microwave plasma gasification for the restoration of urban

MICROWAVE PLASMA GASIFICATION FOR THE RESTORATION OF URBAN RIVERS AND LAKES, AND THE ELIMINATION OF OCEANIC GARBAGE PATCHES

Philip K. Panicker New York University in Abu Dhabi Abu Dhabi, United Arab Emirates

Amani Magid New York University in Abu Dhabi Abu Dhabi, United Arab Emirates

ABSTRACT

This review paper describes techniques proposed for applying microwave-induced plasma gasification (MIPG) for cleaning rivers, lakes and oceans of synthetic and organic waste pollutants by converting the waste materials into energy and useful raw materials.

Rivers close to urban centers tend to get filled with man-made waste materials, such as plastics and paper, gradually forming floating masses that further trap biological materials and animals. In addition, sewage from residences and industries, as well as rainwater runoff pour into rivers and lakes carrying solid wastes into the water bodies. As a result, the water surfaces get covered with a stagnant, thick layer of synthetic and biological refuse which kill the fish, harm animals and birds, and breed disease-carrying vectors. Such destruction of water bodies is especially common in developing countries which lack the technology or the means to clean up the rivers.

A terrible consequence of plastic and synthetic waste being dumped irresponsibly into the oceans is the presence of several large floating masses of garbage in the worlds’ oceans, formed by the action of gyres, or circulating ocean currents. In the Pacific Ocean, there are numerous debris fields that have been labeled the Great Pacific Garbage Patch. These patches contain whole plastic litters as well as smaller pieces of plastic, called microplastics, which are tiny fragments that were broken down by the action of waves. These waste products are ingested by animals, birds and fishes, causing death or harm. Some of the waste get washed ashore on beaches along with dead marine life.

The best solution for eliminating all of the above waste management problems is by the application of MIPG systems to convert solid waste materials and contaminated water into syngas, organic fuels and raw materials. MIPG is the most efficient form of plasma gasification, which is able to process

the most widest range of waste materials, while consuming only about a quarter of the energy released from the feedstock. MIPG systems can be scaled in size, power rating and waste-treatment capacity to match financial needs and waste processing requirements. MIPG systems can be set up in urban locations and on the shores of the waterbody, to filter and remove debris and contaminants and clean the water, while generating electric power to feed into the grid, and fuel or raw materials for industrial use.

For eliminating the pelagic debris fields, the proposed design is to have ships fitted with waste collector and filtration systems that feeds the collected waste materials into a MIPG reactor, which converts the carbonaceous materials into syngas (H2 + CO). Some of the syngas made will be used to produce the electric power needed for running the plasma generator and onboard systems, while the remainder can be converted into methanol and other useful products through the Fischer-Tropsch process. This paper qualitatively describes the implementation schemes for the above processes, wherein MIPG technology will be used to clean up major waste problems affecting the earth’s water bodies and to convert the waste into energy and raw materials in a sustainable and environmentally friendly manner, while reducing the dependence on fossil fuels and the release of carbon dioxide and methane into the atmosphere.

NOMENCLATURE

AC Alternating current CVD Carbon vapor deposition DC Direct current DDT dichloro-diphenyl-trichloroethane EDC Endocrine disrupting chemicals HDPE High density polyethylene HHV Higher heating value

Proceedings of the ASME 2016 10th International Conference on Energy Sustainability ES2016

June 26-30, 2016, Charlotte, North Carolina

ES2016-59632

1 Copyright © 2016 by ASME

IEEE

Institute of Electrical and Electronics Engineers

LDMOS

Laterally-diffused metal oxide semiconductor field effect transistor

LDPE Low density polyethylene MIPG

Microwave-Induced Plasma Gasification

MSW Municipal Solid Waste PAH Polycyclic aromatic hydrocarbons PCB Polychlorinated biphenyls PET Polyethylene terephthalate PP Polypropylene PS Polystyrene PVC Polyvinyl chloride RF Radio frequency WTE Waste to energy tpa Tonnes per annum tpd Tonnes per day

INTRODUCTION

This paper is a review of existing literature on the topics of waste accumulation in water bodies such as rivers, lakes, and oceans; waste-to-energy (WTE) conversion; and gasification technologies. The qualitative information gathered is then used to propose a viable solution to two major environmental problems affecting many countries today. The information presented here will be used to support a subsequent techno-economic modeling study of the same subjects.

Firstly, in many developing countries, the rivers, streams, lakes and other water bodies close to densely populated areas become filled with waste, turning them into dead or sewage streams [1,2]. Secondly, the waste on land gets carried away by precipitation into the rivers and lakes, and much of it flows into the seas and oceans, further leading to the creation of large garbage patches in the oceans. In this paper, microwave-induced plasma gasification (MIPG) will be shown to be the best candidate technology for converting all the wastes in the water bodies mentioned above into energy through gasification, while also providing an environmentally safe and sustainable source of energy, fuels and raw materials.

WASTE ACCUMULATION IN URBAN RIVERS, STREAMS, LAKES AND WATERBODIES

In many developing countries, the cities are experiencing a rapid growth in populations as people migrate from the countryside, leaving their farms and villages, and move to the cities in search of jobs. This growth in urban populations creates a significant pressure on land and housing availability, leading to the formation of highly dense communities and slums. Often these communities are located alongside rivers,

streams, lakes and other water bodies in or near cities. Such dwellings frequently lack proper sanitation and waste-disposal systems and the communities are not provided with waste management services due to the absence of proper planning from the city administration. As a result, people discard their household wastes and empty the discharge from their toilets, bathrooms and kitchens into the water bodies, keeping with the maxim, “out of sight, out of mind”. The slums are an economic necessity for the city, providing shelter to low-income workers, who provide the workforce for the industries in the cities. Before long, traders providing services, sales, food production and sales outlets, and even manufacturing start up and take root within the slums. All of these entities send ever larger amounts of waste into the water bodies.

The accumulation of the waste in the water bodies causes an overgrowth of algae, block up the flow, contaminate the ground water, breed vectors that cause and spread diseases, and severely affect the environment and wildlife. There is often a noticeable and unpleasant stench that develops near the contaminated rivers and lakes. As a result, the waterbodies turn into toxic waste pools.

During rainy seasons, the precipitation carries more waste from the streets and surfaces into the rivers which eventually flow into the seas and oceans. However, waves and tides cause the waste to get deposited along the shores and beaches, creating another ecological crisis. Natural disasters, such as earthquakes, floods, hurricanes, etc. also cause wastes to enter the water bodies including the rivers, lakes and oceans from the land.

Since the waste-filled waterbodies are very repulsive, people avoid dealing with them, and gradually the waste builds up. But, if the cleaning up of the water bodies is delayed, the problems continue to grow until an environmental crisis occurs. The main obstacle for the cleanup is a lack of money with the city authorities. Therefore, the solution to this problem has to be economically viable and provide sufficient returns to the stakeholders.

In order to provide a proper engineering solution to the problem, it is important to look at the composition of the waste. Since the definitions are broad and tend to vary, waste items may be classified as below for the ease of practice [3].

1. Municipal Solid Waste (MSW): Commonly called garbage (US), rubbish (UK) or trash, this includes residential and commercial wastes generated in a community or municipality. It includes food and kitchen wastes, consumer items, containers and wrapping made of paper, cardboard, wood, textiles, glass, plastics, metals, etc. MSW may also include hazardous materials such as batteries, light bulbs, pesticides, cleaning chemicals, etc. Paper constitutes about 15% by mass, while plastics which are non-biodegradable can be as high as 70%. Paper and plastics have significant carbon and hydrogen content.


2. Biomedical wastes: this may constitute medicines, chemical waste from clinics, used medical wastes such as syringes, gloves, sanitary napkins, etc. The pharmaceuticals pose an under-recognized ecological problem. Medicines that people consume that are not absorbed by the body get passed out through excretion. These pharmaceutical chemicals cannot be fully removed by sewage treatment plants and contaminate water reservoirs, rivers, lakes and underground water, and also affect wildlife.

3. Biological wastes: This constitutes human wastes, refuse from butcher shops that are disposed off irresponsibly, dead organisms, etc. that attract pests, pathogens and vectors. They have high carbon content.

4. Biomass: This consist of waste vegetable matter, grass clippings, tree branches, sugarcane waste, rice husks, dead plants, algae in the water, etc. This also has high carbon content.

5. Hazardous wastes: This consists of toxic substances, such as mercury from laboratory instruments, batteries, fluorescent lights, etc., corrosive chemicals, reactive chemicals, flammable and explosive substances, etc.

6. Electrical and electronic wastes: This include parts of circuit boards and electrical or electronic appliances, which may have mixtures of plastics, glass, metals and heavy metals.

7. Industrial and commercial wastes: Wastes from manufacturing industries, automotive wastes and some service industries may be derivatives of petroleum, but will also have heavy metals, common metals, textiles, chemicals, plastics, paper, etc.

8. Others: One recently recognized waste that is ending up in water bodies and hurting animals are microplastics in the form of plastic beads, 500 - 1000 micrometers in size, added to toothpastes, soaps and shampoos.

Table 1 is a small sampling of some constituent substances that are part of wastes that end up in water bodies. Volatile matter includes all substances that evaporate away when the substance is heated. Fixed carbon content of a solid material is the amount of combustible solid carbon remaining after the material is heated and all the volatile substances, moisture and ash have been removed. The percentage by mass of ash, carbon, hydrogen, oxygen, nitrogen, sulfur and chlorine are shown. Finally, the calorific values in terms of the higher heating value (HHV) are indicated. Some common fuels are also shown for comparison with the organic wastes.

TABLE 1 MASS FRACTIONS OF THE CONSTITUENT ELEMENTS OF VARIOUS MSW SUBSTANCES AND THEIR CALORIFIC VALUE IN HHV. [4,5]

Volatile matter

Fixed carbon

Ash C H O N S Cl HHV

(%mass) (%mass) (%mass) (%mass) (%mass) (%mass) (%mass) (%mass) (%mass) (MJ/kg)

Newspaper 88.5 10.5 1 52.1 5.9 41.86 0.11 0.03 n.a. 19.3

Cardboard 84.7 6.9 8.4 48.6 6.2 44.96 0.11 0.13 n.a. 16.9

Recycled paper

73.6 6.2 20.2 n.a. n.a. n.a. n.a. n.a. n.a. 13.6

Glossy paper

67.3 4.7 28 45.6 4.8 49.41 0.14 0.05 n.a. 10.4

Wood (Spruce)

89.6 10.2 0.2 47.4 6.3 46.2 0.07 n.a. n.a. 19.3

PlasticsHDPE 100 0 0 86.1 13 0.9 n.a. n.a. n.a. 46.4LDPE 100 0 0 85.7 14.2 0.05 0.05 0 n.a. 46.6PP 100 0 0 86.1 13.7 0.2 n.a. n.a. n.a. 46.4PS 99.8 0.2 0 92.7 7.9 0 n.a. n.a. n.a. 42.1PVC 94.8 4.8 0.4 41.4 5.3 5.83 0.04 0.03 47.7 22.8

Juice carton (paper+metal)

86 6.1 7.9 n.a. n.a. n.a. n.a. n.a. n.a. 24.4

Hydrogen 0 0 0 0 100 0 0 0 0 141.8

Natural Gas 0 0 0 75.85 24.15 0 0 0 0 54

Diesel 0 0 0.6 83.8 12.1 0 0 3.5 0 44.9Gasoline 0 0 0 86.2 12.8 0 0 1 0 46.5

GARBAGE PATCHES IN OCEANS

Large amounts of floating waste materials, such as plastics, get deposited in oceans from rivers and due to surface runoff [6-13]. In addition, about 20% of the debris in the oceans come from oil rigs and ships that discard or inadvertently drop materials into the water. These materials are then carried away by oceanic currents called gyres. Gyres are caused by the Coriolis effect of the earth. They course through large surface areas of the oceans, linking many continents together in their route. The gyres carry the marine debris and concentrate them in their centers where the flow velocities are small, forming large patches of plastics, chemical waste and organic matter. Most of the floaters are made of plastic bottles, bottle caps, plastic bags, Styrofoam cups, as well as pieces of rejected fishing nets. The floaters may be seen on the surface or just below it and are spread out with a density of about 5 kg/km2. The plastics degrade over time due to the action of waves and ultraviolet rays from the sunlight, creating small particles of plastic about 5 mm wide or smaller, which are termed as microplastics. The debris adsorb toxic chemicals such as pesticides, including DDT, PCBs, carcinogens such as polycyclic aromatic hydrocarbons (PAHs), endocrine disrupting chemicals (EDCs), and other organic pollutants [7]. Large plastic pieces and nets trap turtles, fishes and birds. When the


plastic pieces and microplastics are ingested by fishes and birds, they kills the animals, and the adsorbed chemicals enters the food-chain and cause harm to other organisms. The plastics being non-biodegradable accumulate in the oceans and over half end up sinking to the ocean floor.

Garbage patches are present in all the oceans, with the most well-known being the Great Pacific Garbage Patch in the North Pacific Ocean. Debris fields in the North Atlantic, South Atlantic, South Pacific, Indian Ocean and others have been reported. Also, debris fields are found in the North Sea, English Channel and the Baltic Sea [8]. Garbage patches have been known since the late 1980s and are a subject of intense research. Several solutions have been proposed to clean up the garbage patches. [12,13]

WASTE-TO-ENERGY PROCESSES

Most of the wastes end up in landfills. Some waste organic matter are used to produce biogas (methane) in anaerobic digesters. But, this is a slow process that still produces a lot of solid waste which will eventually go into landfills. Here, the processes involved in treating biomass or waste organic substances to produce energy in a faster way with less land fill are considered, and they may be classified into three types: Pyrolysis, combustion, and gasification [1,14-16]. Organic matter derived from biomass, such as paper, food waste, etc. can be considered a renewable energy source.

Pyrolysis is thermochemical decomposition process in which biomass is heated in an anaerobic or low-oxygen environment, typically at temperatures ranging from 100–650°C, whereby the complex hydrocarbons in the solid material decompose into simpler components including gas, liquid and solid. The intensity and rate of heat addition affects the output of pyrolysis. Pyrolysis may conducted slowly over many days at a low rate of heat addition reaching as high as 400°C such as in the production of charcoal from wood or biochar from biomass in which the carbon content has been increased by breaking down the cellulose, hemicellulose and lignin into simpler compounds, and the volatiles are driven out as gas, and some liquids are obtained by distillation of the volatile substances. Pyrolysis may also be conducted at a fast rate when temperatures reach 450– 600°C within seconds, in which case much more condensable and non-condensable gases are produced. Pyrolysis is only useful for treating selective biomass (wood or wood waste) or organic matter (e.g. coal and petroleum) and not for a diverse, non-homogenous matter such as garbage.

In combustion, the material undergoes an exothermic oxidation in an oxygen rich environment producing temperatures of 700–1,400°C which allow for the processing of municipal solid wastes as well as biomass, such as the waste from paper or wood industries. The combustion of organic matter produces carbon dioxide and water vapor. Solid fuels may leave behind a solid ash residue in the combustion chamber. During the combustion of waste materials, volatile

substances are released to the air and may get oxidized, and as a result smoke, carbon monoxide, sulfur oxides (SO2 and SO3) and other gases are also formed. If the temperatures reach as high as 1,600 °C, then the nitrogen that may be present in the wastes as well as nitrogen from the air may form oxides of nitrogen, collectively called NOx (NO and NO2), in an endothermic reaction. The sulfur oxides react with water in the atmosphere to form sulfuric acid (H2SO4) or sulfurous acid (H2SO3), while NOx reacts with water to form nitric acid (HNO3), all of which will mix and come down with precipitation causing what is termed as acid rain. Acid rain is a highly destructive consequence of industrial pollution, that attack vegetation, animal life and buildings. As a result, incinerator plants that combust waste require the addition of multi-stage gas cleaning equipment, such as scrubbers, cyclones, gas condensing and acid gas removal systems, to remove pollutants from flue gases, including fly ash, mercury, sulfur oxides and NOx, as well as more dangerous material such as dioxins, furans and polychlorinated biphenyls (PCBs). Dioxins, furans and PCBs are a broad class of toxic and carcinogenic substances formed from the improper combustion of certain organic compounds at temperatures of 400-700°C, including waste incinerators.

Nevertheless, today there are many waste incinerator plants that convert municipal solid waste (MSW) to energy around the world and it is currently the most common waste-to-energy conversion method wherein the MSW is used to produce electricity, with the help of steam turbines, and the balance heat is used for process heating or for district heating. These plants are subject to strict emission standards imposed by the environmental agencies. Incinerating the MSW causes a reduction in the volume of the solids by 95% or more, with the remaining solid wastes going in to landfills or being used as raw materials in cement manufacture or for construction. The plants do release carbon dioxide into the atmosphere as a result of the combustion. However, if the MSW were to be sent to a landfill, anaerobic decomposition of the biodegradable components in the waste would produce methane which when released to the atmosphere has a far higher global warming potential (GW) than the carbon dioxide.

The drawback of combustion treatment is that the incinerators can only process certain types of waste. The MSW has to be sorted to remove metals, glass, rocks, etc. Large objects, such as tires, appliances, furniture, tree stumps, etc. have to be broken down into smaller chunks that can be fed into the incinerator. Certain hazardous wastes such as batteries, fluorescent light bulbs, etc. have to be removed and handled separately.

Gasification [14-16] is a process of reacting organic matter with controlled amounts of air and water vapor at high temperatures in excess of 700°C to produce a gaseous mixture consisting of carbon monoxide, carbon dioxide and hydrogen, called synthesis gas (also known as syngas). Syngas can then be converted into electricity and raw materials such as fuels, chemicals, fertilizers, etc. [14]. Within a gasification reactor,


many processes may be occurring at different stages. Dehydration occurs at temperatures of 100–200°C, where water is released from the reactants and turned into steam. In some gasifiers, liquid water may be added to the reactants, while in most cases, the materials may have significant amounts of water within them, such as wood and food waste. Pyrolysis occur at temperatures of 200–400°C, where the volatile substances are released and carbon content of the solids are increased. Partial combustion of some of the reactants occurs at higher temperatures, as the carbon reacts with oxygen to form carbon monoxide and carbon dioxide. Finally, all the reactants and intermediate products combine to form syngas since the residence time can range from a few minutes to an hour. The various reactions are listed below with the heat of reaction indicated alongside. Note that the carbon is released from the organic matter [14]. The last equation below is known as the water-gas shift reaction [17].

C + O2 → CO2 ; ∆Hr = -393.4 MJ/kmol (exothermic) (1)

C + CO2 → 2CO ; ∆Hr = 170.7 MJ/kmol (endothermic) (2)

C + H2O → CO + H2 ; ∆Hr = 130.5 MJ/kmol

(endothermic) (3)

CO + H2O ↔ CO2 + H2 ; ∆Hr = -40.2 MJ/kmol (exothermic) [water-gas shift reaction] (4)

The residue of the gasification process collects at the

bottom of the gasifier. The composition of the residue is dependent on the feedstock material and the type of gasifier, and can consist of ash, char, or a slag that is a mixture of metals, silica and ash which solidifies into a dark, glassy substance that has found many industrial uses, including as a building material.

In addition to syngas, there are other gases of lesser calorific value produced in gasifiers operated at lower temperatures and pressures. Producer gas is composed of carbon monoxide and is produced by passing air over heated coal, coke or wood. Coal gas, also known as town gas, was produced from the partial combustion of coal and it contains a mixture of carbon monoxide, hydrogen, methane, as well as volatile gases produced from the coal and carbon dioxide. Coal gas has higher calorific value than producer gas, but it is has significant amounts of impurities. Water gas is a syngas formed by passing steam over red hot coke, formed by the water-gas shift reaction. It also contains impurities picked up from the coke.

Present day gasifiers may be divided into two main kinds depending on how their energy is generated as thermochemical gasifiers and plasma gasifiers. Thermochemical gasifiers derive their heat from direct combustion (or partial combustion) of the feedstock and the heat generated drive the chemical reactions within the reactor sections. Their operating temperatures can go up to about 1,400°C. Of these there are four types of gasifiers available commercially, namely fixed-bed, fluidized-bed,

entrained-bed and molten salt gasifiers. Even these four classifications have many sub-categories, depending on the form factor of the reactors and various aspects of the feedstock, such as type, energy content, moisture content, volatile matter content, mineral content, ash composition, reactivity, size, density, etc. Plasma gasifiers will be discussed in more detail below.

Thermochemical gasification has been known for over 350 years [1]. In 1659, Thomas Shirley, in the United Kingdom, performed experiments on coal gas derived from coal mines. Various individuals since then have used gasification for town lighting, heating and in factories. During the Second World War, a shortage of petroleum led to many cars and trucks being retrofitted with an onboard gasifier that allowed the vehicles to operate on coal or biomass. Although, the availability of petroleum and natural gas caused gasification to become dormant, the lack of petroleum availability in the early 1970s due to the oil embargo imposed by the members of the Organization of Petroleum Exporting Countries (OPEC) revived an interest in gasification technologies. In the 21st Century, the dread of pollution and climate change has given a new impetus to using gasification technologies in converting coal into “clean coal”. Gasification is also being considered for making use of oil sands, tar, fossil fuel waste and municipal solid waste as fuel in an environmentally safe and sustainable manner.

ADVANTAGES OF GASIFICATION OVER COMBUSTION

Although there are numerous MSW incineration power plants around the world, gasification plants offer many advantages [18,19].

Gasification is a much more versatile method of converting organic matter to energy because it can process feedstocks of any phase, including gaseous (natural gas), liquid (petroleum, vegetable oils, sewage) or solid (coal, sewage sludge, etc.) or combinations of the above. The flue-gas from a gasifier plant has far less harmful products, such as SO2 or NOx. Sulfur in a gasifier forms H2S or COS, which can be converted to H2SO4 or elemental sulfur easily. Nitrogen forms NH3 which can be removed with water. Since oxygen required for partial combustion is much less in a gasifier than in an incinerator, the volume of process gas is lower, the partial pressures of the contaminants is higher in the off-gas, which is favorable for removal of contaminants by adsorption and gas cleaners. The syngas leaves the plant at a higher temperatures which prohibit tar formation.

The syngas from a gasifier is a pure, clean burning fuel, which can be used in fuel cells, internal combustion engines or converted into useful products such as methanol, gasoline, diesel, other fuels, chemicals, fertilizers, etc. The end products from a gasifier containing sulfur and nitrogen are valuable commodities. The solid waste generated in a gasifier has lower volume than an incinerator plant and exits in a molten state


which can be cooled and formed into desirable shapes, making it a useful raw material. These benefits are simply not available with incinerators.

Syngas can also be used directly in an internal combustion engine or a gas turbine, without the need for boilers. Thus, gasifiers can be made smaller and modular, for easy application in a rural or sub-urban location. The water usage in a gasification plant is much lower than that of an incinerator plant.

Gasifiers can be used in combined cycle systems as well, known as the integrated gasification combined cycle (IGCC) power plants. Thus, the production of CO2 per MWh as well as the production of pollution in gasifier combined cycle power plants is significantly less than incinerator plants.

PLASMA GASIFICATION

Plasma gasification is a process in which the organic feedstock, including MSW, coal, and other organic material, is pyrolyzed in a plasma medium, causing them to be decomposed into syngas. A schematic of the plasma arc gasifier is shown in Fig. 1. The plasma temperature can be higher than 5,000°C and can theoretically exceed 13,000°C. However, the temperature at the region where the feedstock is in contact with the plasma can be 2,700–4,500°C. Plasma gasifiers have high thermal efficiencies compared to conventional gasifiers. The temperature of the products downstream of the core plasma reaction zone is still high enough for secondary and tertiary reactions to occur. Oxygen (from air) and/or steam can be introduced at specific locations as required to achieve the required mixture and form the desired products. At such high temperatures, all organic compounds break down into simple gases, such as CO and H2. Inorganic components such as metals, silicates, glass, etc. melt and collect together as a slag at the bottom of the reactor. The molten slag is piped off from the bottom and it cools into a glassy, hard, vitrified substance which is used for manufacturing architectural tiles, construction materials and for fillers in road building.

ADVANTAGE OF PLASMA GASIFICATION OVER CONVENTIONAL THERMOCHEMICAL GASIFICATION

Plasma gasification [1,20,21,22] has all the benefits of thermochemical gasification with many other added advantages. The syngas leaves the gasifier at temperatures in excess of 1 000 °C, which has a high heating value and can be used to run a combustion-turbine system to generate electricity. Such high temperatures allow multi-stage cogeneration as well. The syngas produced is very clean, and can be used for feeding into a fuel cell or for combustion in a clean burner. Upwards of 99% of the carbonaceous mater is converted to syngas. No tar is produced because at the high temperatures, all tars are gasified. No ash, char or residual carbon are produced as well for the same reason.

A big advantage of plasma gasifiers is that it can process a wide variety of materials, including coal, coke, lignite,

petroleum, tars, plastics, biomass, tire waste, MSW, commercial and industrial waste, and sewage sludge [23,24, 25]. Moreover, plasma gasifiers can also pyrolyze discarded electronic gadgets and appliances that use metals, glass, plastics and other organic and inorganic materials in their construction. Thus, sorting of MSW is not required for a plasma gasifier. Moreover, waste materials from electrical appliances, food packaging, and many household and commercial items have plastics, paper or cardboard and metals that are fused together, e.g., chewing gum wrappers, which cannot be mechanically separated. Plasma gasifiers can take in all such waste and gasify them.

Materials that are wet or contain small amounts of water can also be gasified in a plasma gasifier, as water vapor is one of the reactants for the formation of syngas, as understood from the chemical reactions shown earlier. Thus, plasma gasifiers can directly turn sewage sludge and waste, with minimal drying, into syngas, eliminating the need for dumping them into rivers, oceans or landfills. Plasma gasifiers can also take in materials that are classified as biomedical wastes and substances classified as hazardous materials, such as those that are corrosive, poisonous, or toxic.

Plasma gasification can neutralize biohazard wastes, including infectious substances, refuse from hospitals and medical facilities, toxins, poisons, etc. All such matter are decomposed into their elemental stage or converted into clean syngas, thereby eliminating the need for specialized handling, storing or dumping grounds. Pharmaceutical compounds that are washed down into the sewage systems pose a major problem by contaminating rivers, lakes and underground aquifers. The pharmaceutical compounds cannot be completely eliminated by sewage treatment plants and end up in the fresh water sources and even the oceans. Plasma gasification is the best solution for eliminating these compounds by gasifying the sewage sludge.

The carbon dioxide emissions are much lower for a plasma gasifier than conventional thermochemical gasifiers. Plasma gasifiers take in air at the stoichiometric amount for aiding the reactions and do not require oxygen, thus eliminating the need for an oxygen plant. All contaminant gases such as fluorine, chlorine, etc. are reduced to elemental stage and can be easily removed. Similarly, mercury is also removable from the reactor, without contaminating the syngas. Harmful products such as dioxins, furans and PCBs are eliminated by the plasma reactor.

The syngas produced by plasma gasification can be directly burned in a gas turbine, internal combustion engine or combined cycle systems [25], without further purification processes. It can also be used in hydrogen fuel cells that can accept carbon monoxide admixture. The fuel product can be used in a steam cycle or cogeneration system. The syngas can be further processed into hydrogen, liquid fuels or chemicals without extra cleaning. The efficiency of syngas production from biomass is much higher for plasma gasification than conventional thermochemical gasification [25].

Plasma gasification can help bring about a nearly 0% waste to landfill solution. The solid residue from the decomposition is a useful raw material for industry. The carbonaceous matter is


completely converted into syngas. All other substances are decomposed into their elemental stages and can be removed to be used as raw materials. The syngas can be used for generating electricity and for producing other fuels and chemicals. Thus, nearly all the waste is converted to energy and useful raw materials, making this process environmentally friendly and sustainable.

TYPES OF PLASMA GASIFICATION

The plasma gasifiers in usage today can be classified into two types based on the mechanism used for generating the plasma. The first type utilizes a plasma torch, similar in operation to that used for cutting metals in industrial workshops. This type of gasification is called plasma arc gasification or simply plasma gasification. The torch uses electrodes made out of copper, tungsten, graphite or alloys. The torch also uses a working fluid which is pumped through the gap in between electrodes, such as argon, nitrogen, air, oxygen and others as per the conditions required within the reactor, and the working fluid also performs the task of cooling the electrodes and the active regions of the torch. The torch may be powered by direct current or alternating current which may also extend into the radio frequency band. For waste-to-energy gasification, plasma torches rated at tens of MWs have been developed and alternating current power would be preferable as they can be supplied from the electric grid through the appropriate transformers and switchgear. Radio-frequency (RF) plasma torches have been developed that use inductive coupling or capacitive coupling to transmit the power to the torch electrodes, but the current units have been rated at a few kilowatts (kW) mainly used for material synthesis applications such as plasma-enhanced thin-film deposition, material surface treatments, etc. and also in research of gasification, such as biomass pyrolysis [23]. The Westinghouse Plasma Corporation (WPC), a subsidiary of the Alter NRG Corp., a Canadian alternative energy company, has built many waste-to-energy (WTE) plants around the world, including three plants in Japan. One plant in Yoshii city rated at converting 166 short tonnes per day (tpd) of MSW to energy has been in operation since 1999 [24]. WPC is one of the leading companies in plasma torch technologies. The torch is reported to consume about 2 % of the energy generated by the plant. While the thermal efficiency (energy content of reactants going in to total energy content of products) of a conventional WTE plant based on thermochemical gasification or incineration is rated to be between 18-22%, the WPC’s integrated gasification combined cycle (IGCC) plant can deliver over 80% thermal conversion efficiency. In comparison, fluidized bed gasifier/combustor plants that only produce electricity from the waste can reach only about 62% thermal efficiency [24].

FIGURE 1 SCHEMATIC OF THE PLASMA TORCH GASIFICATION SYSTEM. [24]

The second type of plasma generator used for gasification is the microwave-induced plasma generator (MIPG) or simply microwave plasma gasification, which will be described more in a subsequent section [26]. MPIG is a relatively new concept with many interesting research programs in progress around the world, and is the technology being promoted in this paper. At this time, there do not seem to be a commercial microwave plasma gasifier in operation but several are proposed or in the construction phase.

MICROWAVE RADIATION

Microwave radiation [27] lies on the electromagnetic spectrum ranging from 300 MHz (1m wavelength) up to 300 GHz (1 mm wavelength), as shown in Table 2. On the lower end of the frequency band, it overlaps with the higher radio frequency (RF) band, while on the upper end, it overlaps with the terahertz radiation band as well as the (lower) far infrared band. Microwave frequencies are used for communication, radar and heating in microwave ovens [27-29]. On the lower end of the microwave band, the RF signals can be transmitted on open parallel wires or coaxial cables, but they can also be transmitted in waveguides (resembling hollow metallic pipes, with circular or rectangular cross section, whose diameter is equal to one-half the wavelength of the signal) or striplines, which are flat metallic strip conductors sandwiched between two flat conductors acting as electrical grounds, separated by a dielectric medium.

The various bands of the electromagnetic spectrum are regulated by professional societies, such as the Institute of Electrical and Electronics Engineers (IEEE), national communication agencies, such as the Federal Communications Commission (FCC) in the US, and by international agreements, such as the International Telecommunications Union- Radio


Regulations (ITU-R). These agencies have designated bands and allocated them or specific uses. Certain bands have been labeled as industrial, scientific and medical (ISM) radio bands, which are used for applications such as RF process heating, microwave heating and medical diathermy machines used for radiotherapy and chemotherapy. The ISM bands used for microwave heating applications are also shared with amateur radio, communication systems such as Wi-Fi, Bluetooth, etc. as well as radar, as shown in Table 2. Microwave heaters are available that operate at 915 MHz (for commercial and industrial heating applications), 2.45 GHz (most home microwave ovens as well as commercial and industrial heating applications) and 5.8 GHz (industrial heating applications). Of these, 2.45 GHz is most commonly used for microwave ovens, both for home use and industrial or commercial use, as well as for microwave plasma applications, such as for gasification purposes, while 5.8 GHz is less commonly used and costs are therefore higher.

TABLE 2 THE VARIOUS BANDS OF THE MICROWAVE RADIATION, SHOWING THE FREQUENCY RANGES, WAVELENGTHS, AND PHOTON ENERGIES. ALSO, A SHORT LIST OF APPLICATIONS WITHIN EACH BAND IS SHOWN.

Name Photon energy (eV) Wavelength Frequency (Hz)Radio 1.24x10-3 – 12.4x10-18 1 mm – 100,000 km 300x109 – 3Microwave 1.24x10-3 – 1.24x10-6 1 mm – 1 m 300x109 – 300x106

Infrared 1.7 – 1.24x10-3 750 nm – 1 mm 400x1012 – 300x109

Visible light 3.2 – 1.7 390 nm – 750 nm 770x1012 – 400x1012

Ultraviolet 124 – 3 10 nm – 400 nm 30x1015 – 750x1012

X-ray 124 000 – 124 0.01 nm – 10 nm 30x1012 – 30x1015

Gamma ray > 62 100 < 0.02 nm > 15x1018

Name IEEE Classification Band Name FrequencyRadio/Micro-wave UHF Ultra High Frequency 0.3 – 1 GHz (109 Hz)

L Long wave 1 – 2 GHzS Short wave 2 – 4 GHz

C Compromise between S and X 4 – 8 GHz

X Exotic, used in WW II 8 – 12 GHz

Band frequency Wavelength Photon energy (eV) Applications

902 – 928 MHz (915 MHz center)

33.24 cm – 32.31 cm (32.76 cm center)

3.73x10-6 – 3.838x10-6

(3.784x10-6 center)

ISM band; amateur radio (33 cm band); cordless phones and stereo; radio-frequency

identification (RFID); datalinks

2400 – 2483.5 MHz (2.45 GHz

center)

12.49 cm – 12.07cm (12.24 cm center)

9.926x10-6 – 1.03x10-5

(1.013x10-5 center)

ISM band; IEEE 802.11, 802.11b, 802.11g, 802.11n

wireless LAN; IEEE 802.15.4-2006; Bluetooth; radio-

controlled aircraft; microwave ovens; ZigBee; 2-way radios

incl. FRS, GMRS; GPS5.725 –

5.875 GHz (5.8 GHz center)

5.24 cm – 5.1cm (5.17 cm center)

2.37x10-5 – 2.43x10-

5 (2.399x10-5 center)

ISM band; Amateur radio; radar; earth stations

(satellite links)

Mic

row

ave

MICROWAVE GENERATORS

Microwaves [26-29] are most commonly generated using a vacuum tube devices known as the magnetron. Other vacuum tube sources include klystrons (300 MHz to 40 GHz, 100 W to

150 MW), gyrotrons (8 GHz to 800 GHz), travelling wave tubes (300 MHz to 50 GHz, 100W to 100 MW), and others. These devices have been developed primarily for radar and for plasma heating purposes used in fusion research, and various versions of the above devices have been developed that are capable of generating microwave radiation at power ratings from a few kW to 100s of MW. High powered microwave sources which are later versions of the above devices use pulsed power sources and can output microwave power for short durations, lasting as long as a few nano-seconds, repeating at up to a few kilo-hertz, thus achieving power ratings well into the giga-watts range [29]. However, these high powered microwave devices. However, these can only run for short periods of time, lasting from a few seconds to a few minutes. The power conversion efficiency is the ratio of the power of the radio frequency output of the device to the electrical power input. Magnetrons have been reported to have about 65 % efficiency, while klystrons run at between 45-55% efficiency.

Due to the popularity of microwave ovens, magnetrons are very easily available in many power ratings, and various frequencies, and they have become highly efficient, over 65% efficiency reported at optimum operating frequencies. Magnetrons in the 1 kW range can be bought for as low as US$10 [28]. Magnetrons are the most widely used microwave sources for heating applications and are also used in microwave induced plasma gasification reactors. One advantage of using microwave sources for generating plasma is that it is not imperative to have a large microwave source capable of a very high power output in the MW or GW range. Microwaves make it possible to build a reactor with multiple low-powered magnetrons operating in parallel which can give a combined high power rating.

Recently, solid-state microwave sources have made the transition from cellphones and low-powered communications systems to heating applications. One of the first entrants to the market with a solid state microwave source was Freescale Semiconductor, Inc. a US semiconductor company that was spun out of Motorola in 2004 and was acquired in December 2015 by NXP Semiconductors, formerly a division of Philips, the Dutch technology company [30]. Their RF transistor models are based on the laterally-diffused metal oxide semiconductor field effect transistor (LDMOS) technology [31]. Solid-state RF transistors offer many advantages over magnetrons. Magnetrons use a supply voltage of about 2 kV or higher, have a life time of about 500 hours and performance degrades over time. The RF transistors use a DC supply voltage of 28-50 V and can last up to 20 years with no performance degradation. The DC operation enables the transistors to be operated from low voltage power supplies, such as solar photo-voltaic systems, battery packs and vehicular power generator systems (e.g., buses, airplanes, ships, etc.), and eliminates the necessity for heavy and bulky power conversion systems such as transformers, and the associated switching gear. Unlike magnetrons, the properties of the RF generated by solid-state devices can be controlled for phase, amplitude, and frequency and their smaller size makes it


possible to more finely direct the energy towards the target. The MHT1003N model is rated at 250 W, 32 V operating voltage outputting 2.45 GHz at up to 58% overall efficiency and the MHT1002N is rated at 350 W, 50 V supply voltage outputting 915 MHz radiation at 63% power-added efficiency (PAE), which is defined as shown in the equation below, where the numerator is the difference between the RF power output by the source and the RF power input to the source from an oscillator and the denominator is the DC power supplied to the source.

(5)

The Chinese electrical appliance manufacturer, Midea

Group, has several patents for microwave ovens using the LDMOS RF transistors and have demonstrated some working prototypes [32]. It is likely that the price of solid-state microwave sources will drop considerably as more and more models enter the market, making them cheap and viable for building small or modular microwave-induced plasma gasification units. Similar to the magnetrons, many RF transistors can be used in parallel within a reactor to increase the power input to the gasification process.

The electron density within the plasma generated by the microwave source increases with the frequency of the microwave radiation. Higher frequencies also allow heating to occur faster as the electron energy is higher [33]. The waveguides get smaller with increasing frequency. Thus, the 2.45 GHz frequency is more preferable than the 915 MHz. Cost of 2.45 GHz magnetrons are lower due to economy of scale of production, as compared to 915 MHz and the 5.8 GHz sources.

Another reason for choosing the 2.45 GHz frequency over the 915 GHz is the penetration depth which is defined as the distance from the surface of the material at which the irradiated microwave power reduces to 36.8 % or 1/e of the value at the surface [28,33]. Generally, shorter penetration depths mean that more energy is being absorbed by the material. For water at 25°C, the penetration depth is 38.26 cm for a 915 MHz wave 1.44 cm for a 2.45 GHz wave. However, at 95°C, it is 5.7 cm making it more difficult to heat water at higher temperatures. At temperatures above 374°C, supercritical water is transparent to microwave irradiation. Also, ice at -12°C has a value of 1,100 cm, showing why it is difficult to heat frozen food in the microwave.

For paper, the penetration depths are 43.66 cm for 915 MHz and 16.01 for 2.45 GHz. The penetration depths for metals compared to dielectrics are much lower, e.g., the values for waves of 915 MHz and 2.45 GHz for aluminum are 2.7 µm and 1.7 µm, and for iron, they are 5.2 µm and 3.2 µm respectively.

Certain inorganic materials are have large values of penetration depths. At 25°C, glass has a penetration depth of 35 cm, alumina ceramic has 633 cm, Teflon has 9,200 cm, and quartz glass has 16,000 cm, all for 2.45 GHz wave. It means

that these materials can be selectively used for construction or thermal insulation in a reactor.

MICROWAVE HEATING

In conduction or convection, heat travels very slowly and depends on the conductivity of the material or the convection coefficients. The container of the material being heated has to be heated to a higher temperature to create a differential for the heat to flow to the material inside, which creates losses of energy. Microwave radiation passes through transparent materials such as quartz or ceramic and penetrates the material within, direct coupling its energy with the molecules of the material causing the bulk of the material to be heated simultaneously. Thus, heat transfer happens very quickly with much less loss of energy to the surroundings.

Microwaves interact with materials in four different ways [26,28,29,33]. Conductive materials, such as metals, reflect the waves, and appear to be opaque to the microwave radiation. Insulating materials, such as ceramics, Teflon, air and gases only absorb a tiny amount of the energy and appear as transparent materials to the radiation. Many materials absorb much of the wave energy, such as water, food, certain biomass, etc. and convert the wave energy to heat. Magnetic materials, such as ferrites, interact with the magnetic field of the wave and convert it into heat.

Microwaves heat materials in four different ways due to the polarizing effects of the alternating electric and magnetic fields of the wave acting on the molecular or atomic structure of the materials, creating kinetic energy to be generated which is then converted to heat. Electronic polarization occurs in covalent solids with valence electrons whereby the valence electrons are cyclically shifted from equilibrium by the alternating electric field of the wave. This cyclic polarization of the electrons causes heating to occur. Silicon and germanium crystals exhibit this effect.

Dipolar polarization is the most important means of heating that occurs in microwave ovens. Atoms or molecules of most dielectric (insulating) materials possess a permanent dipole moment and they are randomly aligned in the normal state. When the microwave radiation, which has an alternating electric and magnetic field, strikes the material, the dipoles are caused to oscillate by the varying electric field. This causes the material to heat up rapidly.

Interfacial polarization occurs in materials with free electrons that accumulate at interfaces within the material, grain boundaries, or between the free surfaces of mixtures of materials. In the presence of the alternating electric field, the free electrons and charges are caused to oscillate at the interfacial boundaries, causing heat to be generated.

Ionic polarization occurs in ionic substances such as crystals of potassium fluoride, sodium chloride, and other alkali halides, or ionic liquids such as tap water, in which the ions are oscillated by the alternating electric field, causing them to move, collide and to generate heat.


In addition to the polarization mechanism, yet another effect occurs in metals and semiconductor. The oscillating magnetic field of the wave induces oscillating eddy currents to flow on the surface, which creates heat in the material through ohmic (or resistance) heating. This flow of eddy currents on the surface is known as skin effect. Thin metal films heat up rapidly due to skin effect ohmic heating, as a result of which they can be seen to spark within microwave ovens. Graphite and silicon carbide absorb microwave energy in this way, and these substances can be added to the microwave reactor as passive heating elements called suscepters.

In a microwave reactor, all the above forms of heating can happen within the reactants, causing heat to build up inside the bulk of the materials. Materials will conduct or convect heat to the surroundings as they are internally heated by the radiation. Since the matter is confined within the reactor, temperatures shoot up rapidly and ionization occurs thereby initiating the formation of a thermal plasma which is then sustained by the energy of the microwave radiation.

Microwaves generated by high powered gyrotrons are used to create plasmas of very high temperatures in nuclear fusion research. Low powered microwave induced plasmas (100s to a few 1000s of watts) are used as sources of excitation for optical emission spectrometry and mass spectrometry. Spectrometry being an established science can be directly applied for diagnostics within the plasma reactor. Reactants and products can be identified through spectroscopy. It can be used to determine whether more or less oxygen or steam is required for the reaction. Or some materials can be directed for further processing or removed out of the reactor through ports when they are determined to be in the proper chemical constituency.

Other lower temperature, non-plasma microwave applications [26,28] include sintering and processing of metals, ceramics, woods, making of steel and synthesis of organic chemicals [20]. Microwave heating is used for lower temperature pyrolysis of biomass, such as wood, plastics and other wastes [34-36].

ADVANTAGE OF MICROWAVE PLASMA SOURCES OVER PLASMA TORCH SOURCES

In a plasma torch generator, a significant amount of parasitic heat is lost to the body and housing of the torch, as well as to the cooling system. A working fluid is converted to plasma which then transfers the heat to the reactant materials. Thus, there is loss of heat at every stage. The working fluid in some cases may be a gas such as argon, which can add to the running costs. The heat is transferred to the working fluid right outside the mouth of the torch where the plasma is created. Thus, the effective work area is limited in size by the size of the torch. Microwave sources irradiate the reactant materials directly, which is heated inside out, without any initial loss of heat by convection or conduction. Expensive working fluids such as argon or nitrogen are not required. Limited amounts of

oxygen is supplied to the microwave induced plasma according to the stoichiometric ratio of the reactants. [39-46]

The electrodes of the torch experience wear during operation, giving it a short lifespan. The reactor has to be shut down periodically to remove and upgrade the torch components.

The plasma torch cannot be controlled for varying its power consumption, energy output or temperature. The microwave generator, which may be a magnetron, can be controlled easily to modulate the power output and thereby the energy input into the reactants and the temperature of the plasma. Solid-state microwave generators can be controlled for varying the power, phase and frequency. Microwave sources are much more compact and easy to operate in comparison to the plasma torch, or arc discharge unit, which require massive power transformers and switching gear, all of which add to the costs.

Many microwave generators can be added in parallel to a reactor to get the required power rating, with only a minimal increase in complexity of the overall system. Thus, microwave plasma gasifiers can be made smaller and modular and give much better cost savings.

The initial investment costs of the plasma torch gasifying system is very high, running into the hundreds of millions of US dollars. This is the biggest setback for investors and public utility management organizations. As a result, the plasma gasifier plants tend to be large in size to match an economy of scale and return on investment. The plant cannot be easily scaled up or down. On the other hand, microwave sources are very cheap, and many brands and models with high efficiencies are available now. The gasifier plant can be scaled to whatever size is required and costs can be kept low.

Microwave sources allow multiple frequencies to be used, allowing the reactor to be tuned for particular reactants and products. Also, different stages of the reactor can be designed to operate at different temperatures and pressures. For example, a lower temperature primary stage can be set up where some useful volatile gases can be removed or the material can be partially processed and removed before the rest of the matter is fully pyrolyzed to syngas. Thus, microwave induced plasma systems enable much more versatility in the design of the reactor and plant all at a much lower cost.


FIGURE 2 A SCHEMATIC OF A MICROWAVE-INDUCED PLASMA GASIFIER.

MICROWAVE PLASMA SOLUTIONS FOR CLEANING RIVERS, LAKES, BEACHES AND INLAND WATER BODIES

For polluted water bodies with flowing water, such as rivers, streams and navigable water ways, the solution is to set up a microwave-induced plasma gasifier station with a collector system which strains the water and removes solid particles and contaminants. The collector could be in the form of a motorized water wheel that dumps the solids onto a conveyor system while also aerating the water. A boat with a comb and boom could drive the refuse towards the collector station. A mechanized hoe will be required to remove heavy or immobile material, such as tree trunks and matter caught in brushes to clear the flow path. The gasifier reactor is fed from a conveyor system that feeds into a shredder to reduce the particle size of the materials. Smaller particles will gasify faster due to their larger surface area and is able to absorb heat from its surroundings conventionally in the reactor.

The operating frequency of the microwave source can be set at 2.45 GHz for lower costs and ease of availability of sources. A solid-state microwave source would be preferable for a ship-based microwave gasifier due to their lower operating voltages, low energy consumption, lower size and mass, better controllability and absence of high voltage transformers and switching gear, both of which would have added costs and weight. However, magnetrons are cheaper, highly efficient, easily available and serviceable. It would be advisable to use several magnetrons of about 10 kW to bring the power up to required levels, thus saving initial costs. Several such microwave gasifier stations could be set up along rivers at, say, 2 to 3 km intervals.

Such a gasifier station could also accept trash from local inhabitants so that they do not continue to discard their garbage

into the river. A pay-per-use or periodic subscription fee can be charged for users of this waste disposal service. For people who have residences along the sides of the river or lake who dump their kitchen, toilet and other sewage wastes into the river, a vacuum tank truck service could be started with periodic subscription programs so that sewage is collected from a septic tank at the residences [46,47]. Gradually, the sewage collection system could be extended into a drainage system that pipes the sewage directly into the gasification plants. Thus, the gasifier will have a steady inflow of feedstock material from the community that it serves. As the gasifier system scales up to meet the waste processing demand, more gasification reactors can be added to the plant, and the gasifier could handle MSW and industrial or commercial waste materials from the community.

Microplastics present in the river water will have to be strained out and gasified. Larger sizes of garbage can be shredded to the appropriate particle size before gasification. Wet sewage sludge collected can be directly fed into the gasifier, since water is a part of the hydrogen production reaction (water shift reaction) [42]. The quality of the syngas will need to be analyzed or modelled to study the effects of various feedstock. But, based on pyrolysis studies, the syngas product can power a cogeneration system, with some of the electricity being used to power the plant. The plant will have to be connected to the electricity grid to power up the gasifier and to operate it during down times. The excess electricity generated by the plant can be fed into the grid. The solid wastes can be sold to industries that can convert them into useful raw materials. Substance such as chlorine, fluorine, Sulphur, etc. need to be collected and sold to appropriate industrial customers [44]. Thus, all the waste can be converted to energy and raw materials. It is also possible to feed the carbon dioxide back into the gasifier in regulated quantities to be converted back into syngas [45].

Lakes and placid water bodies with heavy pollution can have one or more microwave gasifiers situated at the periphery of the body. Beaches and shores of lakes, rivers, etc. will need the use of street sweeper trucks and crews to collect and deliver the waste from these locations. Microwave gasification can neutralize the pharmaceuticals, insecticides, toxins and other chemicals contained in the sewage sludge. Since the products of the gasifier are all clean, this plant can be placed close to heavily inhabited locations and will act as a waste disposal and cleansing station for the community while providing it with power, sanitation services, useful materials and a source of employment.

MICROWAVE PLASMA SOLUTIONS FOR CLEANING GARBAGE PATCHES IN OCEANS AND TO ELIMINATE OIL AND CHEMICAL SPILLS

Many suggestions have been made to clear the garbage patches in the oceans, including pyrolysis of the plastics and


turning them into oil or diesel onboard ships [12,13]. However, these solutions are not clean because they fail to account for contaminants that are adsorbed by the plastics, including heavy metals, pesticides such as DDT, PCBs, carcinogens, etc. [7]. In addition, pyrolysis of the plastics and biomass from the oceans can release dioxins, furans and other toxins. Also, pyrolysis or combustion processes cannot handle the large content of moisture and salt that come with the waste material, which would require preprocessing prior to the waste-to-energy conversion process.

Therefore, the best option for eliminating the garbage patches while also neutralizing hazardous wastes associated with them is microwave-induced plasma gasification (MIPG) reactors placed on ships. The ships can have systems of booms and combs to collect the debris from the water and convey them into a shredder prior to gasification. This system should be capable of drawing in the microplastics while at the same time preventing fish and aquatic creatures from being sucked into the conveyor system. The sea water can be used for cooling the reactor. Some amount of desalination will have to be done, to get clean water for processes, and the waste heat from the reactor could be used for this purpose.

Once the waste is in the gasifier, the syngas produced can be used to power a gas turbine and steam cycle-cogeneration system to provide power to the onboard systems [25]. The ship will need a small generator running on methanol or diesel to provide initial power. The salt can be converted to sodium or turned into sodium nitrates for use in molten salt energy storage systems. Ocean water contains trace elements, including gold, selenium, silver, antimony, cobalt, nickel, manganese, etc. [49,50] There are also small quantities of rare earths [51 - 54,] such as scandium, yttrium, lanthanum, cerium, neodymium, and other lanthanides. MIPG reduces all dissolved substances into their elemental states. It may be profitable to extract trace elements and rare earths out of the sea water that is used for cooling the reactor on board.

A portion of the syngas can be fed into a Fischer-Tropsch reactor [55] to convert to methanol using a well-established method of synthesis that has been in use since the 1920s. The syngas to methanol synthesis is exothermic and is shown below.

2 H2 + CO → CH3OH ; ΔHr = -92 kJ/mol (6)

Even though methanol only has a calorific value of 20

MJ/kg (HHV), compared to diesel (44.8 MJ/kg HHV) and gasoline (47 MJ/kg HHV), methanol has been gaining much attention as a marine fuel [50,51]. Methanol has been transported by truck and sea for over a hundred years. Thus, the modification needed for storage and handling is minimal or none in many cases. The conversion on the engines, such as larger fuel ports on the injectors, are also minimal and cost effective. It is not derived from crops that may hurt the food supply, such as corn, sugar cane or sugar beet. It is easily and safely storable, and biodegradable. It is a very clean burning fuel on which engines have been known to perform well.

Methanol is formed by a one-step synthesis from syngas, or from methane (syngas to methane to methanol is also possible). Methanol can be further converted by the Fischer-Tropsch process in gasoline and diesel [1]. Thus, it is proposed that the syngas derived by the microwave gasification of ocean garbage patches be turned into methanol, which can also be used to run the engines on board the reactor ship.

One profitable option available for using the energy and carbon produced by the microwave plasma reactor would be diamond synthesis from the carbon plasma [58]. An additional diamond synthesis reactor could be added to the microwave gasifier so that some of the carbon in gaseous state could be bled off to coat materials with thin layers of diamonds or to produce artificial diamonds using the well-known carbon vapor deposition process (CVD). The applications of CVD include the manufacture of cutting tools, thermal conductors for laser diodes and high powered semiconductors, electronic components and optical materials. Thus, the gasifier ship could be an ocean going factory producing valuable commodities out of the ocean garbage patches.

The waste generated on board the ship, including kitchen and toilet wastes [46,47], as well as other solid wastes could also be fed right into the gasifier. Some of the products could then be applied for the CVD diamond synthesis as well.

The microwave gasifier ships can also be used to clean up oil spills. The oil floating on the sea can be collected with booms, while beach sand contaminated with oil will have to be scooped up. All these can then be directly input into the gasifier and converted to syngas and useful raw materials.

All of the above suggestions would mean that the application of microwave plasma gasification for the elimination of the ocean garbage patches could be a very profitable, environmentally conscious and sustainable enterprise.

CONCLUSION

A vast literature survey was done to identify the problems associated with the contamination of rivers, lakes and water bodies located near densely populated centers in developing countries, by sewage, household wastes, biological wastes and synthetic wastes. The waste accumulation creates pollution, breed disease-carrying vectors, and harm wildlife and humans. Plastics and floating synthetic wastes originating on land are carried off by rivers into oceans. Ocean currents, called gyres, carry the waste and accumulate them in specific regions in the oceans. In addition, trash and refuse discarded from ships along shipping lanes are also transported by the gyres. The plastic trash form a wide field of floating plastic and debris that harm or kill the fish, birds and animals. The debris are predominantly visible just below the surface of the water. The plastics disintegrate into tiny particles called microplastics, due to the action of the sun and waves. The plastics also adsorb pesticides and toxins which can kill fishes and enter the food chain. Many


studies have been done and proposals made to clean up the garbage patches but most are not feasible.

Gasification is looked at as a better method of turning MSW and other waste to energy in the form of syngas. Microwave-induced plasma gasification is presented as the most cost-effective, versatile, modular and efficient method of turning waste to energy and raw materials. It is also able to take in all kinds of waste without pre-sorting or pre-treatment, including waste with high moisture content, as well as hazardous and biomedical wastes. A microwave plasma gasification system is presented as the best solution for cleaning up the waste contamination in rivers, lakes and urban water bodies, as well as beaches and seas. Finally, a ship based microwave plasma gasification system coupled with a Fischer-Tropsch processor is presented as the best solution for cleaning up the garbage patches accumulating in oceans.

REFERENCES

[1] Basu, P., 2013, Biomass gasification, pyrolysis and torrefaction: practical design and theory, Academic press. [2] Chandrappa, R., and Das, D. B., 2014, Sustainable water engineering: theory and practice, John Wiley & Sons. [3] Chandrappa, R., and Brown, J., 2012, Solid waste management: principles and practice, Springer Science & Business Media. [4] Sørum, L., Grønli, M. G., and Hustad, J. E., 2001, "Pyrolysis characteristics and kinetics of municipal solid wastes," Fuel, 80(9), pp. 1217-1227. [5] Panda, A. K., Singh, R. K., and Mishra, D. K., 2010, "Thermolysis of waste plastics to liquid fuel: A suitable method for plastic waste management and manufacture of value added products—A world prospective," Renewable and Sustainable Energy Reviews, 14(1), pp. 233-248. [6] Kaiser, J., 2010, "The Dirt on Ocean Garbage Patches," Science, 328(5985), p. 1506. [7] Rios, L. M., Jones, P. R., Moore, C., and Narayan, U. V., 2010, "Quantitation of persistent organic pollutants adsorbed on plastic debris from the Northern Pacific Gyre's “eastern garbage patch”," Journal of Environmental Monitoring, 12(12), pp. 2226-2236. [8] Foekema, E. M., De Gruijter, C., Mergia, M. T., van Franeker, J. A., Murk, A. J., and Koelmans, A. A., 2013, "Plastic in North Sea Fish," Environmental Science & Technology, 47(15), pp. 8818-8824. [9] Van Sebille, E., England, M. H., and Froyland, G., 2012, "Origin, dynamics and evolution of ocean garbage patches from observed surface drifters," Environmental Research Letters, 7(4). [10] Ryan, P. G., 2015, "A brief history of marine litter research," Marine Anthropogenic Litter, pp. 1-25. [11] Gerlach, D. W., "A modest proposal1 for the removal of floating rubbish from the north pacific central gyre," Proc. ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011, pp. 1101-1103.

[12] Tremblay, J. F., 2009, "Mining the sea of plastic," Chemical and Engineering News, 87(33). [13] Sigler, M., 2014, "The effects of plastic pollution on aquatic wildlife: Current situations and future solutions," Water, Air, and Soil Pollution, 225(11). [14] Luque, R., and Speight, J., 2014, Gasification for Synthetic Fuel Production: Fundamentals, Processes and Applications, Elsevier. [15] Brar, S. K., Dhillon, G. S., and Soccol, C. R., 2014, Biotransformation of waste biomass into high value biochemicals, Springer. [16] Young, G. C., 2010, Municipal solid waste to energy conversion processes: economic, technical, and renewable comparisons, John Wiley & Sons. [17] Shah, Y. T., 2014, Water for energy and fuel production, CRC Press. [18] Ouda, O. K. M., and Raza, S. A., "Waste-to-energy: Solution for Municipal Solid Waste challenges- global perspective," Proc. ISTMET 2014 - 1st International Symposium on Technology Management and Emerging Technologies, Proceedings, pp. 270-274. [19] Arafat, H. A., and Jijakli, K., 2013, "Modeling and comparative assessment of municipal solid waste gasification for energy production," Waste management, 33(8), pp. 1704-1713. [20] Kappe, C. O., Dallinger, D., and Murphree, S. S., 2009, Practical Microwave Sythesis for Organic Chemists - Strategies, Instruments, and Protocols, Wiley-VCH, Weinheim. [21] Dias, F., Bundaleska, N., Henriques, J., Tatarova, E., and Ferreira, C., "Microwave plasma torches used for hydrogen production," Proc. Journal of Physics: Conference Series, IOP Publishing, p. 012002. [22] Janajreh, I., Raza, S. S., and Valmundsson, A. S., 2013, "Plasma gasification process: Modeling, simulation and comparison with conventional air gasification," Energy Conversion and Management, 65, pp. 801-809. [23] Tang, L., and Huang, H., 2005, "Biomass gasification using capacitively coupled RF plasma technology," Fuel, 84(16), pp. 2055-2063. [24] Juniper Consultancy Sevice, 2008, "Independent Waste Technology Report: The Alter NRG/Westinghouse Plasma Gasification Report," Bathurst House, Bisley, GL6 7NH, England. [25] Rutberg, P. G., Bratsev, A. N., Kuznetsov, V. A., Popov, V. E., Ufimtsev, A. A., and Shtengel, S. V., 2011, "On Efficiency of Plasma Gasification of Wood Residues," Biomass and Bioenergy, 35(1), pp. 495-504. [26] Shuttleworth, P., Budarin, V., and Gronnow, M., 2013, "The Thermochemical Conversion of Biomass into High-Value Products: Microwave Pyrolysis," The economic utilisation of food co-products, A. Kazmi, and P. Shuttleworth, eds., RSC Publishing, Cambridge, UK. [27] "Microwave," https://en.wikipedia.org/wiki/Microwave. [28] Gupta, M., and Wong Wai Leong, E., 2007, "Microwaves and Metals," John Wiley & Sons (Asia), Singapore.


[29] Benford, J., Swegle, J. A., and Schamiloglu, E., 2007, High power microwaves, CRC Press. [30] "NXP Semiconductors," http://www.nxp.com/. [31] Sechi, F., and Bujatti, M., 2014, Solid-state microwave high-power amplifiers, Artech House. [32] Tang, X., Ou, J., and Liang, C., 2012, "Semiconductor microwave oven and microwave feeding structure thereof," Google Patents. [33] Moisan, M., Barbeau, C., Claude, R., Ferreira, C. M., Margot, J., Paraszczak, J., Sá, A. B., Sauvé, G., and Wertheimer, M. R., 1991, "Radio frequency or microwave plasma reactors? Factors determining the optimum frequency of operation," Journal of Vacuum Science & Technology B, 9(1), pp. 8-25. [34] Appleton, T. J., Colder, R. I., Kingman, S. W., Lowndes, I. S., and Read, A. G., 2005, "Microwave technology for energy-efficient processing of waste," Applied Energy, 81(1), pp. 85-113. [35] Beneroso, D., Bermúdez, J. M., Arenillas, A., de la Peña, F., García, J. L., Prieto, M. A., and Menéndez, J. A., 2015, "Oil fractions from the pyrolysis of diverse organic wastes: The different effects of conventional and microwave induced pyrolysis," Journal of Analytical and Applied Pyrolysis, 114, pp. 256-264. [36] Ludlow-Palafox, C., and Chase, H. A., 2001, "Microwave-Induced Pyrolysis of Plastic Wastes," Industrial & Engineering Chemistry Research, 40(22), pp. 4749-4756. [39] Lupa, C. J., Wylie, S. R., Shaw, A., Al-Shamma'a, A., Sweetman, A. J., and Herbert, B. M., 2012, "Experimental analysis of biomass pyrolysis using microwave-induced plasma," Fuel Processing Technology, 97, pp. 79-84. [40] Lupa, C. J., Wylie, S. R., Shaw, A., Al-Shamma'a, A., Sweetman, A. J., and Herbert, B. M., 2013, "Gas evolution and syngas heating value from advanced thermal treatment of waste using microwave-induced plasma," Renewable Energy, 50, pp. 1065-1072. [41] Chaichumporn, C., Ngamsirijit, P., Boonklin, N., Eaiprasetsak, K., and Fuangfoong, M., 2011, "Design and Construction of 2.45 GHz Microwave Plasma Source at Atmospheric Pressure," Procedia Engineering, 8, pp. 94-100. [42] Djebabra, D., Dessaux, O., and Goudmand, P., 1991, "Coal gasification by microwave plasma in water vapour," Fuel, 70(12), pp. 1473-1475. [43] Gadonna, K., Leroy, O., Leprince, P., Boisse-Laporte, C., and Alves, L. L., 2012, "Study of gas heating by a microwave plasma torch," Journal of Modern Physics, 3(10), p. 1603. [44] Kim, J. H., Cho, C. H., Shin, D. H., Hong, Y. C., and Shin, Y. W., 2015, "Abatement of fluorinated compounds using a 2.45 GHz microwave plasma torch with a reverse vortex plasma reactor," Journal of Hazardous Materials, 294, pp. 41-46. [45] Kwak, H. S., Uhm, H. S., Hong, Y. C., and Choi, E. H., 2015, "Disintegration of Carbon Dioxide Molecules in a Microwave Plasma Torch," Scientific Reports, 5, p. 18436. [46] Liu, M., Woudstra, T., Promes, E. J. O., Restrepo, S. Y. G., and Aravind, P. V., 2014, "System development and self-

sustainability analysis for upgrading human waste to power," Energy, 68, pp. 377-384. [47] Shinogi, Y., and Kanri, Y., 2003, "Pyrolysis of plant, animal and human waste: physical and chemical characterization of the pyrolytic products," Bioresource Technology, 90(3), pp. 241-247. [48] Westphal, G., Kristen, G., Wegener, W., Ambatiello, P., Geyer, H., Epron, B., Bonal, C., Steinhauser, G., and Götzfried, F., 2000, "Sodium Chloride," Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA. [49] Schutz, D. F., and Turekian, K. K., 1965, "The investigation of the geographical and vertical distribution of several trace elements in sea water using neutron activation analysis," Geochimica et Cosmochimica Acta, 29(4), pp. 259-313. [50] Nozaki, Y., 1997, "A fresh look at element distribution in the North Pacific Ocean," EOS Transactions, 78, pp. 221-221. [51] Alibo, D. S., and Nozaki, Y., 1999, "Rare earth elements in seawater: particle association, shale-normalization, and Ce oxidation," Geochimica et Cosmochimica Acta, 63(3), pp. 363-372. [52] De Baar, H. J. W., Bacon, M. P., Brewer, P. G., and Bruland, K. W., 1985, "Rare earth elements in the Pacific and Atlantic Oceans," Geochimica et Cosmochimica Acta, 49(9), pp. 1943-1959. [53] Elderfield, H., Upstill-Goddard, R., and Sholkovitz, E. R., 1990, "The rare earth elements in rivers, estuaries, and coastal seas and their significance to the composition of ocean waters," Geochimica et Cosmochimica Acta, 54(4), pp. 971-991. [54] Elderfield, H., and Greaves, M. J., 1982, "The rare earth elements in seawater," Nature, 296(5854), pp. 214-219. [55] Psarras, P. C., 2015, "Toward an Understanding of Methane Selectivity in the Fischer-Tropsch Process," Cleveland State University. [56] Andersson, K., and Salazar, C. M., 2015, "Methanol as a Marine Fuel Report," Report, Methanol Institute, FCBI Energy. [57] Chudnovsky, B., Reshef, M., Keren, M., and Baitel, S., "Successful Implementation of Methanol Firing at 50 MW Gas Turbine for Long Term Operation," Proc. ASME 2015 Power Conference collocated with the ASME 2015 9th International Conference on Energy Sustainability, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum, American Society of Mechanical Engineers, pp. V001T003A015-V001T003A015. [58] Kamo, M., Sato, Y., Matsumoto, S., and Setaka, N., 1983, "Diamond synthesis from gas phase in microwave plasma," Journal of Crystal Growth, 62(3), pp. 642-644.


microwave plasma gasification for the restoration of urban

Documents