P.F.B. Hansen

High-temperature reaction between sulphur dioxide and limestone. V: The effect of periodically changing oxidizing and reducing conditions

Abstract Sulphur capture by limestone has been studied in a laboratory reactor developed to simulate the periodically changing oxidizing and reducing conditions experienced by limestone particles in a fluidized-bed combustor. Under oxidizing conditions, sulphur is captures ad CaSO 4 . Under reducing conditions, and in the presence of CO, sulphur is captured as CaS. Transformation of CaSO 4 to CaS and vice versa appears to proceed via CaO. Substituting CO with H 2 reducing agent causes an increase in the rate of reductive decomposition of CaSO 4 , and no formation of CaS is observed. Using CH 4 neither reductive decomposition of CaSO 4 nor formation of CaS is observed. The sulphur capacity of 14 European limestones was studied under constant oxidizing as well as under alternating oxidizing and reducing conditions. The relative ranking of the limestones appears to be little influenced by the reaction conditions. Generally, a slight reduction in the sulphur capacity is observed under alternating conditions. The exceptions are limestones with a high content of Fe 2 O 3 , which lowers the sulphur capacity significantly, presumably due to reduced stability of the sulphated limestone under reducing conditions. Rates of CaS formation and of reductive decomposition of CaSO 4 differ greatly for different limestones. Reduction of particle size increases the SO 2 release due to CaS oxidation but decreases the release of SO 2 due to reductive decomposition. Both CaS oxidation and reductive decomposition of CaSO 4 may lead to a diminished degree of desulphurization in real combustors. A temperature optimum observed for desulphurization in fluidized-bed combustors appears to be caused primarily by the competition between sulphur capture and sulphur release, the latter of which becomes increasingly important at high temperatures.

High-temperature reaction between sulphur dioxide and limestone—III. A grain-micrograin model and its verification

Abstract A modified grain model of the reaction between calcined chalk, sulphur dioxide and oxygen has been developed, and the model has been verified by comparison with a large volume of experimental data. A chalk particle is constituted of grains that are non-porous in the uncalcined state. The pore volume of calcined chalk is distributed between macropores corresponding to the interstices between grains and micropores formed in the grains during calcination. The porous grains are assumed to be constituted of non-porous micrograins. Mass transfer in the pores takes place by molecular diffusion and Knudsen diffusion, and micrograins react with sulphur dioxide and oxygen according to a shrinking, unreacted-core mechanism. Since calcium sulphate formed by the reaction has a significantly higher molar volume than calcium oxide, micrograins will grow in volume with increasing degree of sulphation, eventually filling the micropores at a degree of sulphation of approximately 50%. Further reaction in the grains takes place according to a shrinking, partially-reacted-core mechanism, accompanied by an increase in grain volume. The only unknown parameters in the model are the tortuosity factor and the diffusion coefficient in the solid product layer encasing micrograins and grains.

Co-Firing Straw and Coal in a 150 MWe Utility Boiler: In-Situ Measurements

A 2-year demonstration program is carried out by the Danish utility I/S Midtkraft at a 150-MWe PF-boiler unit reconstructed for co-firing straw and coal. As a part of the demonstration program, a comprehensive in situ measurement campaign was conducted during the spring of 1996 in collaboration with the Technical University of Denmark. Six sample positions have been established between the upper part of the furnace and the economizer. The campaign included in situ sampling of deposits on water/air-cooled probes, sampling of fly ash, flue gas and gas phase alkali metal compounds, and aerosols as well as temperature measurements. Material balance closures were carried out at all operating conditions. The experimental data was evaluated together with researchers from the Technical University of Denmark and the results were stored in a data base program developed under the CHEC-research program to predict deposition propensities and high temperature corrosion during co-combustion of straw and coal in PF-boilers. Danish full scale results from co-firing straw and coal, the test facility and test program, and the potential theoretical support from the Technical University of Denmark are presented in this paper.

Catalytic reduction of NO and N2O on limestone during sulfur capture under fluidized bed combustion conditions

Abstract sulfur retention by limestone (CaCO 3 ) under fluidized bed combustion conditions is the net effect of a competition between sulfur capture and sulfur release during which the composition of the Ca surface changes continuously between CaO, CaS and CaSO 4 , the ultimate product being CaSO 4 . The presence of limestone in fluidized bed combustors interacts with the nitrogen chemistry; it acts as a catalyst for volatile—N oxidation to NO and as a catalyst for the reduction of NO and N 2 O, the latter being the subject of this paper. The results show that CaS and CaO are active catalysts for reduction of NO and N 2 O, whereas CaSO 4 and CaCO 3 are poor catalysts. N 2 decomposes readily over CaO under oxidizing conditions which leave NO unaffected. Under reducing conditions, both NO and N 2 O are reduced simultaneously but apparently not at the same active sites. The catalytic activity of limestone decreases during sulfation due to a loss in porosity and a lower activity of the CaSO 4 formed. The highest catalytic activity for NO and N 2 O reduction was found for limestone with a high sulfur capacity. The presence of H 2 O appears not to affect the heterogeneous reactions involving NO and N 2 O.

Catalytic reduction of nitric oxide by carbon monoxide over calcined limestone: reversible deactivation in the presence of carbon dioxide

Abstract The influence of carbon dioxide on nitric oxide reduction by carbon monoxide catalyzed over a porous limestone Stevns Chalk was studied in a fixed bed quartz reactor at 850°C under alternating oxidizing and reducing conditions, and under constant reducing conditions. The presence of carbon dioxide was found to inhibit the catalytic nitric oxide reduction. The nitric oxide reduction by carbon monoxide was found to fit the power law expression − r NO = k · [NO] 1 · [CO] (0.8 – 0.9) · [CO 2 ] −0.7 at ambient pressures, for 300–1200 ppmv NO, 2–6 vol.-% CO and 2–16 vol.-% CO 2 . A simple mechanism for the nitric oxide reduction is proposed and a rate expression which agrees with the experimental results is derived. This study indicates that investigations of nitric oxide reduction by carbon monoxide catalyzed by CaO should be carried out in the presence of relevant concentrations of carbon dioxide.

Hydrodynamics of a commercial scale CFB boiler-study with radioactive tracer particles

This paper presents the experimental results with radioactive tracer particles in an 80 MWth circulating fluidized-bed boiler. Batches of γ-ray emitting tracer particles were injected into the standpipe. The response curves of the impulse injection were measured by a set of successive scintillation detectors located at different parts of the boiler. The results show that there are significant differences of the response signals when boiler loading changes. A model has been developed to obtain information from the experimental data. The particle recirculating rate is obtained by fitting the model to the experimental data. In the same way, the average particle velocities in the furnace are estimated. The impact of loading on the particle recirculating rate and on the hydrodynamics of the boiler is discussed.

Stochastic modeling of long term wave induced responses of ship structures

Abstract This paper presents a procedure for evaluating the long term response distribution in ship structures. A specified travel route through given geographical areas during the lifetime of the ship is defined. The individual scatter diagrams, describing the relative occurrence of different sea states for the geographical areas, are weighted with respect to the relative time occupancy of the ship. A two-dimensional analytical distribution is fitted to the weighted scatter diagram over the lifetime. Frequency response functions for all the sectorial forces are calculated for different ship speeds, heading angles and loading conditions, and fitted to a bi-cubic spline. The wave spectrum is defined by applying a gamma spectrum. The response spectrum is found using a linear frequency domain analysis. From the response spectrum, the peak distribution of the response in each short term period, conditioned on the loading condition, the ship speed and the wave heading direction is computed. The peak distribution over the lifetime is estimated by unconditioning with respect to sea states, maneuvering and loading condition.

Deposition and Corrosion in Straw- and Coal-Straw Co-Fired Utility Boilers

Department of Chemical Engineering, Technical University of Denmark Building 229, DK-2800 Lyngby, Denmark Phone: +45 45 25 28 83, Fax: +45 45 88 22 58, E-mail: ff/hpn/paj/lah/hl/kdj@kt.dtu.dk Midtkraft I/S Power Company, Studstrup Power Station, DK-8541 Skodstrup, Denmark. Phone: +45 86 99 17 00, Fax: +45 86 99 37 20 E-mail: pbh@midtkraft.dk,kha@kt.dtu.dk (1) Currently with Rockwool International A/S, DK-2640 Hedehusene, Denmark. (2) Currently with SunChemical A/S, DK-4600 Koge, Denmark. Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark. Phone: +45 38 14 20 00, Fax: +45 33 63 39 89 E-mail: hss@geus.dk Currently with Danfoss A/S, DK-6430 Nordborg, Denmark. Faelleskemikerne, I/S Fynsvaesrket, Havnegade 120, DK-5000 Odense C, Denmark. Phone: +45 65 90 44 44, Fax: +45 65 90 38 12 Faelleskemikerne, ElsamProjekt A/S, Kraftvaerksvej 53, DK-7000 Fredericia, Denmark Phone: +45 79 23 33 33, Fax: +45 75 56 44 77 E-mail: bos/nh@elsamprojekt.dk Elkraft A.m.b.a., Lautruphoj 5–7, DK-2750 Ballerup, Denmark Phone: +45 44 66 00 22, Fax: +45 44 65 61 04

RELIABILITY ANALYSIS OF A PILE DESIGN

Abstract This paper presents a stochastic pile design for a wellhead platform designed for Danish waters. The relevant failure modes for both ultimate and serviceability limit states are defined. Failure modes for the pile as well as failure in the soil are thereby considered. Reliability criteria are suggested based on requirements formulated by the Nordic Committee for Building Structures. An uncertainty modeling for the environmental conditions, the response and the soil stiffness and strength is suggested. The choice of distribution types by and large follows that suggested by the Nordic Committee for Building Structures. The effects of spatial variation of soil properties are assessed both in terms of varying positions of layer boundaries and in terms of spatially varying soil properties. The model uncertainty in the soil strength for axial compressive and tensile loading is quantified. The most important uncertainties are due to uncertainties in the extreme environmental parameters and load model parameters, and for the soil failure modes also due to the model uncertainty. The uncertainties in soil stiffness, strength and spatial variation play a very small role in all failure modes.

Reliability Analysis of Dynamic Stability in Waves

The assessment of a ship’s intact stability is traditionally based on a semi-empirical deterministic concept that evaluates the characteristics of ship’s calm water restoring leverarm curves. Today the ship is considered safe with respect to dynamic stability if its calm water leverarm curves exhibit sufficient characteristics with respect to slope at zero heel (GM value), maximum leverarm, positive range of stability and area below the leverarm curve. The rule-based requirements to calm water leverarm curves are entirely based on experience obtained from vessels in operation and recorded accidents in the past. The rules therefore only leaves little room for evaluation and improvement of safety of a ship’s dynamic stability. A few studies have evaluated the probability of ship stability loss in waves using Monte Carlo simulations. However, since this probability may be in the order of 10−4 per ship year such brute force Monte-Carlo simulations are not always feasible due to the required computational resources. Previous studies of dynamic stability of ships in waves typically focused on the capsizing event. In this study the objective is to establish a procedure that can identify “critical wave patterns” that most likely will lead to the occurrence of a considered adverse event. Examples of such adverse events are stability loss, loss of maneuverability, cargo damage, and seasickness. The adverse events related to dynamic stability are considered as a function of the roll angle, the roll velocity, and the roll acceleration. This study will therefore describe how considered adverse events can be combined into a single utility function that in its scale expresses different magnitudes of the criticality (or assessed consequences) of the adverse events. It will be illustrated how the distribution of the exceedance probability may be established by an estimation of the out-crossing rate of the “safe set” defined by the utility function. This out-crossing rate will be established using the so-called Madsen’s Formula. A bi-product of this analysis is a set of short wave time series that at different exceedance levels may be used in a codified evaluation of a vessels intact stability in waves.© 2004 ASME