I used the Thermal ducts myself. More on reddit.com

I know the Geothermal running on lava starts at 15 RF/t and can be fully upgraded to around 3.8k RF/t. (3855 RF/t). The math is 15 RF/mb, and by default it consumes 1mb/t. If you hover over the Fuel Buffer bar (thin bar in the middle) you have this pop up: https://imgur.com/61yxBko Take the Total, that's how many mb of lava is in the fuel buffer. Divide that by 40 (it takes 40 ticks at max speed to use the buffer), then multiply by the RF rate (15 RF/mb) and you get the RF/t. Hate having to do the math, but you can derive the maximum possible RF/t output via the GUI if you need to. The Simple Combustion Generator seems to produce 20 RF/t. Not sure how upgrade math works (don't have the upgradable one), but the fuel buffer (as shown above) is how many ticks the fuel will last, and looking up the recipe you can see how much FE any given fuel will produce. Charcoal, for example, makes 32,000 FE with a burn time of 1,600, so 32k/1600 = 20 RF/t. The rest I haven't played with. More on reddit.com

Not sure if this takes lava? Might only be burnable items? More on reddit.com

Combustion first, then petrified fuel with blocks of charcoal, maybe ethanol with liquid combustion after, then fusion. Petrified fuel generators are great because the longer a substance burns for, the more rf per tick it puts out. Ethanol is 800,000 RF per bucket and is made from sugar and water, but needs a special machine to make it. My friend lives geothermal though and it is great. More on reddit.com