As we have learned, in addition to bitul ḥametz, the Sages ordained the active elimination of all ḥametz remaining after breakfast on the morning of the 14th, and any ḥametz that was found during bedikat ḥametz (including the ten pieces of bread that were hidden before the search). Technically one may eliminate the ḥametz in any number of ways, for example: by crumbling it and scattering it into the wind, the sea, or a river (SA 445:1); by pouring bleach or some other substance on it, thus rendering it unfit to be eaten by a dog before the prohibition of ḥametz takes effect, for then it is not considered ḥametz food, so it need not be eliminated (SA 442:9); by placing it, before the prohibition takes effect, in an ownerless public domain; or by flushing it down the toilet, whereby it no longer remains in the house (MB 445:18).
Nonetheless, the holy people of Israel customarily enhance the mitzva of eliminating the ḥametz via burning it. Nothing eliminates ḥametz better than fire. Furthermore, there are poskim who maintain that the mitzva to dispose of ḥametz must be fulfilled by burning.
Those who wish to enhance this mitzva further must make sure to nullify the ḥametz after burning it, since if they nullify it beforehand, the ḥametz will no longer be considered theirs, and they will lose the enhancement of bi’ur by burning. One must therefore be careful to leave enough time after burning the ḥametz to nullify it, for after the fifth hour of the day it is no longer possible to nullify ḥametz (as we learned above 3:6). Hence, as soon as a kezayit of ḥametz has been burned, the enhancement of bi’ur ḥametz by burning has been achieved, and the bitul can be recited.
Some people, when using kerosene to light the fire, are careful not to pour it directly on the ḥametz. They do this so that the fire alone destroys the ḥametz, and the kerosene does not render it unfit for consumption by a dog before it is burned.
Most Rishonim feel that even according to R. Yehuda, the mitzva to burn ḥametz only applies to ḥametz that is left over past midday of the 14th, when it becomes forbidden, similar to the law of notar (uneaten portions of a sacrifice left over until the morning), which must also be burned. However, before midday of the 14th, it is possible that even R. Yehuda would agree that one may destroy the ḥametz any way he wants; this is the opinion of Rabbeinu Tam and Maharam Ḥalawa. On the other hand, Rashi explains that according to R. Yehuda the mitzva is to burn the ḥametz before midday. According to Rosh’s understanding of Rashi, the mitzva to burn the ḥametz only applies during the sixth hour of the day, but according to Tur’s understanding of Rashi, the mitzva to burn the ḥametz applies even prior to the sixth hour. If this opinion is correct, the mitzva to destroy the ḥametz is specifically by burning it. See Berur Halakha on Pesaḥim 27b for a summary of the topic. Even though it is clear according to almost all poskim that there is no mitzva to burn the ḥametz before it becomes forbidden, Rema 434:2 and 445:1 writes that the custom is to burn the ḥametz even earlier. See Or Le-Tziyon 1:33, which explains that the stringency of burning the ḥametz is dependent upon the combination of several opinions: firstly, the authorities who follow R. Yehuda; secondly, Tur’s understanding of Rashi that the mitzva to burn the ḥametz applies even before the ḥametz becomes forbidden; and thirdly, Tosafot’s opinion that the act of nullification renders the ḥametz ownerless, as opposed to Rashi’s understanding that nullification accomplishes the mitzva of destroying the ḥametz (and thus the only way to fulfill the mitzva of destroying the ḥametz according to Tosafot is by burning it). See above, 3:5, on the essence of the mitzva.
According to a simple reading, the mitzva of removing ḥametz (in Shemot 12:15) is to dispose of the ḥametz before it becomes forbidden, as most Rishonim write. Ran and Ritva also write that one fulfills this mitzva by conducting bedikat ḥametz. Rambam writes that the mitzva of removing ḥametz begins on the night of the 14th (MT, Laws of Ḥametz and Matza 3:1). However, Rosh maintains that the mitzva only begins once the ḥametz becomes forbidden.
Sidur Pesaḥ Ke-hilkhato 15:4 states that one should be careful not to pour lighter fluid on the ḥametz itself, so that the ḥametz is eradicated by burning and not by being befouled by the kerosene. Hilkhot Ḥag Be-ḥag 8:10, n. 17, states that one need not be meticulous about this, since the main objective is to turn the ḥametz into ashes, not to befoul its taste. See above, 3:5, n. 8.