MILD Combustion For Clean Energy And Healing The Planet

New Zealand can and must become a world leader in a new technology that could make our electricity supply truly 100% renewable at low cost. Our hydro energy runs short in dry years, wind generation can fail for days at a time, and solar requires costly batteries to provide electricity at night. But flexible biomass energy can fill in many of these gaps.

MILD[1] combustion takes freshly cut trees or branches and heats the fuel to over 500 degrees C. The resulting gas is burnt in a separate chamber with no flame, just an even translucent red-to-yellow glow. It is uniquely clean because any particles or tars are quickly consumed by the hydrogen-rich burning gas.

The fuel bin retains charcoal – more if the wood was dry, less if it was 100 to 150% moisture content as is typical of green wood. This charcoal if spread into the soil will last for centuries (biochar), or can be put to even higher-value uses. Or if more energy is needed, it can simply be burnt for energy by introducing more air and/or water to gasify it (as was done a century ago making town gas - “water gas” - from coke).

MILD combustion has been demonstrated at household scale by Ian Cave. Its ideal use for New Zealand’s energy requirements would be at a regional scale, co-generating electricity and heat for industry or greenhouses, with biochar and/ or liquid fuels as co-products. The plant would be sized to balance the cost of transporting the (heavy) green fuel against the economies of scale of the energy plant. Electricity generation would be varied as required to fill gaps in either bulk or local electricity supply; heat is cheaply stored as hot water.

This requires a change to New Zealand’s electricity market design, which now openly suppresses flexible electricity generation, through its transmission pricing rules and its tolerance of the obvious exercise of market power by the biggest power companies.

The value of MILD combustion of woody biomass to the world economy is arguably greater than billions, or even trillions, of dollars. Both wind and solar power took decades to become cheaper than electricity from new power stations; MILD combustion is inherently simpler, requiring only the effective transfer of heat and gases between parts of the burner.

It would promote very widespread planting of trees for biodiversity as well as timber or fruit and nuts, with a regular harvesting of the wood for energy and biochar. Leaves of some types of trees can be used to feed stock in droughts, or regularly.

The trees directly cool the climate, storing water and carbon in the soil and creating clouds and rain. New carbon accounting will need to recognise the fact that biochar sequesters carbon.

New Zealand’s Covid-driven climate investment needs to be directed by science, which should trump the lobbying of the construction and energy corporates. Research on MILD combustion, especially at regional scales, would support employment ranging from academics, technology and land use researchers down to unskilled labour, and all levels in between.

Demonstrating the use of local trees to provide energy, cool the climate and sequester carbon would greatly enhance New Zealand’s image as a world leader in solving the existential threats of climate change.

[1] Moderate or Intense Low-oxygen Dilution - https://www.researchgate.net/publication/235446478_A_review_of_MILD_combustion_and_open_furnace_design_consideration

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