The moving grate systems of waste-to-energy (WTE) mass-burn combustion chambers are designed for providing efficient flow and mixing of the municipal solid waste (MSW) over the length of the grade. This study presents results from a numerical analysis of the effect of number of reciprocating bars and reciprocation speed on the degree of mixing and residence time of MSW particles on the grate. A particle-based bed model of MSW and a physical model of reverse-acting grate were used in order to quantify the mixing diffusion coefficient of MSW particles. We analyzed the particle mixing with different parameters: particle size (d = 6–22cm diameter), reciprocation speed of moving bars (Rr = 0–90recip./h), and number of moving bars (Nb = 0 to 16 bars). This combination of mathematical modeling and experimental work has shown that: (1) different particle sizes result in different residence times, according to the Brazil Nut Effect (BNE) (2) The number of moving bars (from 0 to 16 bars) of a reverse-acting grate has the net effect of increasing the mean residence time of small and medium sized particles, while decreasing that of large particles. (3) The bar height, h, was found to be one of the major geometric parameters influencing mixing diffusion coefficient, D, and residence time.

The size and shape of New York City municipal solid waste (NYCMSW) and combustion residues (ashes) are numerically analyzed in order to investigate the size reduction of particles on the grate of a waste-to-energy (WTE) combustion chamber. It is also necessary for designing a new combustion chamber, due to the heterogeneous MSW particles. About 360 MSW particles for this study were sampled in the black bags collected in residential areas at five boroughs of New York City. Also about 210 ash particles from a WTE combustion chamber were sampled. Length, breadth, perimeter and area of each MSW and ash particle are measured by means of image analysis that is more accurate than sieve analysis. Based on the image analysis, the particle size distributions (PSiD) and particle shape distributions (PShD) of MSW and residues were created. The mean size of NYCMSW was found to be 12.8cm and standard deviation of the MSW PSiD to be 6.4. Also mean size and standard deviation of the ash PSiD to be 2.4cm and 0.5889, respectively. Also Three types of shape factors (aspect ratio, roundness and sphericity) are used for creating 3 PShDs (aspect-ratio distributions, roundness distributions and sphericity distributions). Based on the similarity of the particle shapes quantified as these shape factors, the particles of MSW and residues are divided into 9 clusters by means of cluster analysis. This cluster analysis showed categorized characteristics of particle shapes that can be used for predicting surface areas of particles and mobility of particles in MSW bed on the traveling grate, both of which are major parameters for simulating combustion process in WTE systems.

The grate systems of waste-to-energy (WTE) mass-burn combustion chambers, which historically stem from coal combustion technology, have an important role in controlling the mixing of heterogeneous MSW during the combustion process. They are designed for providing efficient flow and mixing of Municipal Solid Waste (MSW) in the combustion chamber. This study presents results from a numerical analysis for grate design and chamber operation, i.e., number of reciprocating bars and reciprocation speed that influence the degree of mixing and residence time of MSW particles. A particle-based bed model of MSW and a physical model of reverse-acting grate were used in order to quantify the mixing diffusion coefficient of MSW particles. We analyzed the particle mixing with different parameters: particle size (d = 6–22 cm diameter), reciprocation speed of moving bars (R r = 0–90 recip./h), and number of moving bars (N b = 1–16 bars). According to the size segregation in the particle mixing process in the MSW bed, the undersized waste particles in the MSW bed on the reverse-acting grate have a higher diffusion coefficient than those of oversized and mean size particles. Also the number of moving bars, N b , as well as reciprocation speed, R r , were quantitatively related to a diffusion coefficient equation for MSW particle mixing.

Waste-to-energy (WTE) plants are utilized for the production of heat and electricity from municipal solid waste (MSW) and refuse derived fuel (RDF). Due to high chlorine content (0.5wt.%∼1.0wt.%) in MSW & RDF, high temperature corrosion is often observed on the superheater surfaces and correspondingly leads to a very low efficiency of 15%∼25% in practical WTE plants. To obtain information on the corrosion rate and high temperature corrosion mechanism, a full scale nine-month-long term corrosion test was therefore conducted in a heat and power generating WTE plant in Tianjin, China. The grate boiler with a capacity of 400 tons/d, runs at a burning temperature between 850∼900°C, flue gas temperature between 550∼650°C, steam temperature of 400°C, and steam pressure of 4MPa. The corrosion probes made of same metal alloy with heat exchanger were exposed on the surface of economizer, protector and superheater, respectively. The metal loss by corrosion was determined by measuring the distance from the inside of the ring to the interface between metal and oxide with a measuring microscope. The deposit characteristics as well as elemental compositions were determined using a scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The objective of this work is to evaluate: 1) plant specific corrosion rate on different superheater materials and 2) relationship among chlorine content in the feedstock, chlorine gas emission before air pollutant clean system, and deposits composition. The results showed deposits characteristics depend on the probe location, metal materials, temperature and windward/leeward. Barely chlorine exists in the deposits, except for the outer surface of the deposits at 3 rd SH. The highest corrosion loss for 20G at 3 rd SH was calculated to be 2mm/year, based on the assumption of linear extrapolation of corrosion rate.

Flow, mixing, and, size segregation of Municipal Solid Waste (MSW) particles on the traveling grate of a mass-burn waste-to-energy (WTE) combustion chamber is analyzed for understanding those parameters that control the combustion processes and for designing the chamber. In order to quantify these phenomena, a full-scale physical model of the reverse acting grate was built and used for investigating the effects of the motion of the reverse acting grate under a MSW packed bed with tracer particles ranging from 6 – 22 cm in diameter. Based on these experimental data, a stochastic model of MSW particle within the packed bed on a traveling grate was applied for simulating the MSW particle behavior. The result shows that the motion of the traveling grate, whose speed ranged from 15 to 90 reciprocations/hour, increases the mean residence time of small and medium particles by 68% and 8%, respectively, while decreasing the mean residence time of large particles by 17%. This is because of size segregation of particles known as the Brazil Nut Effect. When the ratio of particle diameter to the height of moving bar, d/h, increases from 0.46 to 1.69, the mixing diffusion coefficient, De at 60/hour., decreases from 96 to 38.4. This indicates that the height of the moving bars should be greater than the diameter of targeted particles.