Hydrothermal conversion

1. Energy from Hydrothermal Treatment of Waste Biomass - Which Process is Greener?
Objective
Supercritical 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, Sweden
2 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*
H2SO4
production and
transport
Landfill of
inorganic
residue*
Anaerobic
digester
(sewage sludge only)
SCWO process
221bars, 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
H2SO4
production
and
transport
1st stage
HTC
34bars,
250oC
Avoided use
of
conventional
diesel
Avoided use
of carbon
black
(turkey waste only)
Electricity* 2nd stage
HTC,
34bars
550oC
Sewage sludge
or turkey waste
Diesel-
transport of
products
Description of Technologies and Environmental Life Cycle Systems
System expansion
Life cycle
system boundary System expansion
Life cycle
system boundary
Patrick, N.T. 1, 2 Svanström, M.1
Fröling, M.2, Peterson, A.2, Tester, J.W.2
Liquid/solid
and 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 Scenarios
SCWO 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
oil
Negative results indicate the processes that
give savings in green house emissions
compared to the traditional alternatives.
-400
-300
-200
-100
0
100
200
300
400
500
600
kgCO2equivalents/1000kgwetfeed
HTC-Landfill gas not recovered
SCWO-heat recovered replaces natural gas use
SCWO-heat recovered replaces district heat
HTC-Landfill gas recovered
SCWO-without heat recovery
Fossil Renewable
-3500
-3000
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
kgCO2equivalents/1000kgwetfeed
SCWO-heat recovered replaces natural gas use
SCWO-heat recovered replaces district heat
Hydrothermal conversion,HTC
SCWO-without heat recovery
Fossil Renewable
SCWO HTC SCWO HTC
HTCSCWOHTCSCWO
Landfill of
residue*
(sewage sludge only)