Wlodzimierz Blasiak

COMBUSTION OF NORMAL AND LOW CALORIFIC FUELS IN HIGH TEMPERATURE AND OXYGEN DEFICIENT ENVIRONMENT

Combustion characteristics of two different gaseous fuels (a low calorific value fuel and methane fuel) have been examined using high temperature and low oxygen concentration combustion air. The momentum flux ratio between the fuel jet and the combustion airflow was kept constant to provide similarity in mixing between the different experimental cases to understand the role of fuel jet property on combustion. Direct flame photography, 2-D Particle image velocimetry (PIV), Light Emission Spectroscopy and chemiluminescent NOX analyzer was used as the diagnostics. These diagnostics allowed information on global flame features, mean and rms components of axial and radial velocity, axial strain rates and vorticity, the spatial distribution of combustion intermediate species, such as, OH and CH, and overall NOX emission levels. The results indicate a slower mixing during high temperature air combustion with low calorific value fuel as compared to methane fuel. The results showed higher turbulence levels and higher axial strain rates for low calorific fuel jets as compared to methane fuel jet during the high temperature air combustion condition. This results in less intense (or mild) combustion conditions with the result of increased flame length and volume and lower NOX emissions. Even for the normal methane fuel high temperature and oxygen deficient combustion conditions provided lower NOX emission. Furthermore, the high temperatures obtained for methane combustion provided lower vorticity and axial strain rates than the low calorific value fuel due to the suppression of vortical structure formation from the stronger heat release. In the case of low calorific value fuel, higher fuel jet velocity into low-density high temperature air leads to longer jet length. This jet causes a local stagnation to the upstream cross-flow to create local higher value of turbulence levels immediately upstream of the jet. The spatial distribution of the flame generated radicals (OH and CH) revealed significant ignition delay of the LCV fuel jet and a far more uniform distribution of the intermediate species. The methane fuel jet showed a prolonged reaction zone and faster ignition at high temperature and oxygen deficient conditions when compared to normal temperature air combustion of methane.

Measurements of mass flux and stoichiometry of conversion gas from three different wood fuels as function of volume flux of primary air in packed-bed combustion

Abstract This paper presents the first experimental series applying a new measurement method to determine mass flux and stoichiometry of the conversion gas as well as the air factor from packed-bed combustion of biofuels. The conversion system employed in this experimental study is characterized by the following concepts: overfired, updraft, fixed horizontal grate, and batch reactor. Three types of wood fuels are studied, namely wood pellets ( 6 mm ), wood chips (4– 50 mm ), and fuel wood ( 300 mm ). The above-mentioned quantities are measured as function of six levels of standard volume flux of primary air in a range of 0.06– 0.46 m 3 n / m 2 s for all the wood fuels. Altogether, 18 (3×6) single tests are carried out. In spite of the fact that single tests are carried out and that the course of a batch run is highly stochastic, it is possible to draw some general conclusions with respect to the particular conversion concept studied. Firstly, the course of a batch conversion of wood fuels is proven to be very dynamic. For example, the dynamic ranges for the air factor of the conversion system is 10:1 and for the stoichiometric coefficients is CH 3.1 O : CH 0 O 0 during a batch for a constant volume flux of primary air. Secondly, despite the fact that the fuel wood was exceptionally dry, only 8 wt % water content on wet basis, the fuel wood studied displayed significantly lower time-integrated mean of mass flux of conversion gas (12– 31 g / m 2 s ) relative to the wood pellets (37– 62 g / m 2 s ) and the wood chips (50– 90 g / m 2 s ). Thirdly, based on the fact that the conversion gas stoichiometry is unsteady it is confirmed that the molecular composition of the conversion gas varies during batch conversion of wood fuels.

Experimental investigation of flow phenomena of a single fuel jet in cross-flow during highly preheated air combustion conditions

During the past decade, new advanced combustion systems that share the same basic concept of using a substantially diluted and high-temperature oxidizer in the reaction volume have gained a great deal of interest regarding their application in industrial and power systems. These novel combustion technologies have proved to offer significant benefits compared to traditional combustion techniques. These benefits include reductions in pollutant emissions and energy consumption, as well as a higher and more uniformly distributed heat flux. This entails the potential to, for example, reduce the size of equipment in industrial units or increase production rates while fuel consumption and the subsequent CO2 emissions are decreased or maintained at the same level. Although the development of these new combustion technologies has occurred fairly recently, it has gained worldwide recognition. During the past few years the technique has been used commercially with several different types of burners. Despite its widespread use, the basic understanding of the chemical-physical phenomena involved is limited, and a better understanding of the combustion phenomena is required for more effective utilization of the technology. The objectives of this work have been to obtain fuel-jet characteristics in combustion under high-temperature, low-oxygen conditions and to develop some theoretical considerations of the phenomena. The effect of the preheat temperature of the combustion air, combustion stoichiometry and the fuel-jet calorific value on flame behavior was investigated. Temperature and heat-flux distribution were also studied using a semi-industrial test furnace to see if similar flame features would be found for the small- and large-scale experiments. Particle Image Velocimetry (PIV) was used for the first time to obtain information on the flow dynamics of a fuel jet injected into a crossflow of oxidizer at either a normal temperature or a very high temperature. Light emission spectroscopy was used to collect information on time-averaged radical distributions in the combustion jet. Jet turbulence, time-averaged velocity distribution, fuel-jet mixing, the distribution of radicals such as CH, OH and C2, and flame photographs were investigated. The results showed delayed mixing and combustion under high-temperature low-oxygen-concentration conditions. The combustion air preheat temperature and oxygen concentration were found to have a significant effect on the burning fuel-jet behavior. The results of the semi-industrial-scale tests also showed the features of even flame temperature and heat flux.

CFD modeling of ecotube system in coal and waste grate combustion

This paper presents a comprehensive numerical simulation for the advanced secondary/over fired air Ecotube system. The computational fluid dynamics technique is applied to simulate and analyze the performance of this new air system in two typical coal and solid waste fired furnaces. Results show that the numerical simulation is successful and the Ecotube technique has good potential for future application in industry.

Conversion of microwave pyrolysed ASR's char using high temperature agents

Pyrolysis enables to recover metals and organic feedstock from waste conglomerates such as: automotive shredder residue (ASR). ASR as well as its pyrolysis solid products, is a morphologically and chemically varied mixture, containing mineral materials, including hazardous heavy metals. The aim of the work is to generate fundamental knowledge on the conversion of the organic residues of the solid products after ASRs microwave pyrolysis, treated at various temperatures and with two different types of gasifying agent: pure steam or 3% (v/v) of oxygen. The research is conducted using a lab-scale, plug-flow gasifier, with an integrated scale for analysing mass loss changes over time of experiment, serving as macro TG at 950, 850 and 760 °C. The reaction rate of char decomposition was investigated, based on carbon conversion during gasification and pyrolysis stage. It was found in both fractions that char conversion rate decreases with the rise of external gas temperature, regardless of the gasifying agent. No significant differences between the reaction rates undergoing with steam and oxygen for char decomposition has been observed. This abnormal char behaviour might have been caused by the inhibiting effects of ash, especially alkali metals on char activity or due to deformation of char structure during microwave heating.

High-Temperature Air Combustion Phenomena and Its Thermodynamics

The fundamentals and thermodynamic analysis of high-temperature air combustion (HiTAC) technology is presented. The HiTAC is characterized by high temperature of combustion air having low oxygen concentration. This study provides a theoretical analysis of HiTAC process from the thermodynamic point of view. The results demonstrate the possibilities of reducing thermodynamic irreversibility of combustion by considering an oxygen-deficient combustion process that utilizes both gas and heat recirculations. HiTAC conditions reduce irreversibility. Furthermore, combustion with the use of oxygen (in place of air) is also analyzed. The results showed that a system, which utilizes oxygen as an oxidizer, results in higher first and second law efficiencies as compared to the case with air as the oxidizer. The entropy generation for an adiabatic combustion process is reduced by more than 60% due to the effect of either preheating or oxygen enrichment. This study is aimed at providing technical guidance to further improve efficiency of a combustion process, which shows very small temperature increases due to mild chemical reactions.

Modeling of kraft recovery boilers

The subject of this work is the modeling of recovery boilers. A three-dimensional, steady-state computer simulation of the flow and combustion process in a recovery boiler is presented here. The numerical algorithm of the original code was modified and extended to cater for certain special aspects of recovery boilers, such as the in-flight combustion of black liquor droplets and char bed burning. To represent the fine structure of numerous air ports, a higher level of grid topology than in the conventional, structured grid system is required. In this work, a highly flexible, unstructured and mixed cell topology grid with local mesh refinement was employed. A number of computations were carried out using various firing arrangements including the ROTAFIRE system. The results show considerable potential and reliability in the use of computer simulations for the improvements and optimization of recovery boilers.

The influence of oxide scale on heat transfer during reheating of steel

Iron ore sinter constitutes a major proportion of blast furnace burden. Hence, its quality and consistency have a significant impact on blast furnace performance. Iron ore fines are the main source for sinter, and the chemical composition of the iron ore fines, together with the thermal conditions that blends are subjected to, plays an important role in forming the primary melt during the sintering process and accordingly determines the sinter structure and quality. Therefore, considerable importance has been placed on the chemical composition and consistency of iron ore fines, particularly in terms of alumina content. Due to depletion of high grade iron ore resources, alumina content in the iron ore fines is expected to increase gradually. Ore with higher alumina content is usually expected to be detrimental in forming the sinter matrix, if sintered alone, due to the low reactivity of alumina bearing minerals and the high viscosity of primary melts. The selective granulation process is a new sintering process for high alumina iron ore fines, and can eliminate the adverse effects of ‘hard to sinter’ or ‘unsuitable – for ironmaking’ ores. In the present work laboratory sintering experiments have been carried out with iron ore fines of different alumina level (2.00 to 5.46 mass-%) to know the influence of alumina on mineralogy, productivity, physical and metallurgical properties of sinter prepared by the conventional and the selective granulation process. With increasing alumina content in sinter of both the conventional and selective granulation process, the fractions of hematite and of silico-ferrites of calcium and alumina (SFCA) as well as the pore phase increased whereas the magnetite and silicate phases decreased. With increase in alumina content sinter productivity and tumbler index (T.I.) decreased, and metallurgical properties like sinter RDI and reducibility improved. However, sinter of the selective granulation process showed better results compared to the conventional process.

Numerical Analysis of Loads Effect on Combustion Performance and NO x Emissions of a 220 MW Pulverized Coal Boiler

This paper presents numerical study on the combustion performance and NOx emissions of a 220 MW pulverized coal boiler. Three different loads have been simulated with combusting coal, 200, 170 and 140 MW, respectively. In order to get as precise as possible numerical analysis results, two-step simulation method has been adopted in this work, namely, air supply system simulation and furnace simulation. After air supply system simulation, the results have been taken as the initial and boundary conditions for furnace simulation. The comparison between the measured values and predicted results from 200 MW case shows much better agreement. According to the simulation results, the adopted two-step simulation method is reasonable and suitable for predicting the characters of the flow and combustion process. It is concluded that the distributions of temperature, O2 and CO concentration inside furnace with different loads shows good similarly. The total NOx emissions decreased with the boiler load reducing, and fuel NOx has the same trend as total NOx, and fuel NOx account for about 66% in total NOx in all the three cases. More important, thermal NOx slowly decreased with the rise of boiler load. More detailed results presented in this paper enhance the understanding of combustion processes and complex flow patterns of front-wall pulverized coal boilers.

Characterisation of heat transfer and flame length in a semi-scale industrial furnace equipped with HiTAC burner

Abstract Abstract This paper investigates the effects of multiple burner nozzles on the combustion characteristics, such as flame volume, heat transfer and NOx emission in a high temperature air combustion (HiTAC) industrial furnace. Experiments were carried out in one semi-industrial furnace located in Kungliga Tekniska Hogskolan (Stockholm, Sweden). Three different types of burners were tested, including both regenerative and recuperative types. Variable flame temperature and oxygen concentration were applied in experiments. Heat transfer characteristics of HiTAC are studied in this paper, and the influences of a variety of inertial fuel/air jets are investigated for both flame length and NOx emission. One improved correlation between chemical flame length and flame Froude number is established for HiTAC with manifold nozzles. NOx emission is also correlated to the flame Froude number. The HiTAC recirculation system effects on flame shape, NOx emission and heat transfer were also examined.