Moving Grate Furnaces Research Articles

Relevance and prediction of N2O in small-scale multi-fuel biomass furnaces using primary NOx reduction measures

The relevance of the greenhouse gas N2O was investigated in two small-scale biomass combustion plants, a 30-kW fixed bed and a 70-kW moving grate furnace, both of which use double air staging. Furthermore, the 70-kW furnace was simulated with ideal reactor models, applying a detailed nitrogen chemistry mechanism to gain insights into N2O formation mechanisms that occur during double air-staged combustion.The experimental and simulated results confirm the existence of correlations between CO, NOx and N2O. For all investigated fuels, N2O reached the highest levels (increase of up to 300 %) when high CO and low NOx concentrations were present. Therefore, caution is advised when looking for minimum NOx emissions, due to the risk of having high N2O emissions, an aspect which is often overlooked.A deeper analysis of the N2O formation mechanisms revealed that N2O is mainly generated from HCN in the oxidation zone and it is thermally dissociated at high temperatures. Finally, a comparison between the results of both plants showed that the concern about high N2O emissions at minimum NOx concentrations is not a reactor-specific problem; therefore, it is necessary to ensure that CO and N2O are kept at a low level during NOx reduction with primary measures.

Phase distribution of PCDD/Fs in flue gas from municipal solid waste incinerator with ultra-low emission control in China

Polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) are the key pollutants of municipal solid waste incineration (MSWI). In this study, the characteristics of 17 toxic 2,3,7,8-substituted congeners in flue gas along six air pollution control devices (APCDs) were investigated in a 400 t/d moving grate furnace located in a typical megacity of Shenzhen, China. The phase distribution and removal efficiency of the different APCDs were analyzed, especially the effect of the selective catalytic reduction (SCR) device. The results showed that PCDD/F TEQs were 59.5%, 67.1%, and 72.5% partitioned into the gas phase (XAD-2 and condensed water) at the economizer outlet, fabric filter outlet, and stack, respectively. Furthermore, the three-year-old catalyst in the SCR tended to remove PCDDs, especially those in the solid phase (filter thimble). More importantly, the PCDF TEQs at the SCR inlet and outlet were 1.045 × 10−3 and 1.568 × 10−3 ng I-TEQ/Nm3, respectively, which meant that the SCR might be ineffective for PCDF TEQ removal. A continuous chlorination of lower chlorinated PCDD/Fs increased the ratio of PCDFs and PCDDs from 0.73 at the SCR inlet to 1.76 at the SCR outlet. This work indicated the asynchronized inefficient removal of PCDD/Fs and nitrogen oxide for this three-year-old catalyst. The obtained results provide suggestions for the entire process of curbing PCDD/F emissions and obtaining ultra-low emission from MSWI.

Energy recovery in China from solid wastes by the moving grate and circulating fluidized bed technologies

In recent years, the Chinese waste-to-energy (WTE) industry is growing at the rate of about thirty new plants each year. The municipal solid waste (MSW) fuel has a low heating value of 4–7 MJ/kg, in comparison to about 11 MJ/kg in U.S. and 8–11 MJ/kg in EU. Combustion of the low heating value fuel on a moving grate (MG), the dominant combustion technology worldwide, is difficult to control and measures have to be taken to remove some moisture prior to combustion. For this and other reasons, an alternative technology, the circulating fluid bed (CFB) has been implemented in China. This paper is a comparative study of the two technologies and was carried out by Columbia University and two senior engineers, representing the MG and CFB technologies of China. Data were derived from industrial operating plants and from the literature. The fuel to MG furnaces is as-received MSW, while the MSW to CFB reactors is pre-shredded using high-torque low-speed shredders. The availability of MG plants, over a 1-year period, is 90% + , while that of CFB facilities is 80% +. Also, the in-plant electricity consumption of MG plants is slightly lower than the consumption of CFB plants. The MG furnace is less compact, than that of a CFB combustion chamber, with a heat flux range from 0.5 to 0.6 MW/m2 of grate surface area, while that of CFB furnace was about 1.7 MW/m2 of furnace cross-section. The bottom ash in a MG process is typically wet-discharged and the recovery of metals is less efficient. A drawback of the CFB process is that the fly ash generated is 5–10% of the weight of MSW combusted, as compared to 1–3% for moving grate plants in China.

Environmental performances of a modern waste-to-energy unit in the light of the 2019 BREF document

The study compares the environmental performances of a new-generation, large scale, combustion-based waste-to-energy unit, active since 2010, with those of different “virtual” units, defined in the light of the Best Available Techniques REFerence document (BREF) for Waste Incineration published by the European Community on December 2019. The average performances of these units have been evaluated in terms of air emissions, material consumptions and energy recovery, based on data related to 355 “existing” European waste incineration lines and those established for the future “new” plants. An attributional Life Cycle Assessment has been used to compare and quantify the environmental performances of the selected units, all equipped with a moving grate furnace and similar air pollution control systems. A sensitivity analysis quantifies how even more severe requests for emission and energy performances as well as the evolution of the European electricity mix until the year 2030 can affect the comparative assessment. The results indicate that the considered large scale waste-to-energy plant has good environmental performances, even in an electricity mix characterised by 45% of renewable sources. This allows an easy compliance with the Best Available Techniques Associated Emission Levels of the new waste incineration BREF document. Possible further improvements of its performances should be focused mainly on a further increase of the energy efficiency, provided that it is economically viable.

Parameters affecting biomass drying during combustion in moving grate furnaces

The most efficient way so far to extract energy from renewable sources is combustion of solid fuel. Solid fuel furnaces of moderate capacity (5–10 MW) equipped with reciprocating grates are most popular. Grate combustion is a well-developed technology; however, to burn biofuel in this type of furnaces in the optimal and safe way, the fuel must be of high quality and have at least constant moisture content. However, increasing demand for biofuel results in increasing prices. To remain in the market and to stay competitive, heat producers choose to utilise moist biofuel of lower quality, whose moisture content can vary and reach up to 60% wt. The burning on the grate of such biofuel is complicated as the drying process occupies most of the space in the furnace. The purpose of this work was to analyse processes taking place in a furnace, such as: primary air supply, influence of flue gas recirculation and radiation from hot surfaces of the furnace to biofuel drying. Analysis of the data obtained would provide technical decisions facilitating optimal fuel combustion in a furnace without additional investments. Analysis of biofuel drying was performed in an experimental setup with a fixed fuel bed. The experiments were performed with wood chips and four different drying fluid temperatures. The results of experimental studies have shown that the drying rate of biofuels upper layers is strongly influenced by radiation from hot surfaces and the moisture content of the sample decreases by 18% wt.

Process and technological aspects of municipal solid waste gasification. A review

The paper proposes a critical assessment of municipal solid waste gasification today, starting from basic aspects of the process (process types and steps, operating and performance parameters) and arriving to a comparative analysis of the reactors (fixed bed, fluidized bed, entrained bed, vertical shaft, moving grate furnace, rotary kiln, plasma reactor) as well as of the possible plant configurations (heat gasifier and power gasifier) and the environmental performances of the main commercially available gasifiers for municipal solid wastes. The analysis indicates that gasification is a technically viable option for the solid waste conversion, including residual waste from separate collection of municipal solid waste. It is able to meet existing emission limits and can have a remarkable effect on reduction of landfill disposal option.

Numerical simulation of Municipal Solid Waste incineration in a moving-grate furnace and the effect of waste moisture content

Mathematical equations governing the physical and chemical processes in grate combustion systems are described and a full-scale municipal solid waste incinerator was simulated. The effects of waste moisture content as well as preheated air temperature have been assessed. Combustion efficiency around 98% was obtained for moisture levels less than 30% while moisture over 35% resulted in poor combustion efficiency. Depending on the waste moisture content, the maximum gas temperature ranges from 1240C to 1510C and the maximum solid temperature inside the packed-bed from 980C to 1230C. The main reaction zone thickness ranges from 250 mm to 350 mm along the bed height.

Converting moving-grate incineration from combustion to gasification – Numerical simulation of the burning characteristics

Waste incineration is a politically sensitive issue in the UK. The major current technology is based on direct combustion of wastes in a moving-grate furnace. However, general public opinion prefers non-direct burning technologies. Waste gasification is one of those nearest technologies available. By reducing the primary air-flow rate through the grate of a packed-bed system, operation of the existing solid-waste incineration equipment can be easily converted from combustion mode to gasification mode without major modification of the hardware. The potential advantages of this are lower dust carry-over in the flue gases, lower bed temperature (and therefore lower NO x formation in the bed), simplified gas-treatment procedures and lower running cost, among other benefits. The major disadvantages are, however, reduced throughput of the wastes and possibly higher carbon in the ash at exit. In this study, numerical simulation of both combustion and gasification of municipal solid wastes in a full-scale moving grate furnace is carried out employing advanced mathematical models. Burning characteristics, including burning rate, gas composition, temperature and burning efficiency as a function of operating parameters are investigated. Detailed comparisons between the combustion mode and gasification mode are made. The study helps to explore new incineration technology and optimise furnace operating conditions.