It is good to do biur chometz [burning of the chometz] by the fifth hour of the day like every year, in order not to get confused between this year and other years (Shulchan Aruch 444:2). However, we don’t do bitul chometz until Shabbos morning, as even if one has intention to annul all his chometz besides for what he plans to eat on Shabbos, perhaps some will be leftover, and he will then have to do bitul again (Mishnah Berurah s.k. 10). If we would do bitul chometz during the time of burning chometz, we would anyway have to do it again on Shabbos morning (Mishnah Berurah s.k. 22).
Those who have the custom to say the “Yehi Ratzon” every year after biur chometz, should say it on erev Shabbos after burning the chometz (Shu”t Teshuvos V’Hanhagos 2:211:4). Although one still intends to keep chometz in his house, every kezayis one destroys is included in the mitzvah of “Tashbisu”.
