Sulfidation Experiments in a Pressurized Thermogravimetric Analyzer With Chinese Calcined Limestones

The sulfidation experiments with two kinds of Chinese calcined limestones were performed in a pressurized thermogravimetric analyzer (PTGA). The effects of reaction temperature (700–950°C), total pressure (0–1MPa), particle size (0.055–2mm), and H2S concentration (0.1–4%) on the sorbent conversions were analyzed. Morphological studies with scanning electron microscope and energy dispersive spectroscopy (SEM-EDS) equipment were made to obtain the pictures of solid surface and of the cross-sectioned samples. Nitrogen adsorption measurements were applied to determine the pore structure properties of the particles. Experimental results show that the sulfidation rate increases with total pressure when the volume fraction of H2S is constant. However, the rate of sulfidation decreases with the increase of total pressure when the H2S partial pressure is constant. Reaction temperature affects the sulfidation greatly, and the reaction rate increases with temperature. The sulfidation is the first order with respect to H2S partial pressure. Moreover, larger particles result in lower conversions and reaction rates. The unreacted shrinking core model was applied to describe the sulfidation to determine the kinetic parameters.

Fouling Tendency of Ash Resulting From Burning Mixtures of Biofuels

Specified mixtures of peat with bark and peat with straw were burned in a lab-scale entrained flow reactor that simulates conditions in the superheater region of a biomass-fired boiler. Deposits were collected on an air-cooled probe that was inserted into the reactor at the outlet. For both mixtures, the deposition behaviour followed a non-linear pattern, which suggests that physico-chemical interaction between the ashes of the different fuels has taken place. The results indicate that it is possible to burn up to 30 wt-% bark (renewable biofuel and pulp mill waste) and up to 70 wt-% straw (renewable biofuel and agricultural waste) in mixtures with peat without encountering increased deposition rates in the reactor. The deposit composition was compared to the fuel ash composition using chemical fractionation analysis and SEM/EDX. While the composition of deposits obtained from pure fuels resembles the feed composition, a considerable change is observed in deposits obtained from mixtures. K and S compounds are attached to Si spheres and the substrate surface. The deposition rate is significantly lowered when removing K, S, Cl and Na in bark prior to burning by washing and mechanical/thermal dewatering.

Retrofitting 25T/h Pulverized Coal-Fired Boiler Into 35T/h Circulating Fluidized Bed Boiler for Burning Mixture of Coal and Bagasse

In China, there are a large number of pulverized coal-fired industrial boilers, whose steam capacities are usually relatively small. These boilers can burn only high-grade coal and have low combustion efficiency. Furthermore, the combustion emissions, such as SO2 and NOx, pollute the environment severely. Therefore it is very important and urgent to adopt economically efficient and environmentally friendly technologies to retrofit these boilers. At the same time, there are many industrial wastes, such as bagasse, wood waste, rubbish, petroleum coke and so on, need burning disposal in China. Fluidized bed combustion technology is a kind of clear combustion technology, which has many advantages, such as excellence fuel flexibility, high combustion efficiency, low pollutant emission and good turndown capability etc. So, adopting fluidized bed combustion technology, retrofitting pulverized coal-fired boiler into fluidized bed boiler can realize pure burning various wastes or co-firing with coal, which should have great economic benefits and social benefits. And the application prospect of the method is also extensive. The State Key Laboratory of Coal Combustion has successfully retrofitted a 25t/h pulverized coal-fired boiler into circulating fluidized bed boiler with in-bed tubes and downward exhaust cyclone. The retrofitted boiler can burn mixture of coal and bagasse and the steam capacity reaches 35t/h. This paper presents the retrofitting measures and the operation status of the boiler after retrofitting.

Opportunities for Implementation of FBC Boilers in Romania

The evolution and current status of fluidized bed technology in Romania are presented. After a period of about 10 years when the FBC technology was rejected as a solution for power generation, due to the new environmental restrictions, the market is open again. Potential users are especially industrial units with high energy consumption, having combustibile wastes resulting from the technological processes. Two case studies are presented. The first is the utility from the city of Motru, producing thermal agent for the town district heating. It is currently equipped with one FB boiler (11.6 MWt hot water) and two pulverized coal boilers (50 t/h steam). The equipment is more than 40 years old, and is no longer acceptable in terms of thermal efficiency and pollutant emmissions. Taking into consideration the important biomass resources from the area, it was decided to build a new CFB boiler firing a mixture of local fuels — lignite and biomass waste. The second case study is ELECTROCARBON S.A. Slatina, the second important polluter from Olt county, producing carbon electrodes for use in metallurgy. Carbon waste resulting from the technological process have been deposited for many years in a nearby landfill, amounting to about 1.2 million tons. In spite of its high calorific value, because of the low volatile content and the grinding difficulties, little use was found for this material. Investigations are carried out to burn it in a fluidized bed boiler located near the site, that would replace two existing heat recovery steam generators producing electricity for the internal consumption.

Appearance of Trace Elements in Co-Firing Fuels

With the implementation of new EU guidelines the levels of maximum allowable emission levels of Cd, Tl, Hg, Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V will be further restricted. This may have implications for co-combustion of coal with waste derived fuels. In this study chemical fractionation, i.e. a stepwise leaching procedure has been applied on coal, peat, sewage sludge, bark, impregnated wood and forest residue. With this method fuels are leached in three steps, i.e. leached with water, ammonium acetate and hydro chloric acid, respectively. Both solubility in different leaching agents of main ash forming matter and the trace elements Cd, Tl, Hg, Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V and Zn were studied. In this way more information became available about the characteristics of co-firing fuels. Thermodynamic calculations were used to show the consequences of the interaction with main ash forming elements on the partition of Cd, Hg, Pb and Zn in the gas/ash phase.

A Novel Approach Towards Waste Treatment in FBC

Waste combustion has the potential to play an important role in the energy production despite its contribution to heavy metals emissions. A new multi-zone temperature combustion technique, known as a Low-High-Low (LHL) temperature method, was developed to reduce pollutant emissions, particularly heavy metals, from FBCs. This paper focuses on the environmental impacts of biowaste combustion at different FBC conditions with emphasis on gas and solid emissions. The biowaste (de-inking sludge) studied contained 15% moisture, 27% carbon, 18% oxygen, and 35% ash. Ash elemental analysis shows a dominance of SiO2, Al2O3 and CaO (38%, 28% and 19%, respectively) with selected alkalis Na2O and K2O (0.3% and 0.2%, respectively). The used biowaste material had a heating value of 10,000 kJ/kg, which indicates that its combustion may be used to treat a portion of the total solid waste produced, while generating energy. The paper reports the following results of LHL vs. Classical FBC: (1) average axial profiles of gas concentrations (NO, NOx, and CO2) as well as their final averages at the exhaust, (2) final heavy metals leachability from generated fly ash. During the multi-temperature combustion experiments (LHL), the final average gas measurements for NO, NOx, and CO2 were 91 ppm, 175 ppm, and 6.1%, respectively. As for the classical FBC experiments, the final average gas measurements were similar (94 ppm, 141 ppm and 5.9% for NO, NOx and CO2, respectively). The final fly ash sample had leachability rates of 0.14 ppm and 0.061 ppm for Cd and Cr, respectively. Such low leachability rates are due to the LHL’s ability to form dense and compact final fly ash structures. On the contrary, 30.7 ppm and 14.3 ppm of Cd and Cr leached out of the porous no-LHL final fly ash structures, respectively. These results confirm that the LHL combustion could be considered as an effective waste-to-energy approach.

Co-Firing of Steam Coal With High Sulfur Content Lignite in a Bubbling Fluidized Bed Combustor

Combustion and emission behavior of 100 % steam coal (SET 1) and a mixture of 80 % by weight steam coal and 20 % by weight local lignite, characterized by high sulfur and ash contents, (SET 2) were investigated in the 0.3 MWt Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) Test Rig. Experiments were performed with limestone addition at various Ca/S molar ratios with fines recycle. In both sets of experiments, parameters other than Ca/S molar ratio were held as nearly constant as possible. On-line measurements of O2, CO2, CO, SO2, NOx emissions were carried out. Comparisons between the emissions show that lower NOx and SO2 emissions are obtained from combustion of steam coal/lignite mixture compared to those from steam coal only despite higher sulfur and almost equal nitrogen contents of the mixture. Calculated combustion efficiencies were found to be around 98 and 96 % for SET 1 and SET 2, respectively. As for the sulfur retention efficiencies, up to three times higher efficiencies were achieved when steam coal is co-fired with high sulfur lignite.

Trace Organic Pollutants Emission From Large-Scale Circulating Fluidized Bed Incinerators of Co-Firing Chinese MSW and Coal

Complex components, high moisture and low caloric value of raw Chinese MSW (Municipal Solid Waste) lead to the difficulties of keeping stable burning and low pollutant emission. Differential Density Circulated Fluidized Bed (DDCFB) incinerator was first developed by Zhejiang University to overcome such difficulties. The research of organic pollutants, especially dioxins and polycyclic aromatic hydrocarbons emission and control from MSW incinerators has been carried out in ITPE since 1998. The aim of this paper is to provide the scheme of a new co-firing CFB incineration technology, and useful data for environmental evaluation of trace organic pollutants emission from incinerators. The art of co-firing CFB is presented briefly in the first part of this paper. The dioxin content in original Chinese MSW is estimated to be 10 pg I-TEQ/g based on the data from Abad et al. Several test runs are conducted in a real-scale (150ton/day) incinerator co-firing MSW and coal to investigate dioxins and polycyclic aromatic hydrocarbons emission to ambient air. Test results show that dioxins input into the incinerator is estimated around 300∼600 mg I-TEQ/y, dioxins output is estimated 3∼100 mg I-TEQ/y, and dioxins emitted to ambient air is around 0.1∼15mg I-TEQ/y. It seemed that most of dioxins in original waste are decomposed by incineration process. For seventeen priority PAHs, its emission to ambient air is around 200∼4160g/y for co-firing runs, which is much more than 80g/y for coal combustion only. This suggests that PAHs emission to ambient air needed to be more concerned than dioxins. That is to say, PAHs emission regulatory should be proposed as soon as possible to restrict all incinerators. Several new incinerators (200 tons/day per unit and 300 tons/day per unit) were built by using new Co-firing CFB technology in past few years. From the environmental evaluation report, dioxins emission in stack gas is in range of 0.0025∼0.06 ng I-TEQ/Nm3, which seemed far below the European limit. The annual dioxin emission to air for 200ton/day or 300 t/d units is around 3∼6 mg I-TEQ. It shows that co-firing CFB incinerator is capable of reducing dioxin emission effectively. Based on industrial demonstration experience of new co-firing CFB incineration technology, it has been proven environmental friendly method for thermal treatment of MSW in developing countries. Some reasons for low dioxin emission of co-firing processes are discussed in this paper.

Combustion Behavior and SO2, NOx Emissions of an Anthracite Coal in a Circulating Fluidized Bed

Combustion behavior and SO2, NOx emissions of anthracite coal in a circulating fluidized bed are reported in this paper. Experimental researches were done on a 1 MWt circulating fluidized bed facility with a 0.31 m × 0.31 m cross section and 11.2 m height combustor. The anthracite coal with 6.28% volatile and 3.76% sulfur content burns steadily during the test. The bed was operated under different temperature, Ca/S ratio and excess air. A limestone containing 75% CaCO3 and 15% MgCO3 was used as the sulfur sorbent. Results show that the SO2 emission varies with operating bed temperature and more than 90% sulfur capture efficiency can be reached while Ca/S is about 3. With Rosemount Analytical NGA2000, N2O, NO and NO2 were also measured in the test. It was found the majority content of NOx was NO and the least was NO2. Those NOx emissions change highly with the excess air number.

Effect of Heat Treatment on the Reactivity and Crystallinity of Coal-Char

The effect of heat treatment on the reactivity and crystallinity of char prepared from the vitrinite of two coals (YX, JJ) was investigated by using XRD and TGA in this paper. The results from TGA show that the reactivity of the chars from YXV and JJV decreases with the increase of heat treatment temperature. The reactivity of YXV char decreases quickly and significantly as heat treatment time increases. However, after heat treatment time of 60 min, it decreases slowly. The effect of heat treatment time on the reactivity of JJV char is small. The results from XRD show that the crystallinity of coal-char is determined by the intensity of heat treatment. When heat treatment time is more than 60 minutes, the turbostratic crystallite of YXV char prepared under 900°C changes remarkably and becomes more orderly. The aromatic layer stacking heights (Lc) of YXV Char when heat treated above 900°C increased with the increase of heat treatment time. The effect of heat treat time on Lc of JJV char is small, but under heat treatment temperature of 1200°C, the crystalline of JJV char grows distinctly. There is a good parallel relationship between the crystalline growth and deactivation of the chars. It can be concluded that the growth of the crystalline is the main reason for the deactivation of coal-char.