Preparation of biogas from waste fabrics

Chemical Engineering Master's Thesis

Abstract

In the last few decades, due to population growth and also the improvement of living standards, the consumption of fibers and the production of textile waste materials have increased drastically. Burial and burning are one of the main ways of managing textile waste materials, which have destructive environmental effects. This is while most of these waste materials can be converted into biological products, including biogas. Biogas is a type of biofuel that is produced during four main stages as a result of biomass fermentation by anaerobic bacteria. In this research, in order to increase the efficiency of biogas production, reduce the crystallinity of the cotton component, and separate cotton fibers from polyester, pretreatment with sodium carbonate solution was used for cotton-polyester and pure cotton fabric, and some characteristics of the remaining polyester were determined. For comparison, the raw cotton and fabric samples were subjected to pre-treatment at different temperatures (50, 100 and 150 °C), in different concentrations of sodium carbonate (0, 0.5 and 1 M) for 120 minutes. Biogas production was performed on raw and pre-processed fabric and cotton samples, as well as a fabric sample made from recovered viscose cotton. The highest amount of methane produced from fabric and cotton, respectively, was 328.9 and 361.1 ml per gram of volatile solids for samples pre-processed at 150°C with a concentration of 0.5 M sodium carbonate for 120 minutes during 40 days. Also, under these temperature conditions and sodium carbonate concentration, polyester fibers were significantly hydrolyzed and separated from the fabric structure, so that the percentage of cellulose in the remaining solid after pre-processing was obtained at about 91%. Methane produced from viscose during 40 days was 381.7 ml per gram of volatile solid. Although the images prepared by scanning electron microscopy of the best pre-processed samples and raw samples of cotton and fabric, do not show a noticeable change except for a slight swelling in the microfibrillar structure of cotton, but the results of FTIR analysis show a decrease in the crystallinity of the cotton component and also the conversion of the cellulose of the first type into the second type. Therefore, the increase in strength Absorption, elimination of impurities, decrease in the crystallinity of the cellulose part and hydrolysis of the polyester part due to alkaline pretreatment can be considered as the factors of increasing the efficiency of biogas and ethanol production in this research. Also, pre-processed samples of cotton and raw fabric were hydrolyzed by cellulase and beta-gluoxidase enzymes for 72 hours to determine the amount of glucose produced. The best samples of enzymatic hydrolysis as well as raw samples were subjected to separate fermentation conditions by Saccharomyces servicii yeast in order to prepare ethanol. The highest efficiency of enzymatic hydrolysis was observed in the cotton sample at the rate of 87.9 and 88.9 percent, respectively, and in the fabric sample at the rate of 79.5 and 81.7 percent, respectively. This is while the efficiency of enzymatic hydrolysis of raw cotton sample was 36.9% and non-preprocessed fabric was 0.28%. The highest efficiency of ethanol production from cotton and fabric was obtained by 69.4% and 59.5%, respectively.

Key words: biogas, fabric, cotton, enzymatic hydrolysis, sodium carbonate

Chapter One: Introduction

1-1                            importance Project

Man has been using textile fibers for various purposes since thousands of years before Christ. Although there is no documented history of the development of the textile industry, initially textile fibers were used to carry food and make mats as shelter. In the later stages of evolution, textile fibers were used as clothes and today they are used in various fields such as clothing, home appliances and industries[1].

Due to the increase in population and the improvement of living standards, the consumption of fibers[1] has increased drastically in the last few decades. So that in 2012, the volume of textile production increased by 1.9% and reached 88.5 million tons. Although these fibers may be used again in some other product after the end of their life, sooner or later they will be thrown away as waste and new fibers will replace worn and old fibers [2 and 3]..

More production means more waste materials, as well as more damaging environmental impacts. Today, textile waste materials [2] are mainly managed by: reuse (second-hand textile goods) [3], reuse in production (as a filler and use in other sectors of the textile industry) [4], recycling [5] (polyester), preparation of compost, burial or burning [6]. Some experts suggest the burning method to convert waste materials into energy, but this method is associated with the release of toxic substances such as dioxins[7], heavy metals, acid, gas and dust particles, which are all harmful to human health and the environment. Also, incineration of waste materials requires advanced equipment, and complete removal of hazardous materials is also impossible. Burying waste materials is the last and most inefficient way to dispose of textile waste materials due to the creation of toxic gases that pollute the environment and the high cost involved [2]. More than 90% of textile fibers can be recycled, which is one of the environmentally friendly ways to dispose of textile waste materials. However, the lack of a cost-effective recycling method on a large scale as well as the wide variety of fibers and colors used in fabric are among the limitations of this method[3].

Due to economic and environmental concerns in the last few decades, many researches have been conducted to find renewable energy sources that can be replaced with fossil fuels. Biogas is one of the biofuels that is obtained through anaerobic digestion [8] of organic substrates and can be a suitable alternative to fossil fuels in the production of heat and power or even be used as fuel for gas vehicles. This biological fuel has many advantages, including renewable capability, reducing the release of greenhouse gases [9] and mitigating the warming of the earth due to these gases, reducing dependence on fossil fuels, flexibility in final consumption and using waste materials as a raw material [4].

About 31.6% of textile production fibers are cotton fibers[10]. Fibers are cellulose-rich solids that can be used as feed in the anaerobic digestion process. However, the proper production of biogas from textile waste materials requires the development of a suitable process [5].

If cotton waste materials are directly used as feed in the biogas process, the desired methane production efficiency will not be achieved. Therefore, in order to increase efficiency, it is necessary to carry out preliminary pre-processing processes[11] on waste materials[6].

With the help of proper pre-processing operations on textile waste materials, goals such as forming a cellulose structure with less crystallinity, reducing impurities in the product and also increasing the available surface of the substrate can be achieved[7].

1-2               Objective

In this research, pre-processing of sodium carbonate to improve biogas production from cotton-polyester fabric was considered as the main goal and optimal conditions for biogas production were obtained. For comparison, the cotton sample was preprocessed under the same temperature and concentration conditions as the cotton-polyester fabric. Investigating the amount of improvement in ethanol production and increasing the level of enzymatic availability of cotton and fabric samples as a result of pre-processing was one of the sub-goals of the project. Main strategies for waste textile management are landfill and incineration that cause many environmental problems while the main part of these wastes can be turned into biological products such as biogas. Biogas is one of the leading biofuels that is produced by fermentation using anaerobic bacteria. In the present study, pretreatment of jean (as a waste textile) with sodium carbonate solution was performed to increase the efficiency of biogas production, reduce the crystallinity of cellulosic part, and separate cotton fibers from the synthetic part of textile, i.e., polyester. This alkaline pretreatment was performed on natural cotton and waste textile samples using various concentration of sodium carbonate (0, 0.