Excess Air Levels Research Articles

Decarbonized combustion performance of a radiant mesh burner operating on pipeline natural gas mixed with hydrogen

With the pressing need to reduce greenhouse gas emissions, blending lower or zero carbon fuels like renewable hydrogen into natural gas is a promising and practical way to achieve clean energy transition. From the perspective of end users and combustion device manufactures, one of the major concerns is the influence of the renewable contents on the combustion devices performance. The possible renewable gas content percentage in pipeline also interests policy makers and gas utility companies. The present study investigates on the influence of hydrogen contents on the operating performance of a surface burner, which is widely adopted in industrial, commercial and residential applications. The interactions among heating load, excess air level and fuel contents are studied by a 3-factor∗3-level experiment design. Evaluated combustion performance characteristics include flame characteristics, burner/exhaust temperature and emissions (NO, NO2, N2O, CO, UHC, NH3). The results showed that hydrogen addition to natural gas slightly increased the burner surface temperature but did not have significant impact on other burner performance parameters. Up to 20% (by volume) natural gas was replaced by hydrogen, and no abnormal effect was observed. Furthermore, tests carried out in a prototype water heater showed similar performance. This study gives a positive sign relative to replacing pipeline natural gas with renewable hydrogen at a low percentage without modifying the burner geometry.

Emission characteristics and source identification of polycyclic aromatic hydrocarbons (PAHs) from used mineral oil combustion

Presently, the characteristic of hazardous polycyclic aromatic hydrocarbons (PAHs) emission from used mineral oil (UMO) combustion in the industrial scenario is poorly explored and elucidated. In this work, the emission characteristic and source identification of 16 PAHs emitted from atomization combustion of five kinds of UMO and two types of base oil were investigated. The influence of temperature (600–1000 °C) and excess air ratio (0.8–1.4) on PAHs emission were assessed. Regarding emission factor (EF) and distribution of specific PAHs, high similarities were observed for combustion of different UMOs. EFs of PAHs were within the range between 61.58 µg g−1 and 105.03 µg g−1 (800 °C, stoichiometric air). The 2- and 3-ring PAHs made up more than 70% of the total PAHs in the flue gas, with the naphthalene accounting for 30.7%–48.1%. Over 80% of the emitted PAHs was partitioned into gas phase. The EFs of PAHs showed a parabolic trend with temperature and peaked at 800 °C. Higher temperature can decrease the total PAHs emission but lead to the enrichment of high-ring PAHs. Adequate air supply can efficiently reduce PAHs formation, especially at high temperatures. More than 95% of the total PAHs was eliminated when the excess air ratio was increased from 0.8 to 1.4 at 900 °C. Temperature influence on PAHs emission can be greatly weakened in high excess air level. The PAHs emitted from UMO combustion were randomly irrelevant to the original PAHs in UMO.

A Hybrid Technique for the Performance Optimization in the Combustion Process of a Power Plant Boiler: An Efficient ANNSSA Technique

In this paper presents a hybrid method for optimization process of combustion in power plant boiler. ANSSA scheme will be joint implementation of Artificial Neural Network (ANN) as well as Salp Swarm Optimization Algorithm (SSA) known ANNSSA. Here, ANN training process will be enhanced by using the SSA calculating. The optimization of economic parameters reduces excess air level and performs combustion efficiency at boiler system. Due to the operation of service boiler, oxygen content of flue gases is one of the significant factors which influence the efficiency of boiler, and influence each other to other thermal parameters of economic like temperature of flue gases combustion, unburned carbon at fly ash slag and consumption of coal power supply. The combustion performance denotes a saving at operating costs of boiler. ANNSSA method evolved for process of combustion to enhance the implementation and efficiency of the power plant boiler. At that time, ANNSSA technique is implemented at MATLAB/Simulink work platform as well as implementation is evaluated using existing techniques.

Effect of Excess Air Levels on PAHs Content in Smoke During Charcoal Combustion in Grilling Process

Charcoal grilling may lead to carcinogenic PAHs contamination of grilled food from incomplete combustion of charcoal. The objective of this study was to determine the effect of complete combustion of charcoal on PAHs content in smoke during the grilling process. Firstly, proximate and ultimate compositions of the charcoal were determined to identify the amount of air required for combustion according to stoichiometry. Different excess air levels consisting of stoichiometric air, 60, 100 and 150% excess air during combustion of charcoal on 16 PAHs released in smoke were studied. Moreover, CO and CO2 concentrations were measured. The use of excess air decreased the emission factor of CO and increased the emission factor of CO2. The 16 PAHs contents in smoke produced from charcoal combusted with stoichiometric air, 60, 100 and 150% excess air were 73.62, 51.78, 27.68 and 19.23 μg/kg dry charcoal, respectively. The use of excess air during charcoal combustion resulted in significantly lower PAHs contents in the smoke. Therefore, the use of excess air during charcoal grilling is one way to reduce the risk of PAHs contamination in grilled food.

Study on the emission characteristics of nitrogen oxides with coal combustion in pressurized fluidized bed

Nitrogen oxides are one of the most significant pollution sources during coal combustion. This experimental study was conducted in a 15 kWth lab-scale pressurized fluidized bed (inner diameter = 81–100 mm, H = 2100 mm) firing with bituminous coals. The effects of operating parameters, including bed temperature (800 °C–900 °C), operating pressure (0.1–0.4 MPa), excess air level (16%–30%) and flow pattern on NOx and N2O emissions were systematically studied during the tests. During each test the interaction effects of all the operating parameters were properly controlled. The results show that most operating parameters have an opposite effect on NOx and N2O emissions, and the N2O emissions mainly depend on the bed temperature. Increasing the operating pressure can significantly suppress the fuel-N conversion to NOx but enhance its conversion to N2O. With the rise of the excess air level and fluidization number, NOx emissions grow distinctly while N2O emissions remain almost unchanged. Total nitrogen oxide emissions increase with the bed temperature while decrease with the operating pressure.

Combustion of Coal Volatiles in the Freeboard of a Fluidized Bed Combustor: an Experimental Approach

Previous attempts to determine the amount of volatile material transferred into and burnt in the freeboard region (FBR) of fluidized bed coal combustors have been few and/or ambiguous. A major reason for this is the difficulty of separating the two combustion processes (volatiles and char) occurring simultaneously within both bed and freeboard sections, and because the instrumentation for measuring these separate phenomena directly is non-existent. Therefore, a calorimetric technique is reported here to produce quantitative information on the fraction of heat released by the volatile component in the FBR. The calorimetric experiments revealed that over one third of the original volatiles may be released and burnt above the bed at low excess air levels ( 850°C) and high excess air levels (up to 40 %). The transfer of coal volatiles in the FBR was found to be influenced by the in-bed operating and fluidizing conditions in the following order: bed temperature (Tb), excess air (XSA), fluidizing velocity (Uo) and coal particle size (dc).

Study on performance and methods to optimize thermal oil boiler efficiency in cement industry

Cement production is an energy-intensive process, so that the cement industry occupies a top position among other energyconsuming industries. Among the equipment used in cement industries, boilers are one of the energy-consuming equipment. Boilers are among the common heating equipment in industrial, commercial, and institutional facilities. In this paper, the performance of thermal oil boiler and useful methods in improving its efficiency and saving energy was investigated. Under normal condition, results showed that the boiler was only working with 55% of its capacity, and in this case, boiler efficiency was 77.48%, based on the heat loss method. Moreover, optimization of excess air level in combustion process as one of the improving performance methods increased the boiler efficiency by about 3%. The volume of fuel was also reduced to about 34.07 m 3 /HR, using economizer as another method. Article history:

Advanced solutions in combustion-based WtE technologies

Thermal treatment of waste by means of combustion in grate-based systems has gained world-wide acceptance as the preferred method for sustainable management and safe disposal of residual waste. In order to maintain this position and to address new challenges and/or priorities, these systems need to be further developed with a view to energy conservation, resource and climate protection and a reduction in the environmental impact in general. MARTIN GmbH has investigated continuously how the implementation of innovative concepts in essential parts of its grate-based Waste-to-Energy (WtE) combustion technology can be used to meet the above-mentioned requirements. As a result of these efforts, new “advanced solutions” were developed, four examples of which are shown in this article.1.Radiant superheaters in the first boiler pass. In this process, radiant superheaters are arranged in sections of the boiler where the flue gas temperatures are very high, so as to achieve higher steam temperatures and thus improve the efficiency of the thermal cycle, which in turn increases the production of electric power.2.Syncom Plus. In the SYNCOM process, combustion air is enriched with oxygen to reduce the flue gas flow and increase the temperatures on the fuel bed, thereby intensifying sintering of the bottom ash. In order to further improve bottom ash quality, a downstream wet mechanical treatment unit (SYNCOM-Plus) is installed to produce granulate. The process achieves higher boiler efficiencies due to a reduced flue gas loss and a high bottom ash quality.3.Low NOX processes. The Low NOX processes help to significantly reduce the content of nitrogen oxides in the flue gas by supplying combustion air/gas in stages and reducing excess air levels. This is achieved by means of primary measures, i.e. the generation of NOX is prevented to a large extent. If required, NOX values can be easily reduced even further, down to very low levels, by combining the VLN (Very Low NOX) and SNCR (Selective Non Catalytic Reduction) processes.4.Dry discharge of bottom ash. With the dry discharge process, it became considerably easier to recover raw materials (ferrous and non-ferrous metals) from solid combustion residues (bottom ash) and to improve the quality of the latter. In addition, savings can be made in terms of operation and residue disposal as no water is used and the residues to be disposed of contain no water.

Monitoring regenerative steel reheating burners using an intelligent flame diagnostic system

The present paper describes the use of an intelligent Flame Monitoring System on regenerative steel reheating burners based on direct measurement and analysis of the flame radiation signals. A series of experiments were conducted on a 500 kW furnace fitted with two burners firing in a regenerative manner. The experiments covered a wide range of burner operating conditions including variations in the burner firing-rate and excess air levels. Gas supply to one of the burners was manually reduced in order to simulate burner imbalance. The flame radiation signals were acquired using a fibre-optic based optical instrument incorporating broad ultraviolet, visible and infra-red photodiodes. The correlation between the dynamic flame signals with respect to the excess air level and nitrogen oxides emissions were made using neural network models following off-line analysis of the acquired signals using different signal processing methods, to yield a set of flame features. The present work indicates that the measurement of flame radiation characteristics, coupled with advanced data modelling techniques such as neural network, provides a promising means of monitoring and optimising burner performance.

Emissions of SO3 from a Coal-Fired Fluidized Bed under Normal and Staged Combustion

This paper reports the measurements of SO3 emissions with and without limestone under unstaged and staged fluidized-bed combustion, carried out on a m2 and 2 m high stainless-steel combustor at atmospheric pressure. The secondary air was injected 100 cm above the distributor. SO3 emissions were monitored for staging levels of 85 : 15, 70 : 30, and 60 : 40, equivalent to a primary air/coal ratio (PACR) of ~0.86, 0.75, and 0.67. Experiments were carried out at 0%–60% excess air level, 1-2 m/s fluidizing velocity, 800–850°C bed temperature, and 20–30 cm bed height. During unstaged combustion runs, SO3 emissions were monitored for a wide range of Ca/S ratios from 0.5 to 13. However, for the staged combustion runs, the Ca/S ratio was fixed at 3. SO3 was retained to a lesser extent than SO2, suggesting that SO2 reacts preferentially with CaO and that SO3 is involved in the sulphation process to a lesser degree. The SO3 emissions were found to be affected by excess air, whereas the fluidizing velocity and bed temperature had little effect. SO3 was depressed on the addition of limestone during both the staged and unstaged operations, and the extent of the reduction was higher under staged combustion.

Real-time, in situ, continuous monitoring of CO in a pulverized-coal-fired power plant with a 2.3 μm laser absorption sensor

A real-time, in situ CO sensor using 2.3 μm DFB diode laser absorption, with calibration-free wavelength-modulation-spectroscopy, was demonstrated for continuous monitoring in the boiler exhaust of a pulverized-coal-fired power plant up to temperatures of 700 K. The sensor was similar to a design demonstrated earlier in laboratory conditions, now refined to accommodate the harsh conditions of utility boilers. Measurements were performed across a 3 m path in the particulate-laden economizer exhaust of the coal-fired boiler. A 0.6 ppm detection limit with 1 s averaging was estimated from the results of a continuous 7-h-long measurement with varied excess air levels. The measured CO concentration exhibited expected inverse trends with the excess O2 concentration, which was varied between 1 and 3 %. Measured CO concentrations ranged between 6 and 200 ppm; evaluation of the data suggested a dynamic range from 6 to 10,000 ppm based on a minimum signal-to-noise ratio of ten and maximum absorbance of one. This field demonstration of a 2.3 μm laser absorption sensor for CO showed great potential for real-time combustion exhaust monitoring and control of practical combustion systems.

Control of NOx emissions of a domestic/small-scale biomass pellet boiler by air staging

The NOx emissions of a UK-manufactured domestic/small-scale 50kWth underfeed stoker biomass pellet boiler were experimentally investigated under different air staging configurations including varying primary to secondary air ratios and different heights of the secondary air inlets above the fuel bed. Two different commercial biomass pellet fuels, i.e. standard Class A wood pellets (fuel-N=0.13wt%) and clean waste wood pellets (fuel-N=0.46wt%), were used in these tests. The total excess air level of the boiler was kept constant for all air staging configurations. The experimental results show that even with the domestic/small-scale biomass boiler, air staging can be effective and lead to considerable NOx reductions, particularly with biomass fuels containing relatively high fuel-N. The height of the secondary air inlets above the bed was found to have important impacts on both NOx and CO emissions. However, the trade-off between NOx emissions and CO emissions needs to be carefully considered when air staging is used to control NOx emissions of domestic/small-scale biomass boilers.

Experimental study on the influence of the thermal input on the reaction zone under flameless oxidation conditions

The reaction zone of a small-scale laboratory combustor operating under flameless oxidation conditions is examined with the aid of hydroxyl radical chemiluminescence (OH*) imaging and measurements of local mean gas temperatures and local mean major gas species (O2, CO2, CO, unburnt hydrocarbons and NOx) concentrations along the combustor axis, as a function of the fuel (methane) thermal input, which was varied between 7 and 13kW. As the fuel thermal input increases, the reaction zone, as typified by the OH* distribution, enlarges and, simultaneously, moves progressively closer to the combustor exit (exhaust) due to the increase in the central jet momentum, while maintaining constant the excess air level. The excess air values used in the present study were low enough to preserve the flameless combustion regime regardless of the fuel thermal input, with the combustor yielding very low NOx (<6ppm@15% O2) and CO emissions (<14ppm@15% O2) regardless of the fuel thermal input. The low NOx emissions, almost independent of the fuel thermal input, are attributed to the suppression of the thermal mechanism promoted by the flameless oxidation regime. Despite being always low, the CO emissions increase with the fuel thermal input presumably because of the lower residence times associated with the higher burner thermal loads.

Development of an intelligent flame monitoring system for steel reheating burners

This article describes the development of a system to indirectly monitor the combustion characteristics of individual burners based on measurement and analysis of the signals detected from photodiodes detecting flame radiation signals. A series of experiments were conducted on a 500 kW pilot-scale furnace and on two 4 MW industrial burners located in two steel reheating furnaces. The flame radiation signals were monitored using a lens that transmitted the flame radiation to ultraviolet, visible and infrared photodiodes through a trifurcated optical fibre. The experiments covered a wide range of burner operating conditions including; variations in the burner load and excess air levels and simulations of burner imbalance. The relationships between the dynamic flame radiation signals and the burner operating parameters and conditions were made off-line using neural network models. The present work indicates that the measurement of flame radiation characteristics, coupled with neural networks, provides a promising means of monitoring and adjusting burner performance.

Operational Optimization of Boilers in Coal-Fired Base Load Thermal Power Plants

This paper describes the performance analysis of steam generators. Various energy conservation measures such as operating excess air level control and insulation condition and its impact on overall heat rate of a thermal power station are discussed. Study results show that the change in operating oxygen level in fl ue gas can give benefi t in unit heat rate to the tune of 10–40 kcal/kWh in 210 MW units and 10–50 kcal/kWh in 500 MW units. Reduction in the boiler skin temperature from 30°C to 10°C above the ambient dry bulb temperature will result in improvement in unit heat rate by 60 kcal/ kWh in 210 MW units and 80 kcal/kWh in 500 MW units.

Forest biomass waste combustion in a pilot-scale bubbling fluidised bed combustor

Combustion experiments of forest biomass waste in a pilot-scale bubbling fluidised bed combustor were performed under the following conditions: i) bed temperature in the range 750–800 °C, ii) excess air in the range 10–100%, and iii) air staging (80% primary air and 20% secondary air). Longitudinal pressure, temperature and gas composition profiles along the reactor were obtained. The combustion progress along the reactor, here defined as the biomass carbon conversion to CO2, was calculated based on the measured CO2 concentration at several locations. It was found that 75–80% of the biomass carbon was converted to CO2 in the region located below the freeboard first centimetres, that is, the region that includes the bed and the splash zone. Based on the CO2 and NO concentrations in the exit flue gas, it was found that the overall biomass carbon conversion to CO2 was in the range 97.2–99.3%, indicating high combustion efficiency, whereas the biomass nitrogen conversion to NO was lower than 8%. Concerning the Portuguese regulation about gaseous emissions from industrial biomass combustion, namely, the accomplishment of CO, NO and volatile organic compounds (VOC) (expressed as carbon) emission limits, the set of adequate operating conditions includes bed temperatures in the range 750°C–800 °C, excess air levels in the range 20%–60%, and air staging with secondary air accounting for 20% of total combustion air.

Estimation of combustion flue gas acid dew point during heat recovery and efficiency gain

When cooling combustion flue gas for heat recovery and efficiency gain, the temperature must not be allowed to drop below the sulfur trioxide dew point. Below the SO3 dew point, very corrosive sulfuric acid forms and leads to operational hazards on metal surfaces. In the present work, simple-to-use predictive tool, which is easier than existing approaches, less complicated with fewer computations is formulated to arrive at an appropriate estimation of acid dew point during combustion flue gas cooling which depends on fuel type, sulfur content in fuel, and excess air levels. The resulting information can then be applied to estimate the acid dew point, for sulfur in various fuels up to 0.10 volume fraction in gas (0.10 mass fraction in liquid), excess air fractions up to 0.25, and elemental concentrations of carbon up to 3. The proposed predictive tool shows a very good agreement with the reported data wherein the average absolute deviation percent was found to be around 3.18%. This approach can be of immense practical value for engineers and scientists for a quick estimation of acid dew point during combustion flue gas cooling for heat recovery and efficiency gain for wide range of operating conditions without the necessity of any pilot plant setup and tedious experimental trials. In particular, process and combustion engineers would find the tool to be user friendly involving transparent calculations with no complex expressions for their applications.

Combustion of Apple Juice Wastes in a Cyclone Combustor for Thermal Energy Generation (ES2009-90152)

The solid waste generated from the apple juice industry (apple bagasse (AB)) was characterized as a fuel, and the potential for its utilization as an alternative energy source was assessed through its combustion in a pilot scale cyclone combustor. A comparative evaluation of the AB and sawdust (SD) properties, as well as of the emissions during the combustion tests, was performed. The high energy content of AB (lower heating value (LHV) equal to 21.09 MJ kg−1), dry and ash-free (daf) basis, which is 26.9% higher than the LHV of SD (16.62 MJ kg−1, daf), and combined with the high volatile matter content (85.36 wt %, daf) improve the ignition and burning of the solids. The emissions of CO, SO2, and NOx and the total organic carbon (TOC) were compared with guideline limits established by Brazilian and international legislation. AB generated much lower CO than sawdust in spite of almost half of excess air levels (13% compared with 26%) and met even the stringent limit of the German regulation for waste incineration. The unburned carbon percentages found in the ash resulted from SD and AB combustion tests were 0.24% and 0.96% in weight, respectively. The absence of sulfur in AB composition represents an advantage with nondetectable SO2. The average level of NOx emission with SD combustion was 242 mg N m−3 and met all the regulation limits. The average NOx emission with AB combustion though was 642 mg N m−3 and met the U.S. EPA regulation but was marginally higher than the Brazilian norm by 15%. TOC concentrations remained below the limits considered even though the TOC level was higher in the AB combustion test. Polycyclic aromatic hydrocarbons (PAH) were not detected or were under the quantification limit of the equipment used in their analysis. Comparing the properties, the burning profiles of SD and AB, and the emissions from their combustion tests, it can be stated that the waste originating from the apple juice industry is suitable for direct combustion, constituting a renewable energy source for this industrial sector.

Operational characteristics of a parallel jet MILD combustion burner system

This study describes the performance and stability characteristics of a parallel jet MILD (Moderate or Intense Low-oxygen Dilution) combustion burner system in a laboratory-scale furnace, in which the reactants and exhaust ports are all mounted on the same wall. Thermal field measurements are presented for cases with and without combustion air preheat, in addition to global temperature and emission measurements for a range of equivalence ratio, heat extraction, air preheat and fuel dilution levels. The present furnace/burner configuration proved to operate without the need for external air preheating, and achieved a high degree of temperature uniformity. Based on an analysis of the temperature distribution and emissions, PSR model predictions, and equilibrium calculations, the CO formation was found to be related to the mixing patterns and furnace temperature rather than reaction quenching by the heat exchanger. The critical equivalence ratio, or excess air level, which maintains low CO emissions is reported for different heat exchanger positions, and an optimum operating condition is identified. Results of CO and NO x emissions, together with visual observations and a simplified two-dimensional analysis of the furnace aerodynamics, demonstrate that fuel jet momentum controls the stability of this multiple jet system. A stability diagram showing the threshold for stable operation is reported, which is not explained by previous stability criteria.

Combustion optimisation of stoker fired boiler plant by neural networks

This paper is concerned with the development of a neural network based controller (NNBC) for chain grate stoker fired boilers. The objective of the controller was to increase combustion efficiency and maintain pollutant emissions below future medium term legislation. Artificial neural networks (ANNs) were used to estimate future emissions from and control the combustion process. Initial tests at Casella CRE Ltd demonstrated the ability of ANNs to characterise the complex functional relationships which subsisted in the data set, and utilised previously gained knowledge to deliver multistep ahead predictions. This technique was built into a carefully designed control strategy, which fundamentally mimicked the actions of an expert boiler operator, to control an industrial chain grate stoker at HM Prison Garth, Lancashire. Test results demonstrated that the developed novel NNBC was able to optimise the industrial stoker boiler plant whilst keeping the excess air level to a minimum. In addition, the ANN also managed to maintain the pollutant emissions within possible future limits for boilers in the size range of 1 to 50 MW. This prototype controller would thus offer the industrial coal user a means to improve the combustion efficiency on chain grate stokers as well as meeting probable medium term legislation limits on pollutant emissions.