Fluidized Bed Combustion Plant Research Articles

Economical Operation and Hazardous Air Pollutant Emissions of Biodegradable Sludge Combustion Process in Commercial Fluidized Bed Plant

Waste sludge is characterized by high moisture, volatile compounds, toxic compounds, and ashes. The efficient operation of a commercial fluidized bed combustion (FBC) plant is important for reducing operational costs. We selected a commercial FBC plant for industrial waste sludge combustion to investigate the mass balance of the FBC process and the performance of the air pollution control device. Based on fuel analysis, the flow rate of incineration air was calculated as 4567 Nm3/h. After FBC combustion, the flow rate of the incineration gas increased to 8493.8 Nm3/h. Analysis of the heat balance showed that some heat potential was lost through leakage during the combustion process. The temperature of the incineration gas decreased to 200 °C at the inlet of the air pollution control device. According to the hazardous air pollutant emission testing of sampling points, the operation factors of lime slurry injection for SOx and HCl in the semi-dry reactor were 64.20 and 4.81 kg/h, respectively. In the wet scrubber, the operation factors of NaOH for SOx and HCl were 23.88 and 3.14 kg/h, respectively. At these operation factors, the available waste generation in the semi-dry reactor and wet scrubber was optimized to 76.6 and 42.57 kg/h, respectively.

Comparing the quantity and quality of glass, metals, and minerals present in waste incineration bottom ashes from a fluidized bed and a grate incinerator

Bottom ash is the primary solid residue arising from municipal solid waste incineration. It consists of valuable materials such as minerals, metals and glass. Recovering these materials from bottom ash becomes evident when integrating Waste-to-Energy within the circular economy strategy. To assess the recycling potential from bottom ash, detailed knowledge of its characteristics and composition is required. The study at hand aims to compare the quantity and quality of recyclable materials present in bottom ash from a fluidized bed combustion plant and a grate incinerator, both located in the same city in Austria and receiving mainly municipal solid waste. The investigated properties of the bottom ash are grain-size distribution, contents of recyclable metals, glass, and minerals in different grain size fractions, and the total and leaching contents of substances in minerals.The study results reveal that most recyclables present are of better quality for the bottom ash arising at the fluidized bed combustion plant. Metals are less corroded, glass contains fewer impurities, minerals contain fewer heavy metals, and their leaching behavior is also favorable. Furthermore, recoverable materials, such as metals and glass are more isolated and not incorporated into agglomerates as observed in grate incineration bottom ash. Based on the input to the incinerators more aluminum and significantly more glass can potentially be recovered from bottom ash from fluidized bed combustion. On the downside, fluidized bed combustion produces about five times more fly ash per unit of waste incinerated, which is currently disposed of in landfills.

Dynamics of large-scale bubbling fluidized bed combustion plants for heat and power production

Bubbling fluidized bed combustion (BFBC) plants for combined heat and power (CHP) production have traditionally been dispatched under slow load changes. As the amount of variable renewable electricity increases in energy systems worldwide, knowledge regarding the transient capabilities of the gas and water-steam sides of BFBC plants is required. The aim of this work is to investigate the dynamic performance of large-scale BFBC plants when accounting for both the gas and water-steam sides. To do so, this paper presents a dynamic model of BFB-CHP plants that result from connecting a model of the gas side to a process model of the water-steam side. The plant model output is validated by comparisons with operational data measured in a 130-MWth BFBC plant that produces electricity, district heating (DH) water and steam for industrial clients. The validation shows that the model can satisfactorily describe both multi-load steady-state operation and load transients. The simulation results highlight the fact that the water-steam cycle achieves stabilization more rapidly after changes in the DH line and steam delivered to customers, as compared to changes in the combustor load. The timescales of the plant outputs for different changes have been calculated, with stabilization times ranging from 2 to 15 min for the power production versus 2–25 min characterizing the DH production. Compared to the stabilization times of the gas side, the water-steam side is an order of magnitude slower, thereby limiting the transient operation capabilities of BFB-CHP plants.

Dynamics and control of large-scale fluidized bed plants for renewable heat and power generation

As the share of variable renewable electricity increases, thermal power plants will have to adapt their operational protocols in order to remain economically competitive while also providing grid-balancing services required to deal with the inherent fluctuations of variable renewable electricity. This work presents a dynamic model of fluidized bed combustion plants for combined heat and power production. The novelty of the work lays in that (i) it provides an analysis of the transient performance of biomass-based fluidized bed combustion plants for combined heat and power production, (ii) the dynamic model includes a description of both the gas and water-steam sides and (iii) the model is validated against operational data acquired from a commercial-scale plant. The validated model is here applied to analyze the inherent dynamics of the investigated plant and to evaluate the performance of the plant when operated under different control and operational strategies, using a relative gain analysis and a variable ramping rate test.The results of the simulations reveal that the inherent dynamics of the process have stabilization times in the range of 5–25 min for all the step changes investigated, with variables connected to district heating production being the slowest. In contrast, variables connected to the live steam are the fastest, with stabilization times of magnitude similar to those of the in-furnace variables (i.e., around 10 min). Thus, it is concluded that the proper description of the dynamics in fluidized bed combustion plants for combined heat and power production requires modeling of both the gas and water sides (which is rare in previous literature). Regarding the assessment of control strategies, the boiler-following and hybrid control (combined fixed live steam and sliding pressure) strategies are found to be able to provide load changes as fast as −5%-unit/s, albeit while causing operational issues such as large pressure overshoots. The relative gain analysis outcomes show that these control structures do not have a steady-state gain on the power produced, and therefore it is the dynamic effect of the steam throttling that triggers the rapid power response. This study also includes the assessment of a turbine bypass strategy, the results of which show that it enables fast load-changing capabilities at constant combustion load, as well as decoupling power and heat production at the expense of thermodynamic losses.

Investigation of the Oxygen Supply and Distribution in a Bubbling Fluidized Bed by Using Natural Ilmenite for Oxygen Carrier Aided Combustion

The application of oxygen carrier materials has one of its origins in the chemical looping combustion processes, which aim at inherent CO2 sequestration by realizing fuel oxidation in the absence of combustion air. This concept can be transferred to conventional fluidized bed combustion processes, the so-called oxygen carrier aided combustion. The replacement of the conventionally applied bed material with an oxygen carrier focuses mainly on large-scale circulating fluidized bed combustion plants. The main stated performance improvements include the reduction of carbon monoxide emissions even at lower excess air ratios and the enhancement of the combustion efficiency. Nevertheless, the ability of shifting oxygen in space and time is of certain interest especially in small-scale bubbling fluidized bed systems. Imperfect fuel mixing, limited residence time in the dense bed zone, as well as limited system complexity often mitigate their combustion performance. Process conditions in bubbling fluidized beds differ from those prevalent in circulating beds regarding bed material particle size distribution, fluidization velocities, or particle volume fraction in the bed. Within this study, the widely used oxygen carrier ilmenite serves as bed material in a laboratory bubbling fluidized bed combustion. The focus was on the oxygen distribution profiles and their shift by ilmenite into the bed during methane combustion. The variation of the excess air ratio, fluidization velocity, and bed height reveal a significant increase of the in-bed CO2 yield (max. 55%) within ilmenite experiments. Moreover, the oxygen conversion profile confirms the shift of the oxygen supply by ilmenite from the lower reactor part to the reduction dominated zone. The amount of shifted oxygen increases at lower excess air ratios. Thus, full conversion is not achieved as kinetics and limited residence time in the bed inhibit the fuel conversion.

Controlling NOx emission from boilers using waste polyethylene as reburning fuel

Literature lacks data on the performance of waste plastics in controlling NOx emissions in practical combustors operating typically in the temperature window of 1000–1200 °C, under a range of fuel-oxygen equivalence ratios, preventing recycling of plastics in this application. In this contribution, we demonstrate that waste plastics, in particular polyethylene, can serve as a reburning fuel for converting nitrogen oxides (NOx) into environmentally-benign combustion products, in practical large-scale combustors such as pulverised coal power plant, circulation fluidised bed combustion plant, entrained flow boilers, and incinerators. The experiments involved a high-temperature vertically-entrained reactor operating in the range of 600–1200 °C, in conjunction with online infrared spectroscopy, chemiluminescence and gas chromatography. Chemical kinetic modelling, supported by DFT calculations with DMol3 package revealed the underlying chemistry of the NOx mitigation reactions, especially the importance of C2H5 and C2H3 radicals. We modelled the process kinetically and performed a techno-economic assessment of the new technology, to prove its financial feasibility. The reaction of pyrolytic fragments with NOx yields excellent removal efficiency of nitrogen oxides of up to 82% and selectivity to N2 up to 85%, within the temperature range of 1000 °C–1200 °C, and fuel-oxygen equivalent ratios of Φ = 0.8–1.2. While we observed the formation of HCN, the overall nitrogen selectivity shifts towards the formation of N2. Both the conversion of NOx and the selectivity to N2 can be improved further by increasing the residence time. The economic assessment indicates that, the use of waste plastics is comparable to other mainstream solid fuels and becomes less expensive when considering renewable benefits.

Condensed phase corrosion of P91 and DSS 2205 steels at advanced oxygen‐fired pressurized fluidized bed combustion plants

AbstractTo achieve the targets of high energy efficiency and reduced CO2 emission, advanced oxygen‐fired pressurized fluidized bed combustion technology is being developed. The generated flue gas condensates are very corrosive, but very limited information is available to select appropriate alloys for the cost‐effective construction and long‐term safe operation of flue gas components. Thus, this study investigated the corrosion performance of P91 and DSS 2205 steels in the simulated condensates at 60°C–150°C. The dominant reactions on the two steels were considerable oxide formation and high chemical dissolution of the formed oxides instead of localized pitting. The increase in temperature leads to an exponential increase in the long‐term corrosion rates of the steels. Benefited from its high Cr and Mo contents, DSS 2205 steel exhibited much better corrosion resistance, and the formed surface scales consisted of inner Fe‐enriched and outer Cr‐enriched oxides in which Cr2O3 was transformed into Cr(OH)3 with the increase in temperature. The corrosion products on P91 steel consisted of inner Cr–Fr–Mo oxides and outer Fe‐enriched oxides, which were porous and unable to protect the steel.

Corrosion of duplex stainless steel 2205 in hot flue gas environments produced at advanced oxy-fired pressurized fluidized bed combustion plants

The selection of constructional material of flue gas components is crucial for the successful deployment and long-term safe operation of the newly developed oxy-fired pressurized fluidized bed combustion plants. In this study, the corrosion performance of a candidate steel, duplex stainless steel 2205 (DSS 2205) was investigated in simulated flue gas mainly composed of CO2/H2O/O2/N2/SO2/HCl with a total pressure of 1.50–1.64 MPa at 270−320 °C. The oxidation kinetics of DSS 2205 followed parabolic law, and the formed surface oxide was a double-layer structure with outer Fe-rich oxide and inner Cr-rich oxide. Besides general oxidation, SO2 and HCl-induced pitting also occurred. Increasing temperature led to enhanced general oxidation and accelerated pitting. Extending exposure duration resulted in increased pit density, but did not noticeably affect the maximum pit depth. These results indicate that the steel is a promising candidate for further pilot- and industrial-scale assessments.

The Use of Biomass as a Renewable Energy Source in a Fluidized Bed Combustion Plant

The Use of Biomass as a Renewable Energy Source in a Fluidized Bed Combustion Plant

Combustion of Refined Renewable Biomass Fuel (RRBF) in a bubbling fluidized bed

The outlet of a mechanical biological treatment plant for mixed municipal solid waste is further processed to produce RRBF (Refined Renewable Biomass Fuel) within the frame of the EU Life+ project MARSS (Material Advanced Recovery Sustainable Systems). The input for the MARSS processing is dry and has small particle sizes below 40 mm, which makes it impossible to be burnt in standard grate firing systems. The main purpose of this study was to examine whether RRBF is a suitable fuel for bubbling fluidized bed combustion for the decentralised production of combined heat and power. RRBF was fed into a bubbling fluidized bed combustion plant with a nominal fuel input of 100 kW in 3 combustion tests. Proximate and ultimate analyses were performed for original RRBF and fly ash. The fuel analyses showed high ash contents between 25 and 37 weight-%, while the lower heating value lay in the range of 10.5–12.9 MJ kg−1. The ash softening temperature was above 1150 °C and therefore no bed agglomeration was observed. Combustion at around 900 °C could be maintained without preheating of the combustion air. The carbon content of the fly ash was about 1 %, which indicates complete combustion in the bubbling fluidized bed with sufficient residence time. The content of phosphorous in the fly ash was above 1 % and therefore this is an interesting material for prospective phosphorous recovery, potentially together with sewage sludge or ash from sewage sludge combustion. RRBF is a suitable solid fuel for fluidized bed combustion.

Enrichment of thallium in fly ashes in a Spanish circulating fluidized-bed combustion plant

This work evaluates the behavior of thallium in a 50MW industrial circulating fluidized-bed combustion plant (CFBC), focusing on the distribution of this element among the bottom and fly ashes separated by the solid retention devices in the plant. The results show that thallium species are mainly retained in the solid by-products and are not emitted to air with flue gases in significant amounts, proving that this technology is a more effective means of preventing thallium emissions than pulverized coal combustion technology (PCC). The mass balance of the thallium content in the solids shows that this element was retained in the ashes separated by the different devices installed in the plant. An evaluation of the ash fractions taken from the strippers, the heat recovery area, the hoppers in the air heater and the electrostatic precipitator, shows that thallium was relatively homogeneously distributed in all the ash samples, independently of their composition, but is slightly related to surface area, which in turn is dependent on particle size and unburned carbon content.

Behavior of Inorganic Matter in a Dual Fluidized Steam Gasification Plant

Ash components of biomass fuels can cause fouling, slagging, and bed material agglomeration during thermal utilization in fluidized bed combustion and gasification plants. The influence of ash components on these problems in dual fluidized bed biomass gasification plants is investigated in an industrial scale plant in Gussing, Austria. Samples of fouling are analyzed, and the results are evaluated. The samples were analyzed by X-ray fluorescence analysis and thermal analyses such as thermogravimetric analysis, differential thermal analysis, and differential scanning calorimetry. Mass balances of inorganic matter are presented, evaluating different loop configurations. The analyses showed high potassium contents compared to the fuel ash composition in fouling of up to 23% by weight. The potassium content of fly ash with a particle size smaller than 200 μm is half that of coarse fly ash with a particle size larger than 200 μm. The thermal analyses showed a large difference between samples of inorganic strea...

Avoiding Mercury Emissions by Combustion in a Spanish Circulating Fluidized-Bed Combustion (CFBC) Plant

This work evaluates the behavior of mercury in an industrial circulating fluidized-bed combustion plant (CFBC) of 50 MW situated in Oviedo, Spain. The results showed that mercury was not emitted in the gas phase in significant amounts, thereby proving this technology as an effective means of preventing mercury emissions. Most of the mercury (85–97%) was retained in the fly ashes. The proportion of mercury emitted in the gas phase was on the order of 0.2–0.3 μg m–3, whereas only 0.02 ng m–3 at most was emitted in the particulate matter. An evaluation of the fly ash fractions separated from the hoppers of the electrostatic precipitator showed the highest proportion of mercury to be present in the fly ashes with the lowest particle size and the highest carbon content. However, this mercury proved to be stable when subjected to leachability tests.

Speciation of mercury in fly ashes by temperature programmed decomposition

Mercury (Hg) is a toxic trace element which is emitted mostly in gas phase during coal combustion, although some Hg compounds may be retained in the fly ashes depending on the characteristics of the ashes and process conditions. To improve the retention of Hg in the fly ashes a good knowledge of the capture mechanism and Hg species present in the fly ashes is essential. The temperature programmed decomposition technique was chosen to identify the Hg species present in fly ashes obtained from two Pulverized Coal Combustion (PCC) plants and a Fluidized Bed Combustion (FBC) plant. The fly ashes were then used as Hg sorbents in a simulated flue gas of coal combustion and gasification. The Hg compounds found in the fly ash from the FBC plant after elemental mercury retention were mainly HgCl2 and HgSO4. The Hg species present in the two fly ashes from the two PCC plants were HgCl2 and Hg0. The Hg species formed in the coal gasification atmosphere was HgS for all three fly ashes. The only Hg compound identified in the fly ashes after the retention of mercury chloride was HgCl2.

Robust Constrained Fuzzy Affine Model Predictive Control With Application to a Fluidized Bed Combustion Plant

In this paper, robust constrained model predictive control of uncertain fuzzy affine systems is considered. Based on piecewise quadratic Lyapunov functions, highly efficient robust constrained predictive control approaches are developed so that the closed-loop stability is guaranteed, and the transient control performance is improved even under input or state constraints. Moreover, by using approximate ellipsoid and S -procedure, the solution to the fuzzy affine model based predictive control can be cast as a convex optimization problem subject to some linear matrix inequalities, rather than a nonconvex problem via bilinear matrix inequalities as in conventional fuzzy affine model-based control. The proposed controllers are thus easier for real-time implementation in industry. Simulation results on the fluidized bed combustion plant have demonstrated the performance of the proposed approaches.

Determination of the Cd-Bearing Phases in Municipal Solid Waste and Biomass Single Fly Ash Particles Using SR-μXRF Spectroscopy

By using an excitation energy of 27.0 keV, synchrotron radiation-induced micro-X-ray fluorescence (SR-microXRF) is employed to extract information regarding the composition and distribution of Cd-bearing phases in municipal solid waste (MSW) and biomass fly ashes. Significance of observation is based on statistics of totally more than 100 individual MSW and biomass fly ash particles from a fluidized bed combustion (FBC) plant. Cd concentrations in the parts-per-million range are determined. In general, although previous leaching studies have indicated Cd to be predominant in the smaller-size ash particles, in the present study Cd is more evenly distributed throughout all the particle sizes. For MSW fly ashes, results indicate the presence of Cd mainly as CdBr2 hot-spots, whereas for biomass fly ashes, which exhibit lower CdX2 concentration, a thin Cd layer on/in the particles is reported. For both ashes, Ca-containing matrixes are found to be the main Cd-bearing phases. Support for this observation is found from independent first-principles periodic density functional theory calculations. The observations are condensed into a schematic mechanism for Cd adsorption on the fly ash particles.

A method for agglomeration detection and control in full-scale biomass fired fluidized beds

The occurrence of agglomeration in fluidized-bed combustion and gasification of biomass and waste can have a very negative impact on the efficiency of reactor operation. The formation of agglomerates seriously affects the otherwise uniform heat distribution in the bed and may finally lead to defluidization and unscheduled shutdown of the plant. A recently developed monitoring method is able to detect the onset of such undesired behavior, making it possible to operate more efficiently. This Early Agglomeration Recognition System (EARS) uses a characteristic process variable (e.g., pressure) measured at high frequency. The sensitivity and selectivity of EARS is illustrated with experimental results and the first industrial test results in an 80-MW th wood-fired fluidized-bed combustion plant is discussed.

Ash Transformations during Combustion of Meat-, Bonemeal, and RDF in a (bench-scale) Fluidized Bed Combustor

Following the recent Bovine spongiform encephalopathy (BSE) experiences, thermal treatment of meat- and bonemeal (MBM) in existing fluidized bed combustion (FBC) plants for refuse-derived fuels (RDFs) has evolved as an interesting disposal and disintegration method. However, only a limited number of studies have previously been performed for combustion of MBM in fluidized beds. The objectives of the present work were, therefore, to determine the bed agglomeration tendencies of these materials during combustion in fluidized beds and to evaluate the effects of dolomite and kaolin addition to the fuel mix, as well as to elucidate the overall ash transformation mechanisms governing the potential bed agglomeration and fouling processes. By controlled agglomeration experiments in a 5 kW bench-scale fluidized bed reactor, the fuel-specific critical agglomeration temperatures in normal quartz bed material were determined for the different fuel/additive mixtures. All collected samples of bed materials, final bed agglomerates, and cyclone ashes were analyzed using SEM/EDS and XRD. The results indicated that the MBM fuels could be expected to be problematic concerning bed agglomeration in normal quartz beds, while kaolin and possibly dolomite addition could be used to reduce this risk to moderate levels. A significant elemental fractionation between the bed material and the cyclone ash was obtained. Apatite (Ca5(PO4)3(OH) or potentially some other calcium phosphates are elutriated from the bed and enriched in the fly ash, while sodium and potassium are enriched in the bed material. The characteristics and the corresponding melting behavior estimations of the necks formed between agglomerated bed particles suggest that silicate melts are responsible for the bed agglomeration. Results from XRD analysis of the fly ash formed from the fuels used in the present study indicated that the risk for melt-related fly ash problems seem relatively small.

Cascade Impactor Sampling to measure Polycyclic Aromatic Hydrocarbons from Biomass Combustion Processes

The increased use of biomass as a fuel in thermochemical conversion processes over the coming years, makes the research of a clean and efficient process necessary. The experiments were carried out in a pilot atmospheric fluidised bed combustion plant of 1 MW[th], where the parameters had been optimised to obtain a clean biomass combustion process. All types of combustion produce organic compounds, especially polycyclic aromatic hydrocarbons (PAHs). The distribution of PAHs was isolated from the eight size particle fractions collected in the fly ash emitted from the biomass combustion process. The particle size distribution was carried out with a multistage cascade impactor system (Andersen Mark III) with a diameter cut ranging from 0·01 to 4·00 nm. It was observed that most of the heavy PAHs are adsorbed on particles having an aerodynamic diameter of less than 2 μm and an unimodal distribution was deduced from the results of the scanning mobility particle size. Polycyclic aromatic hydrocarbons pollutants were studied on a gas chromatograph coupled to a mass spectrometer operating in an ion monitoring mode. High concentrations of heavy PAHs were measured in a small particle size distribution. A high combustion efficiency reduced the PAH emissions. Under stable combustion conditions, the particle emission in the size range 0·1–1 μm was increased and mass concentration was reduced considerably. This was due to a reduction in the number of coarse particles.

Electrostatic precipitator collection efficiency and trace element emissions from co-combustion of biomass and recovered fuel in fluidized-bed combustion.

Particle and trace element emissions from energy production have continuously been subject to tightening regulations. At the same time, not enough is known on the effect of different combustion processes and different fuels and fuel mixtures on the particle characteristics and particle removal device operation. In this investigation, electrostatic precipitator fractional collection efficiency and trace metal emissions were determined experimentally at a 66 MW biomass-fueled bubbling fluidized-bed combustion plant. The measurements were carried out at the inlet and outlet of the two-field electrostatic precipitator (ESP) at the flue gas temperature of 130-150 degrees C. Two fuel mixtures were investigated: biomass fuel containing 70% wood residue and 30% peat and biomass with recovered fuel containing 70% wood residue, 18% peat, and 12% recovered fuel. The particle mass concentration at the ESP inlet was 510-1400 mg/Nm3. Particle emission at the ESP outlet was 2.3-6.4 mg/Nm3. Total ESP collection efficiency was 99.2-99.8%. Collection efficiency had a minimum in particle size range of 0.1-2 microm. In this size range, collection efficiency was 96-97%. The emission of the trace metals As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Tl, and V was well below the regulation values set by EU directive for waste incineration and co-incineration.