Biomass ... and when a country runs out of rubbish

Weather Eye
with John Maunder


Some of the answers to the complexities of the climate system are given in my recently published book Fifteen Shades of Climate: The fall of the weather dice and the butterfly effect.

The following are extracts are from pages 351-359, part 1. Part 2 will follow next week when Norway and Sweden say they have too much capacity for burning waste:

Biomass is plant or animal material used for energy production (electricity or heat), or in various industrial processes as raw substance for a range of products.

It can be purposely grown energy crops (eg miscanthus, switchgrass), wood or forest residues, waste from food crops (wheat straw, bagasse), horticulture (yard waste), food processing (corn cobs), animal farming (manure, rich in nitrogen and phosphorus), or human waste from sewage plants.

Wikipedia says while burning plant-derived biomass releases CO2, it has still been classified as a renewable energy source in the EU and UN legal frameworks because photosynthesis cycles the CO2 back into new crops.

In some cases, this recycling of CO2 from plants to atmosphere and back into plants can even be CO2 negative, as a relatively large portion of the CO2 is moved to the soil during each cycle.

Co-firing with biomass has increased in coal power plants because it makes it possible to release less CO2 without the cost associated with building new infrastructure. Co-firing is not without issues, however.

Often, an upgrade of the biomass is most beneficial. Upgrading to higher-grade fuels can be achieved by different methods, broadly classified as thermal, chemical, or biochemical.

When people began burning wood fuel

Historically, humans have harnessed biomass-derived energy since the time when people began burning wood fuel. Even in 2019, biomass is the only source of domestic fuel in many developing countries.

All biomass is biologically produced matter based on carbon, hydrogen and oxygen. The estimated biomass production in the world is approximately 100 billion metric tons of carbon per year, about half in the ocean and half on land.

Based on the source of biomass, biofuels are classified broadly into two major categories:

  • First-generation biofuels are derived from food sources such as sugarcane and cornstarch. Sugars present in this biomass are fermented to produce bioethanol, an alcohol fuel that serves as an additive to gasoline, or in a fuel cell to produce electricity.
  • Second-generation biofuels utilise non-food-based biomass sources such as perennial energy crops (low-input crops) and agricultural/municipal waste.

There is huge potential for second-generation biofuels, but the resources are currently under-utilised.

Thermal conversion processes use heat as the dominant mechanism to upgrade biomass into a better and more practical fuel.

The basic alternatives are torrefaction, pyrolysis and gasification; these are separated principally by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature).

There are other less common, more experimental or proprietary thermal processes that may offer benefits, such as hydrothermal upgrading.

Some have been developed for use on high-moisture-content biomass, including aqueous slurries, and allow them to be converted into more convenient forms.

A range of chemical processes may be used to convert biomass into other forms, such as to produce a fuel that is more practical to store, transport and use, or to exploit some property of the process itself.

Many of these processes are based in large part on similar coal-based processes such as the Fischer-Tropsch synthesis. Biomass can be converted into multiple commodity chemicals.

As biomass is natural material, many highly efficient biochemical processes have developed in nature to break down the molecules of which biomass is composed, and many of these biochemical conversion processes can be harnessed.

In most cases, microorganisms are used to perform the conversion process: anaerobic digestion, fermentation, and composting.

Glycoside hydrolases are the enzymes involved in the degradation of the major fraction of biomass, such as polysaccharides present in starch and lignocellulose.

Thermostable variants are gaining increasing roles as catalysts in biorefining applications, since recalcitrant biomass often needs thermal treatment for more efficient degradation.

Biomass can be directly converted to electrical energy via electrochemical (electrocatalytic) oxidation of the material.

This can be performed directly in a direct carbon fuel cell, direct liquid fuel cells such as a direct ethanol fuel cell, a direct methanol fuel cell, a direct formic acid fuel cell, an L-ascorbic Acid Fuel Cell (vitamin C fuel cell), and a microbial fuel cell.

The fuel can also be consumed indirectly via a fuel cell system containing a reformer, which converts the biomass into a mixture of CO and H2 before it is consumed in the fuel cell.

The simple proposal that biomass is carbon neutral put forward in the early 1990s has been superseded by the more nuanced proposal that for a particular bioenergy project to be carbon neutral, the total carbon sequestered by a bioenergy crop’s root system must compensate for all the emissions from the related, above-ground bioenergy project.

This includes any emissions caused by direct or indirect land use change. Many first-generation bioenergy projects are not carbon neutral given these demands.

Some have even higher total GHG emissions than some fossil-based alternatives. Transport fuels might be worse than solid fuels in this regard.

Some are carbon neutral or even negative, especially perennial crops.

The amount of carbon sequestrated and the amount of GHG (greenhouse gases) emitted affect how the total GHG life cycle cost of a bioenergy project develops.

Specifically, a GHG/carbon-negative life cycle is possible if the total below-ground carbon accumulation more than compensates for the above-ground total life-cycle GHG emissions.

Biomass releases CO2 gas that was absorbed during its growth cycle, indicating that released gas can be processed by photosynthetic methods. Plants that are the source of the biomass may be a carbon-neutral energy source.

Successful sequestration is dependent on planting sites, as the best soils for sequestration are those that are currently low in carbon.

The assumption that annual cropland provides greater potential for soil carbon sequestration than grassland appears to be over‐simplistic, but there is an opportunity to improve predictions of soil carbon sequestration potential using the information on the initial soil carbon stock as a stronger predictor of ∆C (change in carbon amount) than prior land use.

Forest-based biomass projects

Forest-based biomass projects have received criticism for ineffective GHG mitigation from a number of environmental organisations, including Greenpeace and the Natural Resources Defence Council.

Environmental groups also argue that it might take decades for the carbon released by burning biomass to be recaptured by new trees.

Biomass burning produces air pollution in the form of carbon monoxide, volatile organic compounds, particulates and other pollutants.

In 2009, a Swedish study of the giant brown haze that periodically covers large areas in South Asia determined that two-thirds of it had been principally produced by residential cooking and agricultural burning, and one-third by fossil-fuel burning.

The use of wood biomass as an industrial fuel has been shown to produce fewer particulates and other pollutants than the burning seen in wildfires or open field fires.

The conversion to biomass energy has played a key role in reducing our dependence on fossil fuels.

But is this renewable energy source really as green as we first thought? Kate Ravilious investigates in the January 2020 issue of Environment and Energy using a report in of January 8, 2020.

In May 2019, the UK went an entire fortnight without using any coal to generate electricity. The last time this happened, Queen Victoria was on the throne.

From having had its first coal-free day in summer 2017 to recording its first coal-free week in May 2019, the UK had done an impressive job of weaning itself off the dirtiest fossil fuel.

But, as environmentalists cheer the good news, a truth came to light: biomass power plants — a key renewable energy source and one of the main replacements for coal-fired power — are emitting more CO2 from their smokestacks than the coal plants they have replaced.

In its haste to get rid of coal, the UK may have inadvertently made global warming worse.

The logic behind biomass energy is simple.

Trees and plants absorb CO2 from the air, use photosynthesis to isolate the carbon, and then use it to build tree trunks, bark and leaves.

But when the plant dies, it rots down and much of the carbon is released back into the atmosphere as CO2.

“When we use biomass as an energy source, we are intercepting this carbon cycle, using that stored energy productively rather than it just being released into nature,” explains Samuel Stevenson, a policy analyst at the Renewable Energy Association in London.

Burning fossil fuels releases carbon from geological reservoirs, which would have remained locked up for many millions of years if left untouched.

So switching from fossil fuels to biomass energy seemed like an obvious way for European Union (EU) nations to meet their obligations under the Paris Climate Agreement (signed in 2016). Back in 2009, the EU committed itself to 20 per cent renewable energy by 2020 and included biomass on the list of renewable-energy sources, categorising it as “carbon neutral”.

Several countries embraced bioenergy and started to subsidise the biomass industry. Currently, around half the EU’s renewable energy is based on biomass — a figure that is likely to rise.

Green and leafy energy revolution

In the UK, the Drax Group has led the way with this green and leafy energy revolution. Over the last decade the Drax coal-fired power station in North Yorkshire, which produces around 5 per cent of Britain’s electricity, has seen four of its six generating units being converted to run on biomass.

Today, Drax generates around 12 per cent of the UK’s renewable electricity — enough for 4r million households.

Standing next to the train track at Drax in September 2019, Scott Bentsen watched as 25 wagons of wood pellets were slowly disgorged into one of the four Albert Hall-sized storage domes.

His guide told him that most of the pellets are made from the sawmill residues and waste left over from managed forestry in the US and Canada. This can include tree tops and limbs, misshapen and diseased trees not suitable for other use, and small trees removed to maximise the growth of the forest.

Virtually every day, shipments arrive in ports at Immingham, Hull, Newcastle or Liverpool, each carrying around 62,000 tonnes of wood pellets — enough to keep the boilers going for two and a half days. Unloading the ship takes three days and requires 37 freight train journeys.

You might think that the greenhouse-gas emissions associated with transporting the pellets over such a vast distance must be huge, but I’m told they make up a surprisingly small proportion of the supply chain emissions.

“As long as wood fuels are transported by ship, the distance doesn’t matter too much,” says Scott Bentsen.

The size of the operation at Drax is absolutely staggering. In just under two hours an entire freight train’s worth of wood pellets goes up in smoke.

Although the pellets are made from sawdust and forest thinnings, I am still struck by how colossal the demand for timber must be in order to produce leftovers on this scale. However, Drax says that by creating a market for timber waste it is helping to prevent deforestation.

“Using the low-grade material for wood pellets provides the landowners with additional income, making their land more profitable and helping to incentivise them to maintain and improve the forests rather than using the land for something else,” says a spokesperson for Drax.

Continued next week