Boiler Test Research Articles

Application of high-frequency water injection for the performance improvement of waste-heat-recovery boilers

It has been discussed that injecting water into the intake air can increase thermal efficiency and reduce NOx emissions simultaneously in waste-heat-recovery boilers (WHRB). To increase the evaporation rate of the injected water and recover more thermal energy through the heat exchanger, the water injection amount needs to be controlled properly and droplets in the spray need to be distributed homogeneously with the smallest droplets possible. To achieve this, this study introduces a high-frequency injection strategy that can generate strong in-nozzle turbulence, thus promoting spray atomization and supplying continuous water to the heat exchanger. Three different injection frequencies are applied to investigate water spray characteristics. The water spray structure is visualized using the Mie-scattering technique, and the droplet size is measured using the laser diffraction method. In addition, practical boiler tests are performed to confirm the effect of the high-frequency water injection on the boiler performance. The experimental results showed that the mean droplet size decreased, and the spatial and temporal homogeneity of the spray distribution was improved as the injection frequency increased. As a result of the higher water injection frequency, the evaporation rate and thermal efficiency of the boiler increased, and nitrogen oxide (NOx) emissions decreased.

Theoretical and experimental study of waste-heat-recovery boiler with water injection

The waste-heat-recovery boiler with water injection (HR-B/W) applies the heat exchange between the intake air and exhaust gas with the water injection into the intake air. Previous theoretical studies have discussed that the HR-B/W would increase the thermal efficiency of the boiler by the active heat exchange between the intake air and exhaust gas. It has also been discussed that the increased fraction of water vapor in the air would reduce the flame temperature which in turn decreases the NOx emission. However, the potential performance of the HR-B/W has not been validated through practical boiler tests by considering the evaporation characteristics of the injected water, which plays a critical role in the performance of the HR-B/W. In this study the effects of water injection into the intake air on the thermal efficiency and pollutant emissions of the waste-heat-recovery boiler are investigated using a commercial 24 kW condensing boiler in full load condition. Thermodynamic analysis is performed to evaluate the adequate amount of water injection and trace the physical properties in the boiler upon the water injection amount and evaporation characteristics. The boiler test results showed water injection can increase thermal efficiency to 4.4% point and reduce NOx and CO emissions by 69% and 33% respectively compared to those without water injection. These advantages can be further enhanced if the atomization and evaporation performance of injected water is improved.

Generating simplified ammonia reaction model using genetic algorithm and its integration into numerical combustion simulation of 1 MW test facility

A small reaction model for ammonia is necessary for the design and development of large-scale ammonia combustion systems using computational fluid dynamics (CFD) simulation. The present study generated a simplified reaction model for ammonia using a genetic algorithm (14 species and 45 reactions). Optimization was applied to reproduce the combustion properties of ignition delay times, mole fractions of ammonia and nitric oxide at the exit of a single perfectly stirred reactor (PSR), those of a two-staged PSR, and laminar flame speed. The generated simplified reaction model for ammonia was applied to combustion CFD simulation for a 1 MW boiler test facility. Results of CFD simulation reproduced the characteristics of two-staged combustion well. Nitrous oxide emissions predicted by the simulation agreed well with experimental results.

Evaluation of Particulate Matter (PM) Emissions from Combustion of Selected Types of Rapeseed Biofuels

The manuscript describes the results of an experimental study of the level of PM (particulate matter) emissions arising from the combustion of two selected types of biomass (i.e., rapeseed straw pellets and engine biofuel (biodiesel, FAME)), which were derived from rapeseed. The PM emissions from the combustion of biofuels were compared with those obtained from the combustion of their traditional counterparts (i.e., wood pellets and diesel fuel). Both types of pellets were burned in a 10 kW boiler designed to burn these types of fuels. The engine fuels tested were burned in a John Deere 4045TF285JD engine mounted on a dynamometer bench in an engine dyno, under various speed and load conditions. A Testo 380 analyzer was used to measure the PM emission levels in boiler tests, while an MPM4 particle emission meter was used in the engine tests. The combustion (under rated conditions) of rapeseed straw pellets resulted in a significant increase in PM emissions compared to the combustion of wood pellets. The PM emissions during the combustion of wood pellets were 15.45 mg·kg−1, during the combustion of rapeseed straw pellets, they were 336 mg·kg−1, and the calculated emission factors were 44.5 mg·MJ−1 and 1589 mg·MJ−1, respectively. In the engine tests, however, significantly lower particulate emissions were obtained for the evaluated biofuel compared to its conventional counterpart. The combustion of rapeseed oil methyl esters resulted in a 40–60% reduction in PM content in the exhaust gas on average for the realized engine speeds over the full load range compared to the combustion of diesel fuel.

Integrated use of CFD and field data for accurate thermal analyses of oil/gas boilers

In the present study we develop a complete Computational Fluid Dynamics (CFD) modeling procedure suitable for accurate simulations of industrial boilers fed alternatively with gaseous and liquid fuels. The model is developed and validated by means of data from on-site testing.Two different boilers are considered: a 6-burner steam generator of a refinery for the model definition, and a 32-burner thermal power plant boiler for validation, fed by flare-gas/natural-gas or Heavy-Fuel-Oil (HFO). Selected reliable experimental data coming from performance testing are used for both the set-up of the CFD simulations as well as to compute the Flue Exit Gas Temperatures (FEGTs) employed as validation criteria. The flare-gas testing on the 6-burner boiler allows us to accurately determine that a membrane wall emissivity around 0.60 is appropriate as a boundary condition for radiation. By a sensitivity analysis, the relevance of wall emissivity with respect to fouling thermal resistance on the overall heat transfer inside the furnace is established. For boilers alternatively operating with gaseous fuel and HFO a value of 2.8 kW/(m2·K) has been found appropriate. HFO testing on the 6-burner boiler provides data to develop the additional soot modeling, crucial to properly catch oil flame emissivity, based on the model of Khan and Greeves. This allows the matching of a mean soot mass fraction of 1%–3% in the flame zone, a range found by in-flame measurements in the literature and confirmed in the present study by overall heat exchange data in the whole furnace coming from HFO boiler testing. Finally, the 32-burner boiler is considered for validation: using the same procedure and parameters, excellent agreement is found between experimental and CFD results, both on a 100% load natural gas and on a 107% load HFO performance testing.The study demonstrates how field data can be used to validate CFD simulations and confirms that the use of physically meaningful parameters and models exempts from repeated tuning: no change is needed for emissivity and fouling thermal resistance between gaseous and liquid fuel operation, and even a simple soot modeling can be used, just ensuring physically consistent particulate concentration in the oil flame.

Evaluation of Greenhouse Gas Emission Levels during the Combustion of Selected Types of Agricultural Biomass

This paper presents the results of an experimental study of the emission levels of selected greenhouse gases (CO2, CH4, NOx) arising from the combustion of different forms of biomass, i.e., solid biomass in the form of pellets and liquid biomass in the example of engine biofuel (biodiesel). Both types of biomass under study are rape-based biofuels. The pellets are made from rape straw, which, as a waste product, can be used for energy purposes. Additionally, biodiesel contains rape oil methyl esters (FAME) designed to power diesel engines. The boiler 25 kW was used to burn the pellets. Engine measurements were performed on a dynamometer bench on an S-4003 tractor engine. An analyzer Testo 350 was used to analyze the exhaust gas. CO2 emission studies do not indicate the environmental benefits of using any alternative fuels tested compared to their conventional counterparts. In both the engine and boiler tests for NOx emissions, no environmental benefits were demonstrated from the use of alternative fuels. The measured average NOx emission levels for biodiesel compared to diesel were about 20% higher, and for rapeseed straw pellets, they were more than 60% higher compared to wood pellets. Only in the case of engine tests was significantly lower CH4 (approx. 30%) emission found when feeding the engine with rape oil methyl esters.

Pengujian Boiler untuk Pembangkit Listrik Tenaga Panas Laut

Energy needs are one of the reasons why research on renewable energy is the choice. One of the renewable energies is OTEC (ocean thermal energy conversion) with sea surface heat being an energy source and boilers being a medium for converting heat energy into motion energy so that in the end it becomes electrical energy. Boilers are a vital part of a generator, so boiler testing is a requirement for a generator. The research was carried out by experiment by testing the boiler with various temperatures and pressures so as to produce output power. The results show that temperature and pressure are highly correlated with output power. The application of OTEC shows that temperature and pressure are things that must be considered, the higher the surface temperature, the greater the output power.

Studies on densification and conversion of wastes as fuel briquettes for power generation

Energy in the world is on high emergency now days. Energy resources are abundant but exhausted as per present conditions. Search for alternate energy sources has been trending worldwide. Solar and wind energies are of present interest to minimize cost of investment. But utilization of Municipal and Solid waste as an alternate source of energy is vital in research. Producing fuel briquettes with naturally available waste materials which are biodegradable is an asset. The experimentation is carried out for moisture content, ash content, volatile matter, fixed carbon, sulfur, calorific value and water resistance. The waste materials are mixed with all purpose flour in different ratio’s. Calorific value is evaluated based on ultimate analysis. The effects of various parameters as fuel briquettes for power production were measured of data consisting of characterization (FTIR and XRD), Analysis of materials (Proximate and Ultimate) and Estimation of properties for fuel briquettes (Calorific value, porosity, waster absorption, shatter index, compressive strength, optimization of materials quantity using Response Surface Methodology, Tumbling test, Degree of Densification, Water Boiler Test–Thermal Efficiency, Power Estimation and Power Calculated). The experimental data are obtained by conducting batch experimentations. In the present investigation, the prospective of four materials (leaves) namely red sandal, Wrightiatinctoria, teak and mixed leaves were evaluated to estimate their performance as fuel briquettes for the power production. The result of four biomasses wastes are in fact potential sources of energy which has been validated by the quite substantial amounts of briquettes obtained using these as raw materials and fuel briquettes have environmental benefits that include reduced tree degradation, better management of waste and reduced emissions.

Adaptation of mathematical model of heat and energy characteristics of medium pressure boilers to real operating conditions

The object of this research is the thermal and energy characteristics of medium-pressure boilers at the thermal power plant of a metallurgical plant operating on a common steam header. The fuel used is a mixture of blast furnace and natural gases. The volume fraction of blast furnace gas is 80–95 %. The characteristics of blast-furnace gas are not constant: the elementary composition, humidity and dust content change significantly.The mathematical model developed for the purpose of comprehensive optimization of the modes of joint operation of boilers requires adaptation of the energy characteristics obtained by calculation to real operating conditions. Adaptation of the calculated energy characteristics was carried out individually on the basis of thermal balance tests of boilers and taking into account the design and operational features of each boiler. During the tests, the fraction of the convective component of heat transfer in the furnace for the considered boiler units was determined using a non-stationary heat meter. The absolute value of the heat flux density was determined according to the theory of non-stationary regular heating of the sensitive element (SE). It was found that as the fuel burns out, the level of heat fluxes decreases, and with an increase in the thermal fraction of blast furnace gas, the absolute value of local falling fluxes decreases. The average share of the convective component is 15–20 %. It is proposed to adapt to the real operating conditions of the superheater and tail heating surfaces using empirical correction factors obtained during processing of the results of balance tests.Taking into account the proportion of convective heat flux in the furnace makes it possible to more accurately determine the parameters of the heat carriers along the steam-gas path of the boiler, and, therefore, it is possible to obtain adapted energy characteristics of the boilers. In comparison with analytical calculated dependences, the proposed method provides an individual approach to the operating modes of boilers. Thanks to this, it is possible to reduce the error in determining the optimal solutions to less than 1 %.

Carbonaceous Particulate Matter Emitted from a Pellet-Fired Biomass Boiler.

Biomass pellets are a source of renewable energy; although, the air pollution and exposure risks posed by the emissions from burning pellets in biomass boilers (BBs) are uncertain. The present study examines the organic species in fine particle matter (PM) emissions from an BB firing switchgrass (SwG) and hardwood (HW) biomass pellets using different test cycles. The organic and elemental carbon (OC and EC) content and select semivolatile organic compounds (SVOCs) in filter-collected PM were identified and quantified using thermal-optical analysis and gas chromatography–mass spectrometry (GC–MS), respectively. Fine PM emissions from the BB ranged from 0.4 g/kg to 2.91 g/kg of pellets burned of which 40% ± 17% w/w was carbon. The sum of GC–MS quantified SVOCs in the PM emissions varied from 0.13 to 0.41 g/g OC. Relatively high levels of oxygenated compounds were observed in the PM emissions, and the most predominant individual SVOC constituent was levoglucosan (12.5–320 mg/g OC). The effect of boiler test cycle on emissions was generally greater than the effect due to pellet fuel type. Organic matter emissions increased at lower loads, owing to less than optimal combustion performance. Compared with other types of residential wood combustion studies, pellet burning in the current BB lowered PM emissions by nearly an order of magnitude. PM emitted from burning pellets in boilers tested across multiple studies also contains comparatively less carbon; however, the toxic polycyclic aromatic hydrocarbons (PAH) in the PM tested across these pellet-burning studies varied substantially, and produced 2–10 times more benzo[k]fluoranthene, dibenz[a,h]anthracene and indeno[1,2,3-c,d]pyrene on average. These results suggest that further toxicological evaluation of biomass pellet burning emissions is required to properly understand the risks posed.

Investigation of gas-solid flow and temperature distribution uniformity of 350 MW supercritical CFB boiler with polygonal furnace

Based on the non-uniform gas-solid flow of large-scale circulating fluidized bed (CFB) boilers, an innovative polygon furnace technology was put forward. Combined with cold test, numerical calculation and boiler tests, the effects of three polygonal furnace structures on the uniformity of gas-solid flow and particle concentration distribution were investigated. The research results show the furnace adopts the chamfered-corner structure, the circulating flow rates of three circulation loops are greatly increased, there is no obvious particle agglomeration phenomenon in the chamfered corners center of the furnace, and the non-uniformity of the gas-solid flow and particle concentration distribution can be effectively improved. The maximum relative deviation of three cyclones inlet and outlet flue gas temperatures for 350 MW supercritical CFB boiler is less than 6.0%, and the temperature distribution of the furnace and three cyclones is relatively uniform, the technology demonstration of the first 350 MW supercritical CFB boiler with polygonal furnace is successful.

Исследование режимов работы и методика теплового расчета жаротрубно-дымогарного котла мощностью 3 МВт

The article presents the results from studying the fire-tube boiler operation mode and describes the heat transfer processes occurring in it. The boiler design characteristics are given: specifically, the flame tube length and diameter are 3.2 m and 0.926 m, respectively. An adequate boiler analysis model has been developed, which simulates the heat transfer processes in the boiler using the Boiler Designer software package for computer-aided designing of boilers. The boiler efficiency is 90.92% in firing diesel fuel with a heating value of 42 705 kJ/kg with a flowrate of 0.278 t/h and an excess air factor of 1.37. Recommendations for carrying out thermal design calculations of the fire-tube boiler and further tasks of its research are given. The results obtained from experimental and theoretical investigations are compared with each other. It is shown that the design calculation of a small-capacity fire-tube boiler cannot be carried out strictly in line with the standard boiler unit thermal design method. In accordance with this method, the temperatures at the furnace and boiler outlets should be equal to 1221°C and 222°C. According to the experimental results, the flame tube and the flue tubes receive a significantly larger amount of heat. According to the recommendations, the furnace outlet temperature should not exceed 1200°C. The boiler test results have shown that the flue gas temperature was 195°C. An assessment of the thermal analysis output data has shown that a convection heat transfer component exists in the flame tube, which accounts for up to 30% of the total heat absorption.

Ultrasound on-line measurement of pulverized coal flow in coal-fired boilers

Abstract The pulverized coal flow in the two-phase gas-solid is monitored based on the ECAH (Epstein, Carhart, Allegra, Hawley) theoretical model. The relationship between the volumetric concentration of the pulverized coal and the ultrasound attenuation coefficient is analyzed. The flow velocity and concentration of the coal particles is measured using two ultrasonic probes at an angle of 45° to the flow direction. The results show that the aberration of the ultrasonic on-line measurements of the pulverized coal flow is less than 3 % compared to the coal flow set by the experimental boiler test platform. The general feasibility and reliability of the ultrasonic method of the flow measurements is verified.

CFD simulation of the combustion process of the low-emission vortex boiler

Domestic heat and power engineering needs means and methods for optimizing the existing boiler plants in order to increase their technical, economic and environmental work. The development of modern computer technology, methods of numerical modeling and specialized software greatly facilitates the solution of many emerging problems. CFD simulation allows to obtaine precise results of thermochemical and aerodynamic processes taking place in the furnace of boilers in order to optimize their operation modes and develop directions for their modernization.The paper presents the results of simulation of the combustion process of a low-emission vortex coal boiler of the model E-220/100 using the software package Ansys Fluent. A hexahedral grid with a number of 2 million cells was constructed for the chosen boiler model. A stationary problem with a two-phase flow was solved. The gaseous components are air, combustion products and volatile substances. The solid phase is coal particles at different burnup stages. The Euler-Lagrange approach was taken as a basis. Calculation of the coal particles trajectories was carried out using the Discrete Phase Model which distribution of the size particle of coal dust was accounted for using the Rosin-Rammler equation. Partially Premixed combustion model was used as the combustion model which take into account elemental composition of the fuel and heat analysis. To take turbulence into account, a two-parameter k-ε model with a standard wall function was chosen. Heat transfer by radiation was calculated using the P1-approximation of the method of spherical harmonics. The system of spatial equations was numerically solved by the control volume method using the SIMPLE algorithm of Patankar and Spaulding.Comparison of data obtained during the industrial-operational tests of low-emission vortex boilers with the results of mathematical modeling showed acceptable convergence of the tasks of this level, which confirms the adequacy of the realized mathematical model.

A Novel Approach to Develop the Lower Order Model of Multi-Input Multi-Output System

A mathematical model is a virtual entity that uses mathematical language to describe the behavior of a system. Mathematical models are used particularly in the natural sciences and engineering disciplines like physics, biology, and electrical engineering as well as in the social sciences like economics, sociology and political science. Physicists, Engineers, Computer scientists, and Economists use mathematical models most extensively. With the advent of high performance processors and advanced mathematical computations, it is possible to develop high performing simulators for complicated Multi Input Multi Ouptut (MIMO) systems like Quadruple tank systems, Aircrafts, Boilers etc. This paper presents the development of the mathematical model of a 500 MW utility boiler which is a highly complex system. A synergistic combination of operational experience, system identification and lower order modeling philosophy has been effectively used to develop a simplified but accurate model of a circulation system of a utility boiler which is a MIMO system. The results obtained are found to be in good agreement with the physics of the process and with the results obtained through design procedure. The model obtained can be directly used for control system studies and to realize hardware simulators for boiler testing and operator training.

Pollutant and Corrosion Control Technology and Efficient Coal Combustion

High efficiency and low emissions from coal-fired power stations have been the drive behind the development of present and future efficient coal combustion technologies. Upgrading coal, capturing CO2, reducing emission of NOx, SO2, and particulate matter, and mitigating slagging, fouling, and corrosion are the key initiatives behind these efficient coal technologies. This study focuses on an efficient coal combustion technology utilizing a newly developed fuel additive (Silanite), which addresses most of the aforementioned key points. Silanite, a finely milled multioxide additive, when mixed with the coal without the need to change the boiler installation has been proven to increase the boiler efficiency and flame temperature with reduction in corrosion and NOx and particulate matter (dust) emissions. The process has been developed through bench, pilot (100 kWth), and full scale (233 MWth) and has been found to have a number of beneficial effects that add up to a viable retrofit to an existing power plant as demonstrated by results from 233 MWth boiler tests (under BS EN 12952-15:2003 standard). The benefits proven on commercial and laboratory scale include the following: reductions of 20% in the overall particulates, 42% in loss on ignition, and 8–25% in NOx with about 30% increase in the life span of the tube section of the boiler.

An experimental study of solids deposited on the base of a heated vessel at low pressure

The life expectance of some industrial evaporators is constrained by the rate of corrosion of the heated surfaces, with larger temperatures producing larger corrosion rates. Some process fluids contain dissolved materials that produce solid particles when the fluid is evaporated. These solids can form beds of particles on the heated base of the evaporator vessel or be suspended in the fluid. The presence of these particles may have an effect on the temperature of the heated surfaces and hence their corrosion rate. The effect on temperature of two very different particles are investigated, glass and Caesium Phosphomolybdate particles (Cs3(MoO3)12PO4·14H2O). The later is referred to as CPM. The glass particles had diameters of 500–600μm, a specific gravity of 2.2 and a bed depth variation of 0–32mm. The CPM particles had diameters of the order of 10μm, a specific gravity of 4.0 and a bed depth variation of 0–50mm.The effect of the glass particles was investigated while heating water at a pressure of 50mbar absolute. The CPM particles were investigated while heating a water solution at a pressure of 65mbar absolute. The evaporator used was a model industrial boiler slice. The boiler test section was 1m high, 0.75m wide and 98mm long. The evaporator contained 36 electrically heated tubes to simulate the presence of heated coils. The design of the boiler ensured that the tubes, which were 28.5mm in diameter, were always submerged in a liquid pool. The tube heat flux was maintained at 65kW/m2 and the base heat flux varied within the range 0–45kW/m2.When a base heat flux was applied the glass particles remained within a bed formed on the base of the evaporator, whereas the CPM particles became suspended in the fluid. In both cases, the temperature in the liquid pool is shown to be reasonably constant and close to the free surface saturation value. The bed of glass particles induced boiling on the surface below it at most conditions, increasing the local base temperature and therefore the local corrosion rate. The base remained in free convection outside of the bed, producing lower base temperatures and therefore a lower corrosion rate. The CPM particles did not form a bed on the base of the evaporator and the temperature of the base remained in free convection at temperatures below, or close to, the local saturation value. However, some base heating was evident near the base centre. The presence of CPM type particles is unlikely to affect the corrosion rate.

Shell-side boiling of a glycerol-water mixture at low sub-atmospheric pressures

Experimental data are reported for boiling water and a glycerol-water mixture at a free surface pressure of 50mbar absolute on the shell-side of a thin slice model of an industrial boiler. The boiler test section was 1m high, 0.75m wide, 98mm long and contained 36 electrically heated, horizontal tubes that were 28.5mm in diameter. The design of the boiler ensured that the tubes were submerged in a liquid pool. The height of the liquid pool was set to 2m, submerging the top of the tube bundle in 1.6m of liquid. The heat flux was varied within the range 10–65kW/m2. A near-symmetrical half of the tube bundle contained wall thermocouples. An additional 29 thermocouples were located throughout the liquid pool.For both fluids, the liquid temperature in the pool was found to be reasonably uniform and controlled by the pressure at the free surface. This led to subcoolings of up to 31K on the tube surfaces. The reasonably uniform pool temperature suggests that the liquid re-circulates within it.For water, boiling was initiated in the heat flux range 25–40kW/m2, whereas the glycerol-water mixture initiated boiled within the range 10–25kW/m2. Below these heat flux ranges, both fluids were in natural convection, with the measured wall superheats in reasonable agreement with predictions from a correlation available in the open literature. The difference in the fluids’ boiling onset resulted from the natural convection, heat-transfer coefficients of the glycerol-water mixture being lower than that for water. The boiling wall superheats for water were reasonably well predicted by a correlation available in the open literature.Boiling glycerol-water mixture data, taken at atmospheric pressure and available in the open literature, was used to identify methods for correcting pure fluid boiling heat-transfer coefficients for mixture effects. Mixture boiling superheats were reasonably well predicted by some of these methods. The method that worked best at atmospheric pressure did not work best at low pressure. A method is identified that is reasonable at atmospheric and low pressures.

Use of a turboexpander in steam power units for heat energy recovery in heat supply systems

A method for raising the efficiency of a boiler plant by installing a unit operating by the organic Rankine cycle is presented. Such units allow one to generate electricity to cover the auxiliaries of a heat source at a heat-transfer fluid temperature of no more than 130°С. The results of commissioning tests of boilers revealed that their efficiency is maximized under a load that is close or corresponds to the nominal one. If this load is maintained constantly, excess heat energy is produced. This excess may be used to generate electric energy in a steam power unit with a turboexpander. A way to insert this unit into the flow diagram of a boiler plant is proposed. The results of analysis of turbine types (turboexpanders included) with various capacities are presented, and the optimum type for the proposed flow diagram is chosen. The methodology for the design of turboexpanders and compressors used in the oil and gas industry and their operational data were applied in the analysis of a turboexpander. The results of the thermogasdynamic analysis of a turboexpander and the engineered shape of an axial-radial impeller are presented. Halocarbon R245fa is chosen as the working medium based on its calorimetric properties.

Study of NOx Inhibition Technology Based on Water-Gas Reaction

Pulverized coal combustion is one of the main NOx emission sources. The existing low NOx combustion technology can not meet the requirements of environment conservation. In this paper, a new low NOx combustion technology, the NOx inhibition method based on water-gas reaction, is presented, in which steam is injected into the anoxic flame of pulverized coal to bring water-gas reaction and produce CO and H2 which will inhibit the production of NOx. The produced H2 is especially very active. Water-gas reaction is endothermic, which has an effect of reducing the peak temperature of the flame and is very propitious to the inhibition of thermal type NOx. As the water-gas reaction is also an interim process, the heat absorbed in it will be released when burning with oxygen so that the whole amount of heat inside the furnace is not affected. The principle of the method is proved correct by experiments and industrial scale of 420 t/h boiler tests in which the effect of NOx reduction is obvious. The technology is consistent with that of air-staged combustion.