Brief Communications

The Contribution to Char Burnout from Gasification by H₂O and CO₂ During Pulverized-Coal Flame Combustion

Syngas tempering technology refers to highly reactive gases to consume excess O₂ in the circulating flue gas. An activation environment is created, and the reburning coal is rapidly activated into highly reactive char, increasing its ability to reduce NO. The syngas activation of reburning pulverized coal was studied by a plane flame-carrying flow reactor. Reduced intermediates such as CH_i*, H*, and CO* are produced in the combustion reaction of the main components of the syngas (CH₄, H₂, and CO) with oxygen. The results indicate that after syngas tempering, the char's BET-specific surface area and total pore volume increase by 54.2% and 51.2%, respectively. The relative content of oxygen-containing groups is reduced by almost 5.7%, the relative content of alkyl groups increases by about 14.8%, and the degree of the disorder increases by about 20.68%. The syngas composition has a synergistic tempering effect with O₂. CH₄ has a better tempering effect than H₂, while the effect of CO is not apparent. Among the reducing intermediates, CH_i* has the most significant effect on char, followed by H*. It is instructive to solve the problem that the removal effect of existing pulverized coal reburning to reduce NO_x emissions still needs to be improved.

Oxy-fuel fluidized bed (FB) combustion is considered one of the promising ways to control CO₂ emission from coal-fired power plants. The effect of H₂O on the char conversion during wet flue-gas recycle, in which the H₂O concentration could be around 40%, is still not well understood. To this end, experiments and modeling were performed in this work. Combustion tests with bituminous coal char were carried out in an electrically heated fluidized bed in O₂/CO₂, O₂/H₂O and O₂/CO₂/H₂O for various O₂, CO₂ and H₂O concentrations at the bed temperature of 850 °C. At the same time, the influence of the bed temperature and the char size on char combustion was investigated in O₂/CO₂/H₂O atmosphere. A thermocouple was inserted into the center of the char particle to measure the particle temperature, from which the char combustion characteristics were determined and analyzed. The results indicate that the participation of H₂O in the combustion atmosphere enhances the carbon conversion, and it also reduces the particle temperature. A transient char-particle conversion model, taking into account heat and mass transfer from the bed to the particle and heterogeneous combustion and gasification of char, was developed to quantitatively examine the role of H₂O. The model shows a good ability to predict the measured char-temperature history. Simulations were carried out to establish the role of H₂O in O₂/H₂O and O₂/CO₂/H₂O as in the FB experiments. The model was used to analyze the peak temperature and the burnout time of a char particle, as well as the relative contributions to the consumption of the carbon in the char by O₂ (combustion), and CO₂ and H₂O (gasification). The results indicate that the endothermic char-H₂O reaction is the main reason for the prolongation of the burnout time of char and the decrease in the particle temperature in O₂/CO₂/H₂O as compared in O₂/CO₂. During wet flue-gas recycle, char-O₂ still accounts for a major part of the total carbon consumption, but the contribution of char-H₂O to the overall carbon consumption increases with the H₂O concentration and cannot be ignored (i.e. when the H₂O concentration attains 30%, the contribution of the char-H₂O reaction to the overall carbon consumption is 14%). However, the contribution of char-CO₂ to the char conversion is limited.

The high concentration of CO₂ and low concentration of O₂ at relatively low temperatures of 1123-1223 K are typical reactive atmospheres in the fuel reactor of a chemical looping with oxygen uncoupling (CLOU) unit. Char within this environment can be converted via lean-O₂ oxidation and/or rich-CO₂ gasification. Conventional models, which either considered the oxidation only or considered the oxidation and gasification contributions independently by simply adding the two individual conversion rates together, seem arbitrary. Therefore, it is necessary to quantitatively explore the contributions of the oxidation and the gasification reactions as well as their interactions in CLOU conditions through more detailed particle-resolved simulations.

In this work, a single particle model which described a spherical reacting porous particle with its reacting boundary layer was developed to investigate the conversion characteristics of pulverized coal char in lean-O₂ and/or rich-CO₂ conditions (analogous CLOU conditions). The heterogeneous reaction kinetics developed by Tilghman and Mitchell [35] and the simplified GRI-Mech 3.0 gas-phase reaction kinetics were adopted. It was found that the char conversion rate in CLOU conditions could be basically modeled as the sum of the full oxidation consumption rate and the partial gasification consumption rate, i.e. r_mix=φ₁r_gasi+r_oxid. The interactions between the oxidation and gasification reactions were elaborated in two aspects: on the one side, the oxidation of coal char in CLOU conditions is within the weak zone II burning region, where the oxidation reaction only happens peripherally and is nearly unaffected by the high concentration of CO₂; on the other side, the gasification reaction within the external layer of the coal char particle is inhibited by the oxidation reaction, consequently the gasification reaction only happens inside the coal char particle where O₂ cannot penetrate into. Based on these understandings, a correlation between the coefficient, φ₁, and the effectiveness factor of the oxidation reaction, η, was proposed to describe the overall char conversion rate in CLOU conditions. Through fitting the simulation results under typical CLOU conditions, the quantitative relationship between φ₁ and η was finally attained.

Through a previously validated numerical model, seven combustion scenarios were simulated to study the effects of primary air ratio (PAR) on coal utilization mode and NOx formation characteristics of a wall-fired lignite boiler. Distribution profiles of combustion temperature, concentration of gas components, mole fractions of NO and volatile N-intermediates were discussed. Results show that the effects of PAR on coal utilization mode, i.e. char-O₂ oxidation and char-H₂O gasification process, differ in two separate ranges. When PAR is slightly increased from 0.35 to 0.41, char-O₂ oxidation deteriorates in the lower furnace region while char-H₂O gasification is enhanced. However, when PAR is largely increased from 0.44 to 0.53, char-O₂ oxidation behavior gets re-improved but char-H₂O gasification becomes weaker. Besides, NOx emission at the furnace exit rises monotonously with PAR increasing. Variation of coal utilization mode is attributed to the changes of mixing process between coal particles and secondary air, while the increase of NOx emission is ascribed to the increased initial oxygen around coal particles. The results thoroughly reveal the influence of air distribution between primary and secondary air on coal utilization process and NOx-related characteristics, which can be used as theoretical guidance for the practical operation of lignite-fired boilers.

In thermal energy conversion of biomass, the char conversion rate influences the design of industrial systems. Yet there is no consensus on the relative influences of O₂, CO₂ and H₂O on the char conversion rate. The present study aims at clarifying the roles of these oxidizers on biomass char conversion. Single particles of different particles sizes were oxidized in mixtures of O₂/N₂, CO₂/N₂, H₂O/N₂ and O₂/CO₂/H₂O/N₂ at various temperatures. The results show that for large particles and high temperatures, CO₂ and H₂O gasification reactions play a dominant role, while for sufficiently small particles and low temperatures, the O₂ oxidation reaction plays a dominant role in mixtures of O₂/CO₂/H₂O/N₂. Two different particle models were used to compute char conversions: a detailed single particle model, and a conventional particle model assuming that rates in mixtures of oxidizers can be predicted based on rates of single oxidizers. The results show that the char oxidation rates are strongly overestimated when predicting rates based on single oxidizers, while the detailed model shows a good agreement to the experimental measurements.

The present study investigates conversion of char-N to NO in mixtures of O₂/N₂ and in O₂/H₂O/N₂. Biomass particles of spruce bark were combusted in an electrically heated single particle reactor at 900 °C at various O₂/H₂O/N₂ concentrations. NO concentrations of the product gases were measured during the char combustion stage. The conversion of char-N to NO was significantly higher with H₂O as compared to without H₂O in the gas. Additional fixed bed experiments were conducted to investigate the products of the reaction between H₂O and spruce bark char. The results showed that NH₃ is the primary product in the reaction between char-N and steam. These results explain the observation that more NO is formed during char combustion in the presence of steam: the char-N reacts partly with H₂O to form NH₃, which reacts further to NO.