Investigation on fuel gas production from pulp and paper waste water impregnated coconut husk in fluidized bed gasifier via humidified air and CO2 gasification
Introduction
Dependency on fossil fuels as the main energy sources has led to serious energy crisis and environmental problems [1]. In 2016 the global oil and natural gas consumption rate were about 95 million barrels per day and 9.5 billion cubic meters per day, respectively [2]. However, biomass is abundantly available, renewable and wasted source of energy. As per estimations, the contribution of biomass is only around 10–14% of the total world's energy supply [3]. Also, CO2 is a primary contributor to the global greenhouse effect which is mostly released from the burning of fossil fuels, especially coal [4]. Owing to that, there has been increasing interest in the utilization of biomass for production of environmental friendly biofuels, as it is a CO2 neutral resource in the life cycle. Furthermore, biofuel can also be used in internal combustion engine [5] and spark ignition engine [6] with or without blends of conventional fuel. Moreover, earth is very rich in biomass and this biomass is present in various forms like plant biomass, agriculture waste, solid waste, etc. Most of the biomass is found in the form of agriculture waste like sugarcane molasses, the peel, husk or silk of sweet corn, baby corn, etc. Some of them are used as cattle feed whereas; rest of them like sesame seed straw, mustard straw, etc. are waste for farmers. And thus they burn it in the field, which also generates a lot of greenhouse gases. So, increasing attention is being made to biomass as a substitute for fossil fuel for environmental benign energy generation [7]. The lignocellulosic biomass varies in composition depending on the type of biomass, locality, climatic conditions and the soil where it grows.
Coastal cities of India produce almost 1.5 × 107 metric tons of coconut (Cocos nucifera) per year [8]. With the growing consumption of coconut water, large amounts of wastes after use are produced. Disposal of such high amounts of biomass is a major issue in coastal cities, as they are hard to be eliminated and can cause solid waste pollution and could be a reproductive site for disease-vector bugs. Such biomass should be recycled for reducing the accumulation of trash, adding value to the supply chain and generating profits [9]. Biomass is a composite material mainly composed of hemicellulose 20–40%, cellulose 40–60% and lignin 10–25% [10], which is an ideal renewable resource for the generation of heat and power through thermochemical conversion route if the moisture content is less than 30% by weight [11].
Thermochemical conversion of biomass mainly includes pyrolysis and gasification in which the processes involve the thermal decomposition of organic compounds present in the biomass for its transformation into different forms of useful energy or as fuel gas. Among various thermo-chemical processes, gasification is one of the most promising thermo-chemical conversion routes to recover cleaner energy from biomass [[12], [13], [14], [15], [16], [17], [18], [19]] as it offers high conversion efficiency [20,21]. During the gasification process, biomass is thermally decomposed to small quantities of char and ash, liquid oil and a high amount of gaseous products [2,13,14]. Biomass pyrolysis can be divided into four individual stages: moisture evolution, hemicellulose decomposition, cellulose decomposition and lignin decomposition [22]. Furthermore, biomass gasification includes the reformation of evolved gases in the presence of limited oxygen. The yields of end products of gasification and the composition of gases are dependent on several parameters including temperature, biomass species, particle size, heating rate, operating pressure, reactor configuration and the catalyst used. There are a lot of researches on gasification of biomass using Ni, Zn, Ca, K, Al, Fe, Na, etc based catalyst with gasification medium steam, air, O2 or CO2 [[23], [24], [25], [26], [27], [28], [29], [30]]. Moreover, many researchers did experiments on fluidized bed gasification using sand silica as bed materials [[31], [32], [33], [34]].
However, production of steam and catalyst both require extra cost. Furthermore, the steam can be replaced with humidified air. And, paper and pulp waste water can be used as a catalyst due to rich metal content in it. The primary investigations were performed with NCH and ICH in fixed bed gasification reactor under humidified air gasifying medium and the results were published elsewhere [8]. As a summary, the NCH was treated with paper and pulp industry waste water known as ICH and experiments of gasification in fixed bed gasifier were performed by varying dry air equivalent ratio (ER) and humidified air equivalence ratio (HER) and optimised with temperature. Considering higher yield of H2 concentration in the fuel gas, the optimum condition was found at HER of 0.1 and gasifying temperature 850 °C. Furthermore, the addition of CO2 in the gasifying medium resulted in better quality of fuel gas with more energy in the form of calorific value.
In the present study, NCH and ICH have been taken as raw materials for gasification in bubbling fluidized bed. Experiments have been started from the optimum finding of the previous investigation. In addition to this, several experiments have been performed to optimise ER, temperature, and CO2 air ratios for maximum fuel gas yield and HHV as well after executing fluidized bed gasification process.
Section snippets
Raw material and feed stock preparation
The outer part of unripe coconut was collected from Vishwanath temple shop which is situated inside the premises of Indian Institute of Technology (Banaras Hindu University), Uttar Pradesh, India. It was chopped and sun-dried for five days. Then, it was kept in an air oven at 105 °C for 48 h. After drying, it was ground, sieved and kept in closed container for further investigations. This has been known as NCH. Moreover, some amount of this NCH was mixed with the waste water of paper and pulp
Biomass characterization
Proximate, ultimate analysis and HHV of NCH were investigated in accordance with standard ASTM methods and tabulated in Table 1. It contains moisture 4.27%, volatile mater 69.14% and fixed carbon 21.18%, which are good pre-requisite for thermochemical conversion. Chemical formula of NCH was calculated by considering nitrogen content as unity and found as C6.6H9.8O5.3N from the result of the ultimate analysis of NCH. The following equations were used to calculate stoichiometric air requirement [
Conclusions
In the present work, both NCH and ICH were used as feedstock for fluidized bed gasification process. Furthermore, humidified air with or without CO2 was used as gasifying medium. The result showed the maximum H2 concentration of 55.55 vol % in the fuel gas for fluidized bed gasification which was higher than fixed bed gasification of our earlier study. However, the HHV was low at this condition of HER of 0.1 and gasification temperature 850 °C. The addition of CO2 with humidified air medium
Acknowledgment
One of the authors Mahendra Ram would like to gratefully acknowledge the Ministry of Human Resource Development, New Delhi, India for providing senior research fellowship. Furthermore, the authors are also thankful to the Department of Chemical Engineering and Technology and Central Instrument Facility Centre, Indian Institute of Technology (Banaras Hindu University), Varanasi, Uttar Pradesh, India for providing facilities to execute this research work. He is also thankful to Advanced
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