• Energy Research

Iron-sulfides were observed in deposits collected on a probe inserted at the top of the furnace of a coal-fired power station in Denmark. The chemical composition of the iron-sulfides is equivalent to pyrrhotite (FeS). The pyrrhotites are present as crystals and, based on the shape of the crystals, it was deduced that they were not deposited but instead grew within the deposit. The presence of unburned char particles within the deposits supports the concept that a reducing environment existed in the deposits. Two processes are proposed for explaining the existence of pyrrhotite crystals within a deposit: (1) impact of low viscous droplets of iron sulfide; and (2) sulfur diffusion. Previous research on the influence of pyrite on slagging focused on the decomposition of pyrite into pyrrhotite and especially on the oxidation stage of this product during impact on the heat transfer surfaces. This research shows that the influence of pyrite and its derivatives is also strongly controlled by the flue gas composition in the deposits.

Utilization of biomass as wood or straw in large suspension­fired boilers is an efficient method to reduce the use of fossil fuels consumption and to reduce the net CO2 formation. However, the presence of chlorine and alkali metals in biomass (straw) generate ash with a low melting point and induce large problems of ash deposit formation on the superheater tubes. Full scale studies on biomass ash deposition and removal had been done on biomass grate boilers, while only limited data is available from biomass suspension­firing. The aim of this study was to investigate deposit mass uptake, heat uptake reduction, fly ash and deposit characteristics, and deposit removal by using an advanced online deposit probe in a suspension­fired boiler using wood and straw pellets as fuel. The influence of fuel type and probe exposure time on the ash deposition rate, the heat uptake, the fly ash and deposit characteristics, and deposit removal have been investigated. The final deposit mass signal after a residence time of 3 to 5 days region was 1041, 1475, 1520 and 1670 g/m2 for 35, 65, 80 and 100% straw share respectively in the superheater region (flue gas temperature, 800­900 oC), while the mass uptake was very small in the tube bank region (flue gas temperature, 550­605 oC) during pure wood­firing. It was found that during suspension­firing of pure straw at low boiler load, the overall weight uptake is comparable with grate­firing, even though the amount of fly ash generated was significantly higher during suspension­firing. Deposit removal through debonding was the main mechanism of deposit shedding when no plant sootblower was in operation. Elemental analysis of fly ashes and deposit samples was made in order to determine concentrations of the major elements Al, Ca, Fe, K, Mg, Na, P, Si, S and Cl. It was identified that the straw suspension­firing fly ashes contain high contents of Si and Ca, while grate­firing fly ashes contain higher contents of volatile elements K, Cl and S. Proceedings of the 18th European Biomass Conference and Exhibition, 3-7 May 2010, Lyon, France, pp. 1094-1100

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.

The release to the gas phase of inorganic elements such as alkali metals, Cl, S, and heavy metals in Waste-to-Energy (WtE) boilers is a challenge. Besides the risk of harmful emissions to the environment, inorganic elements released from the grate may cause severe ash deposition and corrosion problems in the boiler, subsequently leading to decreased overall efficiency and costly, unscheduled shut-downs. The objective of this study was to obtain quantitative data on the release of inorganic elements from dedicated, well-characterized waste fractions; in order to understand the release pattern and the link to the formation of fly ash and aerosols in full-scale waste incinerators. The release of metals, S and Cl from four dedicated waste fractions was quantified as a function of temperature in a lab-scale fixed-bed reactor. The waste fractions comprised chromated copper arsenate (CCA) impregnated wood, shoes, automotive shredder waste and PVC (poly-vinyl-chloride). The waste fractions were characterized by use of wet chemical analysis, and, based on the chemical composition of the initial fuel sample and the ash residue after the experiments; the release of inorganic elements was quantified. The lab-scale release results were then compared with results from a related, full-scale partitioning study, in which test runs with the addition of similar, dedicated waste fractions to a base-load waste had been performed in a grate-fired WtE boiler. In general, the elements Al, Ca, Cr, Cu, Fe, Mg, Si and Ti were not released, in consistency with the non-volatile nature of these elements. The elements Pb, Zn, Cl, S, Na, K and As, on the other hand, were released to a significant extent, and the release pattern was found to be both temperature dependent and fuel specific. Possible release mechanisms were discussed in relation to the chemical characteristics of each fuel, and some of the same release mechanisms as previously suggested for e.g. biomass-based fuels in the literature also seemed to apply for the present waste fractions.

Ash deposition on boiler surfaces is a major problem encountered in biomass combustion. Timely removal of ash deposits is essential for optimal boiler operation. In order to improve the understanding of deposit shedding in boilers, this study investigates the adhesion strength of biomass ash from full-scale boilers, as well as model fly ash deposits containing KCl, K2SO4, CaO, CaSO4, SiO2, K2CO3, Fe2O3, K2Si4O9, and KOH. Artificial biomass ash deposits were prepared on superheater tubes and sintered in an oven with temperatures ranging from 500 to 1000 °C. Subsequently, the deposits were sheared off by an electrically controlled arm, and the corresponding adhesion strength was measured. The effect of sintering temperature, sintering time, deposit composition, thermal shocks on the deposit, and steel type was investigated. The results reveal that the adhesion strength of ash deposits is dependent on two factors: ash melt fraction, and corrosion occurring at the deposit–tube interface. Adhesion strength inc...

The high content of alkali chloride in deposits which form during biomass firing in power plants contributes significantly to corrosion of the superheaters. In order to understand the influence of ...

A number of full-scale deposit probe measuring campaigns conducted in grate-fired and suspension-fired boilers, fired with biomass, have been reviewed and compared. The influence of operational parameters on the chemistry of ash and deposits, on deposit build-up rates, and on shedding behavior has been examined. The firing technology and the fuel utilized influence the fly ash and deposit chemical composition. In grate-firing, K, Cl, and S are enriched in the fly ash compared to the fuel ash, while the fly ash in suspension-firing is relatively similar to the fuel ash. The chemical composition of the deposits formed is determined by the fly ash composition and the flue gas temperature; increases in the local flue gas temperature lead to higher contents of Si and Ca and lower contents of Cl in the deposits. The net deposit build-up rates in grate-fired and suspension-fired boilers are at similar levels, 0–100 g/m2·h, while the ash deposit propensity is an order of magnitude larger in grate fired boilers th...

This study investigated deposit formation of biomass fly ash on steel tubes, in a lab-scale Entrained Flow Reactor. Experiments were conducted using model biomass fly ash, prepared from mixtures of K2Si4O9, KCl, K2SO4, CaO, SiO2 and KOH, as well as three different boiler fly ashes: a wood fly ash, a straw fly ash, and a straw + wood cofired fly ash. The fly ashes were injected into the reactor, to form deposits on an air-cooled deposit probe, simulating deposit formation on superheater tubes in boilers. The results revealed that increasing flue gas temperature, probe surface temperature, time, fly ash flux and fly ash particle size increased the rate of deposit formation. However, increasing flue gas velocity resulted in a decrease in the deposit formation rate. A mechanistic model was developed for predicting deposit formation in the reactor. Inertial impaction was the primary mechanism of deposit formation, when pure K2Si4O9, SiO2 or CaO was injected into the reactor, forming deposits only on the upstream side of the steel tube. However, feeding KCl, K2SO4 or KOH into the reactor resulted in deposit formation on both sides of the steel tube, via condensation, thermophoresis, and inertial impaction. Proceedings of the 26th European Biomass Conference and Exhibition, 14-17 May 2018, Copenhagen, Denmark, pp. 440-452

The release of critical ash-forming elements during the pyrolysis and combustion of corn stover has been investigated through controlled lab-scale experiments supported by multicomponent and multiphase thermodynamic equilibrium calculations. Fuel samples were treated under isothermal conditions ranging from 500 to 1150 °C, under both pyrolysis and combustion atmospheres. The volatilized material was quantified by means of mass balances based on char and ash elemental analysis, compared to a corresponding feedstock fuel analysis. Close relations between the observed K and Cl release are found, suggesting that Cl is the main facilitator for K release through sublimation of KCl, determined to begin as the reaction temperature approaches 700–800 °C. K is present in abundance relative to Cl, and the K release is found to cease as the fuel reaches complete dechlorination. In addition, around 50 wt % of the Cl is released at temperatures below 500 °C, presumably as HCl formed through ion-exchange reactions with ...