Study on the effect of collaborative disposal of stale garbage by incineration plants —— Taking a garbage incineration plant in southwest China as an example
1. Introduction
In the past few decades, sending domestic waste to landfill was our main way to deal with domestic waste. However, the landfill method will occupy a lot of land resources, cause secondary pollution of surrounding groundwater and soil, and cause stench in the surrounding environment, which greatly reduces the ecological development value of surrounding areas. However, the way of incineration power generation is used to treat domestic garbage, with little pollution at one time and less than 20% residue after incineration. It can not only reduce the volume of garbage and save land, but also convert incineration heat energy into electric energy, resulting in good economic and ecological benefits. In recent years, incineration power generation has become the first choice for domestic garbage treatment in China.
By the end of 2021, China's municipal solid waste incineration capacity has accounted for 60% of the harmless disposal capacity. Although the waste incineration power generation projects that have been put into operation in China have increased rapidly in recent years, landfill, as a temporary emergency storage place for centralized disposal of waste incineration fly ash, residue and domestic waste during the overhaul of waste incineration plant, plays an irreplaceable role in realizing the final disposal of municipal domestic waste. Waste incineration plant and waste landfill plant are both necessary configurations for the infrastructure of harmless treatment of municipal solid waste. And in many urban areas, a solid waste treatment park model has been formed, in which waste incineration power plants and waste landfills coexist.
At present, many big cities in China are facing the problem of full storage capacity of landfill sites. The Development Plan of Urban Domestic Waste Classification and Treatment Facilities in the Tenth Five-Year Plan points out that in principle, primary landfill facilities will no longer be planned and built, and the remaining storage capacity of existing domestic waste landfill sites will be changed to the bottom to ensure the landfill facilities for backup. In fact, the storage capacity of many municipal solid waste landfills is close to saturation. If the stale garbage in landfills is not treated, there will be nowhere to fill the municipal solid waste when the garbage incineration residue is overhauled in the garbage incineration plant, and the city will face the dilemma of "garbage siege". It is a common problem faced by many cities to deal with the stale garbage in landfills, release a certain storage capacity and relieve the pressure of urban garbage disposal. It is an effective way to realize the resource utilization of the aged garbage in the saturated landfill and move the storage capacity of the landfill by secondary excavation and sending it to the garbage incineration power plant for incineration to generate electricity.
There are also some cases in the industry in which garbage incineration power plants that burn primary garbage are mixed with stale garbage in landfills, but most of them are mixed with stale garbage based on extensive operation experience. In this study, a waste incineration power plant in southwest China was used as the experimental research object to carry out the experiment of mixed burning of stale waste, and systematically studied the proportion of mixed burning and its influence on the operation effect of incineration system. It provides operation reference for similar waste incineration plants in the industry that burn stale landfill waste.
2. Materials and methods
2.1 experimental research object
This paper takes a garbage incineration power plant in southwest China as the research object. The waste incineration power plant is located near the landfill site to be closed, and a solid waste treatment park with incineration and landfill has been formed. The waste incineration power plant is designed to treat 1500t of municipal solid waste every day, and it is equipped with three 500t/d mechanical grate furnaces, three medium-temperature and medium-pressure waste heat boilers and two 15MW steam turbine-generator sets. The plant has been put into operation for several years, and the collaborative incineration of stale waste in the nearby landfill site was not considered in the design of the plant, so the relevant design parameters of the incineration system are designed with reference to the nature of primary waste in the area to be received.
The nearby landfill has been put into operation for nearly 20 years, and it is a valley landfill with a large depth and a storage capacity of nearly 2 million cubic meters. Due to the significant increase of domestic waste in the service area in recent years, the designed treatment capacity of the landfill has fallen far behind the actual demand for waste, and the storage capacity is currently saturated. Although the garbage incineration power plant that has been put into operation nearby undertakes the disposal function of domestic garbage in the service area, during the maintenance of the garbage incineration power plant, this landfill site needs to undertake the emergency protection of domestic garbage in the area. It is urgent to excavate, reduce and recycle the aged garbage in this landfill to make room for emergency landfill of primary garbage.
2.2 Comparison of the nature of primary garbage and stale garbage
Landfill is a natural bioreactor of domestic waste. With the time of landfill, the organic components in the waste are degraded, inorganic and humic, and gradually transformed into stale waste. Therefore, due to the difference of components, the properties of stale garbage are different from those of primary garbage, especially the properties of calorific value and ash. The design parameters of incineration system are based on the nature of primary garbage in the service area when the waste incineration power plant is built. If part of stale garbage is to be incinerated in the later stage of operation, it is necessary to compare the two kinds of garbage from the nature, so as to provide reference for the operation control of this waste incineration power plant. The waste sampling method adopts the quartering method in Sampling and Analysis Methods of Domestic Waste CJ/T313-2009. The primary garbage samples are taken from the garbage dump without fermentation in the garbage storage pit of the garbage incineration power plant. The samples of stale garbage are taken from the middle of the nearby landfill, and the samples here are at least several years old, so the samples are representative.
2.3 Study on synergistic incineration effect
Taking one incinerator production line of waste incineration power plant as the experimental research object, the experiment of mixed burning of stale garbage was carried out, and the fermentation effect, mixed burning ratio and mixed calorific value of stale garbage and primary garbage before entering the furnace were studied respectively, and the influence of mixed burning on boiler evaporation, furnace temperature and slag production was studied.
3. Results and discussion
3.1 Comparison of garbage properties
After the original garbage and stale garbage were fermented in the garbage storage pit for 7 days respectively, the moisture content of the original garbage decreased by about 10%, but it was still higher than the stale garbage. However, the moisture content of stale garbage increased slightly. The main reasons are as follows: after 7 days of fermentation, the organic matter in the primary garbage is decomposed and drained under the action of microorganisms, and the drained water is discharged through the multi-layer grid holes on the side wall of the garbage storage pit, thus achieving the purpose of reducing the moisture content of the primary garbage. However, the stale garbage has passed through the degradation environment in the landfill for many years before the factory, and the organic matter in it has been fully decomposed, and its moisture content has reached a stable state. The fermentation process will cause the problem of moisture absorption and moisture regain of the stale garbage.
It can be seen that the original garbage and stale garbage of man-made plants should be stacked in different areas in the garbage storage pit, and the stale garbage can be put into the furnace without fermentation. The stacking area is only the buffer stacking area before entering the furnace, and it should be in the middle area far away from the holes of the multi-layer grid on the side wall. The primary garbage dump area is located in the sifang district area of the multi-layer grid row holes near the side wall of the garbage storage pit, and it needs to leave enough space for fermentation for 7 days. Because the design of the waste incineration power plant in this study did not consider the co-incineration of landfill stale waste in the later stage of operation, there was no partition wall in the waste storage pit; Therefore, in the daily operation and management, the primary waste clinker after fermentation should be coded at the junction of the primary waste dumping area and the stale waste dumping area.
3.2 Blending proportion and mixing effect
The designed calorific value range of waste incineration power plant in this study is 4200~8500kJ/kg, and the calorific value of primary waste in the service area is within the design range, but the calorific value of stale waste in the nearby landfill is lower than the lower limit of the designed calorific value. If this stale garbage is mixed and burned on a large scale, in order to ensure that the exit flue gas of the incinerator meets the key environmental protection index that the residence time in the area above 850℃ is not less than 2s, auxiliary fuel is often needed to maintain the incineration condition, which will weaken the operation economy of the garbage incineration power plant. Therefore, it can only be mixed with primary garbage in a certain proportion. In order to determine the more economical blending ratio, the blending ratio% of stale garbage can be calculated by using the calorific value formula of mixed materials.
According to the past average calorific value of man-made furnace garbage in our factory, it is about 6900~7200kJ/kg, which is set to 7200kJ/kg, and the blending ratio of stale garbage is not more than 20%. Because of the difference in moisture content and calorific value between stale garbage and original garbage after fermentation, and the calorific value of original garbage is further increased by 51 after 3~7 days of fermentation, it is necessary to use garbage grab to mix and mix two different materials according to the above mixing and burning ratio before the furnace, so as to make the calorific value of the materials into the furnace uniform. In the mixing operation in front of the furnace, the original garbage with relatively high water content and calorific value should be stacked at the bottom, and the stale garbage should be stacked at the top. After being stirred evenly for a short time, it should be put into the furnace.
3.3 Analysis on the influence of furnace temperature and boiler evaporation after entering the furnace
The waste incineration power plant in this study adopts three grate boilers, and the design capacity of a single boiler is 500t/d, the superheated steam parameters are 4.0MPa and 400℃, and the rated evaporation capacity of the waste heat boiler is 43.4 t/h. In order to study the influence of mixed burning on the working condition of incineration system, it is necessary to compare the furnace temperature and boiler evaporation after mixing burning stale garbage at the proportion of 15%~20%. Taking 1# boiler (test furnace) as the experimental research object, the experiment of mixed burning of stale garbage was carried out, and the operating conditions of 2# boiler (control furnace) without mixed burning of stale garbage were compared.
In order to ensure the reliability of the test comparison data, during the test observation period, the artificial furnace feed rate of No.1 boiler and No.2 boiler is 500 5t/d, and no concentrated solution is re-injected. Collecting furnace temperature data, wherein the furnace temperature measuring point selects the measuring point at the intersection of the lower part of the first flue and the secondary air spraying layer, and the flue gas parameters in this temperature measuring area tend to be uniform and stable after being stirred and mixed by the secondary air. The test period is 7 days, during which 8 time points are randomly selected for sinking. See Figure 4 and Figure 5 for the comparison data of furnace temperature and evaporation of waste heat boiler under various working conditions.
When the aged garbage is mixed with 15%~20%, it has little effect on the furnace temperature, and the furnace temperature of the incinerator can stably ensure that the outlet flue gas temperature stays above 850℃. Time is not less than 2s. Even if the calorific value of the overall artificial furnace material is relatively reduced after burning stale garbage, the proper furnace temperature can be maintained by optimizing and adjusting the combustion air during the operation of the incineration system. After the aged garbage is mixed with 15%~20%, the boiler evaporation is slightly reduced in six of the eight comparative working conditions, but it is also higher than the rated evaporation of the design waste heat boiler by 43.4t/h due to the relative reduction of the overall calorific value of the materials. Overall, it has little influence on the operating conditions and thermal economy of incineration system.
3.4 Analysis of influence on slag quantity
During the 7-day test, the average slag yield of No.1 furnace (test furnace) was 17.6%, and that of No.2 furnace (control furnace) was 16.8%. The average slag yield of 1# furnace (test furnace) is slightly higher than that of 2# furnace. The reason may be that the high proportion of incombustible residue in mixed burning stale garbage is related to the initial garbage composition. However, the overall slag yield is 19% lower than the design slag yield of the waste incineration power plant in this study.
4. Conclusion
(1) For the waste incineration power plant designed to incinerate the primary waste, if the aged waste is stored in different areas for a short time in front of the artificial furnace and moisture regain is avoided, the mixture is Cvenly mixed in front of the artificial furnace according to proper operation, and the proportion of aged waste mixed combustion is not more than 20%, which can ensure the uniform heat value of the artificial furnace materials and not affect the operating conditions of the incineration system.
(2) Optimizing the control of combustion-supporting air in the incineration process can ensure that the furnace temperature is stable before and after burning stale garbage and meet the environmental protection requirements.
(3) Although the evaporation of waste heat boiler is slightly reduced and the slag yield is slightly increased after mixing with stale garbage, it has little influence on the operation effect of incineration system as a whole.
(4) Excavating the aged garbage in landfill with saturated storage capacity and sending it to the garbage incineration power plant for co-incineration with primary garbage is an effective way to realize the resource treatment of aged garbage and vacate the storage capacity of landfill, which can realize the economic, environmental and social benefits of aged garbage treatment in landfill.
Source：https://mp.weixin.qq.com/s/GqycLjVrRoMA3qaYC_POfg