Unburned Carbon Emissions Research Articles

Insight into the Competitive and Synergistic Effects during Coal/NH3 Cofiring via Reactive Molecular Dynamics Simulations

In this work, reactive force field molecular dynamics (ReaxFF MD) simulations are performed to study competitive and synergistic effects and their impact on CO, CO2, unburned carbon (UC), and nitrogen oxide (NO) emissions in coal/NH3 cofiring at different conditions. Simulation results show that cofiring results in higher emissions of UC and NO. Increasing the temperature is helpful in reducing UC emissions but has little effect on NO emissions. Although decreasing the O2 equivalent ratio can help in reducing NO emissions, UC emissions increase significantly. The conclusion is that the best O2 equivalent ratio in cofiring is 0.6. Increasing the NH3 cofiring ratio will reduce NO emissions but seriously increase UC emissions. The competition between coal and NH3 for O2 will decrease the coal combustion rate and increase the NH3 combustion rate. The competition effect is further proved by tracing the OH distribution. The synergistic mechanism is revealed. The free radicals and intermediates generated from coal combustion, such as HO2, H2O2, H, and OH, will promote NH3 decomposition and oxidation. The kinetics analysis shows that the competition between NH3 combustion and coal combustion only influences the combustion rate of coal and NH3, while the synergistic effect of coal decreases the activation energy of NH3 combustion. The research results are helpful to further study the mechanism of coal/NH3 cofiring and guide the technology of NH3 blending in coal-fired power plants.

Effect of the Hydrogen Injection Position on the Combustion Process of a Direct Injection X-Type Rotary Engine with a Hydrogen Blend

As a new type of power device, the X-type rotary engine (XRE) is regarded as a major revolution of the internal combustion engine with its special structure and high-efficiency hybrid cycle (HEHC). A 3D CFD model of an XRE with hydrogen–gasoline fuel is firstly built in this paper. The gasoline is premixed with air in the intake of the XRE. The hydrogen is directly injected (DI) into the cylinder with four different injection positions. The effects of the hydrogen injection position on the combustion process, engine thermodynamic performance, and unburned carbon emissions and NOx emissions are investigated. The results show that, due to the interaction between the in-cylinder main flow field and the injected hydrogen gas flow, different hydrogen concentration zones are formed at different injection positions. Furthermore, a larger hydrogen distribution area and being closer to the ignition position led to a faster in-cylinder combustion rate and a higher in-cylinder temperature and pressure. When the injection position is from the front to the back of the combustion chamber such as in position 2, the hydrogen has the widest distribution area and is closest to the ignition position, resulting in its fastest combustion speed. Meanwhile, the peak in-cylinder pressure is 3.73 MPa and the peak temperature is a maximum of 1835.16 K. Especially, the highest indicated thermal efficiency of 26.56% is found in position 2, which is 10.08% higher than that of position 4 (from right to left of the combustion chamber), which was 24.13%. At the same time, due to the best overall combustion effect, position 2 presents the lowest final unburned carbon emission of 0.36 mg, while it produces the highest NOx emission of 9.15 μg. Thus, this study provides important theoretical guidelines for the hydrogen injection strategy of the XRE using hydrogen–gasoline fuel.

Predicting the Conversion Efficiencies of Any Coal Type in CFBCs

This study uses simulations with detailed chemistry to characterize the conversion of the various fuel components—noncondensable gas mixtures, soot, char fines, and coarse char—and to identify the factors that determine unburned carbon emissions (a.k.a. LOI) from CFBCs. It covers virtually the entire operating domain for commercial CFBCs with 20 test cases in 11 full-scale CFBCs for a range of coal quality from brown coals through anthracite. Soot burnout was complete for all cases except one. LOI was entirely determined by incomplete burnout of coarse char particles, except that an anthracite produced fines that burned slower than coarse char. Simulations for a Chinese CFBC accurately depicted how hotter furnace temperatures and load variations affect LOI. The intrinsic char oxidation reactivity determines whether or not char fines and coarse char can effectively compete with noncondensable fuel mixtures for the available O2 at lower elevations, and thereby influence NO production. More reactive chars co...

Pilot-scale development of a low-NOx coal-fired tangential system

A 293 kW pilot-scale facility was used to develop a low-NO x pulverized coal-fired tangential system. Low NO x is achieved by directing the fuel and 20 percent of the secondary combustion air into the center of the furnace with the remaining 80 percent of the secondary combustion air directed parallel to the furnace walls. The separation of secondary combustion air in this manner creates a fuel rich zone in the center of the furnace where NO x production is minimized. This combustion modification technique has lowered NO x 60 percent relative to conventional tangential firing. In addition, CO, UHC and unburned carbon emissions are substantially unaffected by the modification. Also, the modification places a blanket of air on the furnace walls which is beneficial from a wall corrosion and slagging point of view. Finally, the modification shows a decrease in NO x emissions as firebox gas temperature is increased. This characteristic might be beneficially applied in a large-scale system to reduce furnace volume, and thereby capital cost, for a given combustion heat release. Tests are now underway to further optimize and characterize this low-NO x combustion modification technique.