Food and fuel, not food or fuel

Can we use safely use larger quantities of biomass for energy? The conventional answer is no: humankind already uses too much valuable land for non-food purposes. Using more of the world’s productive acreages to produce wood or other biomass for the generation of electricity or heat will increase global environmental stress and reduce food production.

In a very recent paper, Mike Mason and team give a different answer. They probe the use of otherwise unproductive drylands for growing a class of non-food plants for use in anaerobic digesters (AD).[1] AD produces methane which can burnt to produce electricity. Mason’s conclusion is that the world’s drylands cover about 15% of the world’s land area. Planting of carefully chosen aridity tolerant plants on just 10% of this land could produce as much electricity as natural gas does today. As important, the gas from AD can be burned at night or on cloudy days meaning it is a vital complement to solar PV in tropical countries.

 10 month old prickly pear plantation in Kenya

10 month old prickly pear plantation in Kenya

To put Mike Mason’s work in context, I’m going first to look at why wood and plants are usually thought not to be an appropriate replacement for fossil fuels. Put at its crudest, the argument is that burning a tree sends CO2 back into the atmosphere. Replanting a new tree will eventually extract that CO2 again but it might be 60 years before even a fast growing conifer has grown to the same size as the tree that was burnt. So even if we immediately replace every single tree that we use for energy it will be many decades before the world moves back out of ‘carbon debt’.

This may be too crude an analysis. Let’s look at where Drax, the huge power station in Yorkshire that is switching to burning wood pellets, gets its 4m tonnes of fuel from.

1.1m tonnes sawmill residues

1.2m tonnes forest residues

1.0m tonnes woodland thinnings

0.4m tonnes sawmill waste

0.4m tonnes other (straw, miscanthus and other sources)

Total 4.1m tonnes

Drax is very careful. Virtually none of its biomass comes from mature trees cut down solely to keep its giant furnaces burning. The company argues - with much justification in my view - that its purchases are encouraging increased forestation in the southern US, where it sources much of its wood. Removals of wood are running at only about 60% of the natural growth rate in the southern eastern US states.

Nevertheless, Drax’s scale is simply enormous. (It is by far the single most important buyer of wood pellets in Europe, and possibly the world). By one calculation it is using the biological production of over one million hectares of land, or almost as much as the total wooded area of England. (NB Scotland has a lot more). From this, the UK is getting about 3% of its electricity.

If Drax is any example, biomass doesn’t look a good bet as the source of the world’s power in 2050. At root, the reason is that photosynthesis isn’t particularly efficient. We’re lucky to see 1% of the sun’s power turned into burnable carbon in the most efficient plants and trees in well watered zones. Compare this to the 20% of light hitting a good solar panel.

The overall position is far worse than this because much of the world’s area doesn’t support plants. The earth receives about 100,000 TW of energy from the Sun but only about 120 TW is captured by phothosynthesis on land and at sea. Only about half this photosynthesis is carried out by land plants and trees and humankind is currently using about 15 TW in the form of food and other materials.[2] The pessimists in the field think that the ability of humankind to increase this offtake of biomass is very limited. Only a few scientists have previously suggested that more than about 20 TW can be safely abstracted without risk of further environmental stress. Much of the extra 5 TW of photosynthesis will need to be in the form of human and animal food, implying that at most a couple of extra TW might be used to meet human energy needs, such as burning wood in Drax. This is why Mike Mason’s work may turn out to be so important.

Even a couple of TW isn’t negligible. The planet is currently using only about 15 TW and this isn’t ever likely to rise much above 30-40 TW. Nevertheless, as things stand, bioenergy can only provide a fraction of extra energy needs, even under the projections of the most optimistic people in the field.

The work of Mike Mason’s team may change this. In essence, what the group is saying is that we can capture far more energy using photosynthesis than the current research suggests. Agriculturally unproductive drylands, such as in Kenya where the group’s work is largely carried out, can be made photosynthetically useful if the right plants are cultivated and harvested. The paper investigates two types that can capture sunlight well, even in areas with low and highly seasonal rainfall: Euphorbia tirucalli and Opuntia ficus Indica. (The first is usually called Pencil Cactus and the second Prickly Pear).

After harvesting, Mason hypothesises that the best way of extracting the energy and converting it into a useful form is through anaerobic digestion, allowing the plant to rot in a very low oxygen environment, much as grass is broken down in a cow’s stomachs. This produces a biogas that is up to 65% methane, burnable in inexpensive gas engines and turned into electricity. The plants could be combusted but the big advantage of digestion is that the gas produced each day can be stored for burning overnight when PV panels aren’t producing any electricity.

Mason’s team show that it may be possible to generate annual yields of 20 tonnes of dry biomass per hectare using these water efficient plants. That’s about five times the productivity of a hectare of Sitka spruce in the UK.

If Mason is right, cultivating these crops on between 1 and 2% of the world’s land will capture about an extra 1 TW of solar energy without reducing the world’s agricultural production. In fact, he thinks that the by-products of the AD process, including nutrient-rich liquids, can be used to enhance food output. The conflict between food and fuel disappears, he says.

The obvious question to ask Mike Mason is why others haven’t noticed the energy generating capacity of these types of plants before. He’s on record as saying that the reasons include the lack of agricultural interest; these plants have very limited food value and therefore have never been properly cultivated.  

Further experiments to grow and then digest these plants will show whether the results suggested in Mason’s paper can be widely replicated. Do these plants really produce 20 tonnes a hectare on land with low rainfall that is concentrated in a few months of each year? Can they be planted and harvested cheaply? Will the plants be digested properly in AD? Will the impact on food production be as benign as he hopes?

As with so many other apparent breakthroughs, this new approach needs millions of dollars of research money now. Mike Mason had a career as a successful entrepreneur before going into academic work and still keeps his business activities going. There are very few people better qualified to find ways of giving the developing world an extra terawatt of power.

 

 

[1] For people who know about  plants, it may be useful  to know that Mason proposes using so-called CAM plants (crassulacean acid metabolism). CAM plants use relatively little water and do not contain much lignin, a molecule that resists breakdown during AD.

[2] The calories actually eaten by the world’s population represent the energy equivalent of no more than about 2TW of this total. Other biomass is wasted, eaten by  grazing  animals or used as fibre or other  materials.