Design Of Thermochemical Reactors Research Articles

Kinetic modeling of thermal degradation of TDI/MDI-based flexible polyurethane foam under nitrogen and air atmospheres with Shuffled Complex Evolution algorithm: Insights from TG-FTIR analysis

This study investigates the thermal degradation mechanisms and kinetics of TDI/MDI-based flexible polyurethane foam (FPUF) under nitrogen and air atmospheres, employing thermogravimetric (TG) analysis in conjunction with TG-FTIR technique. The information gleaned from these analysis is used to construct kinetic model based on model-free approaches coupled with the Shuffled Complex Evolution optimization algorithm. The results uncovered the five-step complex thermal degradation process of TDI/MDI-based FPUF compared to TDI-based or MDI-based polyurethane in both atmospheres. In nitrogen atmosphere, this involve the dissociation of TDI- and MDI-based urethane bonds, breakage of recombined urea bonds, random chain scission of polyol, and further degradation of incompletely pyrolyzed solid products. In air atmosphere, the process emcompasses dissociation of TDI- and MDI-based urethane linkages, accelerated oxidation of polyol chains, consumption of urea groups, and combustion of partially oxidized intermediates. This work also emphasizes the limitations of solely relying on TG tests for kinetic modeling, necessitating the integration of TG tests and TG-FTIR technique to obtain a comprehensive understanding of degradation mechanisms. The significance of the acquired insights lies in their potential to reshape the design of thermo-chemical reactors, resulting in heightened resource recovery, diminished environmental impact, and a stride towards a circular economy within the polyurethane industry.

Determination of intrinsic kinetic of corncob char gasification with CO2 and steam using multipore diffusion model

Abstract The intrinsic heterogeneous reactivity of biochar in CO2 and steam gasification plays an important role in thermochemical reactor design, adjusting operating conditions, and predicting the quality of biomass gasification products, especially when the combined valorization of syngas and biochar with important textural properties is required. In the present work, the intrinsic heterogeneous kinetics of CO2 and steam gasification of corn biochar is estimated by fitting a multimodal pore size distribution (PSD) with random capillary model evolution with respect to the experimental results by thermogravimetric analysis (TGA). As novelty the independence of the initial biochar textural properties was considered, using two samples with different initial pore size distributions (PSDs): A1 sample with an initial surface area of 54.09 m2/g and A2 sample with 22.14 m2/g. The experimental intraparticle gradient effect is considered by using samples with a particle size of 149 µm conventionally larger than those reported to guarantee chemical kinetic control at 60 µm. The apparent kinetics obtained by TGA revealed a difference of 70 kJ/mol for CO2 gasification and 30 kJ/mol for steam gasification in contrast to initial PSD change. The average activation energies and pre-exponential factor obtained by parametric fitting of the model with respect to the evolution of the conversion for CO2 gasification were E = 210.2 kJ/mol and A0 = 1.13*106 g/m2s, while steam revealed E = 136.64 kJ/mol and 7.1*102 g/m2s. Furthermore, the model reduced the activation energy differences with respect to different PSDs by 5 kJ/mol for CO2 and 18 kJ/mol for steam biochar gasification.

A thermochemical reactor design with better thermal management and improved performance for methane/carbon dioxide dry reforming

A solar thermochemical reactor with better thermal management is proposed to improve the performance for dry reforming of methane. Conical cavity is introduced in the thermochemical reactor to adjust incident solar radiation distribution. Preheating area is adopted to recover sensible heat from gas outlet. Multiphysical model is presented for analyzing the overall performance of the reactor under different inlet flow rates. Also, local ideal reaction temperature required for maximizing local hydrogen production is analyzed according to the reaction kinetics. It is shown that better synergy between real temperature distribution and ideal temperature requirement can be achieved in this new reactor. Compared with conventional reactor, the present reactor exhibits the better performance in terms of reactant conversion, energy storage efficiency and hydrogen yield. Particularly, hydrogen yield is increased by 4.31%–17.12% at inlet flow rates between 6 and 12 L min−1.

Experimental and Numerical Investigation of the Dehydration of Ca(OH)2 at Low Steam Pressures

The CaO/Ca(OH)2 system can be the basis for cost-efficient long-term energy storage, as the chemically stored energy is not affected by heat losses, and the raw material is cheap and abundantly available. While the hydration (thermal discharge) has already been addressed by several studies, for the dehydration (thermal charge) at low partial steam pressures, there is a lack of numerical studies validated at different conditions and operation modes. However, the operation at low steam pressures is important, as it decreases the dehydration temperature, which can enable the use of waste heat. Even if higher charging temperatures are available, for example by incorporating electrical energy, the reaction rate can be increased by lowering the steam pressure. At low pressures and temperatures, the limiting steps in a reactor might change compared to previous studies. In particular, the reaction kinetics might become limiting due to a decreased reaction rate at lower temperatures, or the reduced steam density at low pressures could result in high velocities, causing a gas transport limitation. Therefore, we conducted new measurements with a thermogravimetric analyzer only for the specific steam partial pressure range between 0.8 and 5.5 kPa. Based on these measurements, we derived a new mathematical fit for the reaction rate for the temperature range between 375 and 440 °C. Additionally, we performed experiments in an indirectly heated fixed bed reactor with two different operation modes in a pressure range between 2.8 and 4.8 kPa and set up a numerical model. The numerical results show that the model appropriately describes the reactor behavior and is validated within the measurement uncertainty. Moreover, our study revealed an important impact of the operation condition itself: the permeability of the reactive bulk is significantly increased if the dehydration is initiated by a rapid pressure reduction compared to an isobaric dehydration by a temperature increase. We conclude that the pressure reduction leads to structural changes in the bulk, such as channeling, which enhances the gas transport. This finding could reduce the complexity of future reactor designs. Finally, the presented model can assist the design of thermochemical reactors in the validated pressure and temperature range.

Development of a kinetic reaction model for reduction and oxidation of Si doped Mn2O3 for thermochemical energy storage in concentrated solar power plants

Thermal energy storage plays a significant role in concentrated solar power plants. Particularly, thermochemical energy storage has been proposed as a promising future candidate due to its high energy density. One of the most studied thermochemical reactions are metal oxides redox reactions. The simplicity of using air as both heat transfer fluid and reactant provides important advantages. Among the studied materials, Mn2O3 has been deeply investigated due to its low cost and toxicity, although its reversibility has been shown to be highly deteriorated when exposed to high temperatures. Doping Mn2O3 with 1 at% of Si counteracted this weakness leading to a promising candidate for energy storage. In this work, thermogravimetric analysis of (Mn0.99Si0.01)2O3 demonstrated no significant changes in enthalpy compared to Mn2O3, while the oxidation kinetics and mostly, its cycling stability experienced great improvements. In order to provide tools for future thermochemical reactor design, a complete study of the reduction and oxidation kinetics of (Mn0.99Si0.01)2O3 in a thermobalance was carried out, obtaining satisfactory kinetic parametrization models. Both processes are mainly controlled by nucleation and growth mechanisms, although the diffusion mechanism prevails at high conversion extents (α > 0.8). In order to mimic accurately the effect of the oxygen partial pressure on the reaction kinetics, significant in the case of the oxidation reaction, the developed model includes an innovative contribution. The so called ‘isoconversional activation energy’ was introduced in the model, turning into an accurate prediction tool able to simulate the material kinetics deceleration within the hysteresis zone, providing a reaction rate kinetic model with extended range which has not been provided before.

Effects of partly-filled encapsulated phase change material on the performance enhancement of solar thermochemical reactor

Solar thermochemical reaction is an efficient way for hydrogen production. Due to cloud transients, solar thermochemical reactor endures low chemical conversion efficiency and deactivation of catalyst. To mitigate adverse effect of solar radiation fluctuation, in present paper, a solar thermochemical reactor design partly filled with encapsulated phase change material (EPCM) is proposed. Firstly, a two-dimensional dynamic model of solar parabolic trough receiver reactors (SPTRR) partly filled with EPCM is established and validated against experimental data. Then, the steady-state performance of SPTRR under steady solar insolation is explored numerically, and the findings motivated us to create a novel multi-part filling pattern of EPCM designed to avoid high-temperature deactivation of catalyst. Furthermore, SPTRR’s thermal and chemical behaviors under unsteady condition of solar radiation are analyzed. Finally, an optimal cascaded EPCM (regarding thermal properties) design is selected for the best overall dynamic performance of SPTRR. The results show that under the steady solar radiation, filled with EPCM in multi-part filling pattern, SPTRR can keep high methanol conversion efficiency (0.926) with large filling proportion (0.52) of EPCM, less usage of catalyst (reduced by 48% compared to all-catalyst case) and safe operation. And under the unsteady solar radiation, compared to SPTRR fully packed with catalyst, the delay response time and vibration amplitude of SPTRR with EPCM in 4-part filling pattern can be extended by 63.2% and damped by 48.7%, respectively. The comprehensive unsteady performance indicator of SPTRR with the optimal cascaded EPCM is 23.2% higher than that with non-cascaded EPCM.

Solar electricity via an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions III: Solar thermochemical reactor design and modeling

A two-step solar thermochemical cycle based on Co3O4/CoO redox reactions integrated into an Air Brayton cycle is considered for thermochemical heat storage. The two-step cycle encompasses (1) the thermal reduction of Co3O4 to CoO and O2 driven by concentrated solar irradiation and (2) the re-oxidation of CoO with O2 to Co3O4, releasing heat and completing the cycle. An evacuated horizontal solar thermochemical reactor is proposed with an inclined slope and quartz window for promoting direct irradiation of dense, granular Co3O4/CoO flows. Mechanical analysis of flat and spherical quartz window designs for a 5kWth scale prototype was performed to ensure window stability. Detailed mass and heat transfer analysis for a 5kWth scale prototype was performed coupling Monte Carlo ray tracing for radiative heat exchange to the energy balances for the bed and the reactor. A parametric study of the reactor design was performed with varying cavity depth, particle inlet temperature, and solar concentration ratio. The optimal solar reactor design maximized conversion of Co3O4 to CoO and particle outlet temperature while preventing particle overheating and achieved a Co3O4 to CoO conversion of 0.91, particle outlet temperature of 1385K, maximum flow temperature of 1572K, and absorption efficiency of 0.76.

AUTOTHERMAL AIR REFORMING OF METHANE

The paper suggests the thermochemical reactor design for methane air conversion in which heat is supplied to the catalytic nozzle with particulate intermediate coolant heated by combustion products of part of conversion products in a fluidized bed located above nozzle. Conversion and combustion products are discharged separately. Conversion products satisfy the requirements of a protective environment. A model of the process based on a system of two differential equations, describing the experiment satisfactorily.

Influence of Torrefaction Pretreatment on Reactivity and Permanent Gas Formation during Devolatilization of Spruce

Torrefaction has shown potential for improving biomass properties and converting biomass to a more coal-like fuel. In this paper, both fast and slow devolatilization behaviors of untreated spruce and spruce torrefied at 290 °C and 20–30 min holding time have been studied by conducting experiments in a bench-scale heated foil reactor and a thermogravimetric analyzer, respectively. The former has been applied to estimate the permanent gas formation for biomass entering a fluidized bed reactor, and the latter has been used to determine the effect of torrefaction on (slow) devolatilization reaction kinetics. The reaction kinetics were derived using the Reaction Rate Constant Method and the Senum and Yang Temperature Integral Approximation. The gases produced during fast devolatilization, applying a constant heating rate and a final temperature in the range of 500–1000 °C, were analyzed by Fourier transform infrared spectroscopy, and tars were collected and quantified gravimetrically. Results show that the activation energy and pre-exponential factor increased for the global devolatilization reaction. The former increased by 25% due to torrefaction pretreatment. The yield of the produced permanent gases CO, CH4, and CO2 increased with increasing final devolatilization temperature. CO is the dominant gas at temperatures higher than 600 and 800 °C for untreated and torrefied spruce, respectively. At lower temperatures, CO2 has the highest mass yield. CH4 shows the lowest yield for each final temperature and fuel sample. The results reported in this paper provide basic information for thermochemical reactor design when using (torrefied) spruce as a feedstock. The findings confirm that torrefied spruce is more coal-like than the parent material with respect to activation energy, char production, respective evolution of CO and CO2, and O/C and H/C atomic ratios.

Constructal method to optimize solar thermochemical reactor design

The objective of this study is the geometrical optimization of a thermochemical reactor, which works simultaneously as solar collector and reactor. The heat (concentrated solar radiation) is supplied on a small peripheral surface and has to be dispersed in the entire reactive volume in order to activate the reaction all over the material. A similarity between this study and the point to volume problem analyzed by the constructal approach ( Bejan, 2000) is evident. This approach was successfully applied to several domains, for example for the coupled mass and conductive heat transfer ( Azoumah et al., 2004). Focusing on solar reactors, this work aims to apply constructal analysis to coupled conductive and radiative heat transfer. As a first step, the chemical reaction is represented by a uniform heat sink inside the material. The objective is to optimize the reactor geometry in order to maximize its efficiency. By using some hypothesis, a simplified solution is found. A parametric study provides the influence of different technical and operating parameters on the maximal efficiency and on the optimal shape. Different reactor designs (filled cylinder, cavity and honeycomb reactors) are compared, in order to determine the most efficient structure according to the operating conditions. Finally, these results are compared with a CFD model in order to validate the assumptions.

Thermochemical reactor design and thermal breakdown in middle-rank coals

Experiments conducted at bench scale can throw new light on the operation of pilot and plant scale processes. In the coal gasification/combustion area, such experiments have ranged from determinations of coal reactivities for novel entrained-flow gasifiers, to investigating the fate of injectant coal during blast furnace operation. Meanwhile, the accurate mimicking of conditions within pilot and plant scale equipment requires careful design. This paper attempts an overview of challenges faced in designing experiments where coal thermal breakdown plays an important role. Particular emphasis is placed on characterising the underlying behaviour of the pyrolysing material itself, with as little reference to sample or reactor configuration as possible. [Received: September 11, 2007; Accepted: January 8, 2008]

Optimal design of thermochemical reactors based on constructal approach

AbstractThis work aims to optimize the design of thermochemical reactors such as solid/gas fixed bed reactors working in dynamic mode. The optimization is based on the constructal approach. A thermodynamic criterion, the ratio of reactor output power to its entropy generation, has to be used to take into account the coupled phenomena and the end purpose of such reactors. Two reactors are considered with same overall volume, but different internal configuration of collectors (constructal scales): a “basic” single‐scale configuration (basic reactor), and a “constructal” two‐scale configuration (constructal 1 reactor). Reaction rates notably vary with reactor shape, and an optimal shape factor is found for each reactor, depending on the reactant material density. For both reactors, an important remark is pointed out: the correspondence between the maximum of power/entropy ratio (PER) and the highest reaction rate. Comparison between the optimal basic and constructal 1 reactors shows that the latter presents higher energy efficiency (based on second law analysis), but lower output power than the first. © 2007 American Institute of Chemical Engineers AIChE J, 2007