Research | Burning plastic is over. Microplastics laugh: you are so naive!

Microplastics (Microplastics, maximum diameter ≤ 5mm) are everywhere, and their potential toxicity is attracting more and more attention all over the world. At present, there are two main ways to reduce microplastic pollution, one is to reduce plastic consumption and the production of plastic waste at the source, and the other is to strengthen the recovery and disposal of plastic waste. However, both roads are fraught with difficulties. As a way of disposal, waste incineration and energy recovery account for a large proportion of solid waste treatment systems in both developed and developing countries.

It is widely believed that incineration can permanently eliminate plastic waste and eventually convert polymers into carbon dioxide and inorganic fragments. But new research shows that there is still unburned material in the slag produced by incineration and may be re-flowed into the environment through reuse (brickmaking, paving, etc.) or dumping.

refuse burning

In order to verify whether incineration can completely eliminate microplastics and evaluate the amount of microplastics transported by slag to the environment, in the study "incineration is the Terminator of Plastics and Microplastics" (Is incineration the terminator of plastics and microplastics) published in the journal Journal of Hazardous Materials in 2021, a number of Chinese scholars extracted and identified microplastics in MSW incineration slag for the first time.

Journal of Hazardous Materials periodical

The slags from 16 municipal solid waste incineration plants and 1 slag treatment center from 8 different cities in China were collected. Of the 16 waste incinerators, 12 use grate furnaces and 4 use fluidized beds. Of the 17 sampling sites, 7 have implemented source garbage separation in the areas served, and 10 have not yet been implemented. The garbage entering the incineration plant is mainly domestic waste, and some industrial waste, commercial waste, construction and demolition waste are mixed into packaging, plastic pipes, waterproof materials and wallpaper. The daily treatment capacity of grate furnace is 3308,867 tons / day, and that of fluidized bed is 35000tons / day. The slag production rate of grate furnace is 15.25%, which is higher than that of fluidized bed furnace (8 ~ 10%). The incineration plant selected by the research institute has been built or upgraded in the past ten years. It is technologically advanced and can be used as a representative of a typical incineration plant in China.

During the stable operation of the incinerator, 31 piles of slag were sampled, and the quality of each sample was equal. The suspected microplastic particles in slag samples were extracted through a series of processes (including drying, sieving, NaCl solution treatment, ultrasound, rinsing with deionized water, etc.). All the suspected microplastic particles were photographed, recorded, and identified by μ-FTIR technology, in which the particles with uniform texture, acellular structure, non-cracking under pressure with tweezers and artificial coloring were further identified as microplastics. The combustion loss of slag samples was also tested. After the metal is removed and ground, the sample is heated for combustion loss, and the final weight loss of the cooling residue reflects the content of unburned matter. The conditions were strictly controlled throughout the experiment to avoid the pollution of external plastics.

Incineration slag

In the slag samples, a total of 892 suspected microplastic particles were found, of which 276 were identified as microplastics. The average abundance of microplastics in slag is 116 ±172 / kg. The microplastic abundance of 13 grate furnace sites is 125 ±180 n/kg. Two of the four fluidized bed furnaces were identified as microplastics with an abundance of 84 ±167n/kg. The microplastic abundance of incineration plants in the areas where garbage source classification was carried out (33 ±79 / kg) was lower than that without implementation (228 ±264 / kg), and the difference was statistically significant (p < 0.01). Compared with other soil substrates contaminated by microplastics, the abundance of microplastics in incineration plant slag was significantly higher than that in farmland soil. Therefore, the slag produced by municipal solid waste incineration is a potential source of microplastics. On the other hand, the content of microplastics in slag is two to three orders of magnitude lower than that of sewage sludge (sewage sludge), which means that incineration plays a certain role in eliminating microplastics.

Nine kinds of plastics were found in the slag: polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyester fiber (PES), polyamide (PA), polystyrene (PS), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polyurethane (PU) and epoxy resin (epoxy). All 12 samples from the grate furnace contain PE or PP, proving that PE and PP are the two most widely produced and used polymers. Packaging is the main source of plastic waste. PS accounts for a higher proportion of incinerators with higher input of construction waste because it is mainly used in packaging and construction materials. In the grate furnace samples without garbage source classification, the main categories of microplastics are PP and PS. For the grate ash samples with source classification, the highest proportion of microplastics is PE, followed by PET, PES;PET and PES, the main source of microplastics is waste textiles (fiber products).

Microplastics are divided into four categories according to their shape: particles (granule,43%), fragments (fragment,34%), film (film,18%) and fiber (fiber). Particles refer to irregular and thick plastic, with relatively uniform length, width and thickness; the thickness of fragments is significantly lower than the other two dimensions; the film is a flat object, translucent and more flexible than fragments. Granular microplastics are mainly composed of PP, while fragment microplastics are mainly composed of PE and PS, which may be caused by a large number of packaging. PET and PES account for 70% of the global fibrous plastics, which causes them to become the main microplastic fibers in the slag. The granular and fragmented microplastics in the slag have irregular shapes and rough edges, which also means that the residual plastic fragments in the slag will gradually break up to form microplastics.

Micro plastics

The gas distribution in the fluidized bed can make the waste mix better with the bed materials (such as silica sand, limestone, alumina or ceramic materials), and improve the heat transfer efficiency and contact probability, so the heat loss of fluidized bed slag is lower than that of grate furnace slag. it shows that the combustion efficiency of waste in fluidized bed furnace is higher. In addition, the material density of the fluidized bed is higher than that of most commercial plastics, and the plastic waste with lower density than the bed material is recycled in the furnace, resulting in less microplastic content in the fluidized bed slag. Therefore, the abundance of microplastics in fluidized bed slag is lower than that in grate furnace.

Granular microplastics account for the majority of slag. Previous studies have reported that the main shapes of microplastics in landfill leachate and compost are flakes and fragments, respectively, indicating that the microplastics in slag may be significantly different from those in landfills and composting. It may be because the flakes and fragments in the incineration midplane are more easily destroyed by combustion than particles. In addition, plastic objects of other shapes except flakes and fragments will partially melt into blocks, hindering internal heat transfer, which may cause them to remain in the ash. Small particles have larger surface area, have better contact with air, and have higher heat transfer efficiency than large particles. Ther