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.

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.

Partitioning of Mercury and Other Trace Elements From Coal and Waste-Derived Fuels During Fluidised Bed Pyrolysis

The potential releases of toxic trace elements such as mercury, lead and arsenic call for emission control during fluidised bed (FB) combustion, pyrolysis or gasification of waste-derived fuels and fossil fuels. Control measures for sulphur oxides, nitrogen oxides and particulates effectively remove many other pollutants from the exhaust gases as well, but mercury and several other trace elements are already problematic and this situation will only worsen with time. Besides the effect of temperature, gas atmosphere and halogens, the presence of other species, for example metal oxides, have an effect on under which conditions and in what form trace elements are released from fuels. Understanding the events of trace elements release from solid fuels during the pyrolysis or char combustion stage will provide a key to manipulating their partitioning and controlling their emissions. Pyrolysis experiments were made with coal, sewage sludge and automotive shredder residue (ASR) in a two-stage fluidised bed combustion (FBC) facility. An Ontario Hydro measurement train plus an additional sampling system were used to measure mercury and around fifteen other trace elements in the gases, and also char samples were taken and analysed. Results from these experiments are presented. An issue that is addressed explicitely is the bed material, which may be contaminated with significant amounts of toxic trace elements.

NO and N2O Formation/Decomposition Characteristics During Co-Combustion of Coal With Biomass

Co-combustion technologies of coal with biomass have been applied in many practical coal combustion boilers in order to reduce CO2 emission, fuel cost and so forth. Furthermore, the biomass may be able to enhance the combustion performance and to control NOx and N2O emissions since the biomass contains high volatile matter and evolves NH3 as the main volatile N-species. This study focuses on NOx and N2O emission characteristics during co-combustion of coal with biomass. The main results obtained show that emission amount of NO and N2O during co-combustion is relatively more than that during coal combustion. At least, NO behavior can be simulated by the homogeneous reaction scheme relating to NOx even at constant temperature. However, the N2O behavior will be influenced by heterogeneous schemes due to char particles during co-combustion.

2nd Generation PFB Plant With W501G Gas Turbine

Research has been conducted under United States Department of Energy (USDOE) Contract DE-AC21-86MC21023 to develop a new type of coal-fired, combined cycle, gas turbine-steam turbine plant for electric power generation. This new type of plant — called a 2nd Generation or Advanced Pressurized Fluidized Bed Combustion (APFB) plant — offers the promise of efficiencies greater than 48 percent (HHV) with both emissions and a cost of electricity that are significantly lower than those of conventional pulverized-coal-fired plants with scrubbers. In the 2nd Generation PFB plant coal is partially gasified in a pressurized fluidized bed reactor to produce a coal derived syngas and a char residue. The syngas fuels the gas turbine and the char fuels a pressurized circulating fluidized bed (PCFB) boiler that powers the steam turbine and supplies hot vitiated air for the combustion of the syngas. A conceptual design and an economic analysis was previously prepared for this plant, all based on the use of a Siemens Westinghouse W501F gas turbine with projected gasifier, PCFB boiler, and gas turbine topping combustor performance data. Having tested these components at a pilot plant scale and observed better than expected performance, the referenced conceptual design has been updated to reflect that test experience and to incorporate more advanced turbines e.g. a Siemens Westinghouse W501G gas turbine and a 2400 psig/1050°F/1050°F/2-1/2 in. Hg steam turbine. This paper presents the performance and economics of the updated plant design along with data on some alternative plant arrangements.

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.