Straw-fired Boilers Research Articles

The Effect of Pre-Oxidation on High-Temperature Corrosion Resistance of Superheater Steels

The possibility to raise steam temperatures in biomass- and waste-fired boilers has drawn increased attention, when suitable alternatives for the use of fossil fuels in power production are considered. Currently in straw-fired boilers, for example, maximum steam temperatures are commonly between 450°C and 485°C, depending on the composition of the superheater steel, as compared to 560-600°C in coal-fired boilers. Therefore it is obvious, that increased steam temperatures are required for biomass- and waste-fired boilers in order for them to converge the power production efficiency of coal-fired boilers. In fact, every 10°C rise in the steam temperature results in an approximate increase of 2% in power production efficiency. Unfortunately, higher steam temperatures can easily lead to higher corrosion rates of the superheaters due to aggressive species such as potassium chloride (KCl), which can be found in the deposits at the superheater surfaces. From the material point of view, corrosion can be reduced to a certain extent by modifying the chemical composition of superheater steels to enhance their resistance to corrosion. From the chemical point of view, the challenge can be approached by studying the corrosion reactions and mechanisms. Instead of preventing corrosion with expensive high-alloyed materials, possibilities such as oxide layer manipulation/passivation should be studied more thoroughly to find alternative and more cost-efficient ways to design materials with improved capability to resist corrosion. The aim of this work is to study, whether certain pre-treatment conditions could enhance the high-temperature corrosion resistance of steels used in power plants combusting renewable fuels. More precisely, the effect of such variables as pre-oxidation temperature, time and atmosphere will be addressed. Dual samples of three genuine superheater steels; a low alloy ferritic 10CrMo9-10 steel, a Nb-stabilized austenitic AISI347 steel, and a high alloy austenitic Sanicro 28 steel, were first pre-oxidized under dry conditions at various temperatures (200°C, 500°C, and 700°C) for either 5 or 24 hours. After the pre-oxidation one sample piece of each steel was studied with X-ray Photoelectron Spectroscope (XPS) and with a scanning electron microscope coupled to an X-ray detector (SEM-EDX), whereas the other sample piece was exposed to KCl at 550°C for 168 hours. After the salt exposure the samples were cast into epoxy, cut for cross-sectional samples and analyzed with SEM-EDX. The XPS measurements provided information about the chemical composition and the thickness of the thin oxide layers formed during pre-oxidation, whereas the morphology of the surface and chemical composition of small surface features were studied with SEM (Fig. 1, left). Average oxide layer thickness, structure, and chemical composition were measured and analyzed from the cross-sectional samples with SEM (Fig. 1, right). Figure 1. SEM image with chromium oxide nuclei formed on the steel surface after the pre-oxidation (left) and an SEM image of the cross section of the same steel after exposed to KCl (right). According to the preliminary results, pre-treatment conditions affect the average oxide layer thickness, its structure and chemical composition. Most importantly, the resistance towards KCl-induced high-temperature corrosion was improved with pre-oxidation carried out under certain conditions. The final results are expected to provide more tools to material designers when developing novel materials with improved abilities to withstand material degradation in hostile environments. Figure 1

Lab-scale Investigation of Deposit-induced Chlorine Corrosion of Superheater Materials under Simulated Biomass-firing Conditions. Part 1: Exposure at 560 °C†

Deposit-induced chlorine corrosion was studied under well-controlled laboratory conditions, simulating the conditions in straw-fired boilers and boilers cofiring coal and straw. This was done by exposing pieces of superheater tube (TP 347H FG) covered with synthetic deposits of known Cl content to gas mixtures simulating straw-firing and cofiring of coal and straw, at 560 °C (1040 °F), for 3 days. The corroded specimens, and the reacted deposits, were studied in detail using a scanning electron microscope to determine the corrosion rate, investigate the chemistry and morphology of the corrosion attack, and study the sulfation behavior. Besides the gas compositions, various parameters were studied systematically. Most specimens suffered some internal attack, mostly by selective corrosion and in some cases by grain boundary attack. In all experiments with KCl and KCl−SiO2 deposits, the corrosion products consisted of an oxide scale, containing oxides of Cr and Fe, and on top of that a characteristic mixed layer of iron oxide threads in a potassium sulfate matrix. However, the thickness and shape of this layer was found to be strongly dependent on the experimental conditions. An increase of the percentage of KCl in the deposit resulted in a more uniform and deeper internal corrosion attack. The presence of HCl in the flue gas did not seem to be essential for chlorine-induced corrosion to occur, when a deposit containing KCl was present, but it enhanced the corrosion rate. The degree of sulfation of KCl in the deposits after exposure was quantified by wet chemical analysis and was shown to be dependent on many parameters, including the SO2 concentration in the gas flow, the concentration of KCl in the deposit, and the SiO2 and KCl particle sizes in the deposit. No simple relation was observed between the degree of sulfation in the deposit and the depth of internal attack or the thickness of the oxide or mixed layer. Whereas SiO2 particles were found to be chemically inert with respect to the flue gas and the corrosion attack, CaO particles reacted with HCl from the flue gas, and the resulting CaCl2 played an important role in the corrosion mechanism. As a result, the corrosion rate was strongly enhanced when CaO was present in the deposit, instead of SiO2.

Experiences with Inconel 625 in Biomass and Waste Incineration Plants

Inconel 625 is utilised in both biomass and waste incineration plants in Denmark. In both cases, the performance is good however the morphology of corrosion attack is different which indicates different corrosion mechanisms. In waste incineration plants there is general attack and shallow pitting, and in some cases dendritic attack especially on the fins of waterwalls. The dendritic attack is in the dendritic core. The presence of pits or dendritic attack is linked to the temperature of the metallic surface and the molten salt composition. In a woodchip biomass plant, chromium depletion was observed on the surface of the weld overlay leaving behind a nickel and molybdenum rich porous structure. The corrosion attack was not related to the dendritic microstructure of the weld. In two straw-fired biomass plants, co-extruded Sanicro 63 (alloy 625 type) as well as Inconel 625 weld overlay revealed the same type of attack, again chromium depletion. This indicates that the corrosion mechanism in woodchip and straw power plants are similar. Another interesting result in straw-fired boilers was that Nibas welds (alloy 625 composition) could provoke excessive corrosion in adjacent 18-8 stainless steels indicating a galvanic reaction. The corrosion mechanisms observed in each case are discussed in relation to temperature and corrosive environment.

Towards a CFD-based mechanistic deposit formation model for straw-fired boilers

This paper discusses the application of FLUENT™ in the analysis of grate-fired biomass boilers. A short description of the concept used to model fuel conversion on the grate and the coupling to the CFD code is offered. The development and implementation of a CFD-based deposition model is presented in the reminder of the paper. The growth of deposits on furnace walls and super heater tubes is treated including the impact on heat transfer rates determined by the CFD code. Based on the commercial CFD code FLUENT™, the overall model is fully implemented through the User Defined Functions. The model is configured entirely through a graphical user interface integrated in the standard FLUENT™ interface. The model considers fine and coarse mode ash deposition and sticking mechanisms for the complete deposit growth, as well as an influence on the local boundary conditions for heat transfer due to thermal resistance changes. The model is applied to the straw-fired Masnedø boiler. Results are in good qualitative agreement with both measurements and observations at the plants.

Simultaneous Thermal Analysis (STA) on Ash from High-Alkali Biomass

Improved knowledge of the melting and gas-phase release behavior of ash from biomass fuels is of premium importance, to reduce ash-related problems in straw-fired boilers. In this study, a simultaneous thermal analysis (STA) instrument was used to characterize the behavior of 12 significant different annual biomass ash samples, and several model compounds, such as SiO2, KCl, K2SO4, K2CO3, CaSO4, CaCO3, and some of their mixtures. The STA analyses provided information on mass loss and exothermic/endothermic reactions as a function of sample temperature. The mass loss curves of the biomass ashes during STA heating could, in most cases, be explained from the chemical composition of the ash. Below 800 °C, weight loss is caused by the release of CO2 from CaCO3. At temperatures of 850−1150 °C, KCl evaporates and K2CO3 reacts with SiO2 and CO2 is released. Above 1150 °C and up to 1450 °C, a weight loss from the ash samples may be caused by the release of K2O produced from the K2CO3 decomposition or from silicate...

SEM Investigation of Superheater Deposits from Biomass-Fired Boilers

Straw is used as fuel in relatively small-scale combined heat and power producing (CHP) grate boilers in Denmark. The large content of potassium and chlorine in straw greatly increases the deposit formation and corrosion of the superheater coils, compared to boilers firing coal. In this study, mature superheater deposit samples were extracted from two straw-fired boilers, Masnedø and Ensted, with fuel inputs of 33 MWth and 100 MWth, respectively. SEM (scanning electron microscopy) images and EDX (energy dispersive X-ray) analyses were performed on the deposit samples. Different strategies are adopted to minimize deposit problems at the two boilers. At Masnedø the final superheater steam temperature is 520 °C, no soot blowing of the superheaters is applied and a relatively large superheater area is used. At Ensted, an external wood-fired superheater is used in order to obtain a final steam temperature of 542 °C, while the steam exit temperature of the straw-fired boiler is 470 °C. The mature Masnedø deposit had a thickness of 2 to 15 centimeters and consisted of three distinct main layers. The thick intermediate layer was depleted in chlorine but rich in Si, K, and Ca. This Masnedø intermediate layer was probably generated by in-situ reaction between KCl and Si-rich ash particles, which leads to release of chlorine-containing gases. The innermost layer contained many sublayers of mainly iron oxide, KCl, and K2SO4. The Ensted deposit had a maximum thickness of a few centimeters. The intermediate Ensted layer consisted of melted KCl with inclusions of Ca- and Si-rich particles, and the innermost layer was an iron oxide next to a potassium sulfate layer. Compared to deposits formed on a probe during short-time experiments, the mature superheater deposits contained larger dense inner sublayers of pure KCl and K2SO4. The present study indicates that the innermost layer of the superheater deposits expands by condensation of KCl, even when the deposit has a thickness of several centimeters.

00/01418 Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study

00/01418 Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study

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 K 2SO 4 present adjacent to the metal surface. It is argued that the K 2SO 4 layer present adjacent to the metal surface may lead to reduced corrosion rates at this boiler. A discussion of the deposit structure, the K 2SO 4 layer formation mechanism, and the influence of the inner layer composition on the corrosion of the superheaters is provided.

Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study

The fate of inorganic salts in straw-fired combustion systems was studied in Sandia's Multifuel Combustor. Special attention was drawn to the deposition of potassium, chlorine, and sulphur and the sulphation of potassium chloride to potassium sulphate. The experiments included wheat straw-firing under different combustion conditions and with elevated sulphur levels. Investigations of deposit formation on a simulated superheater tube placed in cross flow with the gas revealed that the deposit was ellipse-shaped and had an inner layer of white condensed salts. The inner layer had a dendritic structure and consisted of potassium chloride and potassium sulphate. Part of the water-soluble potassium in the straw was transformed into potassium silicates and a mixed mineral of potassium, calcium, and silicon. A novel probe was designed to investigate condensation phenomena. SEM-analyses revealed that the structure of the potassium salts consisted of individual angular particles of primarily KCl (1–2 μm) and a sponge-like matrix of sub-micron particles consisting primarily of K 2SO 4, which may represent vapour condensate agglomerates. Mechanisms of deposit formation and the fate of chlorine, potassium, and sulphur in the system are discussed.

Lab-Scale Investigations of High-Temperature Corrosion Phenomena in Straw-Fired Boilers

The corrosion of superheater tube material in straw-fired boilers was investigated in the laboratory. Metal test elements of boiler tube steel (X20CRMV121 and AISI 347FG) were covered with synthetic (KCl and/or K2SO4) and real deposits, and exposed to a synthetic flue gas (6 vol % O2, 12 vol % CO2, 400 ppmv HCl, 60 ppmv SO2, balance N2) in 550 °C electrically heated ovens. Exposure times from 1 week to 5 months were used. The corrosion of the metal test elements was, in general, quite uniform, and the corrosion products consisted mainly of oxides of iron and chromium. All test elements covered with KCl suffered from minor internal attack, and some elements had severe pits with chlorine found in the pit. A dense layer of potassium sulfate and iron oxide was found adjacent to the metal oxide layers on all the metal test elements covered with a deposit containing KCl. The layer had a characteristic structure, with iron oxide threads in a dense potassium sulfate matrix. A mechanism for chlorine corrosion is s...

The formation of submicron aerosol particles, HCl and SO 2 in straw-fired boilers

The concentration of submicron particles, HCl and SO 2 in the flue gas from the combustion of straw from wheat, barley and oil seed rape was studied by field measurements on two utility boilers. The concentration of submicron particles varies from 75–2000 mg Nm -3 § § 1Nm 3 (normal cubic meter)=1m 3 gas measured at 1atm and 0°C. and the mean diameter in the range from 0.2 to 0.6 μm. The principal constituents of the submicron particles are potassium sulfhate and chloride. The concentration and composition of the submicron particles depend on the content of K, Cl and S in the straw. These elements are to a great extent volatilized in the combustion zone. A theoretical analysis indicates that the gas to particle conversion occurs during the cooling of the flue gas by the homogeneous nucleation of K 2SO 4 particles, which act as condensation nuclei for the subsequent condensation of KCl. It is found that the kinetics of the oxidation of SO 2, which is slow below 800°C, is important for the chemical changes of the flue gas. The composition of the submicron particles and the concentration of SO 2 and HCl can be approximately predicted from the assumption that the flue gas composition is determined by a chemical equilibrium for the sulfation reaction at 812±10°C.

Deposition Investigation in Straw-Fired Boilers

In Denmark straw has been applied as fuel at small combined power and district heating plants since the late 1980`s. New equipment have been developed to handle and burn the straw. However, straw is by no means a trivial fuel, problems like fluctuations, deposition in the furnace, corrosion and a poor burn out have been observed at boiler plants. One of the problems has been, the large differences in the amount of deposition in the furnace chamber and on superheater tubes when applying different fuel parcels. Knowledge of how to characterize the straw for predicting the deposition properties in the boiler is needed. We will here present deposition experiments performed at two straw fired power plants with a fuel input of respectively 18 and 30 MW. Some limited experiments have earlier been performed on smaller scale boilers, no clear correlation between laboratory analyses of the straw and deposition in the boilers was found. Combustion experiments on a combined wood and straw fired 18 MW stoker fired boiler have been performed. Strong indications, of condensation of the mineral matter at different places in the boiler chamber, were seen.