Co-firing Process Research Articles

Highly accurate heat release rate marker detection in NH3–CH4 cofiring through machine learning and domain knowledge-based selection integration

Ammonia emerges as a promising substitute for traditional fuels, offering a potential reduction in fossil fuel consumption and the associated emissions. Given its weak reactivity, an effective strategy to harness the potential of ammonia involves cofiring it with methane, to establish a self-sustaining combustion process. The understanding of heat release rate (HRR) in the cofiring process of ammonia and methane is crucial for burner design. This study introduces novel HRR indicators through a machine learning-based approach, focusing on species significantly contributing to HRR. Kinetic analysis is carried out utilizing a one-dimensional freely propagating flame model, applying a mechanism including 59 species and 356 reactions. Domain knowledge-based feature selection narrows down the search space, enhancing computational efficiency during HRR indicator identification. A random forest model is then employed to identify radical/radical combinations and their respective reaction orders, representing HRR optimally based on mean decrease impurity. The proposed HRR markers are [CH4]0·76[OH]1.08, [HCO]0·65[NO]0.24, [CH3]0.65[O]0.63, [NH2][O]0.59, and [NH3]0·93[OH]0.96. The root mean square error for each marker was extracted and compared with literature data, demonstrating higher reliability of the proposed indicators over those previously suggested. Additionally, the paper concludes with a discussion of the feasibility of measuring these markers from an experimental perspective.

Life cycle assessment of ammonia co-firing power plants: A comprehensive review and analysis from a whole industrial chain perspective

Ammonia, a reliable low-carbon alternative fuel with energy storage capabilities, has garnered increasing attention for its application of co-firing in coal-fired power plants as a strategy to mitigate direct carbon emissions. However, various types of ammonia production technologies result in diverse economic feasibility and emission intensities. Simultaneously, each stage, spanning from upstream processes such as raw material extraction to downstream applications, contributes to carbon emissions, which cannot be ignored. It is crucial to select the appropriate assessment method to determine the transformation pathways for co-firing systems. To this end, this review presents a comprehensive life cycle assessment of ammonia co-firing systems from a whole industrial chain perspective, encompassing the entire gamut of processes from fuel production and transportation to co-firing. Studies of the industrial chain perspective and of life cycle assessment methodology that are uniquely tailored for co-firing systems are presented. A nuanced exploration of distinct technologies across the spectrum of system processes ensues, including the advantages, limitations, and trends in advancement, based on carbon emissions and economic criteria. Considering the diverse fuel production, especially ammonia, typologies and intricate processes have undergone comprehensive review. The combustion characteristics, emissions, and economic factors associated with the co-firing process are systematically summarized, drawing upon aspects such as dynamics, experiments, simulations, and demonstration projects. This study illuminates the progression and technology selection of co-firing systems across multiple stages of the whole industry chain, thereby furnishing insights relevant to the low-carbon transformation of ammonia co-firing with coal in power plants.

Combustion characteristics and nitrogen conversion mechanism in ammonia/coal Co-firing process

The coupled combustion of coal with the carbon-free fuel NH3 is an effective technology for reducing CO2 emissions. Based on elemental analysis, proximate analysis, Fourier Transform infrared spectroscopy (FTIR), Carbon-13 Nuclear Magnetic Resonance (13C NMR), and X-ray photoelectron spectroscopy (XPS) characterizations, a coal macromolecular model was constructed. The combustion characteristics of coal/NH3 co-firing were investigated by reactive force field molecular dynamics simulations (ReaxFF MD). The transformation mechanism of elemental N was elucidated. The influence of temperature, oxygen concentration, and NH3 co-firing ratio (CR) on products was investigated. The results showed that during the non-isothermal combustion phase, the coal macromolecules decomposed more rapidly when the heating rate was low. In the isothermal phase, high temperatures promoted secondary reactions. The production of inorganic gases increased with increasing temperature. As the molar fraction of NH3 increased, the production of CO2 and CO was effectively reduced. There were three main pathways for NO formation. As NH3 decomposed to form NH2, NH2 was subsequently converted to NH, and NH reacted with O2 to form NO. The NH2 could also react with O radicals to form HNO, which was then converted to NO. Besides, both thermal NOx and prompt NOx were generated since the N2 was involved in the reaction. In addition, the production of NOx increased significantly at oxygen concentrations greater than 1.4.

Experimental study and kinetic analysis of NO emission characteristics in ammonia/coal co-firing process with different ammonia injection methods

Ammonia/coal co-firing in coal-fired power plants is an effective way to achieve CO2 reduction. In this paper, the ammonia/coal co-firing study was carried out in a 45 kW wall-temperature-controlled staged combustion downstream furnace at 1100 °C. Bituminous coal was used as the experimental object to analyze the effects of ammonia injected at the main combustion zone, reduction zone, with the burnout air, and at the burnout zone on NO release characteristics at different ammonia co-firing ratios. To ascertain the significant reaction of NO production in the ammonia/coal co-firing, chemical reaction kinetics calculations were performed. When co-firing ratios of 10∼30%, the NO emission concentration is lower by the ammonia injection position of the main combustion zone which is 275–346 mg/m3. When co-firing ratios reach to 40% and 50%, NO releases are lower by the ammonia injection position of burnout zone which is 511–574 mg/m3. In addition to exhibiting a greater capacity to decrease NO in environments with low oxygen concentrations and high NO concentrations, ammonia can also, under specific circumstances, have a significant capacity to decrease NO in environments with high oxygen concentrations. Based on the experimental results and chemical kinetic calculations, the NO release pattern and mechanism during ammonia-coal co-combustion with different ammonia injection methods under air staged combustion were investigated, which provides a technically feasible solution to reduce CO2 and NOx emissions from coal-fired power plants.

Property of Cu and PbSrZrTiO3 multilayer actuator fabricated by cofiring with wet reduction

Traditional co-firing processes involving noble metals are costly, thereby prompting a shift to base metals, such as Cu, to reduce manufacturing expenses. This study explored the use of Cu as an internal electrode for fabricating piezoelectric multilayer devices using a Pb0.98Sr0.02(Zr0.48Ti0.52)O3 (PSZT) ceramic material. The compatibility of Cu with PSZT in ambient air and the subsequent wet-reduction was investigated. The addition of polyvinylidene fluoride (PVDF) to the Cu electrode expedited the reduction process; however, this prompted void formation. Thus, epoxy resin was injected to post-reduction to enhance its mechanical property and address this issue. The impact of the PVDF content on the reduction rates, microstructure, and electrical/mechanical properties was analyzed. The experimental results highlighted the successful fabrication of Cu/PSZT devices with performances comparable to those of traditional noble metal (AgPd) counterparts. The polarization and strain of the Cu/PSZT device with 10% PVDF were comparable to those of the AgPd/PSZT. Subsequently, the incorporation of epoxy resin and Cu electroplating greatly improved the mechanical strength of Cu/PSZT to values comparable to that of AgPd/PSZT, as confirmed by the successful fabrication of a piezoelectric ultrasonic device and vibration measurement at 60 kHz. Therefore, this proposed process is a viable alternative for fabricating low-cost and stable multilayer ceramic devices.

Probing into Volatile Combustion Flame and Particulate Formation Behavior during the Coal and Ammonia Cofiring Process: Further Study on the Chemical Structure and Oxidation Reactivity of Soot Particles

Probing into Volatile Combustion Flame and Particulate Formation Behavior during the Coal and Ammonia Cofiring Process: Further Study on the Chemical Structure and Oxidation Reactivity of Soot Particles

Valorizacija otpadne poljoprivredne biomase kao goriva za održivu proizvodnju energije kosagorevanjem

Waste agricultural biomass, like the crop residue, is an abundant indigenous renewable energy source in Serbia. As such, the agricultural residue might be utilized for sustainable power production. Direct co-combustion of biomass with coal/lignite offers a viable option to remove this kind of waste and, in the same time, to use its energy potential, mitigate harmful emissions and retrofit existing coal-fired power plants cost-effectively. However, there are serious issues regarding the waste management and the co-firing process itself, yet to be solved. The main aim of this paper is to present benefits and challenges of co-firing the waste agricultural biomass, as a promising technology for biomass valorization and decarbonization in energy, focusing the agricultural residues energy potential, as well as characteristics, preparation and efficient utilization of the biomass as an alternative fuel in power sector.

Effect of ammonia on N migration and transformation characteristics during coal pyrolysis: Quantum chemical calculations and pyrolysis experiments

The co-firing of the non-carbon fuel, ammonia, with pulverized coal, is an effective way to reduce CO2 emissions in thermal power plants. However, the high nitrogen content of ammonia increases the risk of high NOx emissions during the co-firing process. To achieve low nitrogen combustion of ammonia-coal co-firing, it is necessary to study the impact of ammonia on the N migration and transformation characteristics, during the coal pyrolysis and combustion processes. In this work, the effects of ammonia on the release of N-containing products, during coal pyrolysis, were studied by quantum chemistry calculations and pyrolysis experiments. The theoretical calculation results showed that the mixing of hydrogen-rich ammonia fuel promoted the generation of HCN during coal pyrolysis and increased the energy consumption of HCN generation. Additionally, ammonia-N could be embedded into the coal char structure and exist in the form of N-6. The experimental results indicated that with the temperature increasing, the N content of the coal char and the pyridinic-N increased under the condition of ammonia-coal co-pyrolysis, confirming the microscopic mechanism of ammonia-N embedding into the coal char structure, as indicated by the theoretical calculations. Our work reveals the molecular paths of N migration and transformation and elaborates on the interaction mechanism of ammonia-N and coal-N during the ammonia-coal co-pyrolysis, which has a certain promoting effect on the development of ammonia-coal co-firing mechanism.

Microwave dielectric ceramics with low dielectric loss and high temperature stability for LTCC applications

Preliminary tri-layer architecture ZnNb1.8V0.1O5.75-TiO2+CuO–ZnNb1.8V0.1O5.75 ceramics with high-temperature stability and low dielectric loss have been prepared. In the past, the stacked co-firing process could only be used at normal sintering temperatures. In this paper, the effect of the transition layer in ZnNb1.8V0.1O5.75-TiO2+CuO–ZnNb1.8V0.1O5.75 ceramics with tri-layer architecture sintered at low temperatures (950 °C) on the crystalline structure, microscopic structure, and integrated microwave dielectric performance of specimens has been investigated in depth for the first time. This low-temperature stacked-layer co-firing technique can effectively enable the chemical reaction between ZnNb1.8V0.1O5.75 and TiO2+CuO to take place in a rather narrow region (about 20 μm), i.e., the interface of the heterogeneous layers. In this interface, diffusion of Zn, Nb, V, Ti, and Cu can be observed. This heterogeneous interface can act as an in-situ “glue” that holds the layers together well. The effect of TiO2+CuO content on the microwave dielectric performances of laminated architectural specimens under low-temperature sintering was systematically investigated. Among the existing layer architectures, the stack of ZnNb1.8V0.1O5.75-TiO2+CuO–ZnNb1.8V0.1O5.75 with 0.03 wt% TiO2+CuO exhibits the most excellent microwave dielectric performances. τf can be efficiently tuned close to zero (−7.14 ppm/°C), and remarkably, a low dielectric loss (14,587 GHz) and a high value of εr (24.00) are also achieved. This low-temperature sintered tri-layered architecture is designed to help open new avenues for the development of high-performance microwave dielectrics based on existing material systems, providing effective strategies and methods for 5G applications and multilayer packaging technologies.

Synergistic co-combustion of carbon-rich fraction from coal gasification fine slag and corncob char: Performance, mechanism, kinetics, and thermodynamics

Coal gasification fine slag (FS) is a coal-based solid waste with high carbon content, and landfill-based disposal causes water-soil-gas composite pollution. Co-combustion is a common method of solid waste thermal disposal, and corn cob is used as an auxiliary fuel for co-combustion because of its energy density and ease of handling and processing. In this paper, the thermal degradation characteristics, interaction mechanism, kinetics, and thermodynamics of co-combustion RC (carbon-rich fraction obtained by flotation) and CC (corncob char) were studied. The results showed that physicochemical properties suggested that RC and CC will complement and promote each other in the combustion process. The addition of CC reduced the RC ignition temperature and improved the overall combustion performance. The interaction effect was mainly due to metal catalysis, the effect of the porous carbon structure, the thermal effect, and the hindering effect of ash. The activation energy was reduced due to the blending of RC-CC, and the activation energy was lowest (132.19 kJ/mol) when the RC adding ratio was 60%. The co-combustion process was controlled by the F3 model (chemical third-order reaction kinetics), and it was a typical gas-solid phase reaction with a progressive process from the outsides toward the insides. Thermodynamic analysis showed that the co-firing process was an extremely complex multi-stage reaction. The results of this study provide a theoretical reference for the synergistic resource utilization of coal gasification fine slag and waste biomass.

Effects of ammonia co-firing ratios and injection positions in the coal–ammonia co-firing process in a circulating fluidized bed combustion test rig

Ammonia (NH3) co-firing is a promising technology for reducing greenhouse gas emissions in coal-fired power plants. Prior to commercialization, an experimental study on coal–NH3 co-firing in a pilot-scale circulating fluidized bed (CFB) combustion test rig was conducted for technical verification. The comprehensive combustion characteristics, including pollutant emission, combustion efficiency, and ash properties, of NH3 co-firing with sub-bituminous coal in a CFB combustion test rig and the CO2 reduction according to NH3 co-firing ratios under two different injection positions (dense bed zone (DBZ) and wind box (WB) with primary air) were investigated. When NH3 was injected at the DBZ, NO emissions decreased as the NH3 co-firing ratio increased and CO emissions increased more rapidly than with only coal-fired combustion. Compared with only coal-fired combustion, a 25.4% NH3 co-firing ratio at the WB position simultaneously reduced NO and CO concentrations, achieving the highest combustion efficiency without ash-related problems. However, N2O emissions increased by > 1.5 times, indicating the formation of N intermediates during NH3 burning. Therefore, with minor retrofitting, coal–NH3 co-firing at the WB position is a feasible solution for simultaneously reducing CO2, NO, and CO emissions in commercial CFB combustion plants.

Durability Study of Anode-Supported SOFC with Large-Area Fabricated By 4-Layer Sequential Co-Lamination and GDC Co-Firing Process

Electrolyte-Cathode interfaces are life-threatening regions of solid oxide fuel cells where degradation phenomena occur due to chemical interdiffusion. The Buffer layer is inserted between electrolyte-cathode to limit the cation exchange across the interface. However, state of art buffer layer coated by screen printing does not fully prevent cation migration which results in the formation of insulating phases such as SrZrO3. Herein, a reliable four-layered (NiO-YSZ, NiO-CeScSZ, CeScSZ, and GDC) thin film-based anode supported SOFCs with a large-area (12cm×12cm) were fabricated by sequential co-lamination, in which all cell components perform a distinct role to enhance the interfacial connectivity and packing density of electrolytes. This enhanced connectivity with a highly dense electrolyte (5-6µm) and buffer layer (2-3 µm) accomplished on the porous anode support without any structural defects. The single cell (144cm2) showed a power output of 38W at total current 50A (700°C). The Post-tested cell had exceptional micro-structural characteristics, excellent interfacial adhesion and retained its structural integrity during static and dynamic operation. Also, the formation of insulating phases and decomposition of cathode was effectively prevented, resulting in a remarkable improvement in durability. The cell with four-layered structure exhibits an extreme low-degradation rate of 0.2% kh-1 under 25A current and 21 dynamic load cycling conditions, which satisfies the strict benchmark of durability for commercialization of technology. Key word: Durability, interfacial connectivity, insulating phase, sequential co-lamination Figure 1

Scalable, Patternable Glass-Infiltrated Ceramic Radiative Coolers for Energy-Saving Architectural Applications.

A huge concern on global climate/energy crises has triggered intense development of radiative coolers (RCs), which are promising green-cooling technologies. The continuous efforts on RCs have fast-tracked notable energy-savings by minimizing solar absorption and maximizing thermal emission. Recently, in addition to spectral optimization, ceramic-based thermally insulative RCs are reported to improve thermoregulation by suppressing heat gain from the surroundings. However, a high temperature co-firing process of ceramic-based thick film inevitably results in a large mismatch of structural parameters between designed and fabricated components, thereby breaking spectral optimization. Here, this article proposes a scalable, non-shrinkable, patternable, and thermally insulative ceramic RC (SNPT-RC) using a roll-to-roll process, which can fill a vital niche in the field of radiative cooling. A stand-alone SNPT-RC exhibits excellent thermal insulation (≈0.251Wm-1 K-1 ) with flame-resistivity and high solar reflectance/long-wave emissivity (≈96% and 92%, respectively). Alternate stacks of intermediate porous alumina/borosilicate (Al2 O3 -BS) layers not only result in outstanding thermal and spectral characteristics, causing excellent sub-ambient cooling (i.e., 7.05 °C cooling), but also non-shrinkable feature. Moreover, a perforated SNPT-RC demonstrates its versatility as a breathable radiative cooling shade and as a semi-transparent window, making it a highly promising technology for practical deployment in energy-saving architecture.

Techno-economic Analysis of co-firing waste Refused Derived Fuel (RDF) in coal-fired power plant

Massive efforts have been made to reduce CO2 emissions around the world from the used of fossil fuels by seeking alternative fossil fuels in power plants. The utilization of waste in the energy sector with co-firing technology is one way to reduce the impact on the environment. The Indonesian government is currently issuing SNI 8966:2021 to take advantage by using of biomass waste as raw material for making Refused Derived Fuel or RDF in power plants (BSN, 2021. RDF which has a calorific value of 1800 kcal/kg will be tested in PLTU Indramayu which the coal has caloric value 4100 kcal/kg. The mass ratio for blending is 1% RDF and 99% Coal. This study aims to analyze the feasibility of economical, operational, and environmental if co-firing test carried out with waste RDF at PLTU. RDF is processed from waste using the concept of collaboration with DLH Indramayu. RDF processing site is carried out at PDU Indramayu using the peuyeumization method. Within 11 days, the plant produced RDF 14 tons which will be supplied as mixed fuel for co-firing PLTU Indramayu. Operational observations at the Indramayu PLTU during the 1% BBJP co-firing process showed parameters that were still within operational safe limits, but there was an increase in SO2 and NOx emissions which were still below the KLHK emission standards. The results of the techno-economic analysis show that if there is an increase in LCOE of 0.16-rupiah in the co-firing test with 1% BBJP, while for the Net Present Value (NPV) parameter a value of Rp.40,437,359 is obtained, the Internal rate return (IRR) parameter is 8.25%, the Profitability index (PI) parameter is 1.011 and the last payback period (PBP) is 6.63 years indicating the feasibility of investing in the BBJP processing project. From an economic point of view, the surrounding community provides opportunities for employment and business provision of consumable materials for BBJP processing projects.

Durability Study of Anode-Supported SOFC with Large-Area Fabricated By 4-Layer Sequential Co-Lamination and GDC Co-Firing Process

Electrolyte-Cathode interfaces are life-threatening regions of solid oxide fuel cells where degradation phenomena occur due to chemical interdiffusion. The Buffer layer is inserted between the electrolyte-cathode to limit the cation exchange across the interface. However, state of the art buffer layer coated by screen printing does not fully prevent cation migration which results in the formation of insulating phases such as La2ZrO7 and SrZrO3. Herein, a reliable four-layered (NiO-YSZ, NiO-CeScSZ, CeScSZ, and GDC) thin film-based anode-supported SOFCs with a large-area (12 cm × 12 cm) were fabricated by sequential co-lamination, in which all cell components perform a distinct role to enhance the interfacial connectivity and packing density of electrolytes. This enhanced connectivity with a highly dense electrolyte (5-6 µm) and buffer layer (2-3 µm) was accomplished on the porous anode support without any structural defects. The single cell (144 cm2) showed a power output of 38 W at a total current of 50 A (700 °C). The Post-tested cell had exceptional micro-structural characteristics and excellent interfacial adhesion and retained its structural integrity during steady and unsteady state operation. Also, the formation of insulating phases and decomposition of the cathode were completely prevented, resulting in a remarkable improvement in durability. The cell with a four-layered structure exhibits an extreme low degradation rate of 0.2% kh-1 under 25 A current and 21 dynamic load cycling conditions, which satisfies the strict benchmark of durability for technology commercialization.

Durability improvement of large-area anode supported solid oxide fuel cell fabricated by 4-layer sequential co-lamination and co-firing process

In this work, thin-film based 4-layered (NiO-YSZ, NiO-ScCeSZ, ScCeSZ, and GDC) anode-supported solid oxide fuel cell (SOFC) with a large-area (12cm × 12 cm) is fabricated by sequential co-lamination and GDC cofiring process to improve the cell durability. This results in a highly dense electrolyte (5–6 μm) and buffer layer (2–3 μm) on the porous anode support. The single cell shows a high-power output of 41.5W at 50A and a maximum power density of 1.6 Wcm−2 at 700 °C. After the current load cycling of 21 times from 25 to 50 A, the large-area cell shows a very small degradation of 0.018V, which is attributed to strong interfacial connectivity between the ScCeSZ electrolyte and GDC buffer layer. Subsequently, the long-term test is carried out for 1000 h at 700 °C under a constant current density of 250 mA/cm2. The cell with a 4-layered structure exhibits the lowest degradation rate of 0.2% kh−1 at 25A current, which satisfies the stringent benchmark of longevity for the commercialization of technology.

Physicochemical Characterization of Wood Mixed with Coffee Waste Pellet

Recently, the demand for wood pellets as solid biofuel for co-firing process in the industry has increased. However, the increase in demand is not accompanied by the availability of sawdust (SD) as a raw material. Thus, seeking alternative biomass is needed to substitute wood. One of the alternative biomasses that can be considered as raw material for bio-pellet is the solid waste from the coffee industry. Meanwhile, the coffee industry annually produces large amounts of organic waste such as spent coffee grounds (SCG) and coffee husks (CH). Both solid wastes have organic compounds contained so that they can be utilized as solid biofuel. In this research, We investigated the quality changes of wood pellets before and after the addition of coffee waste in different mass ratios (1:1, 1:3). A densification method with a pressure of 2 tons is used in the wood pellet-making process. Physicochemical characterization of wood pellets such as density, durability, compressive strength, calorific value, and proximate and composition analysis using SCG and CH are conducted. The result shows that the addition of SCG into wood pellet increased the calorific value of wood pellet from 21.03 MJ.kg-1 to 21.71 MJ.kg-1. The calorific value with a ratio of 50: 50 of SD: SCG produces a better calorific value than CH (20.64 MJ.kg-1). The mechanical test of the pellet shows that the addition of SCG and CH slightly decreases the mechanical durability from 99.34% to 97.02%. Thus, increase the appearance density from ρ = 892.46 kg.m-3 to ρ = 1119.33 kg.m-3.

A 39 GHz Dual-Channel Transceiver Chipset with an Advanced LTCC Package for 5G Multi-Beam MIMO Systems

A 39-GHz dual-channel CMOS transceiver chipset with an advanced LTCC packaging and the demonstration of a proposed dynamic multi-beam architecture based on the chipset for 5G MIMO applications. This article presents a 39 GHz transceiver front-end chipset for 5G multi-input multi-output (MIMO) applications. Each chip includes two variable-gain frequency-conversion channels and can support two simultaneous independent beams, and the chips also integrate a local-oscillator chain and digital module for multi-chip extension and gain-state control. To improve the radio-frequency performance, several circuit-level improvement techniques are proposed for the key building blocks in the front-end system. Furthermore, an advanced low-temperature co-fired ceramic process is developed to package the 39 GHz dual-channel transceiver chipset, and it achieves low packaging loss and high isolation between the two transmitting (TX)/receiving (RX) channels. Both the chip-level and system-in-package (SIP)-level measurements are conducted to demonstrate the performance of the transceiver chipset. The measurement characteristics show that the TX SIP provides 11 dB maximum gain and 10 dBm saturated output power, while the RX SIP achieves 52 dB maximum gain, 5.4 dB noise figure, and 7.2 dBm output 1 dB compression point. Single-channel communication link testing of the transceiver exhibits an error vector magnitude (EVM) of 3.72% and a spectral efficiency of 3.25 bit·s −1 ·Hz −1 for 64-quadrature amplitude modulation (QAM) modulation and an EVM of 3.76% and spectral efficiency of 3.9 bit·s −1 ·Hz −1 for 256-QAM modulation over a 1 m distance. Based on the chipset, a 39 GHz multi-beam prototype is also developed to perform the MIMO operation for 5G millimetre wave applications. The over-the-air communication link for one- and two-stream transmission indicates that the multi-beam prototype can cover a 5–150 m distance with comparable throughput.

Investigation on Co-combustion characteristics and NOx emissions of coal and municipal sludge in a tangentially fired boiler

Using the current coal-fired power plant to treat sludge with large annual production is an efficient sludge treatment method. Nevertheless, the majority of the previous studies concentrated on sludge co-firing with low blending ratio and moisture content in utility boiler. Further analysis and discussion are necessary regarding the impact of sludge with high blending ratio and high moisture content on co-firing performance as well as the potential interactions between high moisture content sludge and coal during the co-firing process. The simulation results of the verified computational fluid dynamic (CFD) model can be regarded as numerical experiments. In the present research, CFD was used to analyze the co-firing characteristics of municipal sludge and coal in a 600 MW tangentially fired utility boiler. The impacts of sludge moisture content, blending ratio, blending strategy, and secondary air (SA) distribution on co-combustion characteristics and NOx emissions were investigated, and their potential synergistic effects were assessed. The co-firing of coal and sludge has little impact on the furnace outlet temperature while potentially lowering NOx emissions, provided that the net calorific value of the feeding of the mixture of sludge and coal is kept constant. The reduction of sludge moisture content can significantly improve co-combustion performance, but it also facilitates NOx production. Blending sludge from the position of the medium-upper burners can reduce the number of layers needed to burn the sludge in practical coal-fired boiler. Furthermore, the SA distribution has a major impact on the furnace outlet flue gas temperature and NOx content. The shrunk-middle type is the optimum scheme for SA distribution. This study contributes to a deeper knowledge of the actual co-firing of sludge and coal in the utility boiler and provides a possible theoretical foundation for the widespread, clean, and effective use of sludge.

Influence mechanism of ammonia mixing on NO formation characteristics of pulverized coal combustion and N oxidation in ammonia-N/coal-N

Through ammonia-coal co-firing, ammonia as a carbon-free fuel can be used to effectively reduce CO2 emissions in coal-fired power generation. However, ammonia can also act as an N source that allows NOx to be generated through more reaction paths. Therefore, it is highly necessary to explore the N oxidation mechanism in the ammonia-coal co-firing process to achieve low nitrogen combustion. In this study, the N oxidation mechanism in the ammonia-coal co-firing process was explored through an experiment with a high-temperature furnace, XPS testing, and calculations based on quantum chemical theory. The experimental results show that both temperature and ammonia mixing ratios have a significant impact on NO formation. When the combustion temperature is 1200℃, the NO yield increases gradually as the ammonia mixing ratio increases from 20% to 40% and then to 60%; when the combustion temperature is higher than 1300℃, the NO yield increases first and then decreases as the ammonia mixing ratio increases. In addition, the XPS testing demonstrates that the content of char-N resulting from ammonia-coal co-firing at 1200℃ is higher than that resulting from pure coal combustion at 1200℃. This indicates that the N atoms in ammonia can be adsorbed or embedded in char. The theoretical calculations at the molecular level further reveal that the amino groups in the ammonia-coal co-firing system can be oxidized to produce NO, NO2, and HNO via different paths. Both the experimental data and the theoretical calculations confirmed that NO is the main oxidation product. Moreover, the amino groups can stably exist on the char surface under the effects of different adsorption mechanisms, which is consistent with the characterization test. This work lays a foundation for deepening the understanding of the N migration and transformation mechanism in ammonia-coal co-firing. It provides theoretical support for developing methods to ensure low nitrogen combustion in ammonia-coal co-firing.