VGB POWERTECH 10 (2020) - International Journal for Generation and Storage of Electricity and Heat

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Wood fly ash as cement replacement VGB PowerTech 10 l 2020 by Junckers Industries A/S. About 140.000 ton wood a year is incinerated at this plant. The wood species incinerated are ash, merbau, maple, oak, jatoba and beech, and the incineration temperature in the wood-fired grate boiler is 900 to 1,000 °C. The ash is sprayed with water just after the incinerator in order to avoid spontaneous ignition and dust problems during handling, and the WA sample for this investigation was taken after this spraying. WA2 is fly ash from HOFOR Amagerværket, which fires wood pellets from mainly pine and spruce (waste from wood industry so the fuel can contain bark). The plant incinerates about 350.000 ton wood a year. The incineration temperature is about 1,000 to 1,100 °C in the grate boiler. 2.2 Ash characterization The crystalline phases in of the WAs were identified by X-Ray Diffraction (XRD). The WA was loaded into the sample holder using backloading. A PanAnalytical X-ray diffractometer, sat at the PW3064 Spinner stage for standard powder samples have been used for the measurements. Cu-Kα radiation with a wavelength of 1.54 Å was used as the x-ray source. The samples were measured between 4 °2θ and 100 5 °2θ with a step size of 0.002 °2θ and a sampling time per step of 24.8 s. The XRD plots were qualitatively evaluated using X’Pert High- Score Plus software, with data from the International Centre for Diffraction Data (ICDD). XRF and XRD were performed with powder samples. The WAs were dried at 105 °C for 24 hours prior to the following characterization. The content of major chemical element of the WAs were measured by X-ray fluorescence (XRF) spectrometry by the use of a SPEC- TRO Gmbh X-LAB 2000 with a Pd-tube on samples grounded to a particle size of ≤ 200µm. The equivalent content of oxides was calculated based on the element content. The concentrations of Cd, Cr, Cu, Pb and Zn were measured after pre-treatment of the ash in accordance to DS259; 1.0 g ash and 20.0 ml (1:1) HNO 3 was heated at 200 kPa (120 °C) for 30 minutes. Filtration through 0.45 µm filter and the concentrations measured with ICP-OES in the filtrate. Ash pH was measured by suspending 10.0 g ash in 25 ml distilled water. After 1 hour agitation, pH was measured directly in the suspension with a Radiometer pH electrode. The suspension was filtered, and the water soluble Cl and SO 4 concentrations were measured in the filtrate by Ion Chromatography while water soluble concentrations of Ca, K, Mg and Na were measured with ICP-OES. The acid soluble fraction of Ca and Mg was found by suspending 5 g WA in 1.0 M HNO 3 , agitating the suspension for 1 week and measuring the elemental concentrations in the filtrate with ICP-OES. Loss on ignition (LoI) was found after 30 minutes at 550 °C. Water content Tab. 1. Summary of WA pre-treatments. As received Dried (105°C) was measured as weight loss after 24 hours at 105 °C (calculated as weight loss over the weight of the wet sample). Three replicates of each of these analyses were made. Solubility in water was evaluated: 50.0 g ash suspended in 500 ml distilled water and agitated for 1 min. After settling, the water was decanted. New 500 ml distilled water added. This was repeated so the ash was washed three times. Finally, the suspension was filtered and the ash dried at 105 °C and weighed. 2.3 Pre-treatments The WAs were pretreated to see the effect on the ash mineralogy and following the influence on the compressive strength of mortar. The major ash constituents after the different pretreatments were identified by XRD. The pretreatments were: –– WA1 was dried at 105 °C for 24 hours to see possible changes in chemical compounds from this procedure. –– WA2 was hydrated in the lab to see if the same ash compounds would be obtained in this ash as in WA2, which was hydrated right after incineration. The WA1 was mixed with water similar to WA1 as received and left in a closed bucket for 2 weeks. The wet ash was mixed manually every second day. –– Both ashes contain organic compounds and the ashes were fired in a muffle oven 550 °C to diminish the content of organic material and to investigate if this second heating changes the ash composition. The loss of ignition at 550 °C is as a simple method for estimating the residual carbon in bio ashes [Zhao et al. 2013]. –– Both ashes were washed in water using the same procedure as when evaluating the solubility (paragraph 2.2). –– Both ashes were washed in acid (1.0 M HCl). The same procedure as for evaluating the solubility (see paragraph 2.2) was used, except for the first washing (out of the three subsequent washings) being in this acid instead of distilled water. The mass loss from the acid wash was measured. Ta b l e 1 is a summary of the ashes and pretreatments. 2.4 Mortar sample preparation, compressive strength and setting Mortar was prepared with the different ashes and pre-treated ashes as given in Ta - b l e 1 . For mortar preparation a Portland cement (CEM II/A-LL 52.5R) was used. The sand used in the experiment was a natural sea sand 0 to 4 mm with technical Hydrated Heated (550°C) Water washed (WW) specification following EN 12620. The procedure and recipe base for the mortar samples was from EN 196-1, but we used sea sand and not the standardized sand prescribed in the standard. The mix used in this investigation was: 225 ml water, 450 g cement (ordinary Portland) and 1,350 g sand. In the mortar with WA, 5 or 10 % cement was replaced with ash (corresponding to 22.5 g and 45 g respectively). The mixing and moulding were sought carried out as prescribed in EN 196-1, however low workability experienced during the casting of some of the mixes (mainly with 10 % cement replacement) was so low that higher frequency and longer duration of the vibration was necessary. The ease of casting related to the workability in comparison to the references were noted. The mould consisted of three horizontal compartments so three prismatic specimens were prepared simultaneous for each recipe (40 mm ×40 mm in cross section and 160 mm in length). Demoulding was carried out after 20 to 24 hours and the samples were cured horizontally in a water bath. The ambient temperature during curing was 21 to 25 °C, which was higher than the 20 ±1 °C as prescribed in EN 196-1. After 7 days, the compressive strength was measured. For compressive strength, the mortar prisms were segmented into two in a three-point loading setup, where after the compressive strength was measured for each segment, i.e. six measurements for each mortar recipe. The testing machine for compressive strength was a Mohr & Federhaff AG-49. 3 Results and discussion Acid washed (AW) WA1 X X X X X WA2 X X X X X 3.1 Characterizations of investigated ashes Ta b l e 2 summarizes the overall results from the characterization of the investigated ashes. 3.1.1 Compliance of WAs with EN450- 1 for fly ash in concrete The use of WA is not covered by the current standard governing the use of fly ashes as mineral additions in concrete (EN 450-1). The regulations preclude the use of any material not derived from coal combustion and co-combustion ashes with coal. However, as there is no standard covering WA in concrete, this standard is often used to evaluate the qualities for WAs (e.g. in [Carević et al. 2019], and [Sigvardsen et al. 2019]). It is reasonable to think that a future extension of the current regulations 52
VGB PowerTech 10 l 2020 Wood fly ash as cement replacement Tab. 2. Characteristics of the investigated WAs (~0 indicates below detection limit). General parameters Water solubility (g/kg) Solubility in 1 M HNO 3 (g/kg) Major constituents ( %) (XRF) Heavy metals (mg/kg) (extraction after DS259) pH Loss on ignition 550°C ( %) Water content ( %) Total solubility Water soluble Cl- Water soluble SO 4 2 - Water soluble K Water soluble Na Water soluble Ca Water soluble Mg Acid soluble Ca Acid soluble Mg SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO MnO Na 2 O K 2 O TiO 2 P 2 O 5 SO 3 Cl Zn Cu Pb Cd Cr WA1 13.7 6.6 19.3 270 10.4 84 127 1.1 30 ~0 32.6 73.3 2.6 0.2 0.5 37.8 4.6 2.1 0.8 18.1 0.7 1.8 5.5 0.8 340 130 27 5.1 11 WA2 13.5 2.0 0.3 138 6.4 40.5 56 3.7 18 ~0 2.8 0.2 8.8 1.6 1.4 37.8 3.0 0.7
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