Predicting the temperature and reactant concentration profiles of reacting flow in the partial oxidation of hot coke oven gas using detailed chemistry and a one-dimensional flow model

A numerical approach is presented for predicting the species concentrations and temperature profiles of chemically reacting flow in the non-catalytic partial oxidation of hot coke oven gas (HCOG) in a pilot-scale reformer installed on an operating coke oven. A detailed chemical kinetic model consisting of 2216 reactions with 257 species ranging in size from the hydrogen radical to coronene was used to predict the chemistries of HCOG reforming and was coupled with a plug model and one-dimensional (1D) flow with axial diffusion model. The HCOG was a multi-component gas mixture derived from coal dry distillation, and was approximated with more than 40 compounds: H2, CO, CO2, CH4, C2 hydrocarbons, H2O, aromatic hydrocarbons such as benzene and toluene, and polycyclic aromatic hydrocarbons up to coronene. The measured gas temperature profiles were reproduced successfully by solving the energy balance equation accounting for the heat change induced by chemical reactions and heat losses to the surroundings. The approach was evaluated critically by comparing the computed results with experimental data for exit products such as H2, CO, CO2, and CH4, in addition to the total exit gas flow rate. The axial diffusion model slightly improves the predictions of H2, CO, and CO2, but significantly improves those of CH4 and total exit flow rate. The improvements in the model predictions were due primarily to the improved temperature predictions by accounting for axial diffusion in the flow model

Continuous monitoring of char surface activity toward benzene

Kinetics of thermal decomposition of benzene on lignite-derived char was investigated at 900°C by applying a new method to continuously monitor the char surface activity. Benzene vapor was continuously forced to pass through a micro fixed bed of char with residence time as short as 7.6 ms, and then detected continuously by a flame-ionization detector. Results showed the presence of two different types of char surfaces; consumptive Type I surface and non-consumptive (sustainable) Type II surface. Type I surface of a partially CO2-gasified char had an capacity of carbon deposit from benzene over 20 wt%-char and an initial activity (represented by a first-order rate constant) as high as 160 s-1. Both of them decreased with increasing carbon deposit due to consumption of micropores accessible to benzene, and finally became zero leaving Type II surface that had a very stable activity with rate constant of 4 s-1. The chars without gasification had capacities of Type I surfaces smaller by two orders of magnitude than the partially gasified char, while the Type II surfaces had activities similar to that of the partially gasified char. It was found that Type II surface converted benzene into not only carbon deposit but also diaromatics and even greater aromatics. Composition of the greater aromatics was unknown because they were deposited onto the reactor wall immediately after passing through the char bed

Modelling and simulation of materials synthesis: Chemical vapor depositoion and infiltration of pyrolytic carbon

Numerical simulation of materials synthesis based on detailed models for the chemical kinetics and transport processes is expected to support development and optimization of production processes. Exemplarily, chemical vapor deposition and infiltration of pyrolytic carbon for the production of carbon fiber reinforced carbon is studied by recently developed modeling approaches and computational tools. First, the development of a gas phase reaction mechanism of chemical vapor deposition (CVD) of carbon from unsaturated light hydrocarbons (CH4, C2H4, C2H2, and C3H6) is presented. The mechanism consisting of 757 reactions among 230 species is based on existing information on elementary reactions and evaluated by comparison of numerically predicted and experimentally determined product composition for more than 40 stable gas phase compounds in a CVD flow reactor. The reactor was operated at widely varying conditions: 800-1100 °C and 2-15 kPa. Experimentally observed pressure and temperature effects on the species profiles as function of residence time are well predicted. Second, a model and computer code is presented for the numerical simulation of chemical vapor infiltration (CVI) carbon for the production of carbon fiber reinforced carbon. The chemistry model is based on a multi-step reaction scheme for pyrocarbon deposition, derived from the elementary mechanism, and a hydrogen inhibition model of pyrocarbon growth. This chemical model is implemented in transient 2D simulations of chemical vapor infiltration. The coupled models for mass transport (convection and diffusion), chemical vapor deposition and surface growth, gas-phase and surface chemical reactions are numerically solved by a FEM approach. Three sets of experiments were exemplarily simulated with inlet flows of 20 kPa CH4, 20 kPa CH4 with 4 kPa H2, and 20 kPa CH4 with 10 kPa H2, all at a temperature of 1095°C. The continuous infiltration, pyrolysis, and deposition of methane and its consecutively formed CxHy products lead to temporarily and spatially varying species concentrations and porosity inside the carbon felt. The predicted density distribution agrees well with experimental data

In-Situ Reforming of Tar from the Rapid Pyrolysis of a Brown Coal over Char

Reforming of nascent tar from the rapid pyrolysis of a brown coal over char prepared from the same coal was studied at 750−900 °C. The reforming was very rapid and extensive, allowing only benzene (0.02% on a coal C basis), naphthalene (0.001%), and phenanthrene (0.0001%) to escape from the char bed at an empty-bed gas residence time of less than 170 ms and 900 °C, respectively. Reforming even at 750 °C converted 96% of heavy tar (boiling point temperature >336 °C) into noncondensable gases and coke deposit over the char. Decreasing conversion of the tar into coke with increasing temperature suggested that the tar was reformed in a sequence of coking and steam gasification of the coke rather than direct steam reforming over the char. The reforming at 900 °C gave a negative coke yield due to progress of coke/char gasification faster than the coke deposition. Results of this work thus showed a possibility of complete tar reforming by intensification of contact between the char and volatiles even in the absence of a catalyst

Coproduction of clean syngas and iron from woody biomass and natural goethite ore

Conversion of biomass into clean syngas was studied considering application of low-grade iron ore toreforming of tar. Chipped cedar with moisture content of 0.1–10.1 wt% was continuously pyrolysed at550 C, and the nascent volatiles were subjected to reforming at 690–800 C in a bed of mesoporoushematite derived from a type of natural goethite. The yield of heavy tar (b.p. > 350 C) decreased from18.8 to less than 0.01 wt% during the reforming mainly by its oxidation by the ore and conversion intocoke. The hematite was reduced completely to magnetite and further but incompletely to wustite. Theformation of iron was inhibited by high CO2/CO and H2O/H2 ratios of the gas phase. The coke-loaded magnetite/wustite mixture was, however, an excellent precursor of iron. Reheating the spent ore up to 800 C in the absence of the volatiles reduced the magnetite/wustite to wustite/iron obeying direct and indirect reduction mechanisms. Repeated cycles of such reheating and reforming converted the volatiles and ore into syngas with a total tar concentration as low as 10 mg Nm3-dry and coke-loaded iron, respectively. Contribution of the steam reforming with iron–wustite redox cycles became more important as the reforming-reheating cycles were repeated

Catalytic and Noncatalytic Mechanisms in Steam Gasification of Char from the Pyrolysis of Biomass

Steam gasification of chars from the pyrolysis of a Japanese bamboo and cedar was studied using a reactor that enabled experimental definition of the gas composition in the vicinity of gasifying char particles. Intraparticle diffusion of neither steam nor the product gases influenced the kinetics of gasification. The chars underwent noncatalytic and catalytic gasification in parallel. The noncatalytic gasification, in which kinetic parameters were successfully defined by those for the gasification of the acid-washed char, was first-order with respect to the amount of residual carbon over the entire range of char conversion. In consequence of this, contribution of the catalytic gasification was quantified as a function of the char conversion. Among the inherent alkali and alkaline earth metallic species, potassium (K) played the major catalytic role and its overall activity changed via a maximum in the course of gasification, suggesting the presence of optimum sizes of clusters or particles of K catalyst. The noncatalytic and catalytic reactions obeyed respective Langmuir−Hinshelwood mechanisms that involved inhibition by H2

Mechanism of decomposition of aromatics over charcoal and necessary condition for maintaining its activity

Decomposition of mono- to tetra-aromatics over charcoal was investigated under conditions such as temperature; 700–900 °C, inlet concentrations of aromatics, steam and H2; 7.5–15 g/Nm3, 0–15.5 vol% and 0–15.5 vol%, respectively, gas residence time within charcoal bed; 0.2 s, particle size of charcoal; 1.3–2.4 mm. The charcoal, with an initial surface area of 740 m2/g, was active enough to decompose naphthalene completely even at 750 °C. Aromatics with more rings per molecule were decomposed more rapidly. The aromatics were decomposed over the charcoal by coking rather than direct steam reforming irrespective of temperature and steam/H2 concentrations. The coking, i.e., carbon deposition from the aromatics, caused loss of micropores and thereby activity of the charcoal, while steam gasification of the charcoal/coke formed or regenerated micropores. Relationship between the overall rate of carbon deposition by the coking and gas formation by the gasification within the charcoal bed showed that progress of the gasification at a rate equivalent with or greater than that of the carbon deposition was necessary for maintaining the activity of the charcoal