The challenge of reducing municipal solid waste (MSW) which has been filling landfills around the world for years may have yielded a new solution. Plasma gasification can process carbon-based waste into renewable energy and fuels and can do so without harming the environment.
This type of technology has been used for years to treat hazardous waste, turning it into non-hazardous slag.
There is the promise of a new type of biomass power plant. It will use a waste to energy conversion process using plasma gasification. We will examine both the technology and science surrounding plasma gasification as well as existing and planned projects that use the process.
Plasma gasification is a process that can convert waste to energy into usable products such as electricity, ethanol, vitrified glass and other usable products. It goes way beyond the traditional incinerator and is far cleaner than other gasification processes.
What Is Plasma Gasification?
This is a process that converts matter, either organic or non-organic, to its basic molecular structure. It performs the process in an oxygen-free environment. There is no combustion of the waste in the way an incinerator works.
Organic waste is converted into a synthesis gas (syngas) which contains the chemical and heat energy from the waste. Inorganic waste is converted into an inert vitrified glass slag.
The syngas consists primarily of hydrogen and carbon monoxide which are the basic building blocks of salable items. The syngas can also be used in gas turbines or reciprocating engines to produce electricity or it can be combusted to create steam to be used in a steam turbine-generator.
The Plasma Gasification Waste to Energy Process
- Waste is conveyed from a storage area to entry in the plasma cupola, entering via an airlock chamber.
- Plasma torches at the bottom of the chamber operate at temperatures of around 10,000 degrees F to convert the waste into either a fuel gas or liquid slag.
- The process takes place in an oxygen-deprived environment so there is no combustion. A synthetic fuel gas called syngas is created. The syngas leaves the cupola at the top while the slag runs out the bottom into a pool of water. The slag is converted into granular pieces that are completely inert and can be used as aggregate or shaped into paving bricks.
- The syngas leaves the cupola at about 2,000 degrees F. and immediately is sent to the heat recovery boiler. The temperature is reduced to around 400 degrees F. while making steam that is sent to the power plant to produce electrical energy in a steam turbine-drive electrical generator.
- When the syngas is cooled it is cleaned and scrubbed to remove contaminants. This makes the syngas ready to be used in the power plant.
- Compressors pull the syngas from the plasma cupola and compress it for use in a gas turbine. When the syngas has gone through the gas turbine and the electrical generator to produce electricity.
Plasma Gasification v Incineration
The process of gasification is far superior to incineration and results in an improved environmental impact and energy performance. The way in which incinerators operate is that they burn at high temperatures with the heat generated used to run a boiler and steam turbine so that electricity is produced.
Pollutants such as nitrous oxides and dioxins are produced through the incineration process that involves complex chemical reactions that bind oxygen to molecules. These pollutants are ejected through a smokestack and, unless exhaust scrubbers are in place to clean the gases, directly into the atmosphere.
Gasification takes place as a low-oxygen process which means that fewer oxides are formed. Scrubbers are used as part of the process of producing clean gas and form part of the essential gasification process. With the scrubbers in place the emissions levels are extremely low.
Another by-product of the incineration process is incinerator ash which is highly toxic. This ash is generally disposed of in landfills although one of the main uses for plasma torches in the past has been to melt toxic incinerator ash into a safe slag.
Non-organic materials that are treated in the gasification process is melted down to a glasslike slag. It is a safe material because it forms in a tightly bound molecular structure. This slag looks like the glassy stones displayed below:
Both gasification and incineration have the capability of producing electricity, however the gasification process is far more efficient. Whereas incineration uses the heat to power a steam turbine a gasification system uses gas turbines that can be configured in state-of-the-art power producing integrated gasification combined cycle mode.
How Plasma Gasification Works
Before the gasification process starts the waste must be processed to separate out the recyclable material from the rest. The reason for doing this is that recyclables such as plastics, paper and metals can be sold for a price higher than the fuel that would be produced.
A pre-treatment shreds solid waste into smaller uniform pieces so that it can be fed into the gasifier. The waste is passed into an airlock that prevents gases from escaping into the atmosphere. The plasma gasifier is an insulated air-tight container and the plasma torches are located at the bottom and they are used to create enough heat to gasify the waste feed.
The plasma arc is not applied directly to the waste material which gives the process the classification of plasma assisted gasification. The operating temperatures are significantly higher than a typical flame temperature that is associated with combustion.
The process takes place in an oxygen-starved environment so that a combustible syngas can be produced. This is produced rather than a non-combustible flue gas which is produced during combustion.
Carbon based waste become volatized and turned into synthetic gas (syngas, which is a mixture of H2, CO and CO2). The inorganic materials are vitrified and melted down to be turned into a form of slag which is an obsidian like substance.
When the syngas leaves the gasifier chamber it is put through a series of filtration systems and a water cooling process. All particulate matter is filtered out to clean the gas. The water-cooling process prevents the formation of pollutants such as dioxins and furans.
Following this purifying process the gas goes through a series of scrubbers to remove acids, chlorides, fluorides, sulphates, phosphates, sodium and calcium.
A turbine connected to the process can be used to generate electricity. This electricity may either be used to power the plant or can be sent out as a clean source of renewable energy for use by the public.
More information about the process of plasma gasification can be found on the Phoenix Energy website.
Benefits of Plasma Gasification
Plasma gasification offers a number of important benefits:
- The greatest amount of energy from waste is unlocked.
- Mixing feedstock such as municipal solid waste (MSW), biomass, tires and hazardous waste is possible.
- Methane is not generated by the process.
- Unlike incineration, no leaching bottom ash or flying ash is produced.
- Reduction of landfill.
- Syngas is produced. The syngas may be used as fuel to produce energy or can be processed further to create chemicals, fertilizers or transportation fuel.
Lets break down each of these benefits and look at them a little closer.
Greatest amount of energy from waste is unlocked
Out of all the various types of waste to energy technologies that are in use, plasma gasification produces the most efficient conversion rates.
The high temperatures that are used in the gasifier break apart the chemical bonds of the feedstock and converts them to synthesis gas (syngas). The primary components of the syngas are hydrogen and carbon monoxide and these are the basic building blocks of fertilizers, chemicals, a form of natural gas and liquid biogas that might be used to power transport.
In addition to all of this the syngas can be used to power gas turbines to produce electricity or it can be combusted to produce steam that can be used in a steam turbine-generator.
Mixing Feedstock is possible
Although it is a more efficient process if the waste material is sorted with recyclables and non-organic materials removed first, it is possible to feed the gasifier with all types of waste together. This allows the operators to optimize the tipping fee because they can accept a wider range of feedstock.
Methane Is Not Generated
Methane is one of the most damaging greenhouse gases that are produced and is particularly prevalent as a by-product of landfill. The water-cooling and scrubbing process involved with plasma gasification eliminates the release of methane.
No Ash Creation
A toxic product that comes from incineration is ash, whether it’s bottom ash that leaches toxins into the ground or flying ash that spreads particulates in the atmosphere. The solid waste product that comes from the gasification process is the glass-like slag that is inert.
Reduction of Landfill
This is the most obvious benefit that comes out of the process. Large amounts of solid waste that would be dumped into the ground is no being converted down to its smallest possible components in the form of gas and slag where it will be reused. A landfill site could potentially be completely removed through the operation of a gasification plant.
Syngas Is Produced
Synthetic gas is just one of the usable by-products from a plasma gasification plant. It is a fuel source that can be used to power the facility full time or the gas can be sold to external parties. The creation of renewable energy electricity offers real benefits to the local community.
Not Everything Should Be Gasified
Although a plasma gasification plant has the ability to convert just about any material into something that has a downstream use, in economic terms it is more worthwhile to separate out recyclables. Plastic, metal and paper products can be extracted and sold. As an example, metals can be melted down to a slag using gasification but the process is less efficient and the metal itself has value as a recycled material.
One of the key steps in treating MSW properly before it goes into the gasifier is that it has first gone through a sorting and processing stage. This will separate out any recyclables which can be sold separately. The treatment process will then dry the waste and shred it into uniformly small pieces. Any inorganic materials that have no value as fuel may also be removed.
Plasma Gasification Cost
It has been estimated that a gasification facility with a capacity to consume 750 tons per day would cost around $US150 million. This would be an appropriate sized facility for a small city.
Costs could be recouped year on year through revenues from tipping fees, sale of recyclable material, electricity sales, sales of slag and sulphur.
A plasma gasification plant could get to the point where it is more cost efficient to take garbage to the plasma gasification plant where energy is produced than to simply dump it into a landfill.
Plasma Gasification Waste To Energy Facilities
Listed below are just a few of the plasma gasification facilities that are currently in operation around the world. It is estimated that there are around 300 facilities currently in operation in some form that uses plasma gasification to treat waste.
|Jonquiere, Canada||Aluminum dross||50||1991|
|Alpoca, W. Virginia||Ammunition||10||2003|
|Anniston, Alabama||Catalytic converters||24||1985|
|Honolulu, Hawaii||Medical waste||1||2001|
|Richland, Washington||Hazardous waste||4||2002|
|U.S. Navy.||Shipboard waste||7||2004|
|U.S. Army||Chemical agents||10||2004|
|Bordeaux, France||Ash from MSW||10||1998|
|Bergen, Norway||Tannery Waste||15||2001|
|Landskrona, Sweden||Fly Ash||200||1983|
|Nagpur, India||Hazardous Waste||72||2011|
|Pune, India||Hazardous Waste||72||2009|
|Kinura, Japan||MSW Ash||50||1995|
|Kakogawa, Japan||MSW Ash||30||2003|
|Mihama-Mikata, Japan||MSW/Sewage sludge||28||2002|
|Imizu, Japan||MSW Ash||12||2002|
|Maizuru, Japan||MSW Ash||6||2003|
|Iizuka, Japan||Industrial waste||10||2004|
Plasma gasification is continuing to grow both in the acceptance of the wider community that it is a process that has a positive effect on the environment and from a business aspect where it is a profitable solution. As well as the gasification plants that are already in operation there are also up to 70 plants in the process of being constructed.
The amount of waste that is being treated each day should be looked upon as material that is not being added to landfill. For this reason alone there is a lot to like about plasma gasification as a waste management method.
For more information about Plasma Gasification