Technology Type

Waste-to-Energy Plants

Incinerators are also called Waste-to-Energy (WtE) plants, or Energy-from-Waste plants (EfW).

Incineration Technologies

Incineration is the oldest technology used to process waste, consisting of several types of incinerators such as:

• Grate Incinerators
• Fluidized Bed Technologies
• Rotary Kiln Incinerators

They all involve directly combusting the MSW in an Oxygen-rich environment, typically at temperatures between 700°C and 1,350°C. An exhaust gas composed primarily of CO₂ and water is produced, which flows through a boiler to produce steam to drive a steam turbine generator, producing electricity. Inorganic materials in the MSW are converted to bottom ash and fly ash. These by-products must be disposed in controlled and well-operated landfills to prevent ground and surface water pollution. Although incineration does not eliminate the need for landfills, it does significantly reduce the amount being sent to landfills by about 90% by volume.^[1]

Stoker Type Incineration Plants / Thermal Treatment on Grate

Mass burn is the incineration on a grate of bulk waste that has not undergone sorting, classification or size reduction of any type. The main advantage is that garbage is fed as it comes (except that very large pieces more than around 1m and bulky items have to be shredded) and it is said to be the most efficient solution for handling household waste. Sometimes it is combined with some upfront selective collection according to requirements of the local municipalities. With population size increase and the development of large urban centres the trend is going towards larger mass burn capacity plants. Mass burn is obviously not suitable for smaller volumes where instead alternative technologies could be employed.^[2]
 

Figure 1 - Grate Incinerator Schematics^[3]

The waste is first dried on the grate and then burnt at a high temperature (850 to 950°C) accompanied with a supply of air. With a crane, the waste itself is emptied into an opening in the grate. The waste then moves towards the ash pit and it is then treated with water, cleaning the ash out. Air then flows through the waste, cooling the grate. Sometimes grates can also be cooled with water instead. Air gets blown through the boiler once more (but faster this time) to complete the burning of the flue gases to improve the mixing and excess of oxygen.^[4]

Fludized Bed Incinerators

Fluidized Bed Systems are the second most frequently employed WtE type of technology. Fluidized bed furnaces use an inert material like sand in which fuel is distributed. There are two kinds of fluidized bed furnaces: Bubbling Fluidized Bed (BFB) and Circulating Fluidized Bed (CFB). Between the two variants of fluidized bed combustion technology, CFB combustion is more widely used for power generation than BFB combustion. That is because the level of efficiency is higher, the use of large capacity and the amount of flue gas produced is lower. Stoichiometric air requirements for CFB (1.1 - 1.2) are lower than for BFBC (1.2 - 1.3), which also results in less flue gas produced in a CFB compared to a BFB. The main difference between CFBC and BFBC is that there is a circulation of the used bed material in a CFB, which is also using higher air velocity compared with a BFB.
 

Figure 2 - Fluidized Bed Furnaces

VALMET BFB Furnace	VALMET CFB Furnace

In a BFB furnace, air flow is in the range of 0.9-3.1 m/s, which makes the bed of fuel and inert material fluidized. All the available heat in the fuel can be utilized to maintain the combustion temperature. The sand remains as a one-meter deep bubbling layer at the bottom of the furnace. The hot sand effectively dries and volatilizes the fuel and the volatilized gases and fine fuel particles are then combusted above the bed by secondary air. The residual char and larger fuel particles are combusted inside the sand bed. Excess air is very low, with a dry flue gas oxygen level below 4%. The boiler efficiency depends on the fuel properties but is typically around 90%. BFB furnaces are suitable for a wide range of homogenised fuels with varying heating value and moisture content, such as bark, wood chips, sawdust, forest residue, peat, rice husk, recovered fuel, de-inking and water treatment sludge, and many other recycled products. The feedstock is fed through openings at the bottom or on the side of the FBF boiler that can process particle size sometimes up to 150 mm so that it is not suitable for unprocessed bulk refuse such as MSW.

In a CFB furnace air flow rate is in the range of 4.9-9.1 m/a. A cyclone is placed in the outlet which separates inert and exhaust gases and the inert is then recirculated in the furnace. One main limitation of CFB is that solid waste requires pre-treatment to separate metals to avoid damage to the boiler, to crush waste into smaller pieces of the waste before introduction in the furnace fuel to enable fluidization, which both add cost to the process compared with mass burn on a grate. The main advantage of the CFB Boiler is its fuel flexibility for combusting biomasses and fossil fuels in continuously varying proportions. Suitable fuels comprise fuels derived from agricultural waste streams, recovered fuels, various type of coals, solid petrochemical residues, scrap plastics, petcoke, asphaltene, solidified pitch, sludge or any combination of these.

Both technologies use sand for bed fluidization and the sand will need to be replenished or refilled, because the amount of sand will decrease during operation and it is also required to be refreshed to maintain the sand quality. More generally fluidized bed systems have high efficiency of processing and low residual values. Technology includes energy recovery and flue gas treatment with flue Gas output far below regulatory requirements.^[5,6]

Rotary Kiln Incinerators

Rotary kiln incineration is a two-stage process employed essentially for treating hazardous medical, biological and industrial waste, waste material in a drum can, waste material contaminated with trace amounts of PCB. Temperatures of more than 1,100°C allow to control full destruction of organic compounds, viruses and microbes, low oxygen intake guarantees low emission values.

Companies such as Mitsubishi, Steinmueller and Babcock, or Sumitomo Heavy Industries are promoting the rotary kiln stoker for various kind of industrial waste. Pre-gasification occurs in the rotary kiln where easy-ignition waste such as high calorific waste is gradually incinerated under low-oxygen conditions. The residue dropping from the kiln is post-combusted in the stoker where hard-ignition waste is completely combusted with the formation of pyrolysis gas that undergoes secondary combustion when rising in the in the furnace.

Figure 3 - Rotary Kiln Incinerator Schematics

Comparison of Incineration Technologies

Mitsubishi provides three types of solid WtE technologies with the following compared in Table 1.^[8]

Table 1 - Comparison of different types of Mitsubishi incinerator technologies^[8]

Incinerator Type	Stoker	Gasification & Ash Melting	Rotary Kiln Stoker
Schematics
Capacity per Unit	100-1,000 tonnes per day	100-200 tonnes per day	100-250 tonnes per day
Waste Type	MSW Industrial Waste Biomass	MSW	Industrial waste
Advantages	Construction costs Waste flexibility Proven records	Recycling of valuable metals and slag	Wide range of waste types is acceptable
Disadvantages	Ash generation	Construction costs	Industrial waste only (higher Lower Heating Value)

Incineration Plants Capacities

In 2013, Mass Burn on Grate accounts for 174 million annual tonnes, Fluidized Bed for 12 million annual tonnes, and other technologies (e.g., direct smelting, rotary kiln, etc.) for 2.9 million annual tonnes. Therefore, in terms of  waste combustion capacity, Mass Burn accounts for 92.1% of the global capacity, and Fluidized Bed for only 6.4%. It is noteworthy that  several of the main incineration technology providers only offer mass burn but no alternative incineration technologie.^[9]

By 2018, roughly 2,430 WtE plants are active worldwide, which constitute roughly 360 million tons of disposal capacity.^[10]

The minimum size, from an economic viewpoint, for a Waste-to-Energy plant is around 40,000 t/year. The largest plants have capacities of more than 1 million tonnes per year. Individual combustion lines can have capacities from around 2.5 to 50 tonnes per hour (20,000 to 400,000 tonnes per year, whereby the more typical range is 5 to 30 tonnes per hour (40,000 to 240,000 TPA). A Waste-to-Energy plant is expected to run for at least 8,000 hours per year, roughly 94% of the time.^[11]

The largest mass burn incinerator is the Laogang Renewable Resource Recycling Center, which first and second phases combined are able to burn 3 million tons of garbage per year.  The second phase of the center has eight incinerators that allow it to process about 6,000 tons of garbage daily, corresponding to a 250,000 tonnes capacity per incinerator.  The plant can generate up to 1.5 billion kilowatt hours of power.^[12]

Japan Steel Eng. claims to currently hold the record for largest operating plant per line at 864 tonnes per day, corresponding to about 290,000 tonnes per year, and with a maximum design capacity of 1,200 tonnes per day, which is also the largest in the world.^[13]

Gasification and Rotary Kiln ractors have significantly smaller waste treatment capacities reaching in the upper 10 tonnes per hour, or a maximum of about 80,000 tonnes per year.^[8,13]
 

References

1. Adapted from: Yuzhong Tan, 15^th Apr 2013, College of Engineering, University of California, Berkeley: Feasibility Study on Solid Waste to Energy - Technological Aspects.
2. CNIM personal communication.
3. Vermeulen, Isabel et al.: Sustainable waste processing in a grate furnace and in a fluidized bed incinerator: WtE, recycling and environmental concerns, WIT Transactions on State-of-the-art in Science and Engineering 84 (2014): 83-92.
4. Rachael Lew, 29th Nov 2022, BioEnergyConsult, Moving Grate Incineration: The Most Common WTE Technology.
5. biomassproject.com, accessed Jun 2020 (site no longer available).
6. JFE Engineering Corporation, personal communication.
7. Yang, S., Kong, Q., Zeng, D. et al., Simulation research of a counter-flow rotary kiln hazardous waste incineration system, Int J Coal Sci Technol 9, 60 (2022).
8. MHIEC Company and Waste to Energy Technology.
9. Olivier Morin, 7th Oct 2014, Technical and Environmental Comparison of Circulating Fluidized Bed (CFB) and Moving Grate Reactors, Columbia University, Earth Engineering Center.
10. Richard Ling, Mar 2019, Powering Our Future with Trash, Kleinman Center for Energy Policy.
11. ESWET website, accessed Apr 2020.
12. Xu Lingchao Hu Min Zhong Youyang, 29^th Jun 2019, Asia's biggest solid waste treatment base goes above and beyond, SHINE.
13. NIPPON STEEL & SUMIKIN ENGINEERING Group’s Waste to Energy System.