POWER GENERATION FROM MUNICIPAL WASTES
S.K.ANSARI AND SEEMA ANSARI
Jan 31 - Feb 6, 2011
Solid waste can be defined as material that no longer has any value to the person who is responsible for it. It does not normally include human excretes. It is generated by domestic, commercial, industrial, healthcare, agricultural and mineral extraction activities and accumulates in streets and public places. The words "garbage", "trash", "refuse", and "rubbish" are used to refer to some forms of solid waste.
Solid waste collection by government owned and operative service in Pakistan's cities currently averages only 50 per cent of waste quantities generated. However, for the cities to be relatively clean, at least 75 per cent of these quantities should be collected.
Unfortunately, none of the cities in Pakistan has a proper solid waste management system right from collection of solid waste up to its proper disposal. Much of the uncollected waste poses serious risk to public health through clogging of drains, formation of stagnant ponds, and providing breeding ground from mosquitoes and flies with consequent risks of malaria and cholera. In addition, because of the lack of adequate disposal sites, much of the collected waste finds its way in dumping ground, open pits, ponds, rivers, and agricultural land.
The study revealed that the rate of waste generation on average from all type of municipal controlled areas varies from 0.283 kg/capita/day to 0.613 kg/capita/day or from 1.896 kg/house/day to 4.29 kg/house/day in all the selected cities.
CITY POPULATION (MILLION) 1998 CENSUS POPULATION (MILLION) 2004 POPULATION (MILLION) 2010 SOLID WASTE
GENERATION RATE KG/C/DAY
WASTE GENERATED TONS/DAY TONS/YEAR URBAN AREAS Karachi 9.269 10.818 12.982 0.7969 10,345 3,776,055 Faisalabad 1.977 2.307 2.768 0.5083 1,407 513,546 Hyderabad 1.151 1.343 1.612 0.7319 1,179 430,528 Gujranwala 1.124 1.312 1.575 0.6097 961 350,502 Peshawar 0.988 1.153 1.836 0.6357 1,167 426,008 Quetta 0.560 0.654 0.7848 0.4914 386 140,762 Bannu 0.046 0.054 0.0648 0.5707 37 13,498 Sibi 0.283 0.095 0.114 0.3679 42 15,308 Remaining urban areas 27.261 31.818 38.818 0.5889 22,860 8,343,871 Rulal Areas 88.121 102.853 12.424 0.3679 4,570 134,284 Sub-Total 130.780 152.407 72.978 42,954 15,678,210 Add 10% for Hazardous Waste . 4,296 1,567,821 G. Total . 54,280 15,712,183
The table figures show that 15.7 million tons/annum of wasted materials are generated in Pakistan, but the big difference with the waste generation and the amounts reaching final disposal sites are difficult to recover. In the developed countries, the estimated figure and the recovery are almost the same since most waste must be disposed which later on moves towards the segregation of some components of waste at the source in a number of countries.
The situation is made worse in Pakistan as there are no weighing facilities at disposal sites and no tradition of waste sampling and analysis. Furthermore, the types and quantities of wastes arising and reclaimed vary with the locality and, to some extent, with the season; and areas with more traditional lifestyles tend to generate relatively small quantities of waste, and segregation and reclamation practices are more widespread.
Environmental Problems due to Solid Waste (SW) Ground pollution
As water percolates through SW, it makes leachate that consists of decomposing organic matter combined with iron, mercury, lead, zinc, and other metals from rusting cans, discarded batteries and appliances. It may also contain paints, pesticides, cleaning fluids, newspaper inks, and other chemicals. Contaminated water can have a serious impact on all living creatures, including humans, in an ecosystem.
When waste is burnt, heavy metals like lead, toxic gases and smoke spread over residential areas. The wind also carries waste, dust, and gases caused by decomposition. Putrefaction of waste in sunlight during daytime results in bad smells and reduced visibility.
HEALTH HAZARDS DUE TO SW
* Skin and eye infections are common.
* Dust in the air at dumpsites can cause breathing problems in children and adults.
* Flies breed on uncovered piles of rotting garbage and spread diseases like diarrhea, dysentery, typhoid, hepatitis, and cholera. Mosquitoes transmit many types of diseases like malaria and yellow fever.
* Dogs, cats, and rats living around refuse carry a variety of diseases including plague and flea born fever.
* Intestinal, parasitic, and skin diseases are found in workers engaged in collecting refuse.
Waste-to-Energy Technology with Plasma Gasification for Cities
That day may be far away when most municipal waste managers have little idea what plasma gasification even is. The benefits of the technology - plasma gasification generates zero waste, and produces steam that can be used to create on-site electricity - are so many that municipalities will practically be forced to investigate it as they learn more about it.
This technology produces no emissions and no odors. It delivers power right to the distribution portion of the electrical service system. That helps offload some of the burden on the heavily used public grid. All these factors make the ability to convert waste into energy without any emissions a very exciting proposition.
The plasma gasification process is still in its earliest, most experimental stages. But, some companies in the United States are working on this renewable waste raw material, which provides attraction towards the municipalities to generate electricity from it.
So far, the only plasma-gasification plants currently operating in North America are test facilities designed to explore and weed out any bugs in the technology. But, proponents of the technology are confident that this will soon change. Plasco, for example, is scheduled to build a plasma-gasification plant in Red Deer, Canada, that will take in 300 tons of municipal solid waste a day. It has also signed an agreement to build a second plant in Ottawa that will handle 400 tons of waste every day.
Renewable fuels from municipal waste also attract the Florida company, which signed an agreement to build a plasma-gasification plant in Tallahassee that will each day convert 1,000 tons of mostly municipal solid waste into energy.
After the Plasco project, the other energy producing companies come to fruition with more and more communities adopting this technology. The economics are now beginning to become more realistic for plasma gasification.
On 10th December, 2010, United States announced the Taylor Biomass Energy (TBE) facility groundbreaking ceremony in the town of Montgomery in New York. The Montgomery Project uses the proprietary "Taylor Biomass Energy Solution" as the foundational technology for a three-part, integrated system design that converts the organic biomass portion of Municipal Solid Waste (MSW) to electric power, through gasification. The event marks the start of Phase 1 construction on the new facility, which has received necessary town and state approvals to move forward.
According to the company, the project will generate a net 20-megawatt energy and produce enough electricity to power approximately 27,000 homes based 500 kwh/month usage per residence, with an estimated cost over 20 years of around five cents per kW which is equivalent to Rs4.30 per KW.
Generating energy while reducing trash and producing no pollution is an absolute game changer for this country, and it is happening right there in the Hudson Valley, all the while creating jobs and badly needed economic activity. This project will cost $100 million to make possible this innovative green energy project.
PLASMA ARC GASIFICATION
Plasma arc gasification is a waste treatment technology that uses electrical energy and the high temperatures created by an electric arc gasifier. This arc breaks down waste primarily into elemental gas and solid waste (slag), in a device called a plasma converter. The process has been intended to be a net generator of electricity, depending upon the composition of input wastes, and to reduce the volumes of waste being sent to landfill sites.
Plasma arc gasification is a new garbage disposal solution using plasma technology. This process of garbage disposal is self-sustaining and converts garbage into electricity. Although plasma technology has been around for years, its application to garbage disposal was never seriously considered because the conventional approach of using landfills was less expensive (even with tipping fees and transportation costs).
It was only recently ñ with landfills in scarce supply and with fuel costs on a constant rise ñ that the plasma gasification process has merited deeper consideration.
BASIC OPERATION PRINCIPLE
Relatively high voltage, high current electricity is passed between two electrodes, spaced apart, creating an electrical arc. Inert gas under pressure is passed through the arc into a sealed container of waste material, reaching temperatures as high as 25,000∞F (13,900∞C) in the arc column. The temperature a few feet from the torch can be as high as 5,000-8,000 ∞F (2,760-4,427∞C). At these temperatures, most types of waste are broken into basic elemental components in a gaseous form, and complex molecules are separated into individual atoms.
The reactor operates at a slightly negative pressure, meaning that the feed system is complemented by a gaseous removal system, and later a solid removal system. Depending on the input waste (plastics tend to be high in hydrogen and carbon), gas from the plasma containment can be removed as syngas, and may be refined into various fuels at a later stage or fired on site to provide power.
Syngas is produced exclusively from organic materials with a conversion rate of greater than 99 per cent using plasma gasification.
Other inorganic materials in the waste stream that are not broken down but only go though a phase change (solid to liquid) add to the volume of slag with minimal energy recovery and increased cost for refining. For efficient operation of the plant, a portion of the syngas may be used to run on site turbines to power the plasma torches and feed system.
The basics of plasma technology are straightforward. A high-voltage current is passed between two electrodes to create a high-intensity arc, which in turn rips electrons from the air and converts the gas into plasma or a field of intense and radiant energy.
This is the process behind fluorescent and neon lighting where low voltage electricity passing between electrodes in a sealed glass tube containing an inert gas excites the electrons in the gas. The gas releases radiant energy and electric arc welding or cutting; this electricity passing between electrodes creates plasma that can melt metal.
There are five distinct categories of processes used as the basis for the plasma systems catering for waste management:
* Plasma pyrolysis
* Plasma combustion (also called plasma incineration or plasma oxidation).
* Plasma vitrification
* Plasma gasification in two different variants
* Plasma polishing using plasma to clean off-gases
Plasma gasification is the most common plasma process. It is an advanced gasification process, which is performed in an oxygen-starved environment to decompose organic solid waste into its basic molecular structure. Plasma gasification does not combust the waste as incinerators do. It converts the organic waste into a fuel gas that still contains all the chemical and heat energy from the waste. Also, it converts the inorganic waste into an inert vitrified glass.
Mixed solid waste is shredded and fed into a reactor where an electric discharge similar to a lightning (the plasma) converts the organic fraction into synthesis gas and the inorganic fraction into molten slag. Typically temperatures are greater than 7,000∞F achieving complete conversion of carbon-based materials, including tars, oils, and char, to syngas composed primarily of H2 and CO, while the inorganic materials are converted to a solid, vitreous slag.
The syngas can be utilized in boilers, gas turbines, or internal combustion engines to generate electricity while the slag is inert and can be used as gravel.
Gasification is a process that has actually been used for many years and involves converting complex organic molecules and carbon, in both the liquid and solid state, to simple gases. Most of the gases produced are flammable and are therefore used as fuel in processes or applications where flammable gases are required. The conversion of solids and liquids to gas is usually accomplished by heating the solid, or liquid, in either the presence of very small amounts of air or no air at all. When no air is used, the process is called pyrolysis or destructive distillation.
Before the introduction of natural gas into the United Kingdom in the early 1970s, all gas used in homes and industry came from the gasification of coal. Although gasification is really an old technology, until the invention of plasma gasification in 1995, it was a process that had its fair share of problems and drawbacks.
WHAT IS PLASMA?
Very simply, plasma is the term given to a gas that has become ionized. (An ionized gas is one where the atoms of the gas have lost one, or more, electrons and has become electrically charged.) Gases usually become ionized when heated to very high temperatures (>5000C∞), so plasma is usually a very hot substance.
Plasma is formed by passing an electrical discharge through a gas. Under normal circumstances, gases will not conduct electricity, but when a very high voltage is applied, the insulating properties begin to break down. As electricity starts to flow through the gas, it heats up and it begins to conduct more. Eventually, it becomes so hot plasma is formed.
Well known examples of plasmas are the Sun and lightning. However, these are examples of natural and uncontrolled plasmas. Man made controlled plasmas have been used in industry for many years and for a number of diverse applications, such as chemical analysis and the cutting of metals.
First, garbage is fed into an auger, a machine which shreds it into smaller pieces. These are then fed into a plasma chamber ñ a sealed, stainless steel vessel filled with either nitrogen or ordinary air. A 650-volt electrical current is passed between two electrodes; these rips electrons form the air and create plasma.
A constant flow of electricity through the plasma maintains a field of extremely intense energy powerful enough to disintegrate the shredded garbage into its component elements. The byproducts are a glass-like substance used as raw materials for high-strength asphalt or household tiles and "syngas".
Syngas is a mixture of hydrogen and carbon monoxide and it can be converted into fuels such as hydrogen, natural gas or ethanol. Syngas (which leaves the converter at a temperature of around 2,200 degrees Fahrenheit) is fed into a cooling system which generates steam. This steam is used to drive turbines, which produce electricity - part of which is used to power the converter, while the rest can be used for the plant's heating or electrical needs, or sold back to the utility grid.
Therefore, aside from the initial power supply from the community's electrical grid, the whole machine can produce the electricity it needs for operations. It also produces materials that can be sold for commercial use so, at some point, the plasma gasification system will generate profit for its users.
In plasma gasification, fuel or waste is fed to a reactor vessel where electrically generated plasma at a temperature of 2,000 C∞ is present. When the fuel or waste is exposed to the plasma it is heated to a very high temperature (>2,000C∞), which causes the organic compounds in the fuel or waste to dissociate into very simple molecules such as hydrogen, carbon monoxide, carbon dioxide, water vapor and methane. These simple molecules, that are all gases, are allowed to continuously flow from the reactor to gas cooling and cleaning equipment. Ash and other inorganic material present in the fuel or waste is melted down to a complex liquid silicate that flows to the bottom of the reaction vessel.
Metals that are present also melt and flow to the bottom of the reactor vessel, where they can either mix with the silicate, or if present in a large enough quantity, float on the bottom of it as a separate layer. The liquid melt is allowed to flow continuously from the vessel to a water quench where the liquid silicate melt is cooled to a non-leachable, non-toxic, obsidian like solid silicate. Some metals are not melted. Instead, they vaporize and pass out of the reactor vessel with the gases formed by the organic material. When they enter the cooling equipment for the gases, they condense to fine metal particulates. Halogen and sulphur compounds present in the fuel are converted to hydrogen halides and hydrogen sulphide, and pass out of the reactor with the other gases.
The gas from the reactor has a low to medium calorific value, and is therefore suitable as the fuel for a gas fired power generation unit. However, after leaving the reactor, the gas is still contaminated with a number of undesirable compounds, such as hydrogen chloride and metal particulates, that can cause damage to machinery and the environment. The gas is therefore cleaned up in various processing equipment. The cleaned gas, similar in quality to natural gas, is then fed to a compressor and storage facility ready for use. The most typical use of the gas is as fuel for power generation, although it can also be used as a feedstock for chemical processes.
PLASMA GASIFICATION FEATURES
* Plasma Gasification does not produce hazardous bottom ash and fly ash.
* Plasma Gasification is "fueled" by the "free" waste, and is "powered" by electricity, and can be turned off with the flip of a switch.
* Plasma Gasification unit does not need to be brought up to temperature over 24/36 hours burning expensive fuel oil as does mass burn incineration.
* Plasma Gasification systems require very little maintenance and unlike traditional power plants, do not need to be shut down for weeks at a time for cleaning and maintenance while waste-streams back-up.
* Plasma Gasification is just as efficient in smaller-scale systems as large-scale systems.
* Plasma Gasification can provide a high degree of flexibility over the longer term and it can operate at less than 100 per cent of capacity so there is flexibility when waste-stream decline.
Plasma Gasification plants have a very high destruction efficiency. They are very robust; they can treat any waste with minimal or no pretreatment; and they produce a stable waste form. The arc melter uses carbon electrodes to strike an arc in a bath of molten slag. The consumable carbon electrodes are continuously inserted into the chamber, eliminating the need to shut down for electrode replacement or maintenance. The high temperatures produced by the arc convert the organic waste into light organics and primary elements.
Combustible gas is cleaned in the off-gas system and oxidized to CO2 and H2O in ceramic bed oxidizers. The potential for air pollution is low due to the use of electrical heating in the absence of free oxygen. The inorganic portion of the waste is retained in a stable, leach-resistant slag.
In "plasma torch" systems, an arc is struck between a copper electrode and either a bath of molten slag or another electrode of opposite polarity. As with "plasma arc" systems, plasma torch systems have very high destruction efficiency; they are very robust; and they can treat any waste or medium with minimal or no pretreatment. The inorganic portion of the waste is retained in a stable, leach-resistant slag. The air pollution control system is larger than for the plasma arc system, due to the need to stabilize torch gas.
The waste-to-energy technology produces the following three end-products from processing waste feed stocks:
* A clean synthetic gas (syngas) that is a valuable source of alternative energy;
* An inert vitrified glass that has excellent applications in the construction industry, including: Concrete aggregate, road bed/fill, sandblasting, and recovered metal alloys.
The use of lower heat value plasma gasification syngas as a fuel source for gas engines has been successfully demonstrated with syngases generated from various feedstocks, including the gasification of biomass. Other applications for the utilization of the plasma gasification syngas are as follows: separation of hydrogen from the syngas using state-of-the-art Pressure Swing Adsorption (PSA) technologies, which can provide an excellent source of hydrogen for use with fuel cells; using the syngas as a feedstock for the production of liquid fuels such as ethanol.
While studies have shown plasma gasification syngas to be as clean (perhaps cleaner burning) than natural gas, its heat value per unit (i.e. cubic foot) is lower therefore requiring a larger quantity of plasma gasification syngas to generate the same thermal output. The use of plasma gasification syngas for the production of energy has been successfully proven as viable and cost-effective. Today, syngas from a wide range of sources (including waste-to-energy) has been successfully employed to generate electricity utilizing gas turbine combined cycle systems as well as gas engines. As the cost of natural gas fluctuates the economic benefits associated with the use of waste-to-energy generated plasma gasification syngas will become much more valuable.
Due to differences in density, inorganic waste, which is liquefied, is easily separated into two layers: a metal and a glassy silicate layer. The metal layer can contain relatively pure amounts of iron, copper and aluminum. The glassy product can be used in a variety of commercial applications including concrete aggregate, insulation, roadbed construction, and even in decorative tiles.
The composition of the end-products varies with the composition of the waste being processed. For example, processing medical waste, with a relatively high percentage of paper and plastic (organics), will produce meaningful levels of plasma gasification syngas, and a lesser amount of recoverable metal and glass product. Conversely, processing industrial waste, such as ash from an incinerator or batteries, will produce lower amounts of plasma gasification syngas and relatively more vitrified product (containing metal oxides) and, depending on the nature of the reducing environment in the reactor, recovered metal alloys.
Plasma gasification uses an external heat source to gasify the waste. Almost all of the carbon is converted to fuel gas. Plasma gasification is the closest technology available to pure gasification. Because of the temperatures and drastic conditions involved all the tars, char and dioxins are broken down. The exit gas from the reactor is cleaner and there is no ash at the bottom of the reactor, while there are no byproducts that end up to landfills provided that there are available markets for the produced slag. On the other hand, the use of plasma gasification processes reduce methane emissions produced from the disposal to landfill sites while as a waste to energy treatment method, it enables the displacement of CO2 that would have been emitted had the electricity been generated from fossil fuels.
Carbon monoxide and hydrogen, both as synthesis gas and individually, are important precursors in industrial process. They are the smallest reactive units for synthesizing organic chemicals and play a decisive role in the manufacture of several large-scale organic chemicals. Furthermore, hydrogen in particular could become an important energy source in meeting the demand for heat, electricity and motor fuel for the transportation sector.
Furthermore, chemicals from syngas are very important; particularly ammonia is by far most important chemical made from synthesis gas and consumes five per cent of the world's natural and associated gas production. Even though it is not organic, it is used to make organic chemicals. Methanol is the second important chemical from syngas and is the basis for MTBE, formaldehyde, and acetic acid. It consumes about one per cent of natural and associated gas production.