Energy from Hydrothermal Treatment of Waste Biomass - Which Process is Greener?

ObjectiveSupercritical water oxidation (SCWO) and sub-critical hydrothermal conversion (HTC) are two processes that can be used for energy utilization of wet waste biomass streams with nutrient recovery. SCWO generates heat and phosphorus whereas HTC generates biologically derived diesel oil and nitrogen. The environmental systems performance of the two processes has been investigated using life cycle assessment, LCA, methodology. Two waste biomass streams, turkey slaughtering waste and sewage sludge have been investigated.

Sustainability Challenge

Conclusions

Results

The supercritical water oxidation process completely oxidizes the waste stream, decomposing all organic contaminants and makes possible heat recovery from wet biomass without need for drying. Phosphorus fertilizer can be extracted from the inorganic residue.

GWP for sewage sludge treatment

GWP for turkey waste treatment

1 Chalmers University of Technology, Gothenburg, Sweden2 Massachusetts Institute of Technology, Cambridge, Ma, USA

• Energy scenarios greatly influence what is the best choice of technology regarding environmental performance.

• Both technologies are environmentally attractive when use of fossil fuels are replaced and landfill gas collected.

O2 production and transport*

MgO production and transport*

H2SO4production and transport

Landfill of inorganic residue*

Anaerobic digester(sewage sludge only)

SCWO process221bars, 550oC

Acid extraction of Phosphorus

Diesel

Electricity*

Avoided use of artificial phosphate fertilizer

Avoided use of natural gas

Avoided heat generation*

Spreading of ferric phosphate

Sewage sludge or turkey waste

digested sludge

The hydrothermal conversion process converts, without need for drying, wet biomass into diesel, nitrogenous fertilizer and carbon.

Avoided use of artificial nitrogen fertilizer

H2SO4production and transport

1st stage HTC 34bars,250oC

Avoided use of conventional diesel

Avoided use of carbon black(turkey waste only)

Electricity* 2nd stage HTC,34bars550oC

Sewage sludge or turkey waste

Diesel-transport of products

Description of Technologies and Environmental Life Cycle Systems

System expansionLife cycle system boundary System expansion

Life cycle system boundary

Patrick, N.T. 1, 2 Svanström, M.1Fröling, M.2, Peterson, A.2, Tester, J.W.2

Liquid/solidand oil water separation

A more sustainable society will depend upon more efficiently managed material streams. Materials used by society should preferably not be converted in to waste, but residual materials should be efficiently used as raw materials for beneficial purposes. To achieve this, new technologies and new process solutions might be necessary, and new technical proposals should be evaluated regarding their environmental systems performance.

Energy ScenariosSCWO and HTC have been studied in two different energy scenarios: Fossil scenario where electricity comes from coal power plants and heat is not utilized. Renewable scenario using hydro and nuclear power and where heat is used for district heating substituting mainly bio fuels but partly fossil fuels. A sensitivity analysis was made where the heat substitutes natural gas heating in both scenarios.

*Activities modeled differently in different energy scenarios

oilNegative results indicate the processes that give savings in green house emissions compared to the traditional alternatives.

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kg C

O2

equi

vale

nts

/ 100

0 kg

wet

feed

HTC-Landfill gas not recoveredSCWO-heat recovered replaces natural gas useSCWO-heat recovered replaces district heatHTC-Landfill gas recoveredSCWO-w ithout heat recovery

Fossil Renewable

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kg C

O2

equi

vale

nts/

1000

kg w

et fe

ed

SCWO-heat recovered replaces natural gas useSCWO-heat recovered replaces district heatHydrothermal conversion,HTCSCWO-w ithout heat recovery

Fossil Renewable

SCWO HTC SCWO HTC

HTCSCWOHTCSCWO

Landfill of residue* (sewage sludge only)