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]

Fludized Bed Incinerators

Fluidized Bed Systems are the second most frequently employed Waste-to-Energy (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 1 - 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^[2],[3].

Comparison of Incineration Technologies

Gasification and Rotary Kiln reactors have comparatively smaller waste treatment capacities reaching in the upper 10 tonnes per hour, or a maximum of about 80,000 tonnes per year^[4],[5] compared with stoker type incinerators as shown in Table 1^[4].

Table 1 - Comparison of different types of Mitsubishi incinerator technologies^[4].

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)

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. JFE Engineering Corporation, personal communication.
3. 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).
4. MHIEC Company and Waste to Energy Technology.
5. NIPPON STEEL & SUMIKIN ENGINEERING Group’s Waste to Energy System.