The traditional flame throwers were heavy and dangerous because they carried both napalm or similar fuel, and a tank of high pressure gas to force the fuel through the tube to the igniter, and project the flaming fuel to the target. Lots of places to get leaks, fuel to ooze out and ignite or high pressure parts to rupture.
The former USSR used a somewhat different form of flamethrower with the LPO-50. This still used Napalm as the fuel, but carried in unpressurized tanks. A blank "Shotgun" like charge was screwed into the top of each cylinder, and when the trigger was pulled, the charge ignited and rapidly pressurized the tank, providing the force to propel the fuel towards the target. LPO-50's were capable of firing up to 70m, and considered pretty fearsome weapons for engineers to clear bunkers and so on.
LPO-50 in action
The picture should indicate the issue with man portable flame throwers, however. The soldier si carrying a considerable mass on his back, yet only has 3 shots. Since the purpose is burning corpses, the engineers will need a lot of fuel, as Mark pointed out. Since each tank only holds 3.4l of fuel, you are not even going to make much of a dent in incinerating corpses.
Assuming the field or city is littered with dead bodies and it is dangerous to approach or handle them (although a front end loader works wonders in this scenario), you would probably need a vehicle mounted unit. Modern Russian flame weapons use Thermobaric warheads fired from rockets to create high heat and overpressure, using a metalized fuel-air mixture. Like the flamethrower, it also creates a short, sharp spike in temperature, and is probably not enough to incinerate corpses.
Modern Russian Thermobaric missile launcher
The real issue is the amount of water in the human or animal tissue, so you need sustained heat, and a lot of it. Consider how long it takes to cook a well done steak, and that is over red hot coals or a gas flame at @ 800 degrees C. You need a way to generate that much heat and more to dehydrate the tissues and cause them to ignite and burn.
One possible solution is to simply go high tech and avoid the use of fire in the first place. High energy microwaves will "cook" the target from the inside, and the elevated temperatures can ensure microorganisms are killed, without needing flame, ignition or even necessarily oxygen (projecting a microwave beam into the firing slits of bunkers or down sewers is possible). Of course, microwave projectors will need a lot of energy as well, which explains the use of the heavy truck to carry the projector and generator in the picture.
Current test vehicle for microwave based Area Denial Weapon
The engineers can use robots to place the microwave projectors in difficult or dangerous situations, and other robots can move through the area afterwards to take samples and ensure there is no biological activity