Lone Aslaug Hansen

Influence of deposit formation on corrosion at a straw-fired boiler

Straw-fired boilers generally experience severe problems with deposit formation and are expected to suffer from severe superheater corrosion at high steam temperatures due to the large alkali and chlorine content in straw. In this study, deposits collected (1) on air-cooled probes and (2) directly at the existing heat transfer surfaces of a straw-fired boiler have been examined. Deposits collected on air-cooled probes were found to consist of an inner layer of KCl and an outer layer of sintered fly ash. Ash deposits formed on the heat transfer surfaces all had a characteristic layered structure, with a dense layer of K2SO4 present adjacent to the metal surface. It is argued that the K2SO4 layer present adjacent to the metal surface may lead to reduced corrosion rates at this boiler. A discussion of the deposit structure, the K2SO4 layer formation mechanism, and the influence of the inner layer composition on the corrosion of the superheaters is provided.

Full-scale co-firing of straw and coal

Abstract Co-firing of biofuels and coal in power plants is considered by the Danish utilities as a potential tool in reducing CO 2 emissions. To test this, full-scale measurements were carried out for 1 week on a 250 MW e pulverized coal fired unit using 10–20% straw (thermal basis). With an increased fraction of straw in the fuel, a net decrease in NO x and SO 2 emissions was measured. The SO 2 emission decreased partly due to the lower sulfur content of the fuel per MJ, but also due to higher sulfur retention in the ash. The NO emission decreased solely due to lower conversion of fuel-N. An increased fraction of straw in the fuel blend resulted in a higher potassium content, but no significant increase in slagging or fouling was observed. Only small amounts of deposit at the lower part of the radiant superheater and little slagging at the furnace walls were observed as a result of co-firing straw and coal. No significant effects on the performance of the desulfurization plant were detected, which may be due to the short test period, probably not allowing the desulfurization process to reach steady-state operation.

Quantification of fusion in ashes from solid fuel combustion

Abstract The fusion of ashes produced during solid fuel combustion greatly affects the tendency of these ashes to cause operational problems in utility boilers. In this paper, a new and quantitative laboratory method for assessing the fusion of ashes based on simultaneous thermal analysis, STA, is described. Using STA, melting is detected as an endothermic reaction involving no change in mass. The measurement signals are transferred into a fusion curve showing the melt fraction in the ash as a function of temperature. This is done either by a simple comparison of the energies used for melting in different temperature ranges or by accounting for the relevant melting enthalpies. The method repeatability is good, melting onset determinations and completions generally within 10°C, and melt fractions at given temperatures generally within 10% melt. Results are presented for simple binary salt mixtures, for which the agreement with fusion as determined by phase diagrams is very good, and for straw (salt-rich) and coal (silicate-rich) ashes. Comparing ash fusion curves to index points of current standard ash fusion tests showed initial melting at temperatures typically between 50° and 100°C – but in extreme cases as low as 260°C – below the melting onset as found by the standard fusion tests. Characterizing the fusion by STA provides a more detailed description of the ash fusion as compared to conventional methods, and the onset of ash fusion is more precisely determined. Furthermore, in combination with, e.g. computer-controlled scanning electron microscopy, the method enables identification of the chemical species melting in different temperature ranges. Since ash melting has a major impact on the deposit formation tendency, the presented detailed ash fusion determination improves the prediction of problems related to ash deposition in boilers.

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.

Ash Fusion Quantification by means of Thermal Analysis

A new experimental method for quantification of ash melting has been developed. Using the new method, a conventional STA apparatus is employed, and the melting is detected as endothermic reactions involving no change in mass. The DSC signal is transferred into a melting curve (showing the melt fraction in the ash as a function of temperature) either by simple comparison of the areas below the melting curve or by accounting for the relevant melting enthalpies. The execution of the measurement is simple and the repeatability of the results is very good. The subsequent conversion of the STA curves to a melting curve requires knowledge of the identity of chemical species in the ash and the involved chemistry.

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