Role of Bioenergy in Achieving Sustainability

I was recently invited to give a keynote address at an international conference on Bioenergy and Sustainability. Because of the Covid-19 pandemic the conference was held virtually over Zoom. What follows is an abstract of my presentation; the full lecture can be accessed here.

The word sustainability shares its root with sustenance. In the case of modern society sustenance comes from use of energy, which is derives from many sources: oil, coal, natural gas, hydroelectric, nuclear, wind, solar, and biomass. Annual consumption of global energy is equivalent to 4 cubic miles of oil (cmo), about 3 of which are obtained from fossil sources: oil, coal, and natural gas.

 

The dominance of fossil energy in the global mix has been longstanding—ever since the dawn of the industrial revolution in the mid nineteenth century. As a result, the concentration of carbon dioxide in the atmosphere has increased from 280 ppm to over 400 ppm and continues to rise. CO₂ is a greenhouse gas and it and now threatens life as we know it from the resulting climate change. To avert devastation from climate change or constrained energy supply, the world desperately needs sources of clean, carbon-free energy that together can scale to cmo levels.

 

Much emphasis has been placed in recent years on resources like wind and solar to provide clean electricity. Technological advances have led to dramatic reductions in their costs and their advocates now propose a future powered entirely by them. However, these costs do not include the cost of storage, currently provided by natural gas, nor do they consider the environmental cost of mining for the materials needed for their installation. Scaling them to a 100%-renewables scenario will strain the global supply of commodities like steel, concrete, glass, and aluminum; clearly not a sustainable scenario.

 

Burning biomass has been proposed as a fuel source; indeed, prior to the industrial revolution the world once derived 100% of its energy from bio sources. Unlike wind and solar, bioenergy sources are storable and do not suffer from intermittency. However, biomass use also results in emitting CO₂. The only reason these emissions are not counted is that the regrowth of the biomass would take an equivalent amount of CO₂ out of the air. For this assumption to hold, it is important that we consider harvesting only rapidly growing biomass or annual crops.

 

Global biomass production is substantial; it is estimated that 75 Gt (gigatons, or 10⁹ tons) of biomass are produced annually. Most of the biomass is in the forests and oceans and not readily recoverable, nor is it desirable to cut down this “sequestered” carbon and burn it. The estimate for recoverable biomass resource is only 3 Gt/y. At a heating value of 15 GJ/t (gigajoules/ton) the energy from these 3 Gt of biomass would correspond to only 0.3 cmo. The low energy density of biomass translates into large areas over which the biomass to be harvested and transported to the power plant: 160 sq. miles of fast growing trees each year to power a single 100 MW plant.

Clearly, we cannot rely on biomass to meet global energy demand for clean energy. Yet, there are some applications where energy from biomass is uniquely suited. Production biofuels is one such example, and many conversion of starch in grains into bioethanol is a thriving business—thanks in large part to the support the industry receives from various state agencies. There are also processes for converting lignocellulosic wastes into biofuels, although there deployment has been hampered by high costs. The main reason for using biofuels is to reduce greenhouse gas emissions; however, on a life-cycle basis the biofuels reduce greenhouse gas emissions between 20% and 40%!

Co-firing biomass, particularly waste biomass, may provide only a limited amount of energy, but it would help enormously with waste management since many municipalities are running out of landfill space. Likewise, utilizing agricultural waste in an engineered system rather than open-field burning would go a long way in reducing urban pollution in many countries.

True sustainability demands a scalable source of clean and cheap electricity. Nuclear power can deliver that. It has the smallest environmental footprint and the best safety record, but public concerns over plant safety, long-term storage of waste, and cost are considerable obstacles. Getting the public to embrace nuclear power is a Herculean task, but it must be undertaken. We have to (i) educate the public (ii) stop closing functional nuclear power plants; (iii) expand the fleet of nuclear power plants; and (iv) develop and deploy the next generation of walk-away safe plants that can also use the spent fuel as a resource.