Doc #56 — Wood Gasification for Transport and Stationary Power

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RECOMMENDED ACTIONS (BY URGENCY)

First month:

1. Identify 5–10 prototype workshop locations (pending census data or known shops)
2. Begin prototype construction from Appendix A specifications
3. Begin wood fuel preparation (drying) at forestry sites near prototype locations

First 3 months:

4. Complete and test prototypes on stationary engines and vehicles
5. Develop NZ-specific refinements to the base design
6. Begin training program for fabricators and operators

First year:

7. Nationwide distribution of standardized plans
8. Scale-up construction at all capable workshops
9. Convert priority vehicles (food transport, farm equipment, essential freight)
10. Establish community fuel preparation operations

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1. HOW WOOD GASIFICATION WORKS

1.1 Basic chemistry

Wood gasification is a thermochemical process that converts solid biomass into a combustible gas mixture through partial combustion in a restricted-oxygen environment.

The process occurs in four zones within the gasifier:

Drying zone (~100–200°C): Moisture evaporates from the wood. NZ radiata pine at typical air-dried moisture content (15–25%) loses this moisture before entering the pyrolysis zone. Wet wood (>25% moisture) wastes energy on drying and produces less useful gas — fuel preparation is critical.

Pyrolysis zone (~200–500°C): Wood decomposes into charcoal, tars, and volatile gases in the absence of oxygen. This is the zone that produces problematic tars (see Section 3.4).

Combustion/oxidation zone (~800–1200°C): A controlled amount of air is introduced. Charcoal and some tars burn, producing heat that drives the other zones, along with CO₂ and H₂O.

Reduction zone (~600–900°C): The CO₂ and H₂O from combustion pass through a bed of hot charcoal and are chemically reduced:

• CO₂ + C → 2CO (Boudouard reaction)
• H₂O + C → CO + H₂ (water-gas reaction)

The resulting gas mixture — “producer gas” or “wood gas” — contains approximately:⁶

• Carbon monoxide (CO): 15–25%
• Hydrogen (H₂): 10–20%
• Methane (CH₄): 1–5%
• Carbon dioxide (CO₂): 8–15%
• Nitrogen (N₂): 45–55% (from the intake air)

1.2 Energy content

Producer gas has a calorific value of approximately 4.5–6 MJ/m³, compared to natural gas at ~38 MJ/m³ or petrol at ~34 MJ/liter.⁷ This low energy density is the fundamental reason for the power loss in engines running on producer gas — the fuel-air mixture entering the cylinder contains much less energy per stroke.

Fuel consumption: A typical vehicle gasifier consumes approximately 2.5–4 kg of dry wood per equivalent liter of petrol.⁸ A vehicle that normally uses 10 liters/100 km on petrol will consume roughly 25–40 kg of wood per 100 km on producer gas. This is bulky — a day’s driving might require 50–100 kg of wood, which means either carrying a large wood supply or refueling frequently.

1.3 Gasifier types

Two main designs are relevant for NZ:

Downdraft gasifier (Imbert type): Air enters partway down the reactor and flows downward through the combustion and reduction zones. This design cracks most tars in the hot combustion zone before they reach the gas outlet, producing relatively clean gas. Preferred for vehicle and engine applications. The standard WWII design was the Imbert gasifier, and most modern small-scale gasifier designs are variants of this.⁹

Crossdraft gasifier: Air enters from the side. Faster response time, simpler construction, but produces more tar and is less efficient. More suitable for charcoal fuel (which produces less tar than raw wood).

For NZ applications, the downdraft (Imbert-type) design is recommended for most vehicle and stationary engine use, because NZ’s primary fuel will be raw wood (radiata pine) rather than pre-made charcoal, and tar management is the main operational challenge.

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2. NZ FUEL SUPPLY

2.1 Radiata pine

NZ’s plantation forests are predominantly radiata pine (Pinus radiata), with approximately 1.7 million hectares planted.¹⁰ Annual harvest under normal conditions is roughly 30–35 million cubic metres of roundwood.¹¹ Even a fraction of this harvest, redirected from export to domestic energy, would supply a substantial vehicle fleet.

Radiata pine as gasifier fuel: Pine is a good gasifier fuel — relatively uniform, easy to process, and available in quantity. It has some advantages over hardwoods: lower ash content, predictable behavior. Disadvantages: higher resin content (contributes to tar), lower density (more bulk per unit of energy).

Moisture content is critical. Freshly felled pine has moisture content of 50–60%. This must be reduced to below 20% (ideally 10–15%) for efficient gasification. Air drying takes months depending on climate and piece size. Kiln drying using waste heat from the gasifier or other processes can accelerate this. Under nuclear winter conditions, cooler temperatures and higher humidity will slow air drying — fuel must be prepared well in advance.

2.2 Other wood fuels

• Native hardwoods: Higher density, lower resin, but slower growing and conservation considerations apply. Probably should not be a primary gasifier fuel.
• Willow and poplar: Fast-growing, common in NZ riparian plantings. Acceptable gasifier fuel.
• Wood waste: Sawmill offcuts, demolition timber, prunings. Good fuel if uncontaminated (treated timber should be avoided — CCA treatment releases arsenic).
• Charcoal: Can be made from any wood (Doc #102). Produces cleaner gas with less tar, but charcoal production loses roughly 70–80% of the wood’s original energy content through the carbonisation process.¹² Only justified where very clean gas is needed (e.g., running a generator that powers sensitive equipment).

2.3 Fuel sustainability

Is NZ’s wood supply sufficient for gasification at scale?

NZ’s plantation forests contain an estimated 450–550 million cubic metres of standing timber, with annual growth replacing roughly 30 million cubic metres.¹³ If NZ operated 50,000 vehicles on wood gas (a substantial fleet), each consuming roughly 3,000–7,000 kg of wood per year (depending on use intensity), total annual consumption would be approximately 150,000–350,000 tonnes. For comparison, annual forest growth of ~30 million cubic metres corresponds to roughly 12–15 million tonnes of green wood — so even at the upper end, gasifier fuel demand would represent 2–3% of annual growth.¹⁴

Wood gasification at any plausible scale does not threaten NZ’s timber supply. The constraint is not wood availability but fuel preparation (drying, sizing) and gasifier construction/maintenance.

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3. GASIFIER CONSTRUCTION

3.1 Materials

A downdraft gasifier can be constructed from materials available in NZ:

Reactor body: Mild steel pipe or fabricated steel cylinder. NZ Steel produces suitable steel. Alternatively, repurposed steel cylinders, water heater tanks, or similar pressure vessels.

Fire tube / combustion zone lining: The combustion zone reaches 1000°C+. Mild steel degrades rapidly at these temperatures. Options:

• Stainless steel (available from NZ stocks, not locally produced — finite supply)
• Refractory cement lining (NZ has cement production — Doc #97)
• Ceramic fire brick (potentially producible from NZ clay)
• Accept that the combustion zone lining is a consumable — replaced periodically (every few hundred to few thousand hours of operation)

Gas cooling and filtering: Producer gas exits the reactor at 200–400°C and contains tar, particulates, and moisture that must be removed before entering an engine. Cooling: tube-and-fin radiator or water-cooled heat exchanger, constructable from steel or copper tubing. Copper tubing is preferable for heat exchange efficiency but NZ does not smelt copper domestically — existing copper stocks (plumbing supply, HVAC inventory, scrap) are finite, and steel tubing is an acceptable substitute with lower thermal conductivity. Filtering: progressively finer stages — cyclone separator (steel fabrication), packed bed filter (wood chips, gravel, or fabric), and final filter (fabric or paper). Paper filters are a consumable dependent on ongoing paper production or existing stocks; fabric filters (woven cotton or wool) are reusable and preferred for long-term operation.

Piping and fittings: Steel pipe, standard fittings. Available from NZ industrial stocks and eventually from NZ Steel production.

Fan/blower: A small electric fan or engine-driven blower to draw air through the gasifier during startup. Electric motors for this purpose can be salvaged from household appliances, automotive cooling fans, or HVAC systems; impellers can be cut from sheet steel. Alternatively, a hand-cranked blower eliminates the electrical dependency entirely — a practical option for field use.

3.2 Tools required

Construction requires basic metalworking capability:

• Cutting: angle grinder (requires cutting discs — a finite consumable from existing stocks), oxy-acetylene torch (acetylene and oxygen are finite without gas production infrastructure), or plasma cutter (requires electricity and compressed air)
• Welding: arc welding with stick electrodes (electrode fabrication — see Doc #94) or MIG welding (requires shielding gas — CO₂ or argon — which is finite without industrial gas production)
• Drilling: drill press or hand drill with HSS drill bits (finite consumable; re-sharpening extends life)
• Fitting: standard hand tools

This is within the capability of any competent rural workshop or engineering firm. No specialized equipment is required beyond standard metalworking tools. The key consumable constraints are welding electrodes and cutting/grinding discs — both are high-volume stock items in NZ industrial supply but will deplete without domestic manufacture. Electrode fabrication (Doc #94) addresses the welding consumable; cutting discs may need to be substituted with oxy-fuel cutting as stocks deplete.

3.3 Construction time and skill

A competent welder/fabricator working from detailed plans can build a vehicle-sized gasifier in approximately 40–80 hours.¹⁵ This estimate is based on documented builds; NZ fabricators with no prior gasifier experience would likely take longer for the first unit, with construction time decreasing as experience develops.

Implication: If NZ has hundreds of workshops capable of this work (to be confirmed by the skills census — Doc #8), production of thousands of gasifiers per year is feasible once designs are standardized and distributed.

3.4 The tar problem

Tar — condensable organic compounds produced during pyrolysis — is the main operational challenge. Tar deposits in gas lines, valves, and engines, causing fouling and eventual failure.

Mitigation approaches:

• Gasifier design: Downdraft design routes gas through the hot combustion zone, cracking most tar. Proper sizing of the combustion zone and air inlet is critical.
• Fuel quality: Dry, uniformly sized fuel produces less tar. Wet or oversized fuel is the most common cause of excessive tar production.
• Gas cooling and filtering: Sequential filtering removes remaining tar. Filter maintenance (cleaning or replacing filter media) is a regular task.
• Engine modification: Engines running on producer gas benefit from increased compression ratio — typically raised from the standard petrol ratio of 8–10:1 to 11–14:1 — improving efficiency with the lower-energy fuel.¹⁶ Ignition timing may also need advancement. Modern fuel-injected engines require more modification than older carbureted engines — another reason to prioritize converting older, simpler vehicles.

Honest assessment: Tar is manageable but never fully eliminated. Engines running on producer gas require oil changes at 2–5 times the frequency of petroleum operation, more frequent valve grinding, and periodic decarbonisation of combustion chambers. Engine life is typically reduced by 30–50% compared to petroleum operation due to increased abrasive wear from particulates and acidic condensate in the gas.¹⁷ This is a trade-off worth accepting given the alternative (no fuel at all), but it should be communicated to operators so they plan for higher maintenance workload and earlier engine rebuilds.

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4. APPLICATIONS

4.1 Vehicle conversion

Best candidates for conversion:

• Older petrol engines with carburetors or simple fuel injection (pre-2000 era vehicles tend to be simpler to convert)
• Trucks and heavy vehicles (more room to mount gasifier, greater fuel need)
• Farm vehicles (stationary or low-speed use, close to wood fuel supply)

Worst candidates:

• Modern fuel-injected diesel engines (diesel requires compression ignition; producer gas is spark-ignited — conversion is possible but more complex, requiring a spark ignition system or dual-fuel operation where a small amount of diesel ignites the gas)
• Small cars (limited space for gasifier mounting, relatively low fuel need — better electrified)
• High-speed highway vehicles (power loss makes highway speeds impractical)

Dual-fuel diesel operation: Diesel engines can run on a mixture of producer gas and reduced diesel injection (the diesel acts as a pilot ignition source). This stretches diesel supply while using wood gas for most of the energy. Conversion is simpler than full spark-ignition conversion. May be the preferred approach for NZ’s large fleet of diesel trucks and farm equipment.¹⁸

4.2 Stationary engines

Producer gas is better suited to stationary applications than to vehicles: generators, water pumps, sawmill drives, grain mills, workshop equipment. Stationary gasifiers don’t have vehicle space constraints, can be larger and more efficient, and can operate in more controlled conditions (consistent fuel, steady load). However, compared to petroleum-fuelled stationary engines, wood gas generators still deliver 20–40% less rated power, require 15–30 minutes of startup before producing usable gas, need an operator present for fuel loading and filter maintenance, and produce less stable output under varying loads — voltage regulation or battery buffering is needed for any electrically sensitive application.¹⁹

Priority stationary applications:

• Emergency generators at hospitals, water treatment plants, and communications facilities (backup for grid power — remember, grid is expected to continue but backup is prudent)
• Sawmill drives (where sawmill waste becomes the fuel — elegant closed loop)
• Farm equipment drives (irrigation pumps, grain processing)
• Workshop equipment in locations remote from the grid

4.3 What NOT to power with wood gas

• Aircraft: Do not attempt. The risk of engine failure from tar or fuel interruption is unacceptable.
• Marine engines for offshore passages: Too unreliable for safety-critical applications at sea. Coastal or harbor use might be acceptable with careful maintenance and backup.
• Sensitive electronic equipment: Generator output quality from wood gas engines fluctuates more than from petroleum — use voltage regulation or battery buffer.

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5. SAFETY

5.1 Carbon monoxide

Producer gas contains 15–25% carbon monoxide, which is lethal. CO is colorless and odorless. A leaking gasifier or gas line can kill in an enclosed space without warning.

Non-negotiable safety rules:

• Never operate a gasifier in an enclosed space (garage, shed) without forced ventilation
• All gas-carrying connections must be leak-tight and regularly inspected
• Gasifier vehicles must not be left running in enclosed spaces
• Operators must be trained on CO risk and symptoms (headache, dizziness, confusion → loss of consciousness → death)
• CO detectors should be deployed where available (finite stock — prioritize enclosed workspaces)

The WWII European experience includes documented fatalities from CO exposure in wood gas vehicles.²⁰ This is a real risk, not a hypothetical one.

5.2 Fire risk

The gasifier contains a fire. The gas is flammable. Fuel loading involves opening a hot reactor.

• Keep combustible materials away from the gasifier
• Fuel loading procedure: shut down air supply, wait for gas flow to stop, open slowly
• Fire extinguisher (or at minimum, a bucket of sand/soil) must be available at all gasifier locations

5.3 Burn risk

The gasifier reactor and gas lines are hot (200–1000°C at various points). Operators must not touch hot surfaces. Insulation or shielding of accessible hot surfaces is required, particularly on vehicle-mounted units where passengers or bystanders might contact them.

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6. IMPLEMENTATION PLAN

6.1 Urgency calibration

Wood gasifier construction is not a first-48-hours priority — fuel rationing (Doc #1) is the immediate need, and rationed petroleum keeps essential services running during the transition period. Gasifier construction should begin in the first weeks to months, with the goal of having significant numbers operational by the time petroleum stocks are severely depleted (estimated months to a year under strict rationing, depending on actual stock levels — see Doc #53).

The timeline is driven by petroleum depletion, not by urgency of the gasifier technology itself. If petroleum lasts 6 months under rationing, gasifier construction has 6 months. If it lasts 3 months, the program must move faster.

6.2 Phase 1 (first 3 months): Prototype and training

1. Select 5–10 workshops across NZ with competent fabricators, based on census data (Doc #8)
2. Build prototype gasifiers from this document’s technical specifications (Appendix A)
3. Test on stationary engines and vehicle conversions
4. Refine designs based on NZ-specific conditions (radiata pine fuel characteristics, available steel grades, local expertise)
5. Develop training materials from prototype experience
6. Train trainers — fabricators who can then teach others

6.3 Phase 2 (months 3–12): Scale-up

1. Distribute standardized plans to all capable workshops nationwide
2. Establish fuel preparation infrastructure — wood drying and sizing operations at forestry sites, farms, and community locations
3. Convert priority vehicles: Heavy trucks for food distribution, farm equipment, essential freight
4. Build stationary gasifiers for critical backup generators and workshop equipment
5. Target: Several hundred gasifiers operational within 12 months

6.4 Phase 3 (years 1–3): Mature capability

1. Thousands of gasifiers in operation across the country
2. Community fuel preparation integrated into forestry and agricultural operations
3. Operator training widespread through trade training programs (Doc #156)
4. Design improvements based on field experience — better tar management, optimized reactor geometry, standardized replacement parts
5. Quality control: Inspection and maintenance standards developed from operational data

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7. CRITICAL UNCERTAINTIES

Uncertainty	Impact	Mitigation
NZ workshop capacity	Determines scale-up speed	Census (Doc #8). Prioritize existing capable shops.
Radiata pine gasification characteristics	Pine resin/tar behavior may differ from European precedents	Early prototyping and testing.
Nuclear winter impact on wood drying	Cooler, more humid conditions slow air drying	Kiln drying using waste heat. Plan fuel preparation further ahead.
Steel and welding consumable supply	Gasifier production depends on ongoing steel and electrode supply	NZ Steel continuity (Doc #89). Electrode fabrication (Doc #94).
Operator training speed	Gasifiers need competent operators to run safely and effectively	Train-the-trainer model. Written operating procedures.
Diesel dual-fuel conversion complexity	NZ’s truck fleet is primarily diesel	Prioritize development of dual-fuel conversion procedure.

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APPENDIX A: CONSTRUCTION SPECIFICATIONS

[This appendix would contain detailed engineering drawings, dimensions, materials lists, construction procedures, and assembly instructions for a standardized NZ downdraft gasifier suitable for vehicle and stationary engine use. Content to be developed in conjunction with prototype testing.]

Key design parameters to be specified:

• Reactor diameter and height (sized for radiata pine fuel)
• Air inlet design and sizing
• Reduction zone geometry
• Gas outlet and cooling system
• Filter stages and maintenance schedule
• Vehicle mounting arrangements (towed trailer vs. bed-mounted)
• Engine connection and controls
• Materials list with NZ-available substitutions

APPENDIX B: OPERATING PROCEDURES

[Startup, steady-state operation, shutdown, refueling, filter cleaning, maintenance schedule, troubleshooting. To be developed from prototype experience.]

APPENDIX C: FUEL PREPARATION

[Wood species selection, sizing requirements, moisture content targets, drying methods, storage. NZ-specific for radiata pine and other available species.]

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1. WWII wood gasification is documented in numerous sources. A comprehensive overview: FAO, “Wood Gas as Engine Fuel,” FAO Forestry Paper 72, 1986. http://www.fao.org/3/t0512e/t0512e00.htm — This FAO document remains one of the best practical references on small-scale wood gasification.↩︎

2. Swedish wood gas vehicle statistics: Johansson, T.B. et al., “Energy and Swedish Forestry,” Swedish University of Agricultural Sciences, various publications. The 73,000 figure is from the Swedish National Defence Research Institute (FOA) and is widely cited in the gasification literature.↩︎

3. NZ plantation forest area: Ministry for Primary Industries, NZ Forest Industry Facts and Figures. https://www.mpi.govt.nz/forestry/forest-industry-and-work... — Approximately 1.7 million hectares, of which ~90% is radiata pine.↩︎

4. Performance characteristics of wood gas vehicles are documented in the FAO publication (note 1) and in Reed, T.B. and Das, A., “Handbook of Biomass Downdraft Gasifier Engine Systems,” SERI (now NREL), 1988. https://www.nrel.gov/docs/legosti/old/3022.pdf — Power loss of 30–50% is typical for naturally aspirated engines; fuel consumption figures vary with gasifier design and fuel quality.↩︎

5. NZ’s WWII experience is documented in Baker, J.V.T., “The New Zealand People at War: War Economy,” Historical Publications Branch, 1965. NZ focused on petroleum rationing, substitution with producer gas was minimal compared to European nations.↩︎

6. Producer gas composition varies with fuel moisture, air-fuel ratio, and gasifier design. Typical ranges from Reed and Das (note 4) and the FAO publication (note 1). The calorific value of 4.5–6 MJ/m³ is for dry gas from wood; charcoal gas is somewhat higher.↩︎

7. Producer gas composition varies with fuel moisture, air-fuel ratio, and gasifier design. Typical ranges from Reed and Das (note 4) and the FAO publication (note 1). The calorific value of 4.5–6 MJ/m³ is for dry gas from wood; charcoal gas is somewhat higher.↩︎

8. Performance characteristics of wood gas vehicles are documented in the FAO publication (note 1) and in Reed, T.B. and Das, A., “Handbook of Biomass Downdraft Gasifier Engine Systems,” SERI (now NREL), 1988. https://www.nrel.gov/docs/legosti/old/3022.pdf — Power loss of 30–50% is typical for naturally aspirated engines; fuel consumption figures vary with gasifier design and fuel quality.↩︎

9. The Imbert gasifier was originally developed by Georges Imbert in France in the 1920s and became the standard WWII vehicle gasifier design. Technical details in the FAO and NREL publications cited above.↩︎

10. NZ plantation forest area: Ministry for Primary Industries, NZ Forest Industry Facts and Figures. https://www.mpi.govt.nz/forestry/forest-industry-and-work... — Approximately 1.7 million hectares, of which ~90% is radiata pine.↩︎

11. NZ roundwood production data from MPI (note 3). Approximately 30–35 million cubic metres of roundwood harvested annually in recent years, primarily for export. Standing timber volume estimates from the National Exotic Forest Description (NEFD) published by MPI.↩︎

12. Charcoal yield from wood is typically 20–30% by mass, with energy retention of 20–30% of the original wood energy content. The remainder is lost as heat, volatile gases, and tars during carbonisation. See FAO, “Simple Technologies for Charcoal Making,” FAO Forestry Paper 41, 1987. http://www.fao.org/3/x5328e/x5328e00.htm↩︎

13. NZ roundwood production data from MPI (note 3). Approximately 30–35 million cubic metres of roundwood harvested annually in recent years, primarily for export. Standing timber volume estimates from the National Exotic Forest Description (NEFD) published by MPI.↩︎

14. Standing timber volume estimate from the National Exotic Forest Description (NEFD), published by MPI. The 450–550 million m³ range reflects different assessment years and methodologies. Conversion from cubic metres to tonnes uses an approximate green density for radiata pine of 400–500 kg/m³ (air-dried), though this varies with moisture content. The sustainability calculation is approximate but the order-of-magnitude conclusion — that gasifier demand is a small fraction of growth — holds across reasonable assumptions.↩︎

15. Construction time estimates from documented amateur and professional gasifier builds. See, for example, the GEK Gasifier project (All Power Labs) documentation and various open-source gasifier build projects. Times vary significantly with fabricator experience and available equipment.↩︎

16. Compression ratio recommendations for producer gas engines: Reed and Das (note 4), Chapter 7. Higher octane rating of producer gas (100–105 RON equivalent) permits higher compression ratios without detonation, partially compensating for the lower volumetric energy density.↩︎

17. Engine wear under producer gas operation is documented in Nordström, O., “Wear Tests on Engines Running on Producer Gas,” Swedish Academy of Engineering Sciences, 1944; and in the FAO publication (note 1), Chapter 8. Wear rates depend heavily on gas filtration quality — well-filtered gas can approach petroleum engine life, but field conditions rarely achieve laboratory filtration standards.↩︎

18. Dual-fuel diesel-producer gas operation: Dasappa, S. et al., “Biomass Gasification Technology — A Route to Meet Energy Needs,” Current Science, 2004. Diesel substitution rates of 60–80% are achievable in dual-fuel mode, meaning 60–80% of the fuel energy comes from producer gas with 20–40% from diesel pilot injection.↩︎

19. Stationary engine power derating on producer gas is discussed in Reed and Das (note 4), Chapter 5. The 20–40% derating is typical for naturally aspirated engines; turbocharged engines may recover some of this loss. Startup time depends on gasifier thermal mass and fuel moisture.↩︎

20. CO fatalities from wood gas vehicles during WWII are documented in Swedish and Finnish accident records. Exact numbers are debated but the risk is well-established. Modern gasifier designs and safety protocols significantly reduce but do not eliminate this risk.↩︎