The Coalition for Sustainable Rail, in conjunction with staff from the Natural Resources Research Institute, completed a third round of torrefied biomass fuel tests with the steam locomotives at the Milwaukee County Zoo this past Monday. CSR will be providing more details about the findings of that research next week but, in short, it was a success!
Until then, check out the following video of Zoo train 4-6-2 No. 1924 hauling our test train. The locomotive is burning torrefied biomass fuel.
The deep "humming" noise is the locomotive "drumming." Similar to pulse jet engines, the fuel is generating so much combustable matter that the firebox is consuming more air than it can bring in at one time. As air comes through the firebed, it flash ignites, causing a small "boom." Multiply this by hundreds of bursts of air coming through the firebed at once, you get drumming. This is alleviated through better ash pan design, installing overfire tubes (like most late-model steam engines), or cracking the door a bit. Notice that the noise stops when the engineer opens the door to shovel each scoop of fuel into the firebox.