Transcript Soil - vscht.cz
Thermal treatment of waste
WASTE MANAGEMENT AND TECHNOLOGY Mecislav Kuras
Institute of Chemical Technology in Prague
Waste to Energy Incineration is the combustion
of waste in a controlled manner in order to destroy it or transform it into: - less hazardous - less bulky - more controllable constituents.
Incineration may be used to dispose of a wide range of waste streams including municipal solid waste (MSW), commercial, clinical and certain types of industrial waste.
Incineration is generally the second more frequently selected method of waste management after landfilling. Disposal is a major concern of incineration because landifill space is becoming scarce. Incineration of MSW with energy recovery can be viewed as an attractive alternative to landfilling in many situations.
Waste incineration brief history
The first incinerators were developed in the United Kingdom at the end of 19th century based on the need to manage waste in a sanitary fashion and at the same time to supply energy for industry. After some trials with coincineration of coal and waste, the first municipal solid waste incinerator was constructed in 1876 in Manchester.
After some years incineration spread to other countries: Hamburg, Germany in 1896, followed by Brussels, Stockholm, Copenhagen and Zurich in 1904. British technology was used for the first plants in other parts of Europe.
The basis of today’s
moving grate technology
was developed in the 1920s and 1930s by still existing companies as Martin and Von Roll. The company Babcock &Wilcox presented the combination of a
grate with rotary kiln
. Significant improvement in environmental performance occurs in 1970s when
electrostatic precipitator
removed the majority of the dust, and in the 1980s boosted by the application of air pollution control equipment.
Public awarness resulted in local legislation on emissions and in 1989 the first European Union directives on waste incineration were enforced, revised in 2000 with the application of the concept of best available techniques.
In the former Czechoslovakia the first waste incinerator was constructed in 1933 in Prague, beeing at that time one of the most modern plant in Europe. New big incinerators were installed in 1990s in Prague, Brno and Liberec with overal capacity approx. 600 000 t/year. Several new incinerators are planned to be constructed, two of them are in the period of approval.
Waste incineration statistics
Incinerated waste amounts by waste category in the EU-27 in 2008
(http://epp.eurostat.ec.europa.eu)
Waste incineration statistics
Development of incinerated waste amounts (excl. mineral wastes) by waste category from 2004 to 2008 http://epp.eurostat.ec.europa.eu
Municipal waste treated in 2009 by country and treatment category, sorted by percentage http://epp.eurostat.ec.europa.eu
The incineration proces
The typical (nominal) MSW heating value is generally in the range 10 to 12 MJ/kg and the allowable variation is 8.5 to 14.5 MJ/kg. This a very broad interval compared to systems designated for combustion of a single type of fuel such as coal or wood chips. Reason is very variable character of waste to be incinerated.
In the furnace, the overal result of the incineration proces is that combustible components react with oxygen of the combustion air, releasing significant amount of hot combustion gas. Furthermore the moisture content of the waste is evaporated in the initial stage of the incineration proces and the incombustible parts of the waste form solid residues (bottom ash, fly ash).
During incineration proces in the furnace, the solid constituents of the waste undergo a range of processes as a result of exposure to heat and contact with combustion air: - drying - gasification (formation of combustible gases) - ignition and combustion of gases - burnout of the solids.
Process and energy recovery
The combustion gases pass from the furnace to the afterburning chamber.
To ensure complete burnout the combustion gases must be retained at least 2 second in the afterburning chamber (within the EU 850 °C for municipal waste, 1100 °C for certain types of hazardous waste). No waste is fed into the incinerator before the required temperature has been reached.
The flue gas is cooled in boiler and the presurised water is heated and in high pressure boiler evaporated and steam may be superheated (above its saturated temperature). The purpose – to exploit its energy contents by expansion in steam turbine connected to power generator.
In a combined heat and power plant (co-generation system) typically 25% of steam’s energy is transformed into electrical power. The remaining energy is regaining by condensation of the steam from turbine in a heat exchanger generating hot water for heating purposes.
Waste as a fuel
Waste incineration plants are designated to treat waste with great variation in the composition of the incoming waste. This is the primary difference between waste incineration and other combustion systems, and it has large implication on the desing of the incineration plant.
The practical desing of incineration systems, however, limits the allowable variations of the waste composition. For the desing of a waste incineration plant, the best available data on the amount and composition of each waste type is needed and the effect of expected future changes in the waste management system should be taken into consideration, for example the introduction of source segregation or pretreatment.
The waste being led to the incineration plant often consists of several types of waste, such as household waste, commercial and institutional waste and some industrial waste. The different waste types received at the incinerator have significantly different characteristics.
Tanner ´s triangle for assessment of combustibility of waste
http://www.wtert.eu
Key variables in characterizing waste as fuel
-
moisture content
105 °C)
ash (inorganic)
550 °C) (W) (typically 15-35%, when drying at (A) content (typically 10-25% after ignition at -
combustible (organic)
solids (C) as the difference between the dry solids and the ash content (typically 40-65%)
Principles of waste incineration
Incineration can be viewed as the flame-initiated, high temperature air
oxidation of organic matter
. Incineration is currently practisised to some extend on municipal waste, medical waste and hazardous waste.
Incineration can only destroy the organic compounds, it cannot destroy inorganic (mineral) compounds – which end up as residual ash. Because waste must be oxidised nearly completely (99.99% destruction and removal capacity is required) a large excess of air is used to ensure the sufficient oxygen to do the job.
Emissions from waste incinerators
include unburned organic wastes, products of uncomplete combustion or by –products of combustion, heavy metals, acid gas, ash and others. Emissions of these pollutants can be controlled to very low rates by modern
air pollution control equipment
.
Incineration has several advantages as well as disadvantages when compared with other methods of waste treatment, so it is not always the preferred choice.
Waste incineration The specific benefits of incineration:
- A reduction in the volume and weight of waste especially of bulky solids with a high combustible content. Reduction achieved can be up to 90% of volume and 75% of weight of materials going to final landfill.
- Destruction of some wastes and detoxification of others to render them more suitable for final disposal, e.g. combustible carcinogens, pathologically contaminated materials, toxic organic compounds or biologically active materials that could affect sewage treatment work.
- Destruction of organic components of biodegradable wastes which when landfilled directly generates landfill gas (LFG). - The recovery of energy from organic wastes with sufficient calorific value.
- Replacement of fossil fuels for energy generation with consequent beneficial impact in terms of the „greenhouse“ effect.
Solid residues
The main part of the ash content of the waste leaves the furnace as a solid residue i.e. bottom ash or slag. The remaining ash leaves the furnace as fly ash. The fly ash is normally separated from the flue gas in the flue gas treatment system in an electrostatic precipitator or bag house filter.
There are three types of incinerators:
- moving grate incinerator – mostly for municipal waste - rotary kiln incinerator – for industrial waste (liquid, solid and sludge) - fluidised bed incinerator – solid particles mixed with fuel are fluidised by air In the case of grate incinerator, the bottom ash (slag) drops from the end of the grate into the water trap of the slag pusher. The amount of slag is usually 10-20% by weight of the waste feed, depending on the water composition. Fly ash constitute usually 5-10% of the ash content.
Possible designs of moving grate systems
http://www.wtert.eu
Moving grate incineration
The convential mass burning incinerator based on a moving grate consists of layered burning of the waste on the grate that transport the waste through the furnace. On the grate the waste is dried and then burn at the high temperature while air is supplied. The ash (including noncombustibile waste fractions) leave the grate via the ash chute as slag (bottom ash). The main advantages of the moving grate are that it is well proven technology, can accomodate large variations in waste composition and in heat values and can be built in the very large units (up to 50 t/h). The main disadvantage is the investment and maintenance cost which are relatively high.
The bottom ash (slag) drops from the end of the grate into the water trap of the slag pusher than cooled by contact with cooling water and pass to the conveyor system. The amount of slag is usually 10 - 25 % by weight of the waste feed.
Fluidised bed incinerator
T. Christensen: Solid Waste Technology and Management
Fluidised bed incineration
Fluidised bed incineration is based on a principle where solid particles mixed with the fuel are fluidised by air. By fluidisation the fuel and solids are suspended in an upward air stream, thereby behaving
like a fluid
. The reactor usually consists of a vertical refractory lined steel vessel containing a bed of granural material such as silica sand, limestone or a ceramic material. The fluidisation of the bed is ensured by air injection through a large number of nozzles in the bottom of the incinerator. This causses a vigorous agitation of the bed material in which the incineration of waste takers place in close contact with the bed material and combustion air. This allows for relatively low excess air level, thereby allowing for a high thermal efficiency, up to 90 %. The fluidised bed incinerator is primarily used for homogenous waste typ including liquid waste.
Rotary kiln incinerator
T. Christensen: Solid Waste Technology and Management
Rotary kiln incineration
The mass burning incinerator based on a rotary kiln consists of a layered burning of the waste in a rotary cilinder. The material is transported through the furnace by the rotations of the inclined cylinder. The rotary kiln is usually refractory lined. The diameter of the cylinder may be 1 - 5 m and the lenght 8 - 20 m. The capacity may be as low as 2.4 t/day and is limited to a maximum of approximately 480 t/day. The kiln rotates with a speed of typically 3-5 rotations/h.
The excess air ratio is well above that of the moving grate incinerator and the fluidised bed. Consequently, the energy efficiency is slightly lower and may not exceed 80 %. As the retention time of the flue gas usually is too short for complete reaction to take place in the rotary kiln itself, the cylinder is followed by an after burning chamber, which may be incorporated in the first part of the boiler.
The energy recovery system of a waste fired in combined heat and power plant
T. Christensen: Solid Waste Technology and Management
Energy conversion technology
The energy recovery from a steam producing boiler is known from conventional power plant technology as the Rankine process. The Rankine process allows for energy output in the form of power, steam and various combinations of power, steam and hot water. The energy from the hot flue gases is recovered via the boiler and passed in the internal circuit of steam. The steam energy may be converted to power by menas of a turbine/generator set. The superheated and high-pressured steam from the boiler expands via the steam trurbine and the energy content of the steam is hereby transformed to kinematic (rotation ) energy, which is further transform to electrical energy by the generator. The excess heat of the low pressure steam is via the heat exchanger (condenser) converted to hot water and passed to district heating network or cooled away.
When producing electric power only it is possible to convert an output up to 35 % of the available energy in the waste to power. When producing a combination of heat and power so called
co-generation
, it is possible to utilise more then 90 % of the energy in the waste (27 % electricity output, 60 - 65 % heat output).
Disadvantages of waste incineration
- High capital investments requires longer payback period than final disposal to landfill.
- Because of high capital costs, the incinerator must be tied to long term waste disposal contracts.
-The incinerator is designed on the basis of certain calorific value for the waste. Removal of materials such as paper or plastics for recycling and resource recovery reduce the overal calorific value of the waste and consequently affect incinerator performance - The incineration proces still produce a solid waste residue that requires management and final disposal
T. Christensen: Solid Waste Technology and Management
Current development in waste incineration
In the last years the flue gas cleaning has been improved. The current prioriry is the optimisation of the thermal process to: - increase the energy efficiency - reduce the flue gas flow - minimize the development of hazardous substances like dioxins, CO and NO x - minimize corrosion - improve the ash management
Emmissions from waste incinerator
The most important compounds of emissions from incinerator are:
- acidic gases – hydrochloric acid (HCl), hydrofluoric acid (HF), sulphuric acid (H 2 SO 4 ) - particulates - oxides of nitrogen (NO x ) – formed from waste containing nitrogen compounds or mostly by high temperature fixation of nitrogen in the combustion air. Formation of thermal NO x depends on oxygen availability and the temperature, pressure and residence time of gas in the combustion unit - organic compounds such as dioxins and furans - carbon dioxide – not considered as pollutant, however, contributing to the formation of greenhouse effect
Emmissions from waste incinerator Technologies for their removal
Particulates – electrostatic precipitators, fabric filter (general efficiency more than 99%) Acidic gases – neutralisation with Ca(OH) 2 scrubers (wet, semi-dry, dry) or NaOH in Oxides of nitrogen – catalytic or non-catalytic reduction with ammonia or urea resulting in the transformation of NO x to N 2 .
Dioxins and furans – sorption on activated carbon or decomposition by special catalysts simultaneously with NO x removal.
Controlling emissions to atmosphere
Continuous emissions measurement made on the flue gas at the stack: - particulates – measured directly the amount of light reflected by the particulates (Tyndall effect) - carbon monoxide - hydrogen chloride - sulfur dioxide - nitrogen dioxide - oxygen content
Removal of contaminants NO
x
reduction
NO x formation
- by oxidation of nitrogen in waste - by high temperature fixation of nitrogen in combustion air (depends on oxygen availability, temperature, pressure and residence time of gas in combustion unit)
NO x removal
By catalytic or non-catalytic reduction with ammonia or urea.
Dioxins and furans
Precursors – products of incomplete combustion Removal – from the gas stream by scrubing the gases and by injection of activated carbon into the gas stream.
New approach – catalytic decomposition together with NO x reduction.
Removal of contaminants Acidic gases (HCl, HF, H
2
SO
4
)
Formation – by combustion of materials containing these elements Removal – by scrubing and subsequent reaction with bases (Ca(OH) 2 NaOH. or
Particulates
Removal technology depends on the particle size distribution and the removal efficiency required - fabric filters (baghouse) - electrostatic precipitators
Organic micropollutant emissions from waste incinerator
There is no evidence that incineration with proper flue gas purification is the cause of environmental and health damages, but nevertheless it remains an unpopular and controversional waste management option.
The main concern – polychlorinated dibenzodioxins and dibenzofurans.
Routes by which organic micropollutants can be formed and emitted from incineration processes: 1.
As a result of incomplete combustion of organic wastes present in the original waste
. If PCB is subjected to a destruction with removal efficiency of 99.9999% than the uncombusted fraction comprising 0.00001% (1 mg for every kilogramme incinerated) will be emitted to the atmosphere.
2.
As a result of the synthesis of „new compounds“ in the combustion and post combustion zone of incinerator.
Formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs)
T. Christensen: Solid Waste Technology and Management
T. Christensen: Solid Waste Technology and Management
Alternative thermal processes
- Pyrolysis - Gasification - Hydrogenation and hydrolysis Pyrolysis represents the thermal decomposition of organic molecules in gasification or destructive distillation.
absence
of gasification aids such as oxygen, air, CO 2 , steam, etc. In the temperature range between 150 – 900 o C volatile compounds are expelled and complex molecules are broken down into simpler ones. This process is also called low temperature Generated pyrolysis products are: - pyrolysis gas - pyrolysis coke - oil - tar.
Main product is a gas with heating value 12.5 to 46 MJ/Nm 3 .
The solid residue consists of pyrolysis coke containing varying amount of residual carbon that, unlike gasification, is not converted to gas in this process.
Gasification and pyrolysis
The process of gasification and pyrolysis, discovered at the outset of the XIX century, has only recently (20 – 30 years ago) been proposed for use in the treatment of wastes as an alternative to the „traditional“ thermovalorisation based on combustion process.
Gasification is the conversion of organic fraction of wastes or biomasses into a mixture of combustible gases by means of partial oxidation at high temperatures (400 – 1500 o C). The gas thus produced made up mainly of a mixture of CO and H 2 has calorific potential 4 – 6 MJ/Nm 3 and may be used to fuel internal combustion engines or gas turbines. In addition, the gas may be used as raw material for the manufacturing of chemical products (e.g. methanol).
Pyrolysis is endothermal transformation, in the absence of oxygen, of biomasses or liquid, solid or gaseous fractions of wastes. Pyrolysis can also be applied in the production of bio-oils with an efficacy reaching 80%.
Commercial application especially in Japan.
Waste pyrolysis
The technology of pyrolysis is a form of incineration that chemically decomposes organic materials by heat in the absence of oxygen. Pyrolysis typically occurs under pressure and at operating temperatures above 430 °C. During pyrolysis organic matter is transformed into gases, small quantities of liquid and a solid residue containing carbon and ash. Off-gases are generally treated in a secondary thermal oxidation unit.
There are several variation of pyrolysis system, including rotary kiln, rotary hearth furnace and fluidized-bed furnace. Unit designs are similar to incinerators except that they operate at lower temperature and with less airs supply.
Major applications of pyrolysis are in the treating and destruction of semivolatile organic compounds, fuels and pesticides in soil. Pyrolysis systems may be applied to a number of organic materials that crack or undergo a chemical decomposition in the presence of heat.
The technology is likely more economical on a small scale, such as in treatment of certain types of contaminated soils or hospital wastes.
Gasification
Gasification refers to the conversion of carbon-containing materials at high temperature into gaseous fuels.
Gasification is differentiated from pyrolysis by the addition of reactive gases, which further convert gaseous fuels carbonized residues into additional gaseous products.
Gasification is, strictly speaking, the continuation of pyrolysis proces, where the residual carbon (pyrolysis coke) is oxidized at temperatures above 800 °C with a sub-stoichiometric oxygen.
Steam, carbon dioxide, oxygen or air are often used as gasification agents. Just as pyrolysis, gasification is an independent process, but is still a part of combustion processes. The products generated in the gasification process are determined by the type of agent use, e.g. lean gas, water gas etc.
The necessary reaction energy for the gasification proces is generated by the partial combustion of organic materials in the reactor. Commercial aplication for waste treatment not fully developed.
Pyrolysis of organic materials generates several hundred different polycyclic aromatic hydrocarbons (PAH) but only small quantity of dioxins (PCDD) and furans (PCDF) because oxygen is necessary for these to form.
Plasma
Plasma gasification technology is a novel method for the treatment of wastes at high temperatures in which waste are converted into gas and an inert residue.
The term plasma refers to a conductive, electrically ionised gas.
Several gases such as argon, helium, methane or steam can be used for this purpose. In case of wastes the most commonly used gas is air. The air is rendered electrically conductive by subjecting it to marked differences of electric potential, generating a stable electric discharge (arc) between two electrodes. Resistance afforded by air versus the flow of electrons produces considerably quantities of thermal energy ranging from to 5000 to 10 000 °C.
Two main technologies: - plasma torch - system using graphite electrodes
Plasma technology
Plasma technology is known since 70 ´s. The high temperature and the plasma arch are able to melt almost everything in seconds. Even so a lot of research was done in the past the plasma technology never became a real option in waste management.
The reasons are quite clear:
- to establish a plasma arch and run a plant is very costly - the plasma arch allows only very small amounts to be melted, what makes big amounts of waste unsuitable to be treated by this technology - there are no long-term experiences with this technology The conclusion is that plasma technology is not the right technological option of the „everyday“ waste management.
Plasma technologies
Plasma processes are suited for treatment of a large variety of wastes having a high inorganic fraction and low heat potential. This is due to the fact that a large portion of the heat required for treatment is provided by the plasma and not by oxidation processes.
For the above reason and due to the high operational costs, plasma technologies are mainly applied in the treatment of hazardous or radioactive wastes.
A more extensive distribution of these plants will likely occur once their design and development has been rendered increasingly simple and economical. In the future the system may even constitute a promising alternative to the traditional systems of thermovalorisation, leading to the release of gas emissions with a lower pollutant potential and vitirified solid residue.
Refuse derived fuels (RDF)
RDF is a result of processing solid waste to separate the combustible fraction from the non-combustibles, such as metals, glass and cinder in municipal solid wastes (MSW). RDF is predominantly composed of paper, plastics, wood and kitchen and yard wastes and has a higher energy content than MSW, typically in the range 12 to 15 000 kJ/kg.
Like MSW, RDF can be burned to produce heat or/and electricity. RDF processing is often bound with the recovery of metals, glass and other recyclable materials, thereby improving on paybacks for investment costs.
At present time RDF combustion is less common than mass burning of MSW, but it may change in the future as recovery of recyclable materials and environmental concerns over incinerators emissions become more important.
The major benefints of RDF are
: - It can be shredded into uniformly sized particles or densified into briquets. Easily handled, RDF can be burned or co-fired with another fuel such as wood or coal in an existing facility.
- Fewer noncombustibles such as heavy metals are incinerated. The high temperature of MSW furnace can cause metals to partially volatize, resulting in release of toxic fumes and fly ash.