• Analytical methods for determination of sulfur content in oil shalePikkor, Heliis16th International Symposium "Topical Problems in the Field of Electrical and Power Engineering. Doctoral School of Energy and Geotechnology III" : Pärnu, Estonia, January 16-21, 20172017 / p. 213-215 : ill http://www.ester.ee/record=b4650094*est
• Characterization and enhancement of oil shale fly ash from CFB boilerPikkor, Heliis; Järvik, Oliver; Lees, Heidi; Konist, Alar; Siirde, Andres; Maaten, Birgit6th International Conference on Smart and Sustainable Technologies, SpliTech 20212021 / p. 1-4 https://doi.org/10.23919/SpliTech52315.2021.9566470
• Characterization of oil shale kerogen semi-coke and its application to remove chemical pollutants from aqueous solutionsLees, Heidi; Jõul, Piia; Pikkor, Heliis; Järvik, Oliver; Mets, Birgit; Konist, AlarOil shale2023 / p. 115-132 : ill https://doi.org/10.3176/oil.2023.2.02 https://www.scopus.com/sourceid/17080 https://www.scopus.com/record/display.uri?eid=2-s2.0-85163091047&origin=inward&txGid=86c90fe445512ef1a3beabd8dcc7c567 https://jcr.clarivate.com/jcr-jp/journal-profile?journal=OIL%20SHALE&year=2023 https://www.webofscience.com/wos/woscc/full-record/WOS:000985171900002
• Co-combustion of coal and oil shale blends in circulating fluidized bed boilersKonist, Alar; Pikkor, Heliis; Nešumajev, Dmitri; Loo, Lauri; Järvik, Oliver; Siirde, Andres; Pihu, TõnuOil shale2019 / p. 114–127 : ill https://doi.org/10.3176/oil.2019.2S.03 http://www.kirj.ee/public/oilshale_pdf/2019/issue_2S/OS-2019-2S-114-127.pdf https://www.scopus.com/sourceid/17080 https://www.scopus.com/record/display.uri?eid=2-s2.0-85070496962&origin=inward&txGid=82099f0ee68fdb2ec9adbe39bf7a5176 https://jcr.clarivate.com/jcr-jp/journal-profile?journal=OIL%20SHALE&year=2022 https://www.webofscience.com/wos/woscc/full-record/WOS:000470019100003
• Comparison of the porous structure of oil shale fly ashes from two different CFB boilersPikkor, Heliis18th International Symposium "Topical Problems in the Field of Electrical and Power Engineering". Doctoral School of Energy and Geotechnology III : Toila, Estonia, January 14-19, 2019 : [proceedings]2019 / p. 99-100 : ill https://www.ester.ee/record=b5183874*est
• Determination of the total sulphur content of oil shale by using different analytical methodsMaaten, Birgit; Pikkor, Heliis; Konist, Alar; Siirde, AndresOil shale2018 / p. 144-153 : ill https://doi.org/10.3176/oil.2018.2.04 https://www.scopus.com/sourceid/17080 https://www.scopus.com/record/display.uri?eid=2-s2.0-85047477140&origin=inward&txGid=fd6e2603f349b261f723252eb3ecd918 https://jcr.clarivate.com/jcr-jp/journal-profile?journal=OIL%20SHALE&year=2022 https://www.webofscience.com/wos/woscc/full-record/WOS:000435129900004
• Effect of steam activation on oil shale semi-coke surface propertiesPikkor, Heliis; Konist, Alar; Maaten, Birgit; Järvik, Oliver; Lees, HeidiInternational Multidisciplinary Conference on Computer and Energy Science (SpliTech)2021 / 5 p https://doi.org/10.23919/SpliTech52315.2021.9566397
• Fast pyrolysis as a new approach to the upgrading of oil shale retorting methodsPikkor, Heliis; Maaten, Birgit; Siirde, AndresInternational IX Oil Shale Conference 2017 "Oil Shale Industry in Circular Economy" : 15th-16th November 2017, [Jõhvi], Ida-Viru County, Estonia : summary2017 / p. 23 http://www.ester.ee/record=b4751282*est
• Fluidized bed pyrolysis as a new approach for shale oil productionPikkor, Heliis; Siirde, Andres17th International Symposium “Topical Problems in the Field of Electrical and Power Engineering”. Doctoral school of energy and geotechnology. III : Kuressaare, Estonia, January 15-20, 20182018 / p. 277-279 : ill http://ise.elnet.ee/record=b2950220~S2*est
• Steam activation of oil shale to enhance the porosity of produced semicokePikkor, Heliis; Lees, Heidi; Konist, Alar; Järvik, Oliver; Maaten, BirgitEnergy Sources, Part A : Recovery, Utilization, and Environmental Effects2022 / p. 9064-9073 https://doi.org/10.1080/15567036.2022.2128471 https://www.scopus.com/sourceid/4500151514 https://www.scopus.com/record/display.uri?eid=2-s2.0-85139163161&origin=inward&txGid=5bfc74a1eaaeae40e77a23dee5f96824 https://jcr.clarivate.com/jcr-jp/journal-profile?journal=ENERG%20SOURCE%20PART%20A&year=2022 https://www.webofscience.com/wos/woscc/full-record/WOS:000861649200001
• Surface area of oil shale and its solid pyrolysis products depending on the particle sizePikkor, Heliis; Maaten, Birgit; Baird, Zachariah Steven; Järvik, Oliver; Konist, Alar; Lees, HeidiChemical engineering transactions2020 / p. 961−966 https://doi.org/0.3303/CET2081161 https://www.scopus.com/sourceid/19600161818 https://www.scopus.com/record/display.uri?eid=2-s2.0-85092040065&origin=inward&txGid=c8863b0231097f7e233d33f9fff9e32c
• Surface characterisation of Estonian oil shale semi-cokePikkor, Heliis; Lees, Heidi; Maaten, Birgit; Järvik, Oliver; Konist, AlarChemical engineering transactions2020 / p. 853-858 : ill https://doi.org/0.3303/CET2081143 https://www.scopus.com/sourceid/19600161818 https://www.scopus.com/record/display.uri?eid=2-s2.0-85092026171&origin=inward&txGid=dc9672753a4e2daf1be3b30827100a0e
• Utilization of pyrolytic wastewater in oil shale fired CFBC boilerKonist, Alar; Järvik, Oliver; Pikkor, Heliis; Nešumajev, Dmitri; Pihu, TõnuJournal of cleaner production2019 / p. 487-493 : ill https://doi.org/10.1016/j.jclepro.2019.06.213 https://www.scopus.com/sourceid/19167 https://www.scopus.com/record/display.uri?eid=2-s2.0-85067920306&origin=inward&txGid=80e5d410ddded82c04f720bed488c6d8 https://jcr.clarivate.com/jcr-jp/journal-profile?journal=J%20CLEAN%20PROD&year=2022 https://www.webofscience.com/wos/woscc/full-record/WOS:000483406000043