Underfire Air Research Articles

Development of wood-burning rocket cookstove with forced air-injection

Nearly 40% of the world's population still relies on biomass stoves as their primary cooking source. Indoor cooking with biomass stoves produces harmful emissions such as fine particulate matter (PM2.5) and can result in significant cardiovascular and respiratory disease. Air injection technology, in the forms of over-fire, under-fire, and staged (a combination of under- and over-fire), has been used for decades to reduce emissions in small-scale industrial biomass burners. Recently, researchers have begun integrating air-injection into biomass cookstoves, but the number of studies is limited, and improper air-injection can result in worsening stove performance. In this paper, we develop and analyze three wood-burning biomass rocket stove air-injection strategies using computational fluid dynamics and experiments. We consider over-fire and under-fire air-injection independently, and then develop a staged air-injection system that combines the optimal jet characteristics from the purely over-fire and under-fire systems. Results show that significant performance gains, both in terms of reduced emissions and increased burn-rate are achievable with forced air-injection. For a radial over-fire air injection system, jet momentum flux is found to control fuel and air mixing and is the driving metric for reducing particulate emissions. For the under-fire system, excess air ratio was found to control both the emissions rate and firepower. In a comparison of all three air-injection strategies, over-fire air injection was found to have the largest reduction in emissions, under-fire air injection had the largest increase in firepower and reduction in boiling times, and the staged air injection method achieved both improved emissions and burn-rate. Additionally, the combustion start-up phase was observed to account for the majority of emissions in all forced-draft stoves, and is most sensitive to under-fire injection, remaining an area ripe for improvement.

Assessment of the Impact of Under-Fire Air Introduction on the Pulverised Coal Combustion Efficiency

Assessment of the Impact of Under-Fire Air Introduction on the Pulverised Coal Combustion Efficiency

Emission reduction of NOx and CO by optimization of the automatic control system in a coal‐fired stoker boiler

AbstractTo date research on NOX and CO emission reduction in stoker‐fired boilers has been devoted to combustion modification to the overfire air, diverting air to a selected set of burners, using modified low‐NOX burners, using flue gas recirculation or flue gas treatment with specially controlled catalyst and additives. This study introduces a concept that focuses on the dynamics of the boiler and the automatic control system. The objective of this study was to reduce the NO and CO emissions by restructuring the automatic control system and then tuning the control system with parameters that have been optimized with emission reduction as the objective. Dynamic data were obtained from a step‐input test of either the underfire air or the overfire air. These data were used to model the boiler with a transfer function describing the emissions. The analyzer dynamic response was included in the overall model. The control parameters were determined from this overall emissions transfer function by mathematical optimization. These control parameters constituted the initial values in the automatic control system used for the final tests in the boiler. Additional adjustments to reduce the emissions were carried out during boiler operation. A low controller gain and a fast reset time were found to be the most suitable setting for the control system. The NO emissions controlled by the overfire air and CO emissions controlled by the underfire air produced the best results. Through this process, emission reduction in boiler No. 7 of the steam plant at Vanderbilt University of NO from 0.349 to 0.292 lb/106 BTU and CO emission from 0.135 to 0.0821 lb/106 BTU was obtained from the optimization of the control parameters of the automatic control system. © 2006 American Institute of Chemical Engineers Environ Prog, 2006

Mathematical modelling of MSW incineration on a travelling bed

The rising popularity of incineration of municipal solid waste (MSW) calls for detailed mathematical modelling and understanding of the incineration process. In this paper, governing equations for mass, momentum and heat transfer for both solid and gaseous phases in a moving bed in a solid-waste incineration furnace are described and relevant sub-models are presented. The burning rates of volatile hydrocarbons in the moving bed of solids are limited not only by the reaction kinetics but also the mixing of the volatile fuels with the under-fire air. The mixing rate is averaged across a computation cell and correlated to a number of parameters including local void fraction of the bed, gas velocity and a length scale comparable to the particle size in the bed. A correlation equation is also included to calculate the mixing in the freeboard area immediately next to the bed surface. A small-scale fixed bed waste incinerator was built and test runs were made in which total mass loss from the bed, temperature and gas composition at different locations along the bed height were measured. A 2-D bed-modelling program (FLIC) was developed which incorporates the various sub-process models and solves the governing equations for both gases and solids. Thermal and chemical processes are mainly confined within a layer about 5–9 times in thickness of the averaged particle size in the burning bed. For a large part of the burning process, the total mass loss rate was constant until the solid waste was totally dried out and a period of highly rising CO emission followed. The maximum bed temperature was around 1200 K. The whole burning process ended within 60 min. Big fluctuations in species concentration were observed due to channelling and subsequent ‘catastrophic’ changes in the local bed conditions. Reasonably good agreement between modelling and measurements has been achieved. Yet the modelling work is complicated by the channelling phenomenon in the bed. Numerical simulations without consideration of the channelling effect produced very good agreement with experiments concerning the total mass loss, but significant discrepancy exists for temperature and gas composition profiles. Transient phenomena such as the breaking of waste particles and the “catastrophic” creation of new burning channels occurring during waste incineration is a vital area requiring further investigation at the fundamental level. The underlying theory of bed behaviour must be extended to include these transient events.

Reduction of combustion by-products in WTE plants: O 2 enrichment of underfire air in the M artin S yncom process

The S yncom process involves oxygen enrichment of underfire air, recirculation of flue gas and a combustion control system using infrared thermography of the waste layer on the grate. At the demonstration plant in Coburg, operational reliability and plant availability using S yncom could be proven under real disposal conditions with a waste throughput of 7 t/h. Oxygen enrichment of the underfire air promotes the destruction of pollutants due to the high oxygen partial pressures and temperatures. This is then reflected in very low residual amounts of organic combustion by-products in the bottom ash and flue gas from the S yncom unit. The flue gas concentrations of organic pollutants are reduced, as compared with conventional operation, by over 35% (for CO, total hydrocarbons and PCDD/F) at the boiler outlet. As the flue gas flow is reduced by oxygen enrichment and flue gas recirculation, the resulting reduction in terms of kg of pollutant per Mg of waste is even higher. In the bottom ash, the level of organic residues is reduced, by 45% in the case of loss on ignition and by 55% in the case of TOC and dioxins (I-TE of PCDD/F). This is due to the higher oxygen partial pressures and the fuel bed temperature which is increased by 135 to 1200°C. Other important features of the process include more intense sintering and thus improved immobilization of the bottom ash, as well as reduced flue gas and fly ash flows.

Three-Dimensional Combustion Modeling in Municipal Solid-Waste Incinerator

Three-dimensional flame structures and mixing behaviors of turbulent burning flows in a municipal solid-waste incinerator are investigated by finite-element simulation. The modified κ-ε turbulence model together with wall functions was adopted. Devolatilization of solid wastes was simulated by gaseous methane (CH4) nonuniformly distributed along the inclined grate. The combustion process was considered as two-step stoichiometric reactions when primary underfire air entered and mixed with methane gas in the first combustion chamber. The mixing-controlled eddy-dissipation model was employed for predicting the reaction rates of CH4, O2, CO2, and CO. Results show that the grate is covered by cone-shaped flames that are bent and aligned with the flow directions. Additional mixing in the second combustion chamber can enhance the oxidation and can be improved by provision of more excess air or by the injection of secondary overfire air. Combustion efficiency up to 99.97% and an exit temperature around 1,100–1,300 K can be achieved at 100–150% excess air. Reasonable agreements are achieved between numerical predictions and available in-situ measurements.

Estimation of Burning Rates in Solid Waste Combustion Furnaces

For a furnace burning solid wastes, one of the key areas in design is the sizing of the furnace which is often determined on the basis of empirically selected furnace heat release rate and grate burning rate. An independent estimation of these factors would require the knowledge of physical processes of refuse combustion such as drying and heating of the refuse bed, chemical reactions of waste combustion, supply and distribution of combustion air, and mixing of combustion air with the burning refuse bed. In principle, the gross combustion rate of solid refuse bed is related to the oxygen consumption rate of the burning refuse bed. In this paper an attempt is made to estimate the refuse burning rates in the furnaces by calculating the oxygen distribution in the combustion chamber. A model based on the fundamental principles of conservation of mass, energy and momentum balance is established for this purpose. For different combustion chemistry models, by using a reaction rate coefficient which could be affected by parameters like bed height, porosity of the fuel bed, and underfire air rates, the oxygen distribution in the fuel bed, and hence, the combustion rate can be determined. Comparisons of the theory with the laboratory test data and real plant operating data indicate that the theory can be tuned to fit the data reasonably well by adjusting the reaction rate coefficient.

Modeling of Turbulent Burning Flow and Performance Evaluation of Municipal Solid Incinerator

Finite-element simulations are presented to advance the knowledge of flow structures, species distributions, and mixing behaviors of turbulent burning flows in municipal solid incinerators under various operating conditions. The modified {kappa}-{epsilon} turbulence model together with wall functions was adopted. Devolatilization of solid wastes were simulated by gaseous methane (CH{sub 4}) nonuniformly distributed along the inclined grate. The combustion process was considered as a two-step reaction when primary underfire air entered and mixed with methane gas in the first combustion chamber. A mixing-controlled eddy-dissipation model was used for predicting the reaction rates of CH{sub 4}, O{sub 2}, CO{sub 2}, and CO. Results show that the grate is covered by cone-shaped flames that are bent and aligned with the flow directions. Postflame oxidation is done in the second combustion chamber and can be improved by provision of more excess air or by the injection of secondary overfire air. Combustion efficiency up to 99.98% and an exit temperature around 1,000 C can be achieved at 100--200% excess air. Reasonable agreements are achieved between numerical predictions and available in-situ measurements.

THREE-DIMENSIONAL FLUID FLOW AND HEAT TRANSFER ANALYSIS OF GRATE BLOCKS IN A WASTE-TO-ENERGY POWER PLANT

The cause of high operating temperatures experienced by grate blocks in a waste-to-energy power plant is investigated. A three-dimensional numerical analysis is performed to assess convection from the grate to the underfire air supply and conduction within the grate. A number of geometric complexities, including variable solid thickness, extended surfaces, sharp curvatures, and narrow passages, are accounted for via a finite element approach. The grate temperature is found to be very sensitive to the flow characteristics beneath the grates. Analysis of a modified grate structure was performed, showing a 14% reduction in maximum grate temperature.

TACCOS—A thermography-assisted combustion control system for waste incinerators

A novel type of combustion control system is described for municipal waste incinerators using infrared thermography to obtain information about the temperature distribution in the furnace interior. A scanner camera operating in the mid infrared was installed at the waste-to-energy plant in Coburg, Germany during the experiments. Selection of an appropriate wavelength region allows the direct imaging of the fuel bed through the overlying flame and flue gas atmosphere and the determination of temperature distributions within seconds. The fast response time of the thermography system compared to shielded thermocouples allows the detection of furnace temperature fluctuations on a short time scale of a few minutes. These data can be used for fine tuning of the total underfire air (UFA) flow. Local imbalances in fuel bed temperature are evaluated to redistribute UFA between individual grate zones. The influence of variations in calorific value on the position of the main zone of combustion can easily be measured, thus allowing the sensitive adaptation of UFA distribution to local requirements. Incorporation of the camera system results in an improved overall consistency of the combustion process, as reflected by a reduction of excess air fluctuations and of temperature imbalances across the grate. Measurements in the Coburg plant have shown that flue gas as well as residue burnout quality was improved. Compared with conventional control, emissions of carbon monoxide and unburned hydrocarbons were reduced, and also the fraction of unburned matter in the residue.

An Experimental Investigation of Wood Combustion

This paper presents the results from an experimental investigation of wood combustion. Variables chosen for investigation are fuel moisture content, fuel particle size, excess air, fraction and temperature of under-fire air. Influence of the off-design (part load) operation of the combustion unit on combustion efficiency and particulate emission is also investigated. Data recorded during the experiments include the composition and temperature of the combustion products, particulate emissions, and combustible fraction of the particulate. Based on the experimental data, a linear regression model was developed to investigate the variables affecting the combustion process. A computer model was used to calculate the temperature and composition of the combustion products under adiabatic conditions. Results of the adiabatic model and the experimental regression analysis are compared and discussed. According to the results presented, it is concluded that the combustion efficiency and particulate emissions are most influenced by the factors that increase the volume of the combustion products in the combustion chamber. These variables include excess air, moisture content of the fuel, and the combustion air temperature. Fuel particle size and the fraction of under-fire air did not significantly affect the combustion efficiency and particulate emissions. It is also concluded that the off-design (part-load) operation of the combustion unit, results in higher particulate emissions and lower combustion efficiency.

Combustion Mechanisms in Wood Fired Boilers

The emissions from combustion of wood residue fuel in an experimental spreader-stoker boiler were measured at the Fairplay Test Facility at Oregon State University. Stack gases were monitored to determine levels of excess air, opacity, and particulate loading. Particulate emissions were measured to determine the effects of underfire air flow rate and fuel bed depth on particulate carry over rate. An experiment conducted at four energy release rates and two fuel bed depths indicated that increased bed depth has the effect of reducing particulate emissions and that the effect increases as energy release rate increases. The experiment also showed increased energy release rate has the effect of increasing particulate emissions. The effects were found to be statistically significant.

Combustion characteristics of simulated and shredded refuse

An experimental batch incinerator was used to measure burning and ignition rates in simulated and shredded refuse beds. Provision was made for measurement of refuse weight and fuel bed temperature distributions during combustion as well as the concentrations of CO, CO 2 , H 2 , O 2 , and CH 4 within the fuel bed. The burning rates of the shredded refuse were found to be insensitive to the fineness of shredding but varied with fuel bed density. For loosely packed shredded refuse, overfire air penetrated the bed at low underfire air rates and augmented the burning rate. At high underfire air rates burning rates became ignition limited and burning bed depths were approximately constant during the steady burning regime. No difficulty was experienced in initially igniting the shredded refuse even when underfire air rates of 275 lb hr −1 ft −2 were employed at the start of a run. The fuel bed gas composition measurements suggested that the water gas shift reaction (CO+H 2 O agCO 2 +H 2 ) was close to equilibrium at the top of the bed; this result provides a rough guideline for determining overfire air requirements. For the simulated refuse drying and pyrolysis occurred for prolonged periods after the surface of the fuel element had ignited.

FieldEvaluationofCombustion Air Effectson AtmosphericEmissionsfrom MunicipalIncinerators

Field studies were conducted on two municipal incinerators with capacities of 50 tons and 250 tons per day in evaluating combustion air effects on atmospheric emissions from municipal incinerators. Separate tests were made at several different levels of underfire combustion air over as wide a range as operating conditions would permit. Solid particulate samples were collected under each test condition for comparison purposes. The results of these field tests show that 50 percent underfire and 50 percent overfire air approaches an ideal distribution, which keeps the particulate emissions well below the loadings which result when using a much higher percentage of underfire air. The following gaseous pollutants were measured for each furnace: Oxides of nitrogen, formaldehyde, carbon monoxide and hydrocarbons. Two schematic diagrams are included.