Mitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
1

State-of-the-art Stoker-type Waste Incinerator Begins
Commercial Operation
Mitsubishi Heavy Industries
Environmental & Chemical
Engineering Co., Ltd.
International Project Management
Unit
Business Unit
+81-45-227-1273+81-45-227-1286

In waste treatment, the priorities include achieving stable reduction of waste volume andlowering the environmental impact. Mitsubishi Heavy Industries Environmental & Chemical
Engineering Co., Ltd. (MHIEC) has developed a new stoker-type waste incinerator by improvingand sophisticating the conventional one under the concept of realizing stable volume reduction ofwaste with wide-ranging properties and contributing to lowering the environmental load. InJanuary 2021, our first delivery of the new unit was made to Xiaogan City, Hubei Province, China.
It has started commercial operation. This report summarizes this improved stoker-type waste
incinerator and presents how it has been operated thus far.

|1. Background for development
Figure 1 is a flow diagram of the major treatment processes at Waste-to-Energy (WtE)
plants (with a stoker-type incinerator). A stoker is a device used for combustion, and consists ofmovable grates (which have a grid-like structure and are used to agitate and convey waste). The
stoker-type waste incinerator operates by waste being thrown in by a waste crane onto a stoker andthen burned at high temperature. In MHIEC’s conventional stoker (Figure 2), each grate wasinstalled in such a way as to form an upslope against the direction of waste conveyance. The meritsof this grate installation method include achieving better agitation of waste and securing theresidence time necessary for complete combustion within a compact area. Having been developedby improving the conventional unit while making use of these advantages, our new stoker offers
the features of stable treatment of waste with high water content (as often in the case of overseaswaste) and the scalability for large treatment capacities. These features have made it possible forthe new stoker to be utilized not only in Japan but also across the world.

Figure 1

General flow diagram of the major treatment processes at WtE plants

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
2Figure 2

A diagram of MHIEC’s conventional stoker|2. Features of the improved stoker
2.1

Structure of the improved stoker

Figure 3 is a diagram of our improved stoker. It has a V-shaped structure formed by adownslope grate for accelerating the waste drying process, and an upslope grate with theabove-mentioned advantages.

Figure 3

2.2

A diagram of our improved stoker

High efficiency combustion enabled by the optimal stoker structure andincinerator shape

The merits of adopting a downslope for the drying process are:- Able to smoothly convey waste toward the combustion area.
- Easier to receive radiant heat from the flame, which is necessary for drying waste.
The merits of adopting an upslope for the combustion and post-combustion processes are:- Able to secure the residence time necessary for complete combustion within a compact
area.
- Able to efficiently agitate waste by pushing upward with the grate.
- Easier to receive radiant heat from the flame in the post-combustion area.
By optimizing the stoker structure and incinerator shape, our improved stoker has been
designed in such a way that the surface of the grates in all the processes of drying, combustion andpost-combustion is directed at the center of the flame. Therefore, it is most characterized by
efficient reception of radiant heat during the combustion of waste (Figure 4).

Figure 4 An image of receiving radiant heat with theimproved stoker structure and incinerator shapeMitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)3

2.3

Downsizing of stoker

Because of the addition of the downslope grate, the length of our improved stoker isincreased approximately by a factor of 1.5 compared with the conventional one. As shown in
Figure 5, however, since a stoker is installed in a space beneath the boiler, this increase in thelength causes almost no impact on the combined installation area of the boiler and stoker. On theother hand, if the width of the stoker can be reduced, that of the boiler can also be reduced because
their structures are integrated. The weight and volume of waste are decreased after it is carried on
the downslope grate with the accelerated drying effect, which lowers the carrying load of theupslope grate. As a result, the stoker can have a smaller width than the conventional one, thus
realizing the downsizing. When compared with the conventional unit, our improved stoker isgreater in length but smaller in width, and the grate area is nearly 0.8 times smaller. Because theincinerator chamber containing the stoker and boiler, whose occupancy rate in the WtE plant ishigh, can be downsized, the improvement in the freedom of facility layout for a small site isexpected.

Figure 5 Cross-sectional image of the facility
layout with our improved stoker
As the stoker is installed in a space beneath the boiler, its
increased length has almost no impact on the building.

2.4

Use of modular structure

Our improved stoker is flexible in terms of meeting the required throughput. Specifically, thefeeder, drying grate, and combustion/post-combustion grate are provided in separate modules.
Depending on the required throughput, the width of the stoker can be adjusted by increasing ordecreasing the number of modules to be aligned. The merits of this modular structure include
substantial reduction of on-site installation processes because of the assembly being carried out byattaching each module (Figure 6).

Figure 6

Flexibility in meeting the required throughput with the modular structureMitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)4|3. Plant summary
The plant summary is given in Table 1, and its location is shown in Figure 7. The plant is
the first WtE plant in Xiaogan City, Hubei Province, China. It has two incinerators, each with anominal capacity of 750 tons per day or tpd (1,500 tpd in total) and is equipped with a 35,000-kW
condensing extraction turbine. Since its start of commercial operation, the plant has supported
people’s lives as one of the important environmental infrastructures in the city. As indicated by theplan to build the facilities such as a sludge treatment facility in the neighborhood, this area
including this first WtE plant is envisioned as the future center of venous industry. In this project,
MHIEC is entrusted with the “provision of the stoker and the boiler basic plan”.
Table 1
Completion of construction
Type of incinerator
Nominal capacity
Gas cooling system
Power generation system
MHIEC’s scope of work range

Figure 7

Plant summary
February 2021Full continuous-type incinerator (stoker)
1,500 tpd (750 tpd × two units)*Required capacity: 900 tpd per unit
Waste heat boiler system (6.4 MPa, 450°C)
Condensing extraction- turbine (35,000 kW)
Stoker provision + Boiler basic plan

Plant location|4. Operating conditions
4.1

Treatment capacity and loss on ignition

Figure 8 shows the amount of waste treated during the test operation and the loss on ignitionof bottom ash (wet ash). Because the requirements specified an incinerator that is operable
continuously for 24 hours a day at 120% load, we designed a stoker-type incinerator with anincineration capacity of 900 tpd, although its nominal capacity was 750 tpd. As shown in Figure 8,
the loss on ignition(*) values averaged 1.6% for about three months including the days of treatment
exceeding the incinerator’s design capacity of 900 tpd. This indicates stable continuous operationof the incinerator and sufficient fulfilment of the guaranteed value of loss on ignition (which is lessthan 5% in this project). Low loss on ignition values mean a low amount of decaying organic
matter that causes soil pollution or water contamination at landfill sites, which can help to reduce
the environmental impact.
During the test operation, the lower heating value (LHV) of waste ranged from 6-7 MJ/kg
with the minimum value of about 5 MJ/kg. This indicates that the waste is relatively difficult to beburned, as it has a higher water content and a lower LHV than the typical LHV range of 8-10
MJ/kg in Japan.
*Loss on ignition: represents the weight ratio of the unburnt content in the dried bottom ash

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
5Figure 8

4.2

Amount of waste treated and loss on ignition (Incinerator No. 1)

Handling of low LHV waste (co-firing with the excavated waste)

In this plant, the waste excavated from the neighboring landfill site is co-fired; on some days,
it accounted for more than 30% of all the waste brought for incineration. The excavated waste has
been fermented to some extent, and is muddy with high water content and low LHV. Even when
co-fired with the excavated waste (Figures 9 and 10), the loss on ignition values remained low asshown in Figure 8. The incinerator’s capability of handling the waste with wide-ranging properties
has thus been confirmed.

Figure 9

4.3

Excavation at the landfill site

Figure 10

Excavated waste (in the waste pit)

Stability of exhaust-gas values

The stability of combustion can be indicated by the carbon monoxide (CO) concentration inexhaust gas. Table 2 shows the CO concentration for the five days when on average 900 tpd ormore of waste was treated. In this project, the guaranteed value of the daily average CO
concentration is 50 mg/m3N (i.e., 43 ppm). The daily average during the actual operation was 1-3ppm, with no CO-concentration peaks exceeding 100 ppm. Stable combustion has thus been
confirmed.
Table 24.4

CO concentration in exhaust gas

Date

Amount of waste treatedDec. 19
Dec. 20
Dec. 21
Dec. 22
Dec. 23909 tpd
873 tpd
930 tpd
881 tpd
937 tpd

CO concentration
24-hour average
Number of >100 ppm
1.78 ppm
02.10 ppm
01.89 ppm
02.28 ppm
01.83 ppm
0

Conveyance of large incombustible waste

In the countries with a less strict waste-sorting system than Japan, large incombustible waste
such as bicycles and large truck tires can be delivered together with household waste. In order torealize the worldwide application of our improved stoker, it is important for such waste to beMitsubishi Heavy Industries Technical Review Vol. 59 No. 2 (June 2022)
6

discharged without disrupting the operation of the incinerator, enabling waste treatment to becontinued in a stable manner. There was an incident during the test operation; a stainlesswastewater screen installed in the waste pit (with dimensions of width 1.2 m × length 1.2 m ×height 0.3 m, Figure 11) was detached and was, by mistake, thrown in the incinerator together withother waste. However, the wastewater screen was discharged without getting stuck on the upslopeof the combustion/post-combustion grate. During the test operation, at least two wastewater screens
were thrown in the incinerator. Both were discharged without getting stuck, thus demonstrating thecapability of operating the incinerator continuously.

Figure 11

4.5

Stainless wastewater screen

Number of operating days

Since its start of commercial operation in January 2021, the plant has been in operation as per
the customer’s incinerator operation plan. Table 3 shows the number of operating days for the past
1 year. During this period, the operation was stopped when the operation of the incinerator needed
to be adjusted in accordance with the amount of waste received. The stability of the equipment wasthus confirmed to be sufficient. During this past 1 year, 125 days of continuous operation wasachieved, which well exceeds 90 days of continuous operation required generally in Japan.
Table 3 Number of operating days
Incinerator No. 1
Incinerator No. 2

Number of operating days
282 out of 365
316 out of 365

Availability rate
77%
87%

|5. Future development
Having confirmed that our improved stoker can achieve stable complete combustion of low
LHV waste or overseas waste containing quantities of large incombustible waste, we will
accelerate the promotion of its overseas sale as well as promote the application in Japan plants bymaking use of the advantages of the downsized stoker such as greater freedom of facility layout. InJapan, low excess air combustion with exhaust gas recirculation (EGR) is the mainstream ofhigh-efficiency waste power generation. According to the computational fluid dynamics (CFD)
analysis, our improved stoker can achieve a low-excess-air and low-NOx combustion whencombined with EGR. The application in the actual unit can be done smoothly.
In the test operation, the on-site computer was connected to our computer at the main officethrough the virtual private network, which allowed us to remote-monitor the operating conditionsof the incinerator and collect the operational data. Therefore, we will explore the possibility oflinking with MHIEC’s AI-based remote monitoring and operational support system called
MaiDAS®.
MaiDAS® is a registered trademark of MHIEC in Japan.