Bioenergy has a unique role to play in global decarbonization
- Apr 1, 2021 1:02 am GMT
Biomass — the collective name for renewable materials derived from living organisms, including crops and agricultural residues, by-products from wood-processing industries like paper and construction, and gas from landfills and water treatment plants — is the oldest source of energy harnessed by humans. The history of energy transitions has been one of moving away from biomass and toward fuels with greater power densities, like coal, natural gas, and petroleum. Today biomass contributes a relatively small percentage of total energy supply in most high-income countries. However, many of them are considering expanding its use to help combat climate change, and “traditional biomass” fuels like charcoal, animal dung, and crop residue remain widely-used in many low-income countries.
All sources of energy are fundamentally solar energy, differentiated only by the time scale. Electricity generated by solar, wind, and water flows are all immediate transformations of solar energy: it knocks electrons on solar panels into one another via the photovoltaic effect on solar panels, creates pressure gradients which generate winds that push wind turbines, and drives Earth’s water cycle, forming waterways that can be manipulated to spin hydro turbines. Even fossil fuels owe their beginnings to solar radiation. Ancient plants and marine creatures which were once energized by solar energy were transformed by heat and pressure over the course of a few thousand to hundreds of millions of years. Biomass too is derived from solar energy, but involves harnessing energy stored within chemical bonds which were formed on the order of days to months.
Biomass can be consumed in a number of ways. Most biomass used currently is either burned as solid fuel like wood pellets, charcoal, and briquettes, or turned into liquid biofuels like biodiesel and ethanol. Biodiesel is made by combining alcohol with vegetable oils and animal fats, while ethanol is produced by fermenting plant starches and sugars. Biodiesel can be blended with petroleum diesel in any percentage for use in compression-ignition engines, including 100% biodiesel, although the most common blend is 20% biodiesel and 80% petroleum diesel. Ethanol is blended with petroleum gasoline for use in spark-ignition engines. Most gasoline sold in the USA is blended with 10% ethanol, although some “flex fuel” vehicles can run on E85, which consists of 51-83% ethanol. Solid biomass can also be gasified through a series of high-temperature chemical reactions, which results in gaseous hydrogen and carbon dioxide. The hydrogen produced can be used to decarbonize sectors with few low-carbon alternatives like petrochemicals, refining, and glass. Crucially, producing hydrogen with biomass gasification is less energy-intensive than other common methods like electrolysis or steam-methane reforming.
From a technical standpoint, biomass has the benefit of using some existing infrastructure. Solid fuels can be transported by rail, gaseous and liquid fuels can be transported by pipeline, and all biomass can be co-fired with fossil fuels. However, different fuels are never exact replacements because infrastructure has been designed to optimize performance with one particular fuel. For example, converting coal-fired power stations to burn natural gas, or even coal from a different region, can cause many pieces of equipment to fail sooner than they otherwise would, raising operating costs. Switching boilers from coal to solid biomass, or internal combustion engines from gasoline to ethanol, has similar effects if the percentage of biomass is high. Solid biomass has a lower gravimetric energy density than coal, making it more expensive to transport and store than coal, so it must be consumed close to where it is grown. Similarly, hydrogen has a lower volumetric energy density than natural gas, making it difficult to blend hydrogen into natural gas pipelines beyond 15-20% hydrogen.
Most energy consumed in the poorest countries is categorized as traditional biomass and is burned using open fires or simple stoves for cooking and heating. On the other hand, middle- and high-income consume more biomass in absolute terms, despite being a smaller portion of total energy consumption. Biomass energy is often politically popular because it supports rural employment and energy independence. Europe is the top consumer in the world, using about 1,900 terawatt-hours annually, due to heavy use of wood pellets and other timber products for everything from power generation to space heating, as well as biofuels for transportation. America comes in second, consuming about 1,700 terawatt-hours per year of biomass, predominantly biofuels from corn and soybeans, and Brazil comes in third with about 1,000 terawatt-hours per year, also predominantly biofuels, but produced from sugarcane. Interestingly, China is a small player in biomass energy, despite being one of the biggest producers and consumers of nearly every other energy source.
Biomass is good for the environment in a number of ways. When fossil fuels are burned, carbon that was trapped underground pours it into the earth’s atmosphere, where it becomes a part of Earth’s carbon cycle. Some of it will be deposited in natural carbon sinks like oceans, soil, and plant matter, but some of it will remain in the atmosphere, warming the climate for centuries. In contrast, the carbon dioxide from biomass is continually emitted and deposited as crops are grown, harvested, and turned into fuels. In the case of biofuels, carbon that is emitted during combustion is captured by the crops or trees currently being grown for tomorrow’s biofuels. In the case of gasification, a stream of concentrated carbon dioxide is created, which can be either used productively or sequestered, effectively allowing more emissions in sectors which are difficult to electrify like liquid fuels, building heating, and industrial heat.
Unfortunately, the environmental impact of biofuels is not all good. First, while bioenergy is carbon-neutral in theory, it is not exactly carbon-neutral in practice, because of the gap in time between planting and harvesting resources — turning an old-growth forest (or a poorly-managed new-growth one) into wood pellets might have nearly the same carbon emissions as coal combustion, while harvesting row crops, removing built-up vegetation from forests, or burning the by-products of paper and agriculture produces fewer carbon emissions per unit energy than most fossil fuels. But the phrase “per unit energy” is crucial, because there is much less energy per unit mass in biomass than in fossil fuels. Biomass has a power density of just 0.5-0.6 W/m2, suggesting that even if an entire country was covered with energy crops, it would not provide enough energy for modern levels of consumption, which average around 1-100 W/m2 in wealthy countries. Additionally, biomass combustion produces hazardous local air pollutants like particulate matter and nitrogen oxides. Even though the emissions from biomass combustion are small relative to coal or oil, using biomass for indoor heating and cooking without proper ventilation is a leading cause of respiratory illness and premature death in low-income countries. Finally, intensive agriculture, which is used to grow lots of biomass, can diminish land’s productivity over time. Roughly one-third of topsoil on Earth’s agricultural lands is considered moderately to highly degraded, a concern because it takes centuries to produce just a few inches of topsoil.
Replacing all fossil fuel consumption with biomass would be a disaster for the climate, but biomass will certainly play a role in the decarbonization of the economy. Ethanol produced with current technology might displace some petroleum fuel in sectors that absolutely need liquid fuel like fuel for airplanes, long-haul trucking, and marine shipping, as well as industrial heat and building heat. However, with the spread of electric vehicles for passenger transportation, the major role for biomass will probably be in hydrogen production. Currently, about 25% of the American corn crop is used to produce ethanol, but a recent study by researchers at Princeton University found that using that land to grow crops for gasification instead could play a major role in hydrogen production for economy-wide decarbonization. This might be done by gasifying perennial grasses like switchgrass and miscanthus, which are more sustainable than corn on a lifetime basis because they have high yields, require less fertilizer and other inputs, and sequester more carbon in their roots. But using corn for gasification is still more sustainable than using it for ethanol, since gasification processes are able to utilize the whole corn crop, whereas ethanol typically uses only the kernels.
As with all energy technologies, biomass involves trade-offs — people need to both eat and use energy, but the total land area on Earth is fixed. Humanity needs to grow more food in the next 40 years than it has in the previous 10,000 years combined, while simultaneously minimizing the amount of land under cultivation, but growing crops for bioenergy takes up land that could be used to feed people or livestock. Also similar to other technologies, it is wrong to claim that biomass will either be used widely or not at all. Most likely is that it will play some role in decarbonizing sectors which cannot be electrified, but not much more than that. Energy from biomass requires careful analysis of ecosystem dynamics. In some cases, the status quo will likely remain unchanged, while in others farmland will be converted to grow new crops. In many cases, it will probably make sense to leave areas forested or re-forest farmland, even if that means producing less biomass energy than is physically possible.
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