Effects of Water Leaching on the Ash Sintering Problems of Wheat Straw

Abstract

Biomass energy has been used for decades in lieu of fossil fuels. However, biomass, such as wheat straw, typically contains a high concentration of alkali elements, which is likely to induce unfavorable conditions during combustion, such as slagging, agglomeration, and corrosion in the boiler. This study investigated the effects of leaching on the chemical compounds and sintering temperatures of wheat straw ash before and after leaching by tap water. Ash melting and sintering degree tests were conducted using hot-stage microscopy and a scanning electron microscope, respectively. The results show that the ash content in wheat straw decreased by 26.09% (from 4.14% to 3.06%) following leaching, as did the chlorine (Cl), sulfur (S), and nitrogen (N). Meanwhile, the ash-related elements such as potassium (K), magnesium (Mg), and silicon (Si) reduced after leaching too. Additionally, the higher heating value increased slightly, from 19.25 to 19.53 MJ/kg. At the same time, leaching improved the ash melting temperatures of wheat straw during combustion and minimized the ash sintering degree. Similar results were also shown in scanning electron microscope (SEM) images, which clearly indicated that the leached samples had a lighter sintering degree than the original samples. Overall, the leaching process had a positive effect on the ash sintering problems of wheat straw.Energies 2019, 12, 387 2 of 14 are readily available and renewable for potential energy sources. Currently, cereal residues are considered among the most important biomass energy resources because of their vast production [4]. During 2013, cereal crops such as wheat and rice provided more than 670 million metric tons of food, dominating human food supplies [5].Challenges in both the fuel preparation and burning processes of cereal residues, such as gathering, carriage, pre-treatment, pelleting, and combusting, have attracted the attention of scholars and industries, particularly ash-related problems such as ash sintering, agglomeration, and corrosion induced while burning in boilers [6,[9][10][11][12][13][14][15][16]. High concentrations of Cl combined with alkali metals, such as Na and K, in biomass fuels typically generate sediments during the combustion process. The sediments, such as sinters on the super heater, agglomerate on the water wall, and corrosion of the tube surface occurs not only in fluidized boilers, but also in grate furnaces, and has obvious negative effects on boiler efficiency [10][11][12][13]15].Various measures, such as water or acidic leaching, mineral addition, and co-firing with fossil fuels, have been adopted to solve the aforementioned biomass ash-related problems by optimizing the fuel chemical compositions with low vaporized alkaline compounds [15]. Water or acidic washing (or leaching) directly removes water-soluble elements (e.g., K, Ca, Na, and Cl) from the biomass, which improves biomass fuel characteristics at high temperatures and reduces ash deposits during combustion [17][18][19][20][21][22][23][24]. The heating value i...

Complex Study of Straw Suitability for the Production of Nonindustrial Straw Pellets

“…Many researchers carried out straw leaching at different temperatures, from 25 up to 100 °C . Consumption of distilled, deionized or tap water for straw leaching was characterized by water-to-straw ratio, which was from (0.00525–0.024) L/g (Thompson et al), 0.012 L/g (Wu et al), 0.040 L/g (Jenkins et al), 0.050 L/g (Ma et al), and up to 0.065 L/g (Alabdrabalameer et al) . Straw leaching was often conducted without control of leaching completion; measurements of leachate electrical conductivity or water consumption were not reported. , …”

“…There are different approaches to describing the behavior of leached straw ash: measurements of ash composition with calculations changing slagging and fouling indexes, describing the behavior of leached straw ash in energy installation, and determining characteristic temperatures of ash melting behavior . In the last-mentioned paper, it was shown that due to appropriate water leaching, the ash deformation temperature increased from 920 °C for the original straw to 1320 °C for the leached, and for another sample of straw, from 910 to 1250 °C . In study for the sample of original wheat straw, the ash deformation temperature was 1020 °C, and for the ash of water-leached straw it increased only to 1050 °C.…”

Comparison of Fuels and Effluents Originating from Washing and Hydrothermal Carbonisation of Residual Biomass

“…Washing and a thermo-chemical treatment by hydrothermal carbonisation (HTC) have proven to be effective in reducing the amount of alkali metals and Cl in the upgraded biofuels. Therefore they can be applied to control ash-related issues [8][9][10][11][12][13][14][15][16][17]. Both washing and HTC alter the inorganic fuel composition by the dissolution of soluble inorganics in the process water.…”

Issues of Corrosion and Degradation under Dusty Deposits of Energy Biomass

“…The vast majority of publications on the corrosive influence of biomass concerns the high temperature corrosion of boilers under combustion conditions above 550 • C. The described corrosion processes are related to the activity of potassium and other alkaline metal compounds [25,26] or the presence and activity of fly ash deposits, specific for biomass co-firing [27,28]. Moreover, a number of publications concern surface layers resistant to specific conditions of energy biomass combustion, e.g., [29,30].…”