Bypass Flue Research Articles

HCl removal from cement bypass flue gas at medium-high temperatures using cement bypass dust: Performance, mechanism, and adsorption kinetics

The use of alternative fuels is a growing trend in the cement industry, and it may lead to issues with HCl emissions. This study investigated the HCl removal performance of cement bypass dust (CBPD) at medium-high temperatures (500–700℃). This is not only of great significance for controlling HCl emissions from cement kilns but also alleviates the disposal pressure of this industrial solid waste. The results show that HCl adsorption capacity increased first and then decreased with increasing temperature, reaching a maximum of 676 mg/g at 600°C. The HCl concentrations significantly influenced the reaction rate, yet its impact on the uptake chlorine capacity is comparatively minimal. CBPD without size separation showed the best dechlorination performance and highest final chlorine capacity compared with CBPD of size 0–48 μm and 48–75 μm. Blending inert SiO2 particles into CBPD can greatly increase the breakthrough time. The longest breakthrough time is 7580 seconds, approximately 35 times that of CBPD without dispersion. CaO is the primary substance in CBPD to remove HCl, and the reaction produces CaCl2 and its hydrate. A shrinking core model was used to analyse the experimental data, suggesting a combination control of the chemical reaction and the product diffusion.

Design on a novel waste heat recovery system integrated with the bypass flue and outside primary air preheater for bitumite-fired power plants

Bitumite is widely used in coal-fired power plants. However, due to its high volatile content, the temperature of primary air to coal mills is limited to a lower value because of its high risk of spontaneous combustion. The method to reduce primary air temperature by mixing with cold air leads to irreversible loss and reduces the power generation efficiency of the power plant. To address the issue, a novel flue gas waste heat recovery system integrated with the bypass flue and outside primary air preheater for bitumite-fired power plants was suggested. The thermo- and techno-economic analysis models of the waste heat recovery system were developed, and the design parameters were optimized to maximize profit. The thermo-economic analysis shows that the exergy efficiency of the waste heat recovery system can reach 49.8 %, and the coal consumption rate can be reduced by 4.75 g (kW h)−1 by integrating the optimal waste heat recovery system. The techno-economic analysis shows that the net present value of the power plant during ten years can reach 10.6 M$, and the discounted payback period is 0.6 years. The novel waste heat recovery system can effectively improve the thermo- and techno-economic performance of bitumite-fired power plants.

Continuous DeNOx Technology for Improved Flexibility and Reliability of 1000 MW Coal-Fired Power Plants: Engineering Design, Optimization, and Environmental Benefits

This study endeavors to enhance the operational efficiency of extant coal-fired power plants to mitigate the adverse environmental impact intrinsic to the prevalent utilization of coal-fired power generation, which is particularly dominant in China. It focuses on the assessment and optimization of continuous denitrification systems tailored for a 1000 MW ultra-supercritical pulverized coal boiler. The extant denitrification framework encounters challenges during startup phases owing to diminished selective catalytic reduction (SCR) inlet flue gas temperatures. To ameliorate this, three retrofit schemes were scrutinized: direct mixing of high-temperature flue gas, bypass flue gas mixing, and high-temperature flue gas mixing with cold air. Each option underwent meticulous thermodynamic computations and comprehensive cost analyses. The findings elucidated that bypass flue gas mixing, involving the extraction and blending of high-temperature flue gas, emerged as the most financially prudent and practical recourse. This scheme optimizes fuel combustion heat utilization, significantly curtails fuel consumption, and fosters efficient internal heat transfer mechanisms within the boiler. The evaluation process meticulously considered safety parameters and equipment longevity. The insights derived from this investigation offer valuable guidance for implementing continuous denitrification system retrofits in industrial coal-fired power plants.

Flow and thermal mixing characteristics of a multi-jet bypass flue with main flue baffles

The decrease of inlet flue temperature of denitrification reaction unit is a major challenge that restricts the low load operation of thermal power units. In this study, a new bypass flue heating technology based on multi-pipe and multipoint was proposed. The effect of different baffle openings (RA), directions and position distance (Db) on the mixing characteristics of multiple jets was quantitatively investigated by the experimental system. Combined with the simulation, the changes of the baffle on the multi-jet flow and mixing are revealed. The results show that the mixing temperature increases with increasing RA, but high RA leads to a significant increase in the thermal mixing temperature inhomogeneity (ζT). As RA increases from 0.4 to 0.7, ζT rises from the minimum 0.368 to the maximum 0.559. The mixing temperature and ζT of the baffle in the +x direction are higher than those in the −x direction. When Db increases from 70 mm to 130 mm, the maximum heating effect decreases from 25.97 °C to 18.89 °C, and ζT decreases gradually at y/Dh = 1.25. The findings provide guidance for bypass flue modification and low load denitrification in engineering.

Research on deep utilization technology of flue gas residual heat for large-capacity coal-fired units

The utilization of flue gas waste heat is an important measure to improve the power efficiency of coal-fired units. Traditional method is to arrange the heating surface after the air preheater, which can just recover the low-grade heat, low efficiency. For the engineering practice of a 1000MW double-reheat unit, three feasible schemes are proposed, namely scheme 1 “two-stage gas cooler plus cold air heater”, scheme 2 “air preheater bypass + two-stage flue gas cooler plus cold air heating” and scheme 3 “air preheater bypass + hot primary air temperature adjustment plus two-stage flue gas cooler plus cold air heater”. For the sake of evaluating the actual effects of different schemes objectively, the thermal characteristic calculation model of the four-part rotary air preheater was established by finite difference method, and then the effect of the inlet air temperature and the share of bypass flue gas on the thermal efficiency of the boiler were studied. The equivalent enthalpy drop method was used to establish the thermal characteristic model of steam turbine, and the influence of different heat distribution modes on the heat consumption rate of steam turbine was studied.. Considering the technical economy, the optimal bypass flue gas share of this scheme is 12%, which corresponds to the reduction of coal consumption for power supply of the unit by about 3.23g/kWh. Taking into account comprehensively, scheme 3 is recommended.

Mixing performance of preheated flue gas entering selective catalytic reactor under low loads

The reduced inlet flue gas temperature of the selective catalytic reduction (SCR) denitrification device is a significant problem during the low-load operation of boilers. In this study, an experimental system for turbulent thermal mixing of high and low-temperature flue gases was built to study the thermal mixing method in bypass flue gas technology and the effect of the temperature difference and flow rate variation of high and low-temperature flue gases at low load on the thermal mixing characteristics. The experiments evaluated the thermal mixing characteristics by measuring the temperature field distribution, calculating the inhomogeneity parameters ζT, and investigate the multi-jet distribution characteristics. The investigation was carried out more accurately by combining the simulation results. The results show that the tube wall jet combined with the bottom jet method provides the highest warming effect and possesses the lowest mixing non-uniformity parameter. At the location which is 1.25 times the hydraulic diameter of the duct away from the jet pipe, the temperature difference between the mainstream and the jet drops from 270 °C to 180 °C, causing ζT to drop from 0.46 to 0.37. Decreasing the jet velocity and increasing the main flow velocity promote a decrease in ζT, indicating that the ratio of jet velocity to main flow velocity and the velocity ratio UJUm are crucial factors affecting the thermal mixing characteristics, which directly affects the heat transfer between hot and cold fluids. The ζT decreases from 0.52 to 0.42 as UJUm changes from 3.58 to 2.15. This study provides a reference for ensuring the safe and stable operation of SCR denitrification system during the low-load operation of coal-fired boilers.

Effect of deflector angle and number on evaporation performance of desulfurization wastewater in a spray drying tower

Rotary spray drying technology with bypass flue is widely adopted to realize the zero-discharge of desulfurization wastewater in coal-fired power stations. Significantly, the deflector structure of the drying tower is an essential factor affecting the evaporation performance of desulfurization wastewater. This paper is aimed to study the effect of the angle and number of the deflectors on the evaporation performance in a drying tower of a 600 MW coal-fired station. Results show that the gas distributor divides the flue gas into internal and external parts, and the flue gas in the internal channel is the main factor affecting the evaporation performance. The deflector angles (especially for the internal deflector) have a remarkable impact on the evaporation behaviors, while the deflector numbers just have a minor effect on that. When the deflector number is 24 and the angles of internal and external deflectors are set at 0° and 45°, respectively, a better evaporation performance is obtained with the weighted average time of approximately 0.17 s and the complete evaporation distance of approximately 3.93 m. These optimal settings are adopted in the actual operation, and the industrial measured results show good performance is obtained.

Performance analysis of an efficient waste heat utilization system in an ultra-supercritical coal-fired power plant

With the rise of new energy sources in the power industry, improving the efficiency of coal-fired power plants is essential. Currently, the leading main causes of irreversible energy loss are the energy level mismatches of boilers and turbines. An ultra-supercritical coal-fired power plant thermodynamic system was reconstructed based on the first and second laws of thermodynamics to improve the efficiency of power plants, and an efficient condensate water preheating air integrated bypass flue waste heating efficient utilization system is thus herein proposed. Consequently, high and medium steam turbine extraction flow is returned to the steam turbine to do work, and cold source loss is reduced by increasing the extraction steam flow of the low-pressure cylinder. At the same time, the inlet air temperature in the air preheater is significantly increased by preheating the hot air with condensate water, resulting in a reduced heat transfer temperature difference and significantly improved exergy efficiency. Compared with conventional low-temperature economizer and bypass flue waste heat utilization schemes, the results indicate that the heat transfer exergy efficiency increased significantly from 78.49% to 95.88% in the air preheater, and the heat consumption rate of the turbine generator set decreased by 108.06 kJ/(kW⋅h). The generation efficiency increases by 0.74%, while the standard coal consumption rate decreases by 3.92 g/(kW⋅h). In terms of economic investment, the dynamic payback period is 3.31 years and the net present value is $2.20 M/year. This paper provides an effective energy conservation and emission reduction plan for coal-fired power plants to contribute towards achieving national carbon peaking and carbon neutrality goals.

Performance evaluation of a conceptual compressed air energy storage system coupled with a biomass integrated gasification combined cycle

In this study, a novel design has been developed to improve the energy efficiency of the compressed air energy storage (CAES) system by integration with a biomass integrated gasification combined cycle (BIGCC) system. With the energy cascade utilization principle, the heat from the compressed air cooling is recycled by the heat regeneration system of the BIGCC system in charging process. Besides, different from the normal CAES system, in discharging process, the compressed air from the air storage vessel is set into the combustor of the BIGCC system directly after being heated by the bypass flue, which is arranged in parallel at the heat recovery steam generator of the BIGCC system. By such incorporation, the heat storage equipment can be saved and the electricity consumed by the air compressor of the BIGCC system can be reduced, thus the energy efficiency of the CAES system can be improved. Energy, exergy, economic, and sensitivity analyses were applied to evaluate the performance of the integrated system. The results of the analyses show that with the integration, the round-trip efficiency and exergy efficiency of the CAES system can reach 88.43% and 64.28%, respectively. Besides, the overall efficiency of the coupled system improves by 0.35%.

Boiler Bypass Flue to Adjust Denitration Inlet Flue Temperature Automatic Control System Design and Analysis

Under low-load boiler operating conditions, the flue gas temperature at the inlet of the denitration system is lower than the normal working temperature of the catalyst. In order to ensure the reliable input of the denitration system, a bypass flue and an adjustment damper are added to the boiler economizer. Configure control logic and actual debugging to realize automatic control of the inlet temperature of the denitration system, and ensure that the denitration system can be continuously operating when the boiler is running in the 30% -100% load range.

Analysis of Gas Recirculation Influencing Factors of a Double Reheat 1000 MW Unit with the Reheat Steam Temperature under Control

In this paper, the simulation software EBSILON is used to simulate the reheat units, and the reheat temperature control mode is deeply explored. In the benchmark system, the influence of different load intermediate point temperature on the flue gas recirculation (FGR) is analyzed. Then, the effects of load, coal quality, excess air factor, and feed water temperature on FGR are studied under the premise of intermediate point temperature as design value, and the cause for FGR change is analyzed by comparing the cutoff bypass flue (CBF) system. The results show that under any load, the FGR decreases with the increase of the intermediate point temperature, while under low load, the change of the intermediate point temperature has a greater impact on the FGR rate. When the intermediate point temperature remains constant, the FGR plunge has an increase of load at low load and is almost unchanged at high load; the FGR rate of coal with low calorific value and high moisture content is low and the coal with low volatile and high ash content has great influence on reheat steam temperature; and the excess air factor and feed water temperature are inversely proportional to the flue gas recirculation rate. In the CBF system, the change trend is similar to the reference system, but under the same working condition, the FGR rate is higher than the latter.

Techno-economic evaluation of the novel hot air recirculation process for exhaust heat recovery from a 600 MW hard-coal-fired boiler

To increase the efficiency of thermal power plants, we have recently proposed and analyzed a novel exhaust heat recovery process called hot air recirculation (HAR), applied to a brown-coal-fired power unit as a test case. However, the performance of HAR, applied to a hard coal-fired power unit, is not clearly understood yet. In this study, the HAR process was redesigned to match the actual supply air and flue gas layout of a hard-coal-fired boiler. The thermo- and techno-economic performances of HAR, as well as the conventional bypass flue (CBF) process, applied to a 600 MW hard-coal-fired power unit, were analyzed in detail. The results indicate that, when the boiler exhaust heat is recovered from 122 to 90 °C, the net coal savings, initial capital cost, and payoff period of HAR are 3.49 g/(kW·h), $1.473 million, and 1.34 years, respectively, whereas those of CBF are 2.98 g/(kW·h), $2.528 million, and 3.04 years, respectively. Therefore, it is established that HAR can benefit a hard-coal-fired power plant with a greater saving on coal consumption and a more economical project investment, in addition to providing a safe and reliable operation, as compared to similar processes.

Thermo-economic analysis of typical thermal systems and corresponding novel system for a 1000 MW single reheat ultra-supercritical thermal power plant

In this study, thermodynamic analysis of the bypass flue (BPF) thermal system and the “bypass flue integrated with steam-air heater (BPF-SAH)" thermal system is carried out by taking a 1000 MW single reheat ultra-supercritical coal-fired power plant as a case unit. Although these two typical thermal systems can improve the power supply efficiency of power plant to 43.68% and 43.74%, respectively, there is still excessive exergy loss in the air preheating process on the boiler side. To achieve better heat utilization of flue gas, a novel thermal system with integrated steam-air heaters (SAHs)is proposed. The thermodynamic analysis results show that the power supply efficiency of the integrated SAHs thermal system reaches 43.89%, which is 0.82% points higher than that of the case unit, and the integrated SAHs thermal system overcomes the shortcomings of these two typical thermal systems. Techno-economic analysis comparison of these three types of thermal systems reveals that the integrated SAHs thermal system shows the best techno-economic performance and is feasible in engineering, and the dynamic investment payback period and the net present value of the 30 years lifespan are 2.36 years and 20.412 million USD, respectively.

Simulation study on novel groove separator in a dual‐loop wet flue gas desulfurization spray tower

AbstractThe dual‐loop wet flue gas desulfurization spray tower (DLST) is particularly suitable for achieving “ultralow” emission of SO2 for high‐sulfur coal‐fired power plants. The separator is the key piece of equipment for the DLST. In this study, to overcome the disadvantages of a traditional bowl separator, novel groove separators are proposed and compared with the traditional separator. The effects of different separators on the flue gas flow characteristic, slurry droplet distribution, mass transfer resistance, and desulfurization efficiency were analyzed through numerical simulation based on the Euler–Lagrange method and two‐film theory. The results show that, compared with the traditional bowl separator, the proposed groove separator can improve the uniformity of the gas–liquid flow field in the upper loop, increase the liquid‐film side mass transfer rate significantly, eliminate the empty spaces caused by the high‐speed bypass flue gas, and avoid the “flooding” phenomenon completely. Therefore, the resistance losses caused by the separator can be reduced, the desulfurization efficiency can also be improved by adopting the novel groove separators, and the lower the liquid‐to‐gas ratio, the more obvious the increase in desulfurization efficiency. In general, the advantages of the novel groove separators over the traditional bowl separator are obvious.

Experimental Study on a Flue Gas Waste Heat Cascade Recovery System under Variable Working Conditions

Recovering flue gas waste heat is beneficial to improving the unit efficiency in power plants. To obtain the change rules of performance parameters of a flue gas waste heat cascade recovery system (FWCRS) under variable working conditions, an experiment bench was designed and built. The variation laws of the inlet temperature and exhaust flue gas temperature of a low temperature economizer (LTE), the inlet and outlet air temperature of an air preheater (AP), the heat exchange quantities of the AP, LTE, and front-located air heater and an additional economizer (AE), as well as the waste heat recovery efficiency, the system exergy efficiency, and the energy grade replacement coefficient were obtained as the flue gas flow, flue gas temperature, bypass flue gas ratio, air temperature, and circulating water flow in AE changed. Using an orthogonal test, the flue gas temperature, bypass flue gas ratio and air temperature were proved to be the significant factors affecting the performance parameters of FWCRS, and the bypass flue gas ratio was suggested as an adjusting parameter of FWCRS under variable working conditions.

Numerical simulation on flow and evaporation characteristics of desulfurization wastewater in a bypass flue

Owing to the oversimplification of the evaporation model, existing numerical analysis cannot accurately characterize the evaporation characteristics, which has limited its significance for engineering. Therefore, by user-defined programming, a droplet evaporation model in line with the properties of desulfurization wastewater was implemented in numerical simulation. The effects of guiding measures, nozzle arrangement, nozzle layer, flue gas temperature, moisture and droplet diameter on the droplet evaporation in a bypass flue were explored. The numerical results show that the fluctuation of evaporativity due to moisture can be neglected. The uniformity of the flow field with guiding measures is good and the phenomenon of droplets impinging on the wall surface can be prevented. The droplet collision probability without guiding measures may be nearly 60 times higher than that with guiding measures. A reasonable nozzle arrangement increases the droplet evaporation rate and reduces the probability of droplets colliding with the wall surface. An increased number of nozzle layers enhances the droplet evaporation rate and the average temperature in the flue; by estimation, the average temperature in the flue with three nozzle layers is 10.7 K higher than that for two nozzle layers, and 15.8 K higher than that for one nozzle layer. In the condition of higher temperature and smaller diameter, droplets evaporate more rapidly. These research results can provide a reliable reference for engineering practice.

Experimental research and industrial application on bypass flue gas evaporation technology for FGD wastewater

Experimental research and industrial application on bypass flue gas evaporation technology for FGD wastewater

Experimental Study on Operation Regulation of a Coupled High–Low Energy Flue Gas Waste Heat Recovery System Based on Exhaust Gas Temperature Control

Controlling the exhaust gas temperature (EGT) of coal–fired boilers at a reasonable value is beneficial to ensuring unit efficiency and preventing acid corrosion and fouling of tail heating surfaces in power plants. To obtain the operation regulation of coupled high–low energy flue gas waste heat recovery system (CWHRS) under a given EGT, experimental equipment was designed and built. Experiments were carried out to maintain the exhaust gas temperature under different flue gas flow, flue gas temperature and air temperature conditions. As the flue gas flows, the flue gas temperatures and air temperatures increased, and the bypass flue gas flow proportions or the water flows of the additional economizer were increased to maintain the EGT at about 85 °C. An improved low temperature economizer (LTE) and front located air heater (FAH) system were put forward. As the flow of the crossover pipe increased, the EGT and the inlet water temperature of the LTE increased. As the flow of the circulating loop increased, the EGT and the inlet water temperature of the LTE decreased. Operation regulations of LTE–FAH system under four cases were given. The operation regulations of CWHRS and LTE–FAH system can provide references for power plant operation.

Improved combustion air preheating design using multiple heat sources incorporating bypass flue in large-scale coal-fired power unit

In this study, a new concept of the combustion air preheating system integrated with a bypass flue (BPF) configuration was put forward and its feasibility was evaluated. In the proposed system, the primary air and secondary air are heated separately with cascade heat exchange and the hot and cold bypassing primary air mixing is avoided, which contributes to enhancing the air preheating and waste heat utilization. Thermodynamic analysis was performed based on a typical 600 MW coal-fired power unit incorporating the new concept. The results indicated that owing to the proposed design, the power generation efficiency promotion reaches 0.78% (absolute value) with a standard coal consumption rate reduction of 5.52 g/kWh as compared to the reference unit, which are 0.14% (absolute value) higher and 0.94 g/kWh larger than those caused by the regular BPF retrofitting, respectively. The energy utilization is more rational in the new design according to the energy and exergy analysis, and the exergy efficiency of the primary air heating process is improved from 75.75% (original design) or 79.62% (regular BPF design) to 84.46% due to the proposed concept. The volume and heat exchange area of the APH decline significantly as well in the modified system.

Techno-economic analysis of a novel hot air recirculation process for exhaust heat recovery from a 600 MW brown-coal-fired boiler

There had been several processes for recovering exhaust gas heat at the boiler cold-end, thereby increasing the efficiency of a thermal power plant. However, the existing processes encounter a great challenge on the heat exchanger, wherein the acid corrosion, fouling and abrasion of heating surfaces are inevitable because it is working in very low temperature flue gas. In this paper, a novel hot air recirculation (HAR) process is advanced to avoid these tricky problems. To demonstrate the thermo- and techno-economic performance of the HAR, an in-service 600 MW brown-coal-fired power unit was used as a reference unit for analyses. Meanwhile, the performance of the conventional bypass flue (CBF) process was also calculated for comparison. The results show that, when recovering the boiler exhaust heat from 148 °C to 90 °C, the net coal savings, initial capital cost and payoff period of the HAR are 7.31 g/(kW·h), 2.522 million USD and 1.08 years, whereas the corresponding results of the CBF are 6.79 g/(kW·h), 6.074 million USD and 3.24 years. Overall, the HAR can benefit the power plant from a safe and reliable operation, a greater net coal savings and a better techno-economic performance, exhibiting an obvious superiority in the similar processes.