Primary Air Distribution Research Articles

Numerical analysis of NOx reduction in large-scale MSW grate furnace through in-bed combustion optimization using multi-section fuel bed model with thermally thick treatment

This study focuses on reducing NOx emissions in large-scale municipal solid waste (MSW) grate furnaces through in-bed combustion control. A multi-section fuel bed model with thermally thick treatment is developed to accommodate the configurations of large-scale MSW grate furnaces. The model incorporates detailed sub-grid models to simulate the intraparticle gradient, and the stochastic mixing is accounted to deal with the multi-section fuel beds with vertical drop-offs. The influences of primary air distribution and average grate velocity on combustion status and NOx emissions are investigated numerically and validated in a 600 t/d MSW grate boiler. The results indicate that combining a relatively low primary air ratio with a thickened fuel bed ensures both reduced NOx emissions and high burnout efficiency. By reducing the excess air ratio of primary air from 1.2 to 0.9 and distributing it through a rear-enhanced air mode, the overall NOx emission decreased from 398.50 mg/Nm3 to 215.05 mg/Nm3, although at the expense of an increased unburned carbon content from 1.995 wt% to 3.063 wt%. Further lowering the grate velocity to thicken the fuel bed allowed for leveraging the heterogeneous reduction effect of the char layer, reducing the NOx emissions to 184.95 mg/Nm3. Additionally, the unburned carbon content was also remarkably reduced to 1.560 wt%. The proposed strategies of delaying the primary air supply and thickening the fuel bed offer a cost-effective alternative to post-combustion measures, with which the MSW grate boilers can achieve lower NOx emissions while maintaining efficient and stable combustion.

In-situ measurement of potassium and sodium concentrations in a 16 MW MSW incinerator employing the FES system

To comprehensively understand the release characteristics of alkali metals which can cause corrosion and slagging problems in municipal solid waste (MSW) incinerators, the temporal concentrations of sodium (Na) and potassium (K) in a 16 MW MSW incinerator were measured employing a calibrated flame emission spectroscopy (FES) system. The experimental results revealed that the concentrations of alkali metals increased first and then decreased from the left side to the right side of the MSW incinerator. The maximum concentrations of Na and K were 111 ppm and 184 ppm, respectively. The peak and average concentrations of K near the left wall were 27 % and 58 % higher than those near the right wall at the same height of the MSW incinerator, owing to the different combustion stages of the MSW. The differences of the peak and average concentrations of K at the different heights of the MSW incinerator were 33.7 % and 23.5 %, respectively. Moreover, a positive correlation between the concentration of alkali metals and flame temperature was confirmed during the char phase combustion stage. Since the primary air distribution pattern of MSW incinerator decreased form the left side to the right side, the concentration of alkali metals near the left wall was positively correlated with the primary air flow, and that near the right wall exhibited a positive correlation with the garbage caloric value. This research provided an effectual solution for in-situ measurement of alkali metal concentration and flame temperature in MSW incinerators.

Optimizing biomass combustion in a 130 t/h grate boiler: Assessing gas-phase reaction models and primary air distribution strategies

Grate-firing technology is widely used for biomass combustion for heat and power production due to its good fuel flexibility. However, grate boilers often suffer from high unburnout, low efficiency, and high emissions, and require optimization to improve their performance. The distribution of primary air along the grate length significantly affects combustion and pollutant formation behaviors in the furnace; however, this effect is often under-investigated and poorly quantified. In this study, combustion of corn stover in a 130 t/h grate boiler is investigated experimentally and numerically. To establish a reliable baseline computational fluid dynamics (CFD) model and provide valuable reference for modeling and optimizing this industrial grate boiler, this study first investigates the applicability of the commonly used gas-phase reaction models. The finite rate/eddy dissipation model is found to outperform other gas-phase reaction models such as eddy dissipation model and eddy dissipation concept, which well predicts the temperature and shape of the flame in both the kinetics-controlled primary combustion zone and the diffusion-controlled burnout zone and shows the best agreement with the measured and site observations. Then, the study simulates and quantifies the effects of different primary air distribution modes (uniform, staged and refined-staged) on combustion performance of the boiler. Compared with the uniform primary air distribution, both the staged and refined-staged primary air distribution show an obvious increase in the char burnout ratio, from 76.5 % to 87.8 % and 95.5 %, respectively. Furthermore, the CO concentration at the furnace outlet is remarkably reduced, from 3178 ppm to 2766 ppm and 1917 ppm, respectively. This demonstrates that reasonable air supply in the latter two primary air distribution cases facilitates complete combustion in both the fuel-bed and freeboard. Moreover, in the latter two cases, the temperature distribution in the freeboard becomes more uniform, and the local oxygen-rich regions are eliminated. These are beneficial in suppressing the NO formation, as the emission of which decreases from 172 mg/m3 (at 11 % O2) to 161 mg/m3 and 156 mg/m3, respectively. This work provides important theoretical guidance for the accurate modeling and optimization of grate-firing boilers as well as the design of new grate boilers.

Optimization of the Air Distribution in a Biomass Grate-Fired Furnace

This study utilized a combination of FLIC(1D3.2C) and FLUENT(2021R2) software to optimize the primary air distribution along the grate and the performance of a straw briquette combustion furnace of a 7 MW unit in China used to produce hot air for drying grain. Three air distribution modes, namely front-enhanced, uniform, and rear-enhanced modes, were analyzed to determine their effect on the flue gas components above the grate, the temperature field in the furnace, and the nitrogen oxide concentration at the furnace outlet. The results of the calculations showed that the NOx emissions for the front-enhanced, uniform, and rear-enhanced modes were 133.5 mg/Nm3, 104.4 mg/Nm3, and 76.6 mg/Nm3, respectively. It was found that the rear-enhanced mode can expand the biomass drying, devolatilization, and combustion zone, thus improving the furnace combustion performance and decreasing NOx emissions. These findings can provide useful guidance for optimizing biomass chain-grate-firing furnaces.

Modeling of a Primary Air Distribution in a Plenum Chamber of the 700 MWth Circulating Fluidized Bed Boiler

Modeling of a Primary Air Distribution in a Plenum Chamber of the 700 MWth Circulating Fluidized Bed Boiler

Analysis and monitoring of the combustion performance in a biomass power plant

Combustion air systems are an important aspect of the industrial boiler's performance, and they are being used increasingly to reduce emissions. Generally, improvements in boiler performance and emissions reduction can be achieved by modifying the air system. In the specific case of primary air, its temperature, distribution, and vibration of the grate significantly affect the conversion and mixing of biomass. Consequently, in this work, a comprehensive analysis of a biomass power plant operation, which is a result of an experimental campaign developed for 12 days is presented. In this regard, since the fuel has highly variable physical and chemical properties, disturbances causing an unsteady conversion process and corrosion risks were verified. Furthermore, the influence of the primary air distribution and the grate vibration cycles was evaluated through different tests. The grate vibration periods revealed that increasing the interval without vibrations results in a significant reduction of CO emissions. However, increasing this period may lead to some agglomeration problems in the grate. Concerning the primary air supply, in the three different sections of the grate, the supply of a more significant amount of air in the first two sections of the grate, in the central region of the grate, decreases the CO emissions. Additionally, the possibility to reduce the flue gas temperature using empirical correlations for a quick estimation of acid dew point during flue gas cooling for heat recovery to preheat the primary air was studied. The results revealed a large amount of energy that can be recovered without low-temperature corrosion problems, and 7% of the total input energy can be saved.

Investigation of biomass conversion on a moving grate by pyrolysis gas analysis and fuel bed modelling

Abstract A profound understanding of the fuel conversion on a grate boiler is crucial for an optimised boiler operation with a minimised pollutant emission. This work presents gas measurements and numeric simulations of a 150 kW moving grate boiler. The fuel bed model considers drying, pyrolysis and gasification. In the experiments, temperature profiles above the fuel bed and the fuel bed height are measured. Pyrolysis gas is sampled with a cooled probe and analysed on its CO, CO2, CH4, VOC, H2O, H2 and O2 composition. The experiments show that drying occurs in the first third of the grate with a relatively constant release of water vapour from the fuel bed. The temperature of the dried fuel further increases on the grate. The release of volatiles from the fuel pyrolysis starts towards the end of the drying zone and occurs in a relatively narrow zone. The fuel bed simulations are validated, discussed and used to analyse the influence of parameters on the fuel conversion such as the moisture content of the fuel and the primary air distribution. The model forms a basis to calculate the entry conditions for subsequent gas phase simulations.

Numerical simulation of municipal solid waste combustion in a novel two-stage reciprocating incinerator

A mathematical model was presented in this paper for the combustion of municipal solid waste in a novel two-stage reciprocating grate furnace. Numerical simulations were performed to predict the temperature, the flow and the species distributions in the furnace, with practical operational conditions taken into account. The calculated results agree well with the test data, and the burning behavior of municipal solid waste in the novel two-stage reciprocating incinerator can be demonstrated well. The thickness of waste bed, the initial moisture content, the excessive air coefficient and the secondary air are the major factors that influence the combustion process. If the initial moisture content of waste is high, both the heat value of waste and the temperature inside incinerator are low, and less oxygen is necessary for combustion. The air supply rate and the primary air distribution along the grate should be adjusted according to the initial moisture content of the waste. A reasonable bed thickness and an adequate excessive air coefficient can keep a higher temperature, promote the burnout of combustibles, and consequently reduce the emission of dioxin pollutants. When the total air supply is constant, reducing primary air and introducing secondary air properly can enhance turbulence and mixing, prolong the residence time of flue gas, and promote the complete combustion of combustibles. This study provides an important reference for optimizing the design and operation of municipal solid wastes furnace.

A process simulation model for a 2 ton h-1 incinerator (a combined bed combustion and furnace heat transfer model)

A process simulation model was constructed for a 2 ton h - incinerator. The simulation model was designed to provide system performance parameters according to various operating conditions. In accommodating the wide variation of quality and composition of input wastes, the plant operating parameters such as amount of excess air, preheated air temperature, waste feed rate and primary air distribution over the stoker, etc. must be carefully controlled. The proposed model calculates operating variables of each submodule, by employing steady-state thermal and material balance equations. Combustion of waste bed, and its radiative heat transfer in the combusion chamber are considered. The calculated results of the combustion chamber performance are evaluated, in terms of temperature, locations and width of the flame band, and mean residence time in the secondary combustion chamber. These results are compared with a limited set of field test measurements for verification of the model.

Design Optimization of Coventry Municipal Solid Waste Incinerator

This paper discusses a number of design modifications and changes in operational conditions that have a major influence on the overall performance of the large Coventry municipal incinerator plant (65 MW), which incorporates a heat recovery system for district heating. Four different secondary air-injection systems and various primary air-distribution patterns along the grate were investigated using a computational fluid dynamic model, in an attempt to obtain optimum combustion conditions that would minimize potential emissions of toxic pollutants and reduce maintenance costs at the plant. The modeling work showed that the use of four large high-speed secondary air jets firing towards a common center, together with optimizing the primary air distribution along the grate, produces substantially longer residence times and improves temperature profile at the exit. The novel feature of the proposed secondary air-injection system is the formation of a large and significantly strong recirculation zone, located above the burning refuse bed, in the middle of the furnace shaft. This strong recirculation zone improves the overall performance of the incinerator due to intensive mixing of hot gaseous products evolving from the refuse bed with the combustion air supplied as the secondary air, and greatly improves the gas phase combustion and helps to reduce emissions of chlorinated organic compounds. Results obtained clearly demonstrate that the proposed secondary air-injection system has an excellent potential to satisfy most requirements for emissions levels and gas residence times specified by the European Directives (EEC) for municipal incinerators with a vertical radiation shaft.

Effect Of High Speed Secondary Air Jets On The Overall Performance Of A Large MSW Incinerator With A Vertical Shaft

Abstract This paper discusses a number of proposed design modifications and changes in operational conditions which have a major influence on the overall performance of the Sheffield municipal solid waste incinerator plant. This 35 MW unit incorporates a heat recovery system for district heating. Four different secondary air injection systems and various primary air distribution patterns along the grate were investigated using computational fluid dynamics in an attempt to eliminate the existing slagging problems and at Ihe same time obtain optimum combustion conditions which would minimise potential emissions of toxic pollutants and reduce maintenance costs at the plant. The modelling work showed that the use of four tangential high speed secondary air jets together with optimising the primary air distribution along the grate, produces substantially longer residence times, improves the temperature profile at the exit, reduces the concentration of toxic pollutants and increases the combustion efficiency of the incinerator. The novel feature of the proposed secondary air injection system is the formation of a large size and significantly strong recirculation zone located above the burning refuse bed, in the middle of the furnace shaft. The presence of this strong recirculation zone improves the overall performance of the incinerator due to intensive mixing of the hot gaseous products (CO, volatile matter and hydrocarbons) evolving from the refuse bed with the combustion air supplied as the secondary air, and thereby greatly improves the gas phase combustion and helps to reduce emissions of chlorinated organic compounds; i.e. Dioxins. Results obtained to date clearly demonstrate that the proposed secondary air injection system has excellent potential to help satisfy all the requirements for the emission levels and gas residence times specified by the EC Directives for municipal incinerators with a vertical radiation shaft. The principal conclusion reached from this modelling study is that upgrading of the existing municipal solid waste incinerators to meet some current environmental criteria can often be achieved with modification of the present conventional secondary air jet system designs together with the optimisation of the plant overall performance as a function of key process operating variables.