AGRICULTURAL WASTE MANAGEMENT

The common traditional approach to manage agio-waste has been its release to the environment either with or without any treatment. Such practices increase the risk of environmental pollution and public health hazards (Wright et al., 1998). The dumping of organic waste causes volatilization of ammonia which affects the overall quality of air (due to the emission of gases like NH3, NO, and CH4), water (due to release of high nitrogen and phosphorous), and soil (because of loading of potassium and phosphorous). The stagnation of AWs provides a medium for the growth and multiplication of flies, which are the causative agents of various diseases (Fabian et al., 1993). In order to clear lands, agricultural waste is openly burnt in the fields, which emit CO„ CH4, and other pollutants in the air, thus contributing the climate change. The improper management of waste is an alarming problem of the century. There is a dire need to recognize waste as a potential resource rather than leaving it as discarded products. This will not only provide us a renewable energy source but also proves helpful in the prevention of contamination of air, water, and land resources. It has been found that some AWs have a potential of establishing markets. Most of them can be extensively used as soil nutrient recycling and soil upgrading purposes. These wastes possess the potential of replacing the synthetic fertilizers. Besides this, biomass wastes possess the high value with respect to material and energy outputs; as a result this can be used as a raw material for large-scale industries (Zafer et al., 2014). In the developing countries it has been found that, AWs have great potential to be used as raw material that has a capacity of generating 50 billion tons of oil and may offer renewable energy to millions of houses which still lack access to basic facility of electricity (Buyukgungor et al., 2009).

To circumvent these problems, significant efforts are being made by many governments and non-govemment agencies throughout the globe. Biotechnology has provided a wide platform to manage waste agricultural biomass and to convert it into a material resource.

APPLICATION OF BIOTECHNOLOGY IN THE MANAGEMENT OF WASTE

Biotechnology had played an important role in the management of agricultural waste in terms of economical and environmental benefits. Biotechnological applications and approaches had enabled one to reduce the toxic and hazardous effects of waste and by converting them into value added products that are environmental friendly. Some of the biotechnological applications viz. biofiltration, biosorption, bioreactor, bioleaching, composting, and phytoremediation, etc., had contributed immensely in mitigating agricultural waste and making it beneficial for mankind. Since agricultural residues are starch and cellulose rich, it therefore possesses an inexhaustible raw material for the generation of energy. Some of the potential applications of agricultural residues that had been exploited with the aid of biotechnology had been discussed in this review.

AGRICULTURAL WASTE AS A RAW MATERIAL FOR CONVERSION OF ENERGY

Agricultural biomass has a potential to be used as a resource for energy and other value added products. There are several routes for the conversion of agricultural biomass into energy. The two major routes are thennochemical conversion and biochemical conversion (Figure 13.1).

Schematic representations of different routes of conversion of waste into energy

FIGURE 13.1 Schematic representations of different routes of conversion of waste into energy.

THERMOCHEMICAL CONVERSION

Thennocheinical conversion is characterized by very high temperature and is best suited for low moisture containing biomass. The three principal methods of thennocheinical conversion are combustion, gasification, and pyrolysis which are discussed one by one.

COMBUSTION

There are certain properties of agricultural residues which lend them as an important tool for biotechnologists for generation of electricity. The potential of power generation varies from few kilowatts up to several megawatts. According to the study conducted in the year 1980 it has been reported that capacity of power generation in sugarcane growing countries was about 70% more than the total electricity generated in these countries from all sources on that time. The biomass residues are characterized by high volatile matter which makes it quite easy to burn and ignite (Hallet al., 1991). The agricultural residues mainly consist of plant materials that had been periodically harvested. During the growth and development, these plants have lost CO, to atmosphere through the process of photosynthesis. This adds the extra benefit of using agricultural residue as partial substitution of fossil fuels as their combustion is CO, neutral which may have otherwise aroused the issue of global warming (Werther et al., 1995; Oganda et al., 1996).

Besides the generation of electricity, biotechnologists have also used the agricultural biomass for production of low-pressure steam ranging from 0.1 to 1 MPa and high-pressure steam which ranges from 4 to 10 MPa. The low pressure steam finds its use in certain industrial processes like food preparations, lumber, and pulp manufacture and kiln operations. The high- pressure steam are being used to expand turbines either connected to electric generators or connected to direct machine drives. To achieve this end, biotechnology had provided a helping hand in the designing and operation of systemic machinery. With the advancement of biotechnology, different types of combustors for agricultural residues have been designed from time to time. Among these, grate-fired systems is one of the first combustors used in industrial settings, while fluidized bed systems are the versatile units prevailing fr om the last 10 years. Suspension burners being one of the recent combustors designed for the special application of diy fuel. Depending upon the purpose of the process and the size and property of available fuel, the selection of a suitable system is made (Tilman et al., 1997).

GASIFICATION

It is a process of choked or incomplete combustion that uses heat and pressure to convert any carbon rich material into synthetic gas known by several names like wood gas, syngas, producer gas, etc. This synthetic gas mainly composed of CO and H still retain the combustion potential and is passed through the pipes to other places where it is used for at large scale piuposes. Gas produced by this method is known by several names as w'ood gas, syngas, producer gas, etc. In this way, gasification adds value to agricultural waste by converting them to marketable fuels. The generation of syngas depends upon the availability of types of waste. Usually, in farmlands, the feedstock’s like cane stock are used for it, but in the urban areas, wastes like garbage and tires are also utilized for it. According to one of the studies conducted in the US (Department of Energy), gasification can be considered for the production of hydrogen for transportation fuel. Apart from being a building block for a broad range of chemical products, it is also be supplied to power-generating fuel cells and power-generating turbines (Singhania et al., 2017).

PYROLYSIS

Pyrolysis is defined as the process of the irreversible thermochemical decomposition of material at increased temperature (300 to 600°C) in the absence of oxygen to vaporize a portion of the material, leaving a char behind (Figure 13.2). In the process of pyrolysis low energy materials are converted into high-energy compounds, which include liquids like bio-oil and solids like biochar, and gases like syngas (the production of syngas from pyrolysis is actually the gasification is discussed above). The compounds thus produced are used for a wide range of purposes, starting from the burning of the cooking stove to the running of automobiles (Laird et al., 2009). The three forms of pyrolysis products, i.e., liquid (bio-oil), solid (biochar), and gas (syngas) are apportioned according to the condition, type of reaction, and availability of biomass. Broadly pyrolysis is categorized into three categories: fast, slow', and gasification. In the process of fast pyrolysis, bio-oil is produced; thus, this process is employed when the production of liquid fuel is concerned, hr contrast to fast, slow' pyrolysis maximizes the production of biochar

(Venderbosch and Prins, 2010; Butler et al., 2011; Jarboe et al., 2011). The bio-oil produced from pyrolysis is characterized by certain properties like non-oxidative reactivity and presence of oxygenates like organic acids. The oil is also high composed of high moisture and high heteroatom content. Therefore, in order to use the bio-oil as transportation fuel, prior upgrading operations like distillation and hydroprocessing are required (Elliot, 2007; Mortensen et al., 2011; Balat, 2011; Sorrell et al., 2010).

The other technique that is involved in the thermal conversion of biomass waste is catalytic pyrolysis. In the process of catalytic pyrolysis zeolite catalyst is added and mixed with the waste which is then subjected to pyrolysis, hi some cases, catalyst is not directly mixed with biomass instead are placed under the pyrolysis reactor in a fixed form (Carlson et al., 2009). Vapors from the mixture get absorbed in the pores of the zeolite. Within the pores, these vapors undergo rearrangement reactions to produce olefins and aromatic compounds (Carlson et al., 2011). In this process proteins and oxygenated compounds get converted into hydrocarbons without adding any amount of hydrogen. The nitrogen from proteins gets converted to ammonia (Wang and Brown, 2013).

Theiinochemical conversion of agricultural biomass through different processes

FIGURE 13.2 Theiinochemical conversion of agricultural biomass through different processes.

 
Source
< Prev   CONTENTS   Source   Next >