The reciprocating biomass conversion reactor (RBCR) is a novel means to produce bio-oil by fast-pyrolysis whose core is a re-purposed 4-cycle internal combustion engine. An inert gas and a small volume-fraction of pulverized biomass are input into the RBCR intake, and the crankshaft is cycled by an external energy source to supply the process heat. The biomass is converted during the compression stroke, and then the bio-products are exhausted. The cycle is: intake, compression/heating, expansion/cooling, and exhaust. This strategy is unique in its reciprocating approach and the potential benefits of the RBCR relative to the state of the art are increased efficiency, throughput, and bio-oil quality.