Hydrothermal co-liquefaction of sewage sludge and formic acid

The biocrude produced by non-catalytic hydrothermal liquefaction (HTL) has a too high heteroatom content to be used directly as a fuel [1-3]. Several studies found that different H2 initial pressures during HTL experiments were ineffective in improving the biocrude yield [4-6], nevertheless it was observed that higher initial pressures of H2 brought an improvement of the concentration of light compounds in the biocrude, decreasing the average molecular weight of biocrude oil. To date H2 is mainly generated from fossil sources and realization of a possible hydrogen economy from alternative sources presents many challenges. The use of a liquid hydrogen storage media could represent for the HTL process an alternative solution to avoid the mass transport resistance associated to gaseous H2, furthermore Biller and Ross [7] observed that the addition of selected organic acids had similar effects than the addition of molecular H2. According to these premises, we studied the influence of formic acid as hydrogen donor on the yield and quality of the biocrude produced by HTL of sewage sludge (SS). Formic acid was selected because it can decompose to hydrogen and carbon dioxide under mild conditions [8] and it can be produced by electrochemical reduction of carbon dioxide, an appealing process to capture atmospheric carbon dioxide under the form of a useful chemical [9].