Synthesis of methanol from gaseous products of pyrolysis of sewage sludge

Objectives. To study the influence of compositional variability (different ash and organic matter contents) of sewage sludge on the characteristics of synthesis gas (syngas) and to determine the yield of products in the entire chain of conversion of sewage sludge to methanol through the stage of syngas production by two-stage pyrolysis.
Methods. Syngas was produced by a two-stage pyrolysis method. After heating sewage sludge from 20 to 1000°C in an oxygen-free medium, heterogeneous thermal cracking of the volatile products was carried out in a biochar medium at 1000°C. The syngas was converted to methanol on a CuZnAl catalyst in an isothermal flow heat-pipe reactor at a feedstock feed rate of 600 h−1, an internal reactor pressure of 5 MPa, and temperatures in the catalyst bed of 205, 215, and 225°C. The resultant syngas having a CO2 content of less than 0.5 vol % and a H 2/CO ratio of 1.8 was used as feedstock for methanol production.
Results. The experimental studies of syngas production from sewage sludge demonstrated the active formation of syngas during twostage pyrolysis in the temperature range of 140–600°C regardless of the ash content of the sludge. The H2/CO ratio in the syngas produced by two-stage pyrolysis of sewage sludge was shown to depend on the H/O atomic ratio in the sludge composition. Crude methanol was obtained at maximum yield and purity at a temperature of 225°C in the catalyst bed. The overall conversion of carbon monoxide was 43.6%.
Conclusions. Variability in the composition of sewage sludge significantly influences quantitative parameters to a large extent in terms of the specific volume yield of syngas and insignificantly terms of its composition. No qualitative influence was exerted by the difference in the types of sewage sludge on syngas production. The experimental studies showed that 1 kg of sewage sludge with a relative moisture content up to 5 wt % can produce 1.1 nm3 of syngas and a further 220 g of pure methanol.

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