Characterization and Utilization of Bio-char

1.1 Municipal Sewage Sludge (MSS)

Sludge consists of various elements such as waste, organic and inorganic compounds, which are disposed into the atmosphere and are harmful to it. Sludge contains different pollutants and solid waste, including heavy metals, large organic solids, calcium, magnesium, metal sulphides, heavy metal organic complexes, precipitated soaps and detergents, biomass, and precipitated phosphates. Methods of sludge treatment involve stabilization and dewatering residue. Various types of unit processes and operations are used for the management of the sewage sludge process.

However, due to the wide variability in sludge characteristics and differences in acceptability of treated sludge for ultimate disposal, a universal solution is impossible.

Fig. 1.1: Municipal Sewage Sludge (MSS)

1.1. Traditional method of handling municipal sewage sludge

a. Landfilling

b. Incineration

c. Open Dumping

d. Fertilizer

Fig. (a) Landfilling

Fig. (b) Incineration

Fig. (c) Open Dumping

Fig. (d) Fertilizer

1.2 Biomass waste

Biomass waste refers to the aggregates obtained from agricultural land such as crop leftovers, animal fodder, organic industrial waste, and human and animal waste. These are materials obtained from plants, which require sunlight to flourish.

Biomass can be derived from various general and special-purpose sources such as wood from forests, forest leftovers, sugarcane bagasse, rice husks, and kitchen waste. Biomass waste, which contains various advantageous materials, has not been utilized until now. Previously, waste from the aforementioned sources was simply dumped into landfills, or burned in open areas, causing various pollution issues and environmental degradation.

Energy obtained from biomass is largely due to the carbon dioxide contained in the material, which is a result of the sunlight trapped in these materials and the photosynthesis process. If biomass waste is left for an extended period, it will break down, thereby releasing the stored energy and the carbon dioxide contained within it. If released in a quick, directed, and regulated manner, this energy can be used in various beneficial ways.

Fig.1.2 Biomass waste

1.3 Alternate methods of MSS and biomass waste disposal

Pyrolysis:

Pyrolysis is a process of chemically decomposing organic materials at elevated temperatures in the absence of oxygen. The process typically occurs at temperatures above 400o C and under pressure.

Co-pyrolysis:

Co-pyrolysis is similar to the pyrolysis process but is carried out using two or more materials as feedstock.

Fig. 1.3 Process setup

CHAPTER 2

PYROLYSIS AND CO-PYROLYSIS

2.1 Pyrolysis

The word "pyrolysis" is derived from the ancient Greek dictionary, where 'pyro' means fire and 'lysis' means separating. Thus, the word itself reveals its purpose and meaning. Pyrolysis is a chemical process that decomposes organic materials at high temperatures in the absence of oxygen and under controlled pressure conditions. It is commonly used to convert organic materials into a solid residue containing ash and carbon, along with small quantities of oil and gases. Extreme pyrolysis, which employs severe conditions, produces carbon as a product and is also referred to as carbonization. Pyrolysis performed using various feed materials produces different quantities of yield. The amount of residues also depends on the initial carbon content, organic and inorganic material content, and heavy metal content of the feed materials. The carbon composition in the char and the oil obtained are directly proportional to the initial carbon content of the feed or raw materials used in the initial stages. Unlike other processes, pyrolysis does not involve reactions with water, other reagents, or oxygen.

2.2 Types of pyrolysis

There are three main types of pyrolytic processes, depending on the process time and biomass temperature. The names of these pyrolysis processes are as follows:

1. Slow Pyrolysis
2. Flash Pyrolysis
3. Fast Pyrolysis

2.2.1 Slow pyrolysis

The process is characterized by the length of solids and the gas residence time, low temperature, and slow biomass waste heating. The temperature here ranges from 0.1o to 2o C per second, with the prevailing temperature nearly reaching 500o C. The residence time of biomass waste ranges from minutes to several days, while that for gases may be over eight seconds. During slow pyrolysis, char and tar are released as the main products as the biomass waste slowly devolatilizes.

2.2.2 Flash pyrolysis

It occurs at temperatures ranging between 400oC and 600oC and at rapid heating rates. The vapour residence time for the flash pyrolysis process is less than two seconds. The comparative production of char, gas, and tar is less than that of slow pyrolysis.

2.2.3 Fast Pyrolysis

The fast pyrolysis process is primarily used to produce bio-oil and gas. During fast pyrolysis, the biomass waste is heated to temperatures ranging from 650oC to 1000oC. The temperatures are recalculated depending on the desired amount of bio-oil and gas products. The char is obtained in large quantities as a residue and has to be removed frequently.

2.3 Advantages of pyrolysis

• It is a simple and inexpensive technology for processing a variety of feedstocks.
• It potentially decreases the amount of waste going to open landfills, thereby reducing land pollution and greenhouse gas emissions.
• It reduces the risk of water pollution.
• It has the potential to decrease one's dependence on imported energy resources by generating energy from domestic waste.
• Waste management using various pyrolysis techniques reduces the risk of mismanagement of open lands. Some of these techniques are cheaper compared to open dumping and landfilling.

2.4 Applications

• Pyrolysis is used widely in various industries to produce activated carbon, charcoal, and other substances from biomass waste and wood.
• The gas produced during the pyrolysis of waste can be used in steam and gas turbines for producing electricity.
• Pyrolysis plays a vital role in carbon-14 dating and mass spectroscopy.
• The biochar obtained from pyrolysis is also used as fertilizer and is used to increase the fertility of barren landfills, thereby helping to boost the agricultural production of farmers and the country, thus reducing one’s dependence on other allies.

2.5 Co-pyrolysis

Co-pyrolysis is a process which involves two or more materials as feedstock. Many studies have indicated that the use of this process has improved the characteristics of oil, as it increases the oil yield, reduces the water content, and increases the calorific value of the oil. Besides the use of this technique, it has also reduced the production cost and has enabled tackle waste management and pollution issues caused by the disposal of waste in open areas.

Co-pyrolysis is a process that does not involve solvents, catalysts, additional pressure, process waste, or complicated equipment. It is a process that saves waste treatment costs, solves various environmental problems, significantly reduces waste, enhances energy security and its feedstock is available worldwide.

Fig. 2.5 Co-pyrolysis Overview

CHAPTER-3

LITERATURE REVIEW

3.1 Literature Review

Recent trends suggest that biochar has gained immense importance in the field of global environment. Primarily, biochar has been used as a soil amendment, carbon sequestration, and also as an adsorbent for wastewater treatment. In the past, specific applications for biochar were not well-defined. However, literature suggests that biochar has great potential for absorbing environmental contaminants. Biochar is a carbon-rich solid product derived from the pyrolysis or co-pyrolysis of biomass and MSS, with minimal or zero oxygen. One piece of literature suggests that chars are obtained from three different mixtures, and comparing the data samples provides a brief idea about the char adsorption capacity. The goal of literature studies is to perform efficient biochar upgrading. For optimal biochar production or upgrading, sewage sludge has been proven to be a good feedstock. Applications for biochar as an adsorbent include the removal of heavy metal ions from wastewater, as well as dye wastewater treatments. The production of biochar also carries various advantages such as management of municipal sewage sludge. Multiple literature sources are available that characterize the biochar, primarily focusing on physical and chemical properties. Different parameters have been used to characterize the biochar, which is crushed and sieved into various ranges. An additional characterize parameter is through a CHNOS elemental analyzer, which determines which elements the biochar contains.

Thermogravimetric analysis of biochar was performed using an integrated thermal gravimetric analyzer. Well-known literature suggests that sewage sludge is a very good feedstock material for the production of biochars. However, a problem arises when the total/leachable contents of some heavy metal elements in biochar exceed the corresponding norms. In this work, these issues were addressed through the addition of other biomass (such as rice straw and sawdust) for the co-pyrolysis with SS. The addition of RS/SD reduced biochar yield, yet the contents of organic matter in biochar significantly improved. Thermal stability, surface area, and pore volume of biochar depend on the addition of biomass and specifically, the addition of SD. The total contents of heavy metals in the biochar products, especially Cu, Zn, and Ni, were reduced. Unfortunately, the reduction of heavy metal mobility in biochar, according to the toxicity characteristic leaching procedure, was not observed. Sewage sludge has certain characteristics such as high moisture content, ash, high density, and viscosity, while having a low heat value. Conversely, crop straw contains lower ash concentrations and can be made into high heat value pyrolysis products.

The potential of residue biochar, derived from the co-pyrolysis of dewatered sewage sludge (with 80% moisture content) and pine sawdust, for the adsorption of methylene blue (MB) from an aqueous solution, was studied. The biochar was characterized by using a scanning electron microscope, X-ray fluorescence, and Brunner-Emmet-Teller (BET) techniques. Adsorption experiments were carried out to investigate the effects of various parameters on MB adsorption and to evaluate the surface area and maximum adsorption capacity for MB. The observed adsorption process followed the second-order kinetic equation, suggesting that the adsorption might be a chemisorption process. The experimental adsorption isotherm data were well-fitted with both the Langmuir model and the Freundlich model.

Adsorption experiments were performed in batch mode to evaluate the effects of different parameters on the adsorption of MB. In each experiment, 100 ml of the dye solution with 1 g/L of adsorbent was added to a 250 ml conical flask. The sample was then shaken at 190 rpm for 240 minutes. Other literature surveys provide brief insights into product yield and biochar characteristics. It is known that pyrolysis results in three products: biochar, bio-oil, and syngas. Bio-oil and biochar yields were calculated on a wet biomass basis, while the non-condensable gas yield was calculated by difference, and the liquid organic yield was calculated by subtracting the water content from the bio-oil yield.

One piece of literature offers insights into the proximate analysis, pH, and Cation Exchange Capacity. The contents of volatile matter and ash were determined using the American Society Testing and Materials' methods. Volatile content was determined by weight loss after heating the char in a covered crucible to 950°C and holding it for 7 minutes. Ash content was determined by weight loss after combustion at 750°C for 6 hours with no ceramic cap. The pH of biochars was measured using a pH meter at a 1:5 solid/water ratio after shaking for 30 minutes.

The next parameter is the surface properties of biochar. The surface morphology of the biochars was examined using a scanning electron microscope. SEM micrographs of biochars, produced at different temperatures, show that the biomass had softened, melted, and fused into a mass of vesicles. These vesicles resulted from volatile gases released within the biomass. As the temperature increased, more volatile gases were released from the biomass. Finally, to obtain the adsorption data, we used the Brunauer-Emmett-Teller (BET) surface area and Langmuir adsorption isotherm.

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