Investigation of the Effect of Biomass-Limestone Interaction on the Ash Composition for Co-combustion of a Lignite Coal and Olive Cake in a Circulating Fluidized Bed Combustor


VAROL M., Atimtay A.

29th Annual International Pittsburgh Coal Conference, Pittsburgh, United States Of America, 15 - 18 October 2012, pp.1412-1438

  • Publication Type: Conference Paper / Full Text
  • City: Pittsburgh
  • Country: United States Of America
  • Page Numbers: pp.1412-1438
  • Akdeniz University Affiliated: No

Abstract

In this study, it is aimed to investigate the effect of biomass+limestone mixtures on ash composition. Olive cake was used as a biomass resource because of its high alkali metal (Na and K) content (K2O content of olive cake ash was about 50% by wt.). Bursa/Orhaneli lignite and Can limestone was used in the tests. The Ca/S ratio was kept at 2. The tests included co-combustion of Bursa/Orhaneli lignite and olive cake mixture containing 50% by wt. olive cake with and without limestone addition. Co-combustion tests were conducted in a circulating fluidized bed combustor at 850oC. The combustor column has an inside diameter of 108 mm and a height of 6 m. A Deposit Sampling Probe (DSP) was designed and used in the upper part of the combustor column during the tests to simulate a heat exchanger tube. The probe was cooled to 550oC with air. The particles accumulated on the surface of the probe were collected at the end of the combustion test. After the probe was taken out of the combustor column, the particles collected on the probe were very slowly and carefully scraped from the surface into a sampling pot. Then, they were sent for XRD and SEM-EDS analysis. At the end of the combustion test, bottom ash (BA) was also collected. Bottom ash samples were taken for XRF, XRD and SEM-EDS analyses. Ash deposition rate was calculated. Ash balance was done. In order to compare the results from co-combustion tests in the CFBC, Bursa-Orhaneli lignite and olive cake samples were combusted alone in a furnace according to the ASTM standards (ASTM D1374-04 Standard Test Method for Ash in the Analysis Sample of Coal and Coke from Coal for coal samples, ASTM E1755-01 Standard Test Method for Ash in Biomass for olive cake). The ash samples obtained were subjected to XRF and XRD analyses. The results of fuel ash analyses were compared with the analysis results of ashes obtained from the co-combustion tests (both the bottom ash and the ash collected on the deposit probe).

The results of this study showed that although SiO2, MgO, and CaO are the major oxides in the bottom ash for the co-combustion test without limestone, CaO, SO3, SiO2, and MgO are also the major oxides in the bottom ash from the tests with the addition of limestone. However, the CaO content in bottom ash has increased due to limestone addition. Quartz (SiO2), Lime (CaO), Anhydrite (CaSO4), Arcanite (K2SO4) and Forsterite (Mg2(SiO4)) are the phases encountered in the bottom ash for the co-combustion test without limestone. The Quartz (sand) which is used as bed material is the dominant phase in the bottom ash. Quartz (SiO2), Calcite (CaCO3), Lime (CaO), Anhydrite (CaSO4), Arcanite (K2SO4) Potassium Calcium Sulfate (K2Ca2(SO4)3) and Forsterite (Mg2(SiO4)) are the phases encountered in the bottom ash for the co-combustion test with limestone. Limestone addition to the fuel mixture might transfer K element from Arcanite to Potassium Calcium Sulfate in the bottom ash. While K was in the form of Arcanite for the co-combustion test without limestone, it mostly appears in the form of Potassium Calcium Sulfate in the bottom ash for the co-combustion test with limestone. The main phase determined in the deposits obtained from the Deposit Sampling Probe is also Potassium Calcium Sulfate. Since the surface of the deposit sampling probe is at 550oC, it is expected that the K oxides in the fuel mixture condenses on the surface of the probe and transforms into the phase of Potassium Calcium Sulfate. SEM-EDS results are strongly consistent with XRF and XRD results for the bottom ash samples and the ash from the Deposit Sampling Probe.