Municipal solid waste: Incineration

previous THERMAL PROCESSING OF WASTE-J. C. JONES
2. Municipal solid waste
Part II: Incineration
2.1 Introduction

We saw in the previous chapter that MSW tends to have a low natural bulk density, and to incinerate a consignment of MSW is to convert it to an ash having about a tenth of the volume. The ash is abundant and harsh, having a strong propensity to adhesion to plant surfaces which soon corrode as a result. The ash from MSW incineration contains metallic elements which might be recoverable. The most important function of incineration is of course destruction of micro-organisms. Obviously incineration of MSW results in carbon dioxide release, but there is a counter argument to this. Such wastes as paper and cardboard if taken to a landfill instead of being incinerated start to release methane by decomposition after time of the order of years, and it is well known that methane is a much more powerful greenhouse gas than carbon dioxide.

More often than not, incineration of MSW will be set up so that some of the heat is put to use, for example in hot water supply and district heating. The term Waste-to-Energy (WTE) then applies. It will be usual for heat from the incinerator to cross a boundary at a heat exchanger, in which case one fluid ‘belongs’ to the incinerator operator and the other ‘belongs’ to the purchaser of the heat. Nothing with mass changes hands, and energy has been sold simply and solely as such. At larger facilities (e.g., the Detroit incinerator – see next section) there will sometimes be steam turbines for generation of electricity which can be sold on. When MSW is processed to make a saleable fuel perhaps in pelletised form, that is refuse-derived fuel (RDF).

This chapter will be concerned with incineration and with extension to WTE. RDF will feature in the third of the group of chapters on MSW.

2.2 Examples incinerators and analysis of their operation

2.2.1 Preamble

Our purpose in this chapter will be best served by detailed examination of some major MSW incineration facilities and selections will be from different parts of the world. A waste incinerator has not fulfilled its entire role once it has destroyed the waste: the post-combustion gases have to treated before release into the atmosphere, and it is this aspect of waste incineration which most often attracts criticism and objection from environmental groups. Accordingly for each incinerator we review both combustion performance and pollutants in the combustion products will be considered.

2.2.2 The Detroit incinerator

What is believed to be the largest waste incinerator in the world is in Detroit3. It has been in service since 1989 [1]([1] http://www.flickr.com/photos/tedguy49/269580555/). It does not belong to the City, having been leased by it from a private owner throughout its existence, and the question of how much longer these arrangements will continue is currently the subject of debate and lobbying. Of Dutch design, the incinerator processes between 2200 and 3000 US tons of waste per day.

The Detroit incinerator was conceived during the presidency of Gerald Ford. His predecessor President Richard M. Nixon, during whose second term in office the 1973 oil embargo took place, had emphasised the potential of city waste as a fuel for electricity generation. The money to build the Detroit facility was raised in the 1980s [2,3]([2] http://detroit1701.org/Detroit%20Incinerator.html, [3] http://www.modeldmedia.com/features/incinerator0809.aspx), and by the time it came into service in 1989 the oil supply-and-demand situation was quite different from that in 1973. A view that the raison d’etre of the Detroit incinerator was expired by the time it opened for business therefore has at least limited validity.

The incinerator provides electricity for 30000 households in Detroit. It used to provide steam for Detroit Thermal [4]([4] http://www.detroitthermal.com/), suppliers of heat to about 100 buildings in Detroit’s central business district. Detroit Thermal now use natural gas instead to raise steam. A simple calculation apropos of these figures is in the boxed area below.

Taking the mid range of the daily amount of waste processed to be 2500 tonne and taking the calorific value to be 10 MJ kg-1, the energy released in a day’s incineration is: 2500 × 103 kg × 107 J kg-1 = 2.5 × 1013 J If electricity generation is at 30% efficiency this becomes: 7.5 × 1012 J Now the average domestic daily electricity consumption in the US [5]([5] http://tonto.eia.doe.gov/ask/electricity_faqs.asp) is approximately 30 kW hours = 1 × 108 J 30000 homes will require: 108 × 3 × 104 J = 3 × 1012 J

Order-of magnitude agreement is pleasingly evident in the calculation. The gap between the calculated figures presumably represents in part the steam that was formerly bought by Detroit Thermal and will now be on the market.

On the pollution control front, the incinerator facility experienced major difficulties only about a year after it came into operation [6]([6] Toledo Blade newspaper, 5th May 1990.) when on account of the amounts of mercury it was releasing into the atmosphere it was closed down by the authorities for a period of days. Permission to resume was dependent upon a commitment to install improved pollution control plant. The facility produces about 1000 tonne per day of ash. Difficulties with the ash from MSW combustion have already been described.

2.2.3 The Tuas South Incineration Plant (TSIP), Singapore

Tuas is an industrial zone in western Singapore. The waste incinerator plant there is the largest of four such plants in Singapore and receives household and industrial waste. Constructed by Mitsubishi and commissioned ten years ago, its nameplate capacity is 3000 tonnes per day. This puts it in the same ‘league’ as the Detroit incinerator considered in the previous section. Electricity is generated [7]([7] http://app.mewr.gov.sg/web/Contents/Contents.aspx?Yr=2000&ContId=500) by means of a steam turbine using a Rankine cycle. This uses waste water from industrial processing, which is cleaned by membrane filtration before use. TSIP therefore does not draw on the potable water supply. The waste which the Tuas South facility receives is fairly low in calorific value, about 6 MJ kg^-1 [8]([8] http://www.tuaspower.com.sg/tuas.asp). A reader will be aware from Chapter 1 that MSW can be twice this in calorific value. Calculations similar to those in the previous section reveal that 3000 tonnes per day of waste of calorific value 6 MJ kg^-1 burnt to raise steam for entry to a Rankine cycle with 35% efficiency would generate electricity at approximately 75 MW. About a fifth of this is used at the facility and the remainder sold on. Electricity generation on a very much larger scale takes place at Tuas Power Station, a separate facility currently being expanded. This uses a variety of conventional fuels.

Two further points will be made in relation to TSIP. One is that corrosion in the boiler has at times been severe and this has been attributed [9]([9] http://eprints.usq.edu.au/535/) to hydrogen chloride arising from the burning of PVC in the fuel waste received. This of course is also a potential problem in relation to dioxins. The other point of interest is that, since metal components are not removed before admittance to the incinerator the solid residue contains both ash and ‘slag’, that is, metal possibly partly oxidised originating from the metal items such as cans in the waste. Iron in the slag is recovered with a magnet for recycling and the remainder disposed of with the ash. In the city state of Singapore space is at a premium, and the ash and slag from TSIP are in fact taken to an offshore landfill at Pulau Semakau. This also receives any MSW generated in Singapore not disposed of at one of the four incinerator plants.

2.2.4 The Gojogawa Incineration Plant, Nagoya Japan

A very detailed account of this plant is given in [10]([10] http://www.city.nagoya.jp/_res/usr/52635/16_Gojogawa_Incineration_Plant_City_of_Nagoya.pdf), and points can be gleaned which are of general interest. Japan relies almost entirely on imported fuel. She has no crude oil to speak of and although she has coal no longer mines it buying it instead from countries including Australia and Indonesia. One therefore expects that a waste incinerator which reliably produces electricity would be viable in Japan.

The Gojogawa plant, constructed over the period 1995 and 2004, is smaller than the incinerator plants discussed previously in this chapter. It receives 560 tonne per day of waste. Using the same figure for the calorific value of MSW which featured in the previous section it can be estimated that the plant will produce electricity at 12 kW. The actual value [10] is 14.5 MW.

Chemical analysis figures for the waste received at the facility under discussion are not available. However, dry MSW usually contains about 50% carbon and about 7% hydrogen. The calculation in the boxed area below develops this discussion.

560 tonne per day of waste as received equivalent to 400 tonne per day of dry waste Supply per hour = 16.5 tonne of which: 8.25 tonne carbon 1.2 tonne hydrogen moles carbon burnt per hour = 8.25 × 103/0.012 = 6.9 × 105 requiring an equivalent number of moles of oxygen. moles hydrogen (expressed as H2) burnt per hour = 1.2 × 103/0.002 = 6 × 105 requiring 3 × 105 moles of oxygen Total oxygen requirement per hour = 106 mol Total air requirement per hour = 4.76 × 106 mol Volume at 1 bar 298 K = 120000 m3 If say 25% excess air is used, volume of air per hour = 149000 m3

Now we are told in [10] that there were three draft fans at the incinerator, and that their combined capacity is 188000 m³(1 bar, 298K) per hour. The largest of the three provides for a variable delivery of air, the value having been incorporated into the combined figure being the maximum. It will therefore not be working at full capacity all the time, and there is likely to be a small degree of interdependence of the performance of the largest fan and those of the other two which are not themselves controllable. Having regard to such factors and also to approximations made in the composition of the waste, agreement to within about 20% of the specified and calculated air supply rates is a very good result.

Other features of interest at the Gojogawa incineration plant include removal of dioxins from the postcombustion gas by adsorption on to activated carbon. Sulphur dioxide, which of course forms an acidic solution with water, and hydrogen chloride are removed in the conventional way by neutralisation with lime.

2.2.5 Further examples

These are given in the table below. Comments follow the table.

Place	Details	Reference
Oahu, Hawaii.	≈1500 tonne per day of MSW processed, and 7% of the electricity for Oahu generated.	[11] http://www.stopwmx.org/kailua.html
Nantes, France.	Up to 500 tonne per day. Electricity and heat sold on.	[12] http://www.memagazine.org/backissues/membersonly/august99/features/trash/trash.html
Port Talbot, Wales	30 tonne per day of MSW disposed of.	[13] http://www.swanseafoe.org.uk/crymlyn-burrows-incinerator-part1.html
St Gallen Switzerland	≈ 125 tonne per day.	[14] Belevi H., Moench H. ‘Factors determining the element behaviour of MSW incinerators Part 1 Field Studies’ Environmental Science and Technology 34 2501-2506 (2000)
Stoke-on-Trent, England	≈ 500 tonne per day	[16] http://ukwin.org.uk/category/campaign-updates/staffordshire/
Berlin, Germany	1400 tonne per day	[17] Technical Bulletin, ABB AG Power Technology Systems, Mannheim, 2006.
Tokyo	The Shin Koto incineration plant, with a capacity of 1800 tonnes per day, is the largest in Tokyo	[18] www.info.gov.hk/gia/general/200909/14/P200909140245.htm

The facility at Oahu is believed to have a limited future. This is because landfill space for the ash is becoming used up. In France there are endeavours to dispel the idea that waste incineration is an unaesthetic or even sordid activity by introducing an artistic dimension. Part of the incinerator site at Nantes is given over to a display of modern sculpture. An incinerator close to Paris is illuminated after dark to give it visual impact, almost as if it were a cathedral! There have been difficulties with the Port Talbot incinerator (row 3 of the table), and the local authority which operates it has initiated legal proceedings against the firm which, under contract, built it. The figure for the St. Gallen facility represents only something like 2% of the MSW incinerated in Switzerland, where in 2000 its disposal at landfills ceased by law [15]([15] http://www.swissworld.org/en/environment/waste_management/incineration/). The Berlin facility is the primary MSW disposal route for that city, as in Germany since 2005 only incinerator residue can be land-filled, not untreated waste. The incinerator in Berlin predates by a few years perestroika and is in a state of obsolescence. Extensive upgrading and retrofitting are under way. The Shin Koto incineration plant (final row of the table) has recently been visited by officials from Hong Kong with a possible view to the building of one like it there.

2.3 Small-scale waste incinerators

Circumstances under which small-scale MSW incineration is required include remote communities and passenger shipping terminals. In the latter ‘household’ waste generated during a long voyage needs to be disposed of. As an example of the former, in Canada two separate settlements of indigenous people of the Cree race benefit from MSW incinerators built to meet their needs. These have capacities of respectively 3 and 8 tonne per day [19]([19] http://www.ecosolutions.com/). The same firm which supplied them has installed in the port of Belize an incinerator for waste from passenger ships. It can take a load of up to about 200 kg. Other situations where small incinerators for disposal of waste find application include military bases and mines.

2.4 Concluding remarks

In these days of concern on two fronts – depletion of conventional fuels and build-up of carbon dioxide in the atmosphere – incineration of MSW is at first consideration attractive. That it is ‘renewable’ nobody would deny and that it is largely carbon-neutral was shown in the previous chapter. In the 1880s, when oil and coal in the US were both very much growth industries, there was interest in ‘energy from waste’ and implementation of the idea in NYC as already noted. Yet at the present time whenever proposals to build an incinerator are made there is widespread opposition, as is the case in Leeds, England at the moment. One can be confident that in a country like the UK a newly commissioned incinerator facility will be state-of-the-art with all possible care and attention to emissions and to disposal of solid residue.

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