Department of Chemical Engineering
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Item A mechanistic study on the reaction pathways leading to benzene and naphthalene in cellulose vapor phase cracking(Elsiever, 2014-10) Srinivas, AppariThe reaction pathways leading to aromatic hydrocarbons such as benzene and naphthalene in gas-phase reactions of multi-component mixtures derived from cellulose fast pyrolysis were studied both experimentally and numerically. A two-stage tubular reactor was used for evaluating the reaction kinetics of secondary vapor phase cracking of the nascent pyrolysates at temperature ranging from 400 to 900 °C, residence time from 0.2 to 4.3 s, and at 241 kPa. The products of alkyne and diene were identified from the primary pyrolysis of cellulose even at low temperature range 500–600 °C. These products include acetylene, propyne, propadiene, vinylacetylene, and cyclopentadiene. Experiments were also numerically validated by a detailed chemical kinetic model consisting of more than 8000 elementary step-like reactions with over 500 chemical species. Acceptable capabilities of the kinetic model in predicting concentration profiles of the products enabled us to assess reaction pathways leading to benzene and naphthalene via the alkyne and diene from primary pyrolysates of cellulose. C3 alkyne and diene are primary precursors of benzene at 650 °C, while combination of ethylene and vinylacetylene produces benzene dominantly at 850 °C. Cyclopentadiene is a prominent precursor of naphthalene. Combination of acetylene with propyne or allyl radical leads to the formation of cyclopentadiene. Furan and acrolein are likely important alkyne precursors in cellulose pyrolysis at low temperature, whereas dehydrogenations of olefins are major route to alkyne at high temperatures.Item Kinetic modeling of non-catalytic partial oxidation of nascent volatiles derived from fast pyrolysis of woody biomass with detailed chemistry(Elsiever, 2015-06) Srinivas, AppariThe gas-phase partial oxidation (POx) of nascent volatiles (NV) derived from the fast pyrolysis of cedar sawdust at 700 and 800 °C was studied both numerically and experimentally. A detailed chemical kinetic model (DCKM) was applied to simulate the POx in a two-stage reactor: the first stage was designed for fast biomass pyrolysis and the second effected the POx of the NV. Analytical pyrolysis experiments were also conducted to approximate the molecular composition of the NV, which is required input for computations using DCKM. The DCKM was modified by an empirical kinetic model for the decomposition of an ill-defined portion of the NV. The kinetic model coupled with a plug-flow reactor model reproduced the observed trends in the product yields with respect to both temperature and oxygen-to-fuel ratio, for not only the major products, but also minor products such as aromatic hydrocarbons which are typically found in the refractory post-gasification tar.Item 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(Elsiever, 2015-04) Srinivas, AppariA 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.Item Chemical structures and primary pyrolysis characteristics of lignins obtained from different preparation methods(J-Stage, 2014) Srinivas, AppariThis work aims at investigating correlations between primary pyrolysis characteristics of lignin and chemical structure of lignin feedstock. Three different types of lignin samples were prepared through enzymatic hydrolysis, organosolv extraction, and Klason procedure. Analysis by FT-IR and solid state 13C-NMR revealed that the lignin samples exhibited different contents of aromatic carbons, connection carbons, methoxyl carbons, and aliphatic side chains. The three lignin samples were pyrolyzed in a two-stage-tubular reactor at 650 ℃, and pyrolysis products were analyzed with gas chromatographs on-line. More than fifty compounds including inorganic gases, light hydrocarbons (LHs), aromatic hydrocarbons (AHs), phenol derivatives and light non-phenolic oxy-compounds (NPOCs) were gaschromatographically separable and quantified. The influence of the lignin structures on the pyrolysis characteristics was studied, and the correlation of product distribution and lignin chemical structures was examined. The total carbon selectivity into char and tar was increased with increasing lignin aromaticity. Methoxyl group and aliphatic substituents likely contributed for enhancing char formation, while hydrogen in lignin enhanced tar formation. Yields of LHs and NPOCs were increased with increasing aliphatic carbons of the lignin samples. AHs were formed from gas-phase recombination of LHs such as olefins, diolefines and alkynes, rather than directly from aromatic structures in the original lignin likely because of high energy required to cleavage carbon-oxygen bond existed in major structural units such as syringols or guaiacols.Item In-situ reforming of the volatiles from fast pyrolysis of ligno-cellulosic biomass over zeolite catalysts for aromatic compound production(Elsiever, 2014-11) Srinivas, AppariCharacteristics of in-situ catalytic reforming of the products derived from fast pyrolysis of biomass were studied with an originally designed analytical pyrolysis technique. The volatile products derived from ligno-cellulosic biomass as well as cellulose, xylan, and lignin were converted using a two-stage tubular reactor at 550 °C over various zeolite catalysts with different acidities and pore structures. HZSM-5 exhibited the best performance for converting the cedar derived volatiles to arenes, which mainly composed of benzene, toluene, and naphthalene with a selectivity of 26% on carbon basis. The HZSM-5 had a little effect in increasing the yields of the arenes for xylan and lignin, while it had a significant effect for cellulose, showing that more than 30% of carbon in cellulose was converted into arenes. A reaction pathway analysis for reforming of volatiles suggested that alkyne and diene such as acetylene, propyne, and cyclopentadiene are the important precursors of the major aromatic hydrocarbon products such as benzene, toluene, and naphthalene. The formations of those intermediates were also confirmed experimentally in an early stage of the in-situ reforming.Item Quantification of Carbon Content in cogeneration boiler waste and Its Effect on pozzolanic Reactivity(Elsiever, 2017) Srinivas, AppariSupplementary cementitious materials are extensively used in concrete because of the enhancement of fresh and hardened properties compared to ordinary Portland cement. Plants ingest orthosilicic acid from ground water, which is later polymerized as reactive silica in the plant cells. When bagasse and rice husk are burnt as fuel in the cogeneration boiler, reactive amorphous silica is formed because of combustion process and it is present in the residual ashes. In addition to silica, residual biomass such as sugarcane bagasse ash and rice husk ash have higher carbon content due to presence of unburnt particles and it significantly influences the pozzolanic performance of residual biomass. Furthermore, higher carbon content has adverse effects on concrete such as reduction in workability and strength. Even though, a number of earlier research studies have suggested bagasse ash and rice husk ash as an excellent pozzolanic material, still determination of carbon content using more accurate method is not reported in the existing literature. A detailed study on the presence of carbon content in the residual biomass is imperative to achieve maximum reactivity as well as superior alternative cementitious material. In this paper, carbon content in the residual biomass was determined using more appropriate methods and conventional loss on ignition method. Results from the study showed considerable difference in carbon content between sophisticated method and loss on ignition method. Influence of loss on ignition on pozzolanic performance and workability of residual ash blended specimens were investigated and compared with control specimens. Moreover, fast pyrolysis and detailed kinetic model approach was found to be more precise as compared to loss on ignition method.Item Study of the Combined Deactivation Due to Sulfur Poisoning and Carbon Deposition during Biogas Dry Reforming on Supported Ni Catalyst(ACS, 2017) Srinivas, AppariThis paper presents a detailed study of catalyst deactivation as a result of simultaneous sulfur poisoning and coke deposition during biogas dry reforming. Experiments are performed at 700 and 800 °C with 5 and 10 ppm of H2S in model biogas with CH4/CO2 = 1.5 and 2.0. To assess the relative effect of chemisorbed sulfur in deactivating the supported Ni catalyst as compared to that of coke deposition, the experiments are performed with and without H2S in the feed. The catalyst deactivation is found to be faster in the presence of H2S. The deactivation due to sulfur chemisorption is not reversible at 700 °C, while at 800 °C the catalytic activity of Ni starts to recover on removing H2S from the feed stream. The results show that the exit CO mole fraction goes up for the sulfur poisoned catalyst which suggests that the reverse water gas shift and coke gasification reactions are not suppressed to the same degree as the reforming reaction. The fresh and the spent catalysts are characterized using XRD, BET, EDS, and TEM. The characterization of the spent catalyst shows that dry reforming of model biogas, with and without the presence of H2S, leads to the formation of multiwalled carbon nanotubes for the chosen operating conditions.Item A circular framework for the valorisation of sugar industry wastes: Review on the industrial symbiosis between sugar, construction and energy industries(Elsiever, 2018-12) Srinivas, AppariThis paper provides a comprehensive review of literature on the properties of sugar industry waste, their varied uses in energy and construction sector, performance and limitations. An efficient upcycling of sugar industry waste in energy production would help the energy sector to reduce its dependency on non-renewable fossil fuels. Literature demonstrates that in the recent years there has been an increased research interest in thermochemical conversion of sugarcane bagasse to produce cleaner energy, rather than its landfilling or combustion. On the other hand, utilisation of secondary by-products from sugar industry in the production of cement, bricks, paver blocks, activated binder and other construction products helps to cut down the carbon footprint of the construction industry, while improving the properties of the final products. From the perspective of the sugar industry, such an arrangement eliminates disposal problems and creates additional revenue. Although independent research studies investigating the use of sugar industry by-products exist, few studies consider these valorisation options together to minimise waste and to create an efficient material flow chain. This study identifies various material and energy recovery pathways from published literature and connects the materials and processes to form a continuous material supply chain with minimum wastage. From the findings, a symbiotic framework has been developed with primary and secondary by-products from the sugar industry serving as source materials for energy production and sustainable construction products.Item Sustainable alternatives to carbon intensive paddy field burning in India: A framework for cleaner production in agriculture, energy, and construction industries(Elsiever, 2019) Srinivas, AppariField burning of rice straw as a method of disposal by farmers is a major concern as the smoke and harmful emissions from it causes dangerous levels of pollution. Seven out of ten cities with the poorest air quality in the world are in India, and paddy field burning has been cited as one of the main reasons. Hazardous emissions from paddy field burning is combined with greenhouse gas emissions from coal fired power plants and cement manufacturing units, causing severe respiratory illnesses and other fatal health issues. The recent literature indicates an increased interest in rice straw valorisation, possibly in response to the increasing concerns over paddy field burning. However, a systematic study that compiles and compares various rice straw valorisation routes is lacking in the existing literature. This paper provides a comprehensive review of literature on various alternative uses of rice straw in energy and construction industries. Effective use of rice straw as a biomass fuel in energy sector not only reduces the field burning, but also decreases the dependency on non-renewable fossil fuels. Moreover, the proposed utilisation of secondary by-products from rice straw-based energy production in the construction industry can help to attain the goals of resource cascading. The availability of rice straw in major rice producing states of India and the feasibility of establishing rice straw-based power plants are also examined in this study. The potential reduction of harmful emissions from such resource recycling has been determined as per the guidelines issued by the intergovernmental panel on climate change. From the review of literature and the findings of the present study, a sustainable framework has been developed with well-defined cleaner production pathways to create a seamless material flow chain between the agricultural, energy, and construction sectors.Item Numerical study on the steam reforming of biomass tar using a detailed chemical kinetic model(JIE, 2015) Srinivas, AppariSteam reforming (SR) and partial oxidation (POx) of nascent volatiles (NV) generated from fast pyrolysis of cedar wood chips in a two-stage reactor were studied numerically. A detailed chemical kinetic model (DCKM) consisting of more than 8000 elementary step-like reactions and more than 500 chemical species was used to simulate pyrolysis at 750 °C and reforming of the NV at 9 00 °C in the first and second stages, respectively. The molecular composition of the NV, which is one of the required boundary conditions for computations using the DCKM, was approximated based on analytical pyrolysis experiments. Global reactions accounting for the decomposition of t he ill-defined portion of t he NV and soot reforming were also tested to improve t he model capabilities. The DCKM with the global reaction coupled with a plug-flow reactor model could fairly reproduce the experimentally observed trends for the effects of oxygen and steam partial pressures on the yields of major products such as hydrogen, carbon monoxide, and tar residual rate.