A Simulation Study for Fluidized Bed Combustion of Petroleum Coke With CO2 Capture

Petroleum coke is regarded as a difficult fuel because of its high sulphur content and low volatile content. However, its low price and increased production, means that there is a powerful economic stimulus to use it for power generation. In this work, a process simulation has been performed as part of a feasibility study on the utilization of petroleum coke for power generation with low-cost CO2 capture. The proposed system employs a pressurized fluidized bed combustor and a calciner. In the combustor itself, the petroleum coke is burned and most of the CO2 generated is captured by a CaO sorbent under pressurized condition to form CaCO3. The CaCO3 is transported into the calciner where limited proportion of the petroleum coke is burned with pure O2, and calcines the spent sorbent back into CaO and CO2. A nearly pure CO2 stream is obtained from the calciner for subsequent disposal or utilization. The predicted overall efficiency of the combustion is near 40%. The proposed system would also be suitable for firing other high carbon and low ash fuel, such as anthracite.

Factorial Analysis on the 2-D Transient Solid Velocity in Fluidized Bed Combustor

The transient solid velocity analysis in fluidized bed combustor (FBC) freeboard has been critical in the past two decades (Haidin et al 1998). The FBC cold model (6-in ID) was designed and fabricated. The solid transient velocity in FBC freeboard was measured and analyzed with the assistance of the advanced instrumentation. The laser-based Particle Image Velocimetry (PIV) was applied to the FBC cold model to visualize the transient solid velocity. A series of transient particle velocity profiles were generated for factorial analysis. In each profile, the particle velocity vectors for 100 position points were in the format of Vx and Vy. Analysis of Variance (ANOVA) was used to determine the significant factors that affect the transient particle velocities, time, and position coordinates. Then, the 1010factorial design method was used to develop a specific empirical model of transient particle velocity in FBC freeboard which was in the shape of Vx = f1(t, x, y), and Vy = f2(t, x, y). This unique factorial analysis method was proved to be very effective and practical to evaluate the experimental conditions and analyze the experimental results in FBC systems.

Control of Ash-Related Operational Problems in BFB Combustion of Biofuels and Waste

When moving towards CO2 neutral bio fuels and waste derived fuels, new challenges are set for combustion facilities and technical boiler solutions. A common feature for both bio- and waste fuels is a big variety in composition, often high levels of alkali metals, chlorine and moisture which make these fuels difficult to burn in facilities designed for conventional fuels such as coal, peat and wood. The problems that might occur due to high alkali and chlorine levels in the fuels, are slagging, fouling, corrosion and bed sintering. The Fortum BioMAC BFB boilers are designed especially for difficult, unconventional fuels such as rice husk, olive waste, straw, construction residue, de-inking sludge, etc. The design of each individual boiler is made based on advanced theoretical prediction tools and extensive fuel testing in laboratory and in pilot scale combustion facilities. The theoretical tools consist of a multi-phase multi-component chemical equilibrium model that estimates the slagging/fouling, sintering and corrosion propensity of the fuels/fuel mixtures and of a computational fluid dynamics part. CFD calculations are used to optimize the flow pattern and the temperature of the boiler in order to avoid hot temperatures in the vicinity of refractory linings and cooled surfaces. The chemical equilibrium calculations predict the melting behavior of the fuel ash, which is used as an indicator for the placement of the superheaters. The bottom ash removal is controlled for efficient removal of coarse material, screening and recirculation. The ash related problems of important bio and waste fuels, the analytical procedure of the evaluation of the usability of the fuels and the adaptation of the boiler design are discussed in the paper.

Lignite-Fired Combined Cycle Power Plant With ACPFBC: Concept and Thermodynamic Calculations

A 200 kWth test plant was constructed by BTU Cottbus for the purpose of developing a special variant of coal conversion based on 2nd generation PFBC. This concept, primarily to be used for generating power from lignite, employs a circulating type fluidized bed and is characterized by a design that combines the two air-blown steps “partial gasification” and “residual char combustion” in a single component. The subject of this paper is to develop an overall power plant concept based on this process, and to perform the associated thermodynamic calculations. In addition to the base concept with one large heavy-duty Siemens gas turbine V94.3A fired with Lausitz dried lignite (19% H2O), further versions with variation of Siemens gas turbine model (V94.3A and V64.3A), the water content of the fuel fired (raw lignite with more than 52% H2O or dried lignite) as well as the method of drying the coal were investigated. Common assumptions for all versions were ISO conditions for the ambient air and a condenser pressure of 0.05 bar. As expected, the calculations yielded very attractive net efficiencies of almost 50% (LHV based) for a variant with the small V64.3A gas turbine and up to more than 55% for the large plants with the V94.3A gas turbine. It was further demonstrated that thermodynamic integration of an advanced, innovative coal drying process (e.g. fluidized-bed drying with waste heat utilization) causes an additional gain in net efficiency of about three percentage points compared with the variant of firing lignite that was first dried externally. In addition to the basic function of the coal conversion system, it was necessary to also assume preconditions such as complete carbon conversion, reliable hot gas cleaning facilities and fuel gas properties that are acceptable for combustion in the gas turbine. Put abstract text here.

Study of Operation of a Pilot CFB-Reactor in Dynamic Conditions

The development of a high efficiency CFB power plant (once-through supercritical CFB technology) and the use of alternative fuels require advanced methods of control and knowledge of the dynamic behavior of the furnace. Dynamic response analysis is needed for design of control algorithms in load changes. The operation of a pilot CFB-reactor in dynamic conditions and in load changes is analyzed experimentally and by modeling at different process conditions. Reactivity parameters for different fuels can be extracted from simple dynamic measurements and then used in computations studying operation in load changes. Dynamic studies are also required to see the necessary requirements for the fuel quality and fuel feed system to maintain stable operation. For high volatile coals the fuel feeding must be steadier to keep the variation in the outlet oxygen concentration at some range than with coals with low reactivity or alternatively greater air coefficient is needed to prevent too low O2 concentrations, which can cause an increase in CO emissions. The fuel quality can be characterized by the fluctuation of oxygen concentration in flue gases in steady operation conditions, which depends on the fluctuations in the combustion and in the fuel feed and on operational conditions. The amplitude of the fluctuations was studied. For advanced controls, it is necessary to know the factors affecting the process dynamics, such as reactivity and the behavior of char inventory in bed. This information is also necessary in developing and optimizing the CFB boiler considering emissions, combustion process and furnace scale up.

Solids Flow Pattern in the Bottom Zone of a Rectangular Cross-Section Circulating Fluidized Bed

Solids flow pattern in the bottom zone of a rectangular cross-section CFB was investigated by using hot particles as the tracer. The experiments were carried out in a cold model circulating fluidized bed. The riser has an inner cross-section of 0.3 m by 0.5 m and a height of 5.8 m. The solids were returned into the riser at a height of 0.75 m above the air distributor within an angle of about 40 degree. Quartz sand was used as the bed material. The hot particles were also quartz sand but with a little smaller size. Specially designed miniature electrically heating devices were installed flush with the inner bed wall or inside the bed. At each run, about 10–15 cm3 hot particles were slowly pulled into the bed. The temperature response around the device was measured with four copper-constantan thermocouples. Based on the experimental results, a 3-D core-annulus model describing the solids flow pattern in the bottom zone of the CFB riser is proposed.

Performance Results With ALSTOM’s Circulating Moving Bed Combuster™

ALSTOM is developing and testing a new and more efficient coal combustion technology, including a new type of steam generator known as a “circulating moving bed (CMBTM) combustion system combustor.” The CMBTM combustion system technology involves a novel method of solid fuel combustion and heat transfer. In this design, a heat exchanger will heat the energy cycle working fluid, steam or air, to the high temperature levels required for advanced power generation systems. This will produce a step change in both performance and capital costs relative to today’s pulverized coal and fluid bed boiler designs. In addition to high temperature Rankine cycles, the CMBTM combustion system is an enabling technology for hydrogen production and CO2 capture from combustion systems utilizing innovative chemical looping airblown gasification and syngas decarbonization. ALSTOM’s 3MWth Multi-Use Combustion Test Facility has been modified to allow operation in CMBTM combustion system mode. This paper summarizes the results of this program, which includes performance results from pilot plant testing. Participants include the U.S. DOE, ALSTOM, the University of Massachusetts, and the Massachusetts Institute of Technology. The total program cost is $2,485,468 with the DOE’s National Energy Technology Laboratory (NETL) providing 60% of the funding under Cooperative Agreement No. DE-FC26-01NT41223.

The Local Heat Balance in a Stationary Fluidized-Bed Furnace Burning Biomass Fuels

This work investigates the influence of biomass fuel properties on the local heat balance in a commercial-scale fluidized bed furnace. Experiments with different wood based fuels were performed in the Chalmers 12 MWth circulating fluidized bed boiler, temporarily modified to run under stationary conditions. A two-phase flow model of the bed and splash zone is applied, where the combustion rate in the bed is estimated by global kinetic expressions, limited by gas exchange between oxygen-rich bubbles and a fuel-rich emulsion phase. The outflow of bubbles from the bed is treated as “ghost bubbles” in the splash zone, where the combustion rate is determined from turbulent properties. It is found that a large amount of heat is required for the fuel and air to reach the temperature of the bed, in which the heat from combustion is limited by a low char content of the fuel. This implies that a substantial fraction of the heat from combustion of volatiles in the splash zone has to be transferred back to the bed to keep the bed temperature constant. It is concluded that the moisture content of the fuel does not considerably alter the vertical distribution of heat emitted, as long as the bed temperature is kept constant by means of flue gas recycling.

Biomass Ash – Bed Material Interactions Leading to Agglomeration in FBC

In (bubbling) fluidized-bed combustion and gasification of biomass, several potential problems are associated with the inorganic components of the fuel. A major problem area is de-fluidization due to bed agglomeration. The most common found process leading to de-fluidization in commercial-scale installations is “coating-induced” agglomeration. During reactor operation, a coating is formed on the surface of bed material grains and at certain critical conditions (e.g., coating thickness or temperature) sintering of the coatings initiates the agglomeration. In an experimental approach, this work describes a fundamental study on the mechanisms of de-fluidization. For the studied process of bed de-fluidization due to sintering of grain-coating layers, it was found that the onset of the process depends on: a) a critical coating thickness, b) on the fluidization velocity when it is below approx. four times the minimum fluidization velocity and c) on the viscosity (stickiness) of the outside of the grains (coating).

Model-Based Analysis of Transient Behavior of Large Scale CFB Boilers

The main objective of this study was to investigate the load change capability and effect of the individual control variables, such as fuel, primary air and secondary air flow rates, on the dynamics of large-scale CFB boilers. The dynamics of the CFB process were examined by dynamic process tests and by simulation studies. A multi-faceted set of transient process tests were performed at a commercial 235 MWe CFB unit. Fuel reactivity and interaction between gas flow rates, solid concentration profiles and heat transfer were studied by step changes of the following controllable variables: fuel feed rate, primary air flow rate, secondary air flow rate and primary to secondary air flow ratio. Load change performance was tested using two different types of tests: open and closed loop load changes. A tailored dynamic simulator for the CFB boiler was built and fine-tuned by determining the model parameters and by validating the models of each process component against measured process data of the transient test program. The know-how about the boiler dynamics obtained from the model analysis and the developed CFB simulator were utilized in designing the control systems of three new 262 MWe CFB units, which are now under construction. Further, the simulator was applied for the control system development and transient analysis of the supercritical OTU CFB boiler.