Qunyi Zhu

Kinetic study of corn straw pyrolysis: Comparison of two different three-pseudocomponent models

With heating rates of 20, 50 and 100 K min(-1), the thermal decomposition of corn straw samples (corn stalks skins, corn stalks cores, corn bracts and corn leaves) were studied using thermogravimetric analysis. The maximum pyrolysis rates increased with the heating rate increasing and the temperature at the peak pyrolysis rate also increased. Assuming the addition of three independent parallel reactions, corresponding to three pseudocomponents linked to the hemicellulose, cellulose and lignin, two different three-pseudocomponent models were used to simulate the corn straw pyrolysis. Model parameters of pyrolysis were given. It was found that the three-pseudocomponent model with n-order kinetics was more accurate than the model with first-order kinetics at most cases. It showed that the model with n-order kinetics was more accurate to describe the pyrolysis of the hemicellulose.

Analysis of coals and biomass pyrolysis using the distributed activation energy model

The thermal decomposition of coals and biomass was studied using thermogravimetric analysis with the distributed activation energy model. The integral method resulted in Datong bituminous coal conversions of 3-73% at activation energies of 100-486 kJ/mol. The corresponding frequency factors were e(19.5)-e(59.0)s(-1). Jindongnan lean coal conversions were 8-52% at activation energies of 100-462 kJ/mol. Their corresponding frequency factors were e(13.0)-e(55.8)s(-1). The conversion of corn-stalk skins were 1-84% at activation energies of 62-169 kJ/mol with frequency factors of e(10.8)-e(26.5)s(-1). Datong bituminous coal, Jindongnan lean coal and corn-stalk skins had approximate Gaussian distribution functions with linear ln k(0) to E relationships.

Influence of Staged-Air on Airflow, Combustion Characteristics and NOx Emissions of a Down-Fired Pulverized-Coal 300 MWe Utility Boiler with Direct Flow Split Burners

Cold airflow experiments were conducted to investigate the aerodynamic field in a small-scale furnace of a down-fired pulverized-coal 300 MW(e) utility boiler arranged with direct flow split burners enriched by cyclones. By increasing the staged-air ratio, a deflected flow field appeared in the lower furnace; larger staged-air ratios produced larger deflections. Industrial-sized experiments on a full-scale boiler were also performed at different staged-air damper openings with measurements taken of gas temperatures in the burner region and near the right-side wall, wall heat fluxes, and gas components (O(2), CO, and NO(x)) in the near-wall region. Combustion was unstable at staged-air damper openings below 30%. For openings of 30% and 40%, late ignition of the pulverized coal developed and large differences arose in gas temperatures and heat fluxes between the regions near the front and rear walls. In conjunction, carbon content in the fly ash was high and boiler efficiency was low with high NO(x) emission above 1200 mg/m(3) (at 6% O(2) dry). For fully open dampers, differences in gas temperatures and heat fluxes, carbon in fly ash and NO(x) emission decreased yielding an increase in boiler efficiency. The optimal setting is fully open staged-air dampers.

Improving Combustion Characteristics and NOx Emissions of a Down-Fired 350 MWe Utility Boiler with Multiple Injection and Multiple Staging

Within a Mitsui Babcock Energy Limited down-fired pulverized-coal 350 MW(e) utility boiler, in situ experiments were performed, with measurements taken of gas temperatures in the burner and near the right-wall regions, and of gas concentrations (O(2) and NO) from the near-wall region. Large combustion differences between zones near the front and rear walls and particularly high NO(x) emissions were found in the boiler. With focus on minimizing these problems, a new technology based on multiple-injection and multiple-staging has been developed. Combustion improvements and NO(x) reductions were validated by investigating three aspects. First, numerical simulations of the pulverized-coal combustion process and NO(x) emissions were compared in both the original and new technologies. Good agreement was found between simulations and in situ measurements with the original technology. Second, with the new technology, gas temperature and concentration distributions were found to be symmetric near the front and rear walls. A relatively low-temperature and high-oxygen-concentration zone formed in the near-wall region that helps mitigate slagging in the lower furnace. Third, NO(x) emissions were found to have decreased by as much as 50%, yielding a slight decrease in the levels of unburnt carbon in the fly ash.

Overall Evaluation of Combustion and NOx Emissions for a Down-Fired 600 MWe Supercritical Boiler with Multiple Injection and Multiple Staging

To achieve significant reductions in NOx emissions and to eliminate strongly asymmetric combustion found in down-fired boilers, a deep-air-staging combustion technology was trialed in a down-fired 600 MWe supercritical utility boiler. By performing industrial-sized measurements taken of gas temperatures and species concentrations in the near wing-wall region, carbon in fly ash and NOx emissions at various settings, effects of overfire air (OFA) and staged-air damper openings on combustion characteristics, and NOx emissions within the furnace were experimentally determined. With increasing the OFA damper opening, both fluctuations in NOx emissions and carbon in fly ash were initially slightly over OFA damper openings of 0-40% but then lengthened dramatically in openings of 40-70% (i.e., NOx emissions reduced sharply accompanied by an apparent increase in carbon in fly ash). Decreasing the staged-air declination angle clearly increased the combustible loss but slightly influenced NOx emissions. In comparison with OFA, the staged-air influence on combustion and NOx emissions was clearly weaker. Only at a high OFA damper opening of 50%, the staged-air effect was relatively clear, i.e., enlarging the staged-air damper opening decreased carbon in fly ash and slightly raised NOx emissions. By sharply opening the OFA damper to deepen the air-staging conditions, although NOx emissions could finally reduce to 503 mg/m(3) at 6% O2 (i.e., an ultralow NOx level for down-fired furnaces), carbon in fly ash jumped sharply to 15.10%. For economical and environment-friendly boiler operations, an optimal damper opening combination (i.e., 60%, 50%, and 50% for secondary air, staged-air, and OFA damper openings, respectively) was recommended for the furnace, at which carbon in fly ash and NOx emissions attained levels of about 10% and 850 mg/m(3) at 6% O2, respectively.

Numerical Simulation of Flow, Combustion, and NO x Emission Characteristics in a 300 MW Down-Fired Boiler with Different OFA Ratios

A computational fluid dynamics model of a 300 MW down-fired furnace equipped with over-fire air (OFA) has been developed using Fluent 6.3. The validated CFD model is applied to investigate the effects of several operating conditions at full load with different OFA ratios. Simulation results indicate that as the OFA ratio rises from 0% to 40%, the NO x emission falls and the combustible material content in the fly ash increases. On the whole, n, the optimal OFA ratio for the studied down-fired boiler is 20%. This study provides a basis to assess in depth future operations of down-fired boilers.

Industrial Application of an Improved Multiple Injection and Multiple Staging Combustion Technology in a 600 MWe Supercritical Down-Fired Boiler

To solve the water wall overheating in lower furnace, and further reduce NOx emissions and carbon in fly ash, continuous improvement of the previously proposed multiple injection and multiple staging combustion (MIMSC) technology lies on three aspects: (1) along the furnace arch breadth, changing the previously centralized 12 burner groups into a more uniform pattern with 24 burners; (2) increasing the mass ratio of pulverized coal in fuel-rich flow to that in fuel-lean flow from 6:4 to 9:1; (3) reducing the arch-air momentum by 23% and increasing the tertiary-air momentum by 24%. Industrial-size measurements (i.e., adjusting overfire air (OFA) damper opening of 20-70%) uncovered that, compared with the prior MIMSC technology, the ignition distance of fuel-rich coal/air flow shortened by around 1 m. The gas temperature in the lower furnace was symmetric and higher, the flame kernel moved upward and therefore made the temperature in near-wall region of furnace hopper decrease by about 400 °C, the water wall overheating disappeared completely. Under the optimal OFA damper opening (i.e, 55%), NOx emissions and carbon in fly ash attained levels of 589 mg/m(3) at 6% O2 and 6.18%, respectively, achieving NOx and carbon in fly ash significant reduction by 33% and 37%, respectively.

Influence of Inner Secondary Air Vane Angle on Combustion Characteristics and NOx Emissions of a Down-Fired Pulverized-Coal 300 MWe Utility Boiler

Using an IFA300 constant temperature anemometer system, cold air experiments on a quarter-scaled burner model sited in a 300 MWe down-fired boiler were conducted to investigate the influence of various inner secondary air vane angles on the flow characteristics in the burner nozzle region. By increasing vane angles, no central recirculation zone appeared, the axial velocity decay rate increased, radial velocities increased at the jet boundary, and tangential velocities clearly increased in the inner secondary air zone. Industrial experiments were also performed on a down-fired pulverized-coal 300 MWe utility boiler with swirl burners. Gas temperature, concentrations of gas components (O2, CO, and NOx) in the burning region, and carbon content in the fly ash were measured with inner secondary air vane angles of 42°, 47°, 53°, and 60°. By increasing vane angles up to 53°, NOx emission and boiler efficiency increased; when vane angles were increased to 60°, NOx emission increased, but boiler efficiency decreased.

Combustion and NOx Emission Characteristics with Respect to Staged-Air Damper Opening in a 600 MWe Down-Fired Pulverized-Coal Furnace under Deep-Air-Staging Conditions

Deep-air-staging combustion conditions, widely used in tangential-fired and wall-arranged furnaces to significantly reduce NOx emissions, are premature up to now in down-fired furnaces that are designed especially for industry firing low-volatile coals such as anthracite and lean coal. To uncover combustion and NOx emission characteristics under deep-air-staging conditions within a newly operated 600 MWe down-fired furnace and simultaneously understand the staged-air effect on the furnace performance, full-load industrial-size measurements taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in flue gas, and carbon in fly ash were performed at various staged-air damper openings of 10%, 20%, 30%, and 50%. Increasing the staged-air damper opening, gas temperatures along the flame travel (before the flame penetrating the staged-air zone) increased initially but then decreased, while those in the staged-air zone and the upper part of the hopper continuously decreased and increased, respectively. On opening the staged-air damper to further deepen the air-staging conditions, O2 content initially decreased but then increased in both two near-wall regions affected by secondary air and staged air, respectively, whereas CO content in both two regions initially increased but then decreased. In contrast to the conventional understanding about the effects of deep-air-staging conditions, here increasing the staged-air damper opening to deepen the air-staging conditions essentially decreased the exhaust gas temperature and carbon in fly ash and simultaneously increased both NOx emissions and boiler efficiency. In light of apparently low NOx emissions and high carbon in fly ash (i.e., 696-878 mg/m(3) at 6% O2 and 9.81-13.05%, respectively) developing in the down-fired furnace under the present deep-air-staging conditions, further adjustments such as enlarging the staged-air declination angle to prolong pulverized-coal residence times in the furnace should be considered to improve the deep-air-staging combustion configuration.

Airflow and Combustion Characteristics and NOx Formation of the Low-Volatile Coal-Fired Utility Boiler at Different Loads

In order to solve the problems of lower heating rate of the primary air/fuel mixture and higher NO x emission of the boiler, both the small-scale cold air experiment of a single burner and industrial-scale experiments of the centrally fuel-rich (CFR) swirl burner on a retrofitted 300 MW wall-fired boiler fed with low-volatile coal under deep air staging were performed. The aerodynamic characteristics, flue gas temperature, and temperature distribution of the furnace and carbon content in the fly ash were measured at loads of 300, 230, and 150 MW. Results illustrate that a central recirculation zone appeared close to the outlet the CFR burner under deep air staging. Compared with the enhanced ignition-dual register (EI-DR) burner, the gas temperature and the heating rate of the CFR burner are much higher, so it can guarantee the timely ignition and stable combustion of the pulverized coal. As for the CFR burner, with a decreasing load, the heating rate of the gas temperature decreases and the ignition position of the primary coal/air mixture becomes backward. The overall temperature of the furnace also decreases with decreasing load, as does the difference between the temperatures in the burning region and the lower position of the burnout region. After the retrofitting of the combustion system, the exhaust gas temperature decreases from 149 to 146 °C, the NO x emissions at the air preheater exits decrease from 1354 to 778 mg/m3 (6 % O2), and the boiler thermal efficiency increases from 89.75 to 90.57 % at the rated load.