties, as obtained from quantitative structure-activity relationship (QSAR) models included in the Vega platform, were determined. These portions, emission factors, and benzo[a]pyrene equivalence concentrations were further on used to estimate the potential environmental impact of ash disposal. The PAHs emission factors are higher compared to values in the air pollutant emission inventory guidebook of the cooperative program for monitoring and evaluation of the long-range transmission of air pollutants in Europe (EMEP/EEA). The overall emission factors of 16 PAHs for combustion of lignite and coal waste are determined to be 0.15 and 249.97 mg kg−1 of fuel, respectively. Based on the ratios of benzo[a]pyrene equivalence concentrations of each ash and correspondent fuel, the disposal of fly ash from the cyclone of fluidized bed boiler represents the highest risk to the environment among tested ashes.

The role of alkali metal in N2O and NO emission during high-sodium Zhundong coals oxy-fuel circulating fluidized bed combustion

To explain the role of alkali metal in N2O and NO emission during Zhundong coals oxy-fuel circulating fluidized bed combustion, the influences of alkali metals on the fuel-N and recycled-N were investigated in the present work. Results were obtained based on the experiments performed on a 50 kW oxy-fuel circulating fluidized bed combustor system. First, the N2O and NO emission characteristics during high-sodium Zhundong coals oxy-fuel combustion were obtained. Though the emission characteristics varied with coal types due to the different coal properties, it can be concluded that high-sodium coal can achieve low N2O concentration. High inlet oxygen concentration was conducive to the decrease of N2O and NO concentrations. The simulated recycled SO2 can enhance the reduction of NO. Then, the influences of alkali metals on the fuel-N and recycled-N were identified via injecting NaCl/Na2SO4 during low-sodium coal combustion. Both NaCl and Na2SO4 can decrease CO and N2O concentrations but increase NO concentration. While there hardly exist any interactions between NaCl/Na2SO4 and simulated recycled NO.

Control of the Fluidized Bed Combustor based on Active Disturbance Rejection Control and Bode Ideal Cut-off

The fluidized bed combustor (FBC) boiler faces many control challenges such as high-order dynamics and strong nonlinearity. A hybrid control structure combining active disturbance rejection control (ADRC) and Bode ideal cut-off (BICO) is proposed to handle with these control difficulties and speed up the output responses. An empirical tuning procedure is summarized for the hybrid control structure. Simulation results show that the hybrid control structure has the best tracking performance compared with other control strategies. In addition, the hybrid control structure can obtain the satisfactory disturbance rejection performance when the FBC system has the coal quality variation or input disturbances. The superiority of the hybrid control structure can ensure the satisfactory control performance and shows a great potential in industrial applications.