Freeboard Temperature Research Articles

Secondary air induced flow structures and their interplay with the temperature field in fixed bed combustors

Air staging in solid fuel combustion features widely in small scale domestic boilers to large scale moving grate combustors. Whilst the effects of air staging on the combustion characteristics of these systems are generally known, there is very little insight available into the role of secondary air on the flow and temperature field in the freeboard of batch-type fixed bed biomass combustors. Three-dimensional gas phase simulations using the Transition 𝑘kl-𝜔 and Finite Rate/Eddy Dissipation models were validated against freeboard temperatures and emissions (CO2 and O2) measured on the same set-up. Results show that secondary air at Qs/Qt ≥0.25 induces two recirculation zones, upstream and downstream of its injection point with maximum freeboard temperatures generally observed around these recirculation zones, if Qs/Qt drops to 0.12-0.18 only an upstream recirculation zone is observed until it diminishes by Qs/Qt = 0.06. However, there is a trade-off caused by what appears to be a cooling effect if Qs/Qt is increased between 0.25 and 0.71. Modelling results show that in reacting cases, unlike non-reacting modelling on the same geometry, the strength of the secondary air induced upstream recirculation zone appears significantly stronger.

Role of Air Staging in a Batch-Type Fixed Bed Biomass Combustor under Constant Primary Air

Staged combustion of biomass is the most suitable thermo-chemical conversion for achieving lower gaseous emissions and higher fuel conversion rates. In a staged fixed bed combustion of biomass, combustion air is supplied in two stages. In the first stage, primary air is provided below the fuel, whereas in the later stage, secondary air is supplied in the freeboard region. The available literature on the effects of air staging (secondary air location) at a constant primary air flow rate on combustion characteristics in a batch-type fixed bed combustor is limited and hence warrants further investigations. This study resolves the effect of air staging, by varying the location of secondary air in the freeboard at five secondary to total air ratios in a batch-type fixed bed combustor. Results are reported for the effects of these controlled parameters on fuel conversion rate, overall gaseous emissions (CO2, CO and NOx) and temperature distributions. The fuel used throughout was densified hardwood pellets.Results show that a primary freeboard length (distance between fuel bed top and secondary air injection) of 200 mm has higher fuel conversion rates and temperatures as well as lower CO emissions, at a secondary to total air ratio of 0.75 as compared to primary freeboard length of 300 mm. However, NOx emissions were found to be lower for a primary freeboard length of 300 mm as compared to 200 mm. An increase in secondary to total air ratio from 0.33 to 0.75 resulted in higher freeboard temperatures and lower CO as well as NOx emissions. The outcomes of this study will be helpful in the effective design of commercial scale biomass combustors for more efficient and environmentally friendly combustion.

Progress Variables to Resolve the Steady State Period in a Batch-Type Fixed Bed Combustor

ABSTRACT Lab scale combustion of solid fuels, such as biomass, is mostly performed using fixed bed combustors. Steady state data detailing various progress variables (temperature and emissions) are widely available within the literature. However, there is very little information available on how the onset of the steady state operation is identified especially in relation to batch operation, which is subject to depleting fuel bed. This paper uses experiments coupled with a systemic post-processing approach based on MATLAB in order to identify the onset of the steady state operation. The results show that by using both the percentile mean deviation and slope of certain measured progress variables (including the fuel bed temperature, freeboard temperature, fuel mass loss, and emissions) the steady state regime can be accurately predicted. The use of freeboard temperature data alone is not sufficient. Based on the methods developed, radiation error in thermocouple data was found to be significant in both non-staged and staged air combustion modes of fixed bed combustor. Moreover, the radiation error was lower for downstream thermocouples in contrast to upstream thermocouples.

Role of Primary Freeboard on Staged Combustion of Hardwood Pellets in a Fixed Bed Combustor

In staged fixed bed biomass combustion, primary air is supplied beneath the fuel bed with secondary air then provided above in the freeboard region. For fixed bed configurations, the freeboard is further divided into a primary freeboard length (LI), which is upstream of the secondary air and a secondary freeboard length (LII), measured from the secondary air all the way to the exhaust port. Despite extensive research into fixed bed configurations, no work has been successfully completed that resolves the effects of changing LI on fuel conversion, both in the fuel bed and within the freeboard of batch-type biomass combustors. In this study, experiments on a 202 mm diameter and 1500 mm long batch-type combustor have been conducted to determine the effects of changing primary freeboard length over three secondary to total air ratios (Qs/Qt) and two total air flow rates (Qt). The impact of these conditions has been studied on (i) intra-bed fuel conversion, measured through burning rate (kg/m−2 s−1), fuel bed temperature (°C) and ignition front velocity (mm-s−1), as well as (ii) post-bed fuel conversion in the freeboard, expressed through freeboard temperatures and emissions (NOx ppm, CO2%, CO ppm, O2%). The fuel used throughout the above experiments was Australian hardwood pelletised biomass. Results show that changes to primary freeboard length over LI = 200 mm, 300 mm and 550 mm, or LI/D = 1.00, 1.48 and 2.72, respectively, affect both intra-bed and freeboard (post-bed) performance indicators. The highest values of burning rate, ignition front velocity and fuel bed temperature were observed for interim values of LI/D = 1.48 at Qs/Qt = 0.25 and Qt = 0.358 kg/m−2 s−1. Primary freeboard lengths of LI/D = 1.00 and 1.48 were found to have higher freeboard temperatures, NOx and CO2 as well as lower CO and O2 values as compared to LI/D = 2.72 at Qs/Qt = 0.50 and 0.75. Increasing Qs/Qt from 0.25 to 0.50 for LI/D = 1.00 and 1.48 initially increased freeboard temperatures, with an accompanying increase in NOx and CO2 as well as decrease in CO values. However, further increase in Qs/Qt to 0.75 lead to lower freeboard temperatures for all primary freeboard lengths.

Experimental verification of the impact of the air staging on the NOx production and on the temperature profile in a BFB

The results of an experimental research on air staging in a bubbling fluidized bed (BFB) combustor are presented within this paper. Air staging is known as an effective primary measure to reduce NOX formation. However, in the case of a number of industrial BFB units, it does not have to be sufficient to meet the emission standards. Then selective non-catalytic reduction (SNCR) can be a cost-effective option for further reduction of the already formed NOX. The required temperature range at the place of the reducing agent injection for an effective application of the SNCR without excessive ammonia slip is above the temperatures normally attained in BFBs. The aim of this paper is to evaluate the impact of staged air injection on the formation of NOX in BFB combustors and to examine the possibility of increasing the freeboard temperature. Several experiments with various secondary/primary air ratios were performed with a constant oxygen concentration in the flue gas. The experiments were carried out using wooden biomass and lignite as fuel in a 30 kWth laboratory scale BFB combustor. Furthermore, the results were verified using a 500 kWth pilot scale BFB unit. The results confirmed that the air staging can effectively move the dominant combustion zone from the dense bed to the freeboard section, and thus the temperatures for an effective application of the SNCR can be obtained.

Analytical study on the Heat-Transfer Characteristics of a Fluidized Bed Reactor Heated by Multi-Stage Resistance

A fluidized bed heated by multi-stage resistance heaters was utilized innovatively to realize independent heating of different inside zones. A steady-state physical and mathematical model was developed to study the effect of dense phase, freeboard, fluidized carrier gas on temperature distribution in the fluidized bed. The results showed that compared with the experimental values, the differences between the calculated temperature, heat transfer coefficient and fluid velocity at the outlet of fluidized bed by the model were within 15%, 17%, and 14% respectively, indicating that the model had high reliability. The model inversion illustrated that the heat transfer coefficient at the outlet of the fluidized bed increased gradually in the order of the fluidized carrier gas temperature, dense phase temperature, fluidized carrier gas velocity and freeboard temperature. The heat transfer simulation took the height of the dense phase and the freeboard as the input parameters of the model by hundreds of iteration, the calculated results confirmed again that the existence of particles in the dense phase increased its heat transfer efficiency with the gas and the wall, ultimately making the dense phase as the main heat transfer zone.

Experimental study of the NOX reduction through the staged oxygen supply in the oxy-fuel combustion in a 30 kWth bubbling fluidized bed

This paper presents a comprehensive experimental study of the NOX reduction through the staged injection of oxygen in the oxy-fuel combustion of lignite and wooden pellets in a 30kWth bubbling fluidized bed experimental facility. A number of experiments was performed examining the impact of relevant operational parameters on the efficiency of NOX reduction. Various secondary to primary oxygen ratios were selected while keeping the overall concentration of oxygen in dry flue gas the same for constant ratio of O2/CO2 concentration in fluidizing gas and for constant fluidized bed temperature. Besides that, the impact of the overall oxygen stoichiometry was studied.The results showed that oxygen staging can be used successfully to reduce NOX emissions. Within the experiments, the NOX reduction efficiency as high as 50% was achieved (compared to the operation without oxygen staging) for the ratio of secondary and primary oxygen 0.5 in case of lignite and 1 in case of wood combustion. The extensive staged injection of oxygen also caused a significant raise of the freeboard temperature. There, it was measured about 900 °C for the lignite combustion and 880 °C in the fluidized bed, and about 950 °C for the combustion of wood and 880 °C in the fluidized bed. Oxygen staging is a promising path to reduce NOX in the oxy-fuel combustion in bubbling fluidized beds or to provide convenient conditions for the application of non-selective catalytic reduction in the freeboard section of the fluidized bed facility.

Effect of different gasifying agents (steam, H2O2, oxygen, CO2, and air) on gasification parameters

Two sensitivity analyses were performed in an Aspen simulation of fluidized bed gasification for five different gasifying agents such as steam, hydrogen peroxide (H2O2), pure oxygen (O2), carbon dioxide (CO2), and air. In the first sensitivity analysis, the modified equivalence ratio (MER) was varied (0.22-0.36). For the varied modified equivalence ratio (MER), %hydrogen, H2/CO molar ratio, and hydrogen yield were the highest in steam-gasification, but %carbon monoxide, %methane, CO yield, and the lower heating values (LHV) were the highest in CO2-gasification. In the second sensitivity analysis, the freeboard temperature was varied (500-900 °C). With increasing freeboard temperature, %hydrogen and %carbon monoxide increased while %carbon dioxide and %methane decreased for all the gasifying agents. Also, with increasing freeboard temperature, the LHV decreased and the hydrogen yield, CO yield, and the gas production rate increased for all the gasifying agents, but the H2/CO molar ratio increased only in oxygen, air, and CO2-gasification.

Analysis of Coke Oven Gas Injection from Dome in COREX Melter Gasifier for Adjusting Dome Temperature

Coke oven gas (COG) injection from the dome in a COREX melter gasifier is a good method to not only reduce the amount of solid fuel used for gasification, but also to adjust the freeboard temperature that should be maintained within a certain range. In this paper, the characteristics of COG injection used to adjust dome temperature are studied through a static model. The results show that an increase in melting rate causes a decrease in dome temperature and generator gas volume. However, with an increase in coke rate, dome temperature increases, while generator gas volume decreases. For low coke rate conditions, the dome temperature is generally found to be lower than 1050 °C, and COG is blasted for combustion and heat releasing. For high coke rates, the dome temperature is generally higher than 1050 °C. Under the premise that oxygen reduction cannot meet the demand, COG is used for decomposition. After the COG injection, the amount and reduction capacity of the generator gas can meet the needs of reduction in shaft furnace. The findings of this work can be used as a theoretical basis to guide plant operations for COG injection.

Oxy-fuel combustion study of biomass fuels in a 20 kWth fluidized bed combustor

Oxy-fuel combustion is one of the promising carbon capture technologies considered to be suitable for future commercial applications with stationary combustion plants. Although more and more biomass and waste are now being burned in stationary combustion plants, research on oxy-fuel combustion of biomass has received much less attention in comparison to oxy-fuel combustion of coal. In this work, a series of tests was carried out in a 20 kWth fluidized bed combustor under oxy-fuel conditions firing two non-woody fuels (miscanthus and straw pellets) and one woody fuel (domestic wood pellet). The effects of the combustion atmosphere (air and oxy-fuel) and oxygen concentration in the oxidant of the oxy-fuel combustion on gas emissions and temperature profiles were systematically studied with the overall excess oxygen coefficient in the combustor being maintained roughly constant throughout the tests. The experimental results showed that replacing the air with an oxy-fuel oxidant of 21 vol% O2 and 79 vol% CO2 resulted in a significant decrease in combustion temperature and ultimately led to the extinction of the biomass flame due to the larger specific heat of CO2 compared to N2. To keep a similar temperature profile to that achieved under the air combustion conditions, the oxygen concentration in the oxidant of O2/CO2 mixture had to be increased to 30 vol%. A drastic decrease in CO emissions was observed for all three biomass fuels (up to 80% reduction when firing straw) under oxy-fuel combustion conditions providing that the oxygen concentration in the oxidant of O2/CO2 mixture was above 25 vol%. NOx emissions were found to decrease with the oxygen concentration in the oxy-fuel oxidant, due to i) the increase of bed temperature, which implies more volatile-N released and converted in the dense bed zone and ii) the less dilution of the gases inside the dense bed zone, which leads to a higher CO concentration in this region enhancing the reduction of NOx. Similar NOx emissions to those obtained with air combustion were found when the oxygen concentration in the oxy-fuel oxidant was kept at 30 vol%. Further analysis of the experimental results showed that the gas emissions when firing the non-woody fuels were controlled mainly by the freeboard temperature instead of the dense bed region temperature due to the characteristically high volatile matter content and fines of this kind of biomass fuels.

Co-Combustion of Tannery Sludge in a Bench-Scale Fluidized-Bed Combustor: Gaseous Emissions and Cr Distribution and Speciation

In this study, eleven coal mono-combustion tests and four co-combustion tests of coal and tannery sludge were conducted on a 35kW fluidized bed combustor. The combustion behavior and emission characteristics of the fuels were investigated on a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB). The effects of an excess air ratio, primary air rate, secondary air ratio and fuel type on flue gas emissions were studied. The results showed that the fluidization status and temperature distribution had direct influences on CO emission. Sufficient fluidization and high temperature effectively reduced CO emission. NOx emission was relatively sensitive to the excess air ratio and increased with increasing excess air ratio. By comparing BFB and CFB, we found that CFBs have an advantage in optimizing combustion and controlling emissions by enhancing mixing and increasing freeboard temperatures. The Cr speciation and distribution among different ash types were extensively investigated in four co-combus...

Methodologies for Processing Fixed Bed Combustor Data

ABSTRACTExperiments are conducted on a laboratory-scale fixed bed combustor. This article discusses some of the data analysis methodologies that define the onset of steady state conditions, which are widely reported in the literature but poorly defined. The effects of using different methods to determine the onset of the steady state performance, and to correct thermocouple data so as to account for radiative effects, are also presented. Results show that using a combination of CO and NO emissions with free-board temperatures (at multiple axial positions) is more effective than other emissions, using temperatures (only) or the fuel burning rate (kg m–2 s–1). An important indicator is to analyze the percentile deviation of temperatures and emissions, compared to only using the time evolution of these variables. To characterize the significance of correcting thermocouple data for radiative (wall) losses, two numerical models (aspirated and bare bead) were compared. Radiative effects on thermocouples were found to be most prominent nearer to the downstream secondary air inlet due to the cooler wall temperature. Due to the likely radiative effects from freeboard deflectors, the sensitivity of these outcomes to the presence of downstream deflectors is also presented.

The Combustion Performance of Pelletized Jatropha Seed Residue in a Vortexing Fluidized-Bed Combustor

Jatropha curcas is considered a promising energy crop. The response surface methodology is introduced to study the combustion performance of pelletized Jatropha seed residue in a vortexing fluidized-bed combustor, and regression models for the bed temperature (Tb), the combustion fraction (Yb), and the NOx emissions are provided. The result shows that the combustion efficiency is above 99.5%, indicating that JSR pellets burn very well in the combustor. CO can be eliminated by stage combustion accompanied by a controlled freeboard temperature, but NOx increases correspondingly. Tb and Yb have similar trends, showing the interaction between them. The variation in the NOx emissions can be attributed to the shift in volatile-N combustion. In summary, the optimum operating conditions lie slightly above the line of Tb equal to 800 °C.

Combustion of Municipal Solid Waste in a Pilot Scale Fluidized Bed Combustor

Combustion study of municipal solid waste combustion in a pilot scale fluidized bed combustor had been carried out. The work was aimed at demonstrating sustainable combustion of municipal solid waste by employing operating parameters gained from previous studies. The primary and secondary air factor used were AF = 0.8 and AF = 0.6 respectively. The fluidization number was 5Umf and both in-bed and freeboard region temperature distributions were monitored continuously. Results on the combustion studies revealed that the initial bed temperature could be sustained due to high thermal capacity of sand but later dropped due to problem related to the mixing of bulky and heterogeneous components of municipal solid waste and sand.

Influence of ash agglomerating fluidized bed reactor scale‐up on coal gasification characteristics

To study the influence of fluidized‐bed reactor scale‐up on coal gasification characteristics, a model of the ash agglomerating fluidized‐bed reactor has been developed using an equivalent reactor network method. With the reactor network model, the scale‐up effects of a gasifier were studied in terms of the characteristics of the chemical reactions in the jet zone, the annulus dense‐phase zone and the freeboard zone. Results showed that the changes occurred in the inequality proportion of the volume of the jet zone during the reactor scale‐up. Taking into consideration the utilization of a portion of the backflow gas, the expansion of the jet zone volume and the coal particle residence time, the temperature of the jet zone was increased from 1592 to 1662 K. Also, both the annulus dense‐phase zone temperature and the freeboard zone temperature decreased, causing subsequent decrease in the carbon conversion efficiency. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1821–1829, 2014

Controlling the freeboard temperature applying a novel design of fluidized bed

An experimental study on controlling the temperature of gases in the freeboard applying a novel fluidized bed configuration has been carried out. A bubbling fluidized bed reactor of 300mm ID is equipped with a jet pipe of 38.1mm ID to adopt the novel configuration. A part of air is fed through the jet pipe to be issued vertically near the bed surface creating a large fountain of particles in the freeboard. The remaining part of air is fed through the distributor plate to fluidize bed solids. A third stream of hot air at 300°C is introduced in the freeboard beyond the main bed to simulate a heat generation source. The hot air is evenly distributed over the reactor cross section using a special pipe. Due to contacting the hot air with the entrained particles, its temperature is dampened while the bed solids heat-up. After a certain period of time the bed temperature profile evens out. The obtained results demonstrate that the fountain particles are very effective in transporting heat from the freeboard gases to the bed that minimizes the temperature difference between them, i.e., better control of the freeboard temperature. The bed stabilizes at a higher temperature while the freeboard temperature is better controlled with increasing the jet velocity and with decreasing bed particles sizes. Moreover, by applying the novel configuration the freeboard temperature can be effectively controlled to a higher extent along the reactor height compared with the splashing zone of conventional operation.

Analysis on Inherent Characteristics and Behavior of Recycling Dust in Freeboard of COREX Melter Gasifier

The generator gas of melter gasifier inevitably carries a lot of dust, which would be transported to the freeboard for recycling, as a result of the existence of the fluidized bed. The recycling dust, with an attractive concentration of C and Fe, is considered as a useful secondary material, thus it is important to investigate the inherent characteristics and behavior of recycling dust in the freeboard. In the present work, the recycling dust characteristics are investigated through the granulometry, chemical, mineralogical and micro analysis. The recycling dust has an irregular shape and an uneven granulometric distribution with the following primary minerals: Fe, FeO, SiO2, 2CaO·SiO2 and 2CaO·Al2O3·SiO2. Furthermore, a three-dimensional mathematical model at steady state is developed to describe the recycling dust combustion and the freeboard temperature distribution, which significantly influences the recycling dust behavior. The region of high temperature above 3000 K can be observed in front of DB due to the combustion of C in the recycling dust, with a combustion ratio of almost 100%. Based on the above analysis, the recycling dust behavior is complex. Fe and FeO in the recycling dust may be oxidized and finally smelts into liquid iron, while other minerals smelt into molten slag. It is worth noting that the phenomena of circulation and accumulation of alkali metals, ZnO and S could occur, despite that most of them are discharged out of the melter gasifier through the generator gas and molten slag.

Incineration of kitchen waste with high nitrogen in vortexing fluidized-bed incinerator and its NO emission characteristics

Some municipal solid waste (MSW) can be used as the fuel. Combustion of MSW with high nitrogen content is successfully conducted in a lab-scale vortexing fluidized-bed incinerator (VFBI). Pigskin with 16.5 wt.% nitrogen content was used to simulate the high nitrogen content kitchen waste, and silica sand was used as the bed material. The effects of operating conditions, such as the bed temperature, freeboard temperature, excess oxygen ratio, and static bed height on the CO and NO concentrations at the exit of combustor and cyclone were investigated. The experimental results show that the freeboard temperature is the most important factor for CO emission. The order of operating conditions impact on the NO emission is: (1) excess oxygen ratio; (2) bed temperature; (3) freeboard temperature; and (4) static bed height. Utilizing cyclone can significantly reduce the CO emission concentration when the CO concentration released from the freeboard is higher than 50 ppm. On the other hand, the cyclone has no significant effect on the NO emission. Despite having high nitrogen content, a low conversion from fuel-N to NO was attained. Compared with other types of combustors, VFBI reduces the CO and NO emission concentrations much better when burning MSW with high nitrogen content.

Combustion behavior and pollutant emissions of batch fluidized bed combustion

The combustion experiments were carried out in a pilot-scale vortexing fluidized-bed combustor (VFBC) with a batch fuel feeding system. Effects of particle sizes and feeding intervals on the temperatures within the combustor and the pollutant emissions at the exit of the combustor were studied. Rubber balls were used as the fuel to simulate the hazard waste. Silica sand was used as the bed material. The experimental results show that the combustor temperatures change periodically with the feeding cycles and the highest temperature appears in the splashing or freeboard regions. The mean bed and freeboard temperatures increase with the particle size, and decrease with the feeding interval. However, the mean bed and freeboard temperatures show a inverse trend. The mean CO concentration in the flue gas decreases with fuel particle size, but increases with feeding interval. There is an inverse dependency between the CO and NOx concentrations at the furnace exit. Results from this study would be helpful in developing more comprehensive physical model for VFBC with batch feeding system and setting optimal operational parameters in practical applications.

Evaluation of the performance of a commercial circulating fluidized bed boiler by using IEA-CFBC model: Effect of primary to secondary air ratio

The performance of a commercial circulating fluidized bed boiler in the Yeosu thermal power plant, which has been operating since October 2011 by KOSEP, has been evaluated by using the IEA-CFBC model. To validate the calculation procedure of the model, the calculated results were compared with the operation values such as the tem- peratures, pressures, emissions of SO2 and NO, particles size distribution and unburned carbon fraction of the CFB boiler at a certain actual condition. The calculated results were comparable to measured values from the CFB boiler, so these could conform to acceptable formats with a good accuracy. The effect of the primary to secondary air ratio on the performance of the CFB boiler was also determined. As the primary air ratio increased, the solid fraction and temperature in the furnace freeboard increased. As a result, the solid circulation rate and the heat absorption in the furnace increased with increasing the PA ratio. In the case of the amount of heat absorption, the wall tube of the furnace ab- sorbed much more generation heat in the furnace than the wing wall tube. The SO2 emission decreased due to increase of the limestone hold up in the furnace, and the combustion efficiency somewhat increased with increasing the PA ratio. Therefore, from these results, we could expect to control the boiler performance such as the furnace temperature, steam temperatures of superheater or reheater, gaseous emissions and combustion efficiency through the changing the PA ratio of the CFB boiler.