SUBJECTS:
Wiithin the waste industry incineration is the controlled high temperature destruction of combustible material. It is usually done for 3 reasons:
During incineration the oxyegn in air is used to convert or oxidise wastes to simple gases and solids. The vast majority of incinerable wasts are organic in nature, ie. they contain carbon.
The solid products of incineration are referred to as ash, and this comprises metals, oxides and non-combustibles.
The key to effective incineration is control.
Control of factors necessary for effective incineration.
Effective incineration requires:
The control of these factors is essential.
Temperature. The higher the temperature the more effective the the incineration and the lower the possibility of any unburnt waste being released or hazardous by-product being formed. Temperatures of 900 - 1100°C will destruct hydrocarbon waste. Temperatures of 1100 to 1300°C are needed for chlorinated solvents and other wastes which are difficult to incinerate. Temps over 1200°C are needed for bond breakup. At temperatures below 900°C hazardous by products can be formed these include dioxins and dibenzofurans. At temperatures below 800°C incomplete combustion is likely to occur and soot formation will result.
Residence time. It is necessary to hold the waste at high temperatures for sufficient time to ensure destruction. The longer the material is held at high temperature the more likely it is to be destroyed. The residence time for gaseous substances is a minimum of 2 seconds. For solids the residence time could be minutes or even hours.
Turbulence enables waste and air to be well mixed. Turbulence should be maximized so that contact between the waste and the oxygen In air Is as high as possible.
Excess oxygen must be present to ensure that the oxidative processes predominate and the pyrolytic processes are minimized. Generally oxygen should be present at 50-100% higher concentration than is theoretically required to decompose the waste.
DRE for a particular organic compound is calculated from the following
DRE = Win - Wout x lOO%
Win
Where
Win = concentration of that compound in the waste feed x volumetric flow rate of stack gas.
Wout = concentration of that compound in stack gas x volumetric flow rate of stack gas
This procedure is used to assess the efficiency of high temperature incinerators and is often applied to the more difficult wastes such as PCBs - if these are being adequately destroyed then all other combustible materials in the feed will also be destroyed.
The latest high temperature incinerators achieve DREs of 99.9999% of PCBs.
Waste Reception
Waste Storage
Kiln Designs
a) Rotary Kiln
Multi waste capability - liquids, drummed solids, bulk solids. Long solids residence time. Continuous operation - loading and ash out. Flexible operation - variable rotation speed.
b)Multiple Hearth
Excellent for burn out of large objects - eg transformers and contaminated equipment. Long residence times for drummed wastes. Long gas phase residence times possible.
c) Moving Hearth Types
Continuous loading and ash out, so attractive br commercial applications. Used for Clinical waste - good control over agitation of high ash wastes. Cheaper to build than rotary kiln - lower intrinsic particulates emissions. Severe problems with early moving grate types.
d) Single Static Hearth
The most common type for small installations - often manual loading and ashing out - typical of small hospital incinerators. No agitation of waste, so easy to create pockets of unburned refuse -the "baked alaska" effect. Older units commonly have no emission control equipment, inadequate afterburner.
e) Fluidised Bed
Often used for sewage sludge also plastics and pharmaceutical industry wastes. Cylindrical vessel containing an inert bed of granular material which is fluidised by an air feed. Waste is pumped into or above the bed. Simple design and low capital costs. Good combustion efficiency. But operating costs relatively high and not suitable for irregular, bulky or tarry washes. Figure 1 shows a general diagram of a fluidised bed incinerator.
Figure 1: Fluidised Bed incinerator
Applicability of incinerator systems to waste type
The suitability of the different types of incinerator systems for different types of wastes is summarised in Table 2.
Table 2: Applicability of Systems to Waste Types
|
|
|
|
|
|
|
|
|
Solids |
|
|
|
|
|
|
|
granular homogeneous |
|
X |
X |
X |
X |
|
|
irregular bulky (pallets) |
|
X |
X |
|
|
|
|
low melting points (tars) |
X |
X |
X |
X |
X |
|
|
organics with fusible ash |
|
X |
X |
|
|
|
|
Gases |
|
|
|
|
|
|
|
organic vapour laden |
|
X |
X |
|
|
X |
|
Liquids |
|
|
|
|
|
|
|
aqueous contam with organics |
X |
X |
X |
|
|
X |
|
organic liq inc.halogenated |
X |
X |
X |
|
X |
X |
|
Solids/Liquids |
|
|
|
|
|
|
|
Waste with halogens |
X |
X |
X |
|
X |
X |
|
aqueous/organic sludges |
X |
X |
X |
X |
X |
X |
All Incinerators produce emissions, these fall into two basic categories.
Gas phase emissions from hazardous waste incinerators comprise carbon dioxide, water, excess air, oxides of nitrogen, sulphur and phosphorous and halogen acids.
The major problem from an atmospheric emissions viewpoint are the acidic gases: - Sulphur dioxide and hydrochloric acid. These are the products of incineration of sulphur containing and chlorinated compounds respectively. Both gases are water soluble and can thus be removed from the effluent gas stream with water sprays - a number of strategies exist to accomplish this. On dissolving these gases produce an acidic solution which needs to be neutralised normally prior to sewer discharge. This neutralisation step is usually made easier by using alkaline solutions in the water sprays. This has the further advantage of increasing the scrubbing efficiency of such a system.
Particulate emissions are very different in nature, they comprise solid particles of either unburnt feed materials or ash formed in the incinerator and can also be removed by a number of methods.
The cleaned gases are cool and do not possess the kinetic energy required to pass up the stack for atmospheric discharge, consequently an induced draught fan and/or reheat facilities are usually deployed.
Heat Recovery
Gases leave the afterburner at 1100 to 1 200C - there is considerable potential for heat recovery. Recovered heat can be used to reheat a wet plume, so making it invisible. Recovered heat can be used for power generation, partly reclaiming fuel costs. Corrosion is the big problem associated with heat recovery.
Monitoring and Control
Comprehensive plant condition monitoring covers temperature measurement at all critical points in the system, measurement of oxygen levels, and measurement of main discharge consent parameters, usually on a real time basis.
This is essential for:-
Using waste of one type or another to supply useful energy is a well established method of obtaining added value before final disposal. This will be especially important where final disposal options become more limited and in situations where environmental and economic costs (including collection and transport) of recycling are high and where the pratical optimum for materials recovery has been reached. For example, if 25% recycling of MSW is reached, that leaves some 75% , which in some cases, can be used for energy recovery.
The four main ways of energy recovery from waste are:
Of these, energy from landfills and sewage sludge digestion are the most important sources of energy from waste. For example, Luggage Point STW in Brisbane, and Roghan Rd Landfill in Brisbane where approximately 600m3 of gas per hour is being collected.
What are the advantages and disadvantages of energy recovery from incineration?
Incineration, even incineration for energy recovery is not popular in Australia. There are probably historical reasons for this, as there has always appeared to be plenty of land available for landfill, though interestingly there have been municipal waste disposal incinerators in some smaller cities. For example, Burley-Griffin, the architect responsible for designing Australia's capital Canberra designed and built incinerators in the early 1900's. Two of them to my knowledge have been converted into theatres for staging plays.
The real problem of incineration has been with the destruction of hazardous wastes, or as they are now known as scheduled wastes. These include PCBs, HCB, other chlorinated hydrocarbons and organochlorine pesticides. These are wastes which are organic, resist degradation, are toxic, and accumulate. Historically these wastes were exported to the Rechem incinerator in Pontypool, or disposed of by the floating incinerator Vulcanus. The Vulcanus last visited Australia in 1982 and the Minister for the Environment stopped the export of hazardous wastes in June 1994. Interestingly there is still the export of a potentially hazardous waste - photographical laboratory chemicals which go to Thailand where the silver is extracted.
With exports stopped the only thing to do was store them. Currently there are an estimated 100 000 tonnes of scheduled waste stored in Australia awaiting disposal. This includes 8000 tonnes of HCB at one site near Botany Bay. It does not include 45 000tonnes of soil contaminated with 100tonnes of HCB buried under a car park, nor does it include an unestimated but rumoured amount of soil in rural areas which were once cattle dips and contain DDT and arsenic - sometimes at quite high levels. The investigation level for arsenic in garden soil is 10mg/kg. We have some parcels of soil with over 150 000mg/kg. These are very few, but in some cases have residences built on them.
The government approach to the problem is quite typical - set up a taskforce. Such a taskforce was set up in 1987 and from then till 1992 they produced 3 reports. They also oversaw the establishment of a draft strategy and the development of national regulations for the management of scheduled wastes. In their initial work they favoured the establishment of a high temperature incinerator but this was rejected. Part of the problem of course is that the usual method of dealing with these wastes is by incineration and the NIMBY syndrome applied. The development of new technologies also went against the incineration option. (There was also the NIME and NITEY syndromes - not in my electorate and not in this electoral year.)
A new taskforce waste established in 1993 called a National Advisory Body (NAB for short, though they don't have any arrest powers). One of the key elements of their role has been consultation which they have done through a range of public formus. Attendance at these ranged from about 15 to 150. This group appears to be more efficient than the previous taskforce - they have produced 19 reports in 2 years. They have devised, and are still devising different strategies for different wastes. The main work to date has been on PCBs, with other groups looking at HCBs and pesticides. For PCBs they recommend landfilling of solid PCBs below 50mg/kg. Above that they should be destroyed, with strict provisions about approval of methods, restriction of gaseous and liquid emissions, and control over residues
Because incinerators are not to be used, this has led to the development of new technologies. Three which have been started in Australia - the Eco Logical hydrogenation process, base catalysed dechlorination (BCD), and plasma arc conversion. The Eco Logic process was developed in Canada, and a $2million (£1million) plant has been established in Western Australia. The process involves the treatment of gaseous or liquid wastes in a reactor at 850°C in a hydrogen atmosphere. The halogens are displaced producing a mixture of methane, hydrocarbons and hydrogen chloride. The HCl is removed by scrubbers and the hydrocarbons are burned with the heat used to preheat the waste stream.There first job was to treat 200tonnes of agricultural chemicals, mainly DDT. The capacity of the plant is about 30tonnes per day of high strength PCB and pesticide wastes. The plant can be moved from site to site so is useful for transporting to the wastes, thus eliminating the problems of transporting contaminated wastes.
The US EPA developed the BCD process and two plants in Australia are operating batch reactors with a capacity of about 2500tonnes per year. One unit is transportable with a setup time of about 5 hours. This procss uses a hydrogen donor catalyst and sodium hydroxide which when heated removes the halogens as simple salts with the remaining hydrocarbons to be used as fuel. A second reactor is used for treating organochlorine contaminated soils
The Plascon process was developed in Australia by the CSIRO and a private firm. The process an arc of ionised argon gas at 15,000°C which goes into a reaction zone at 5,000°C. The waste is injected into this and pyrolisis occurs within about 20 milliseconds splitting the compounds into atoms and ions. The stream of gas, now at about 2,000°C is rapidly quenchd to ambient temeratures to prevent recombination into complex molecules. The gas stream is scrubbed and the remainder, mainly CO and H2 is flared. Trials on a mixture of 34% chlorophenols, 47% phenoxies and 19% toluene resulted in very low stack gas emissions. The chlorophenols were about 1mg/m3 and dioxins/furans 0.006 - 0.009 ng/m3 - less than one tenth the limits enacted in Germany in 1990 for incinerator emissions of 0.1ng/m3. The plant cost about $900,000 or £450,000, and can handle about 1 tonne per day. Trails have been carried out on full strength PCBs, CFCs and similar compounds. The plant is portable, and a second plant has been completed.
Given that these plants operate at or near capacity, then a quick calculation shows that one plant each of the three systems will treat about 37tonnes of waste per day. Given Australia's stockpile of 100,000tonnes that means there is at least 7 years work ahead, but this does not include the contaminated soils, nor any new wastes being produced.
SUMMARY
Incineration of wastes is not used in Australia nearly as much as overseas. This is partly historical because of the large expanses of land (apparently) available for landfills. However, with increasing pressure on land and the potential for energy recovery it could b a developing technology in Australia. The development of a high temperature incinerator for disposal of intractable wastes is highly unlikely, espcially given the development of new technologies in Australia.
Information
Consult Cheremisinoff (1992) on Reserve in the Library.
Next Week
The Lab Session this week is a forum on regional waste management. Next weeks lecture looks at life cycle assessment, while the lab will be a field trip to measure beach litter.