Criticism of the Prevailing Belief that Wood Combustion is CO<sub>2</sub> - Neutral

Frank Diederichs; Frank Diederichs

ISSN: 2641-3094

Global Journal of Ecology

Commentary Open Access Peer-Reviewed

Criticism of the Prevailing Belief that Wood Combustion is CO₂ - Neutral

Frank Diederichs*

Rigas Fereou 3, 66100 Drama/Sitagroi, Greece

Author and article information

*Corresponding authors: Frank Diederichs, Rigas Fereou 3, 66100 Drama/Sitagroi, Greece, E-mail: [email protected]; [email protected]

ORCiD : https://orcid.org/0000-0001-9374-425X

doi : 10.17352/gje.000108

Received: 23 January, 2025 | Accepted: 10 February, 2025 | Published: 11 February, 2025

Cite this as

Diederichs F. Criticism of the Prevailing Belief that Wood Combustion is CO₂ - Neutral. Glob J Ecol. 2025;10(1):001-003. Available from: 10.17352/gje.000108

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© 2025 Diederichs F. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Main article text

Any fuel of plant origin that has grown by absorbing CO₂ from the atmosphere via photosynthesis burns in a CO₂-neutral way since virtually its entire carbon content is recycled into the atmosphere during combustion. This or very similar formulations are often used to explain the supposed CO₂ neutrality of burN_ing plant biomass. This statement would be true if the CO₂ produced was released into the atmosphere at the same rate as it disappears from it. But as is well known, this is not the case nowadays. At present, CO₂in the atmosphere is mainly produced by burN_ing fossil fuels such as coal, oil, and natural gas. It is mainly removed from the atmosphere through photosynthesis and distribution in seawater. However, these processes are far from sufficient to remove CO₂ from the atmosphere at a sufficient rate to achieve CO₂ neutrality.

Obviously, the above idea of CO₂ neutrality is based on the completely arbitrary assumption that only the CO₂ produced by burN_ing wood (as an example of plant biomass) is recycled in a sustainably managed forest via tree growth, while the CO₂ from other combustion sources is not available for photosynthesis. In the atmosphere, however, more or less all of the CO₂ emissions from all sources are mixed so it must be assumed that the amounts used to regrow the forest also correspond to this atmospheric mix from different sources. This means that due to its low percentage in the mix, the CO₂ from wood burN_ing only contributes a small degree to regrowth, while CO₂ from fossil sources, due to their much greater relative frequency in the mix, are also the main contributors to photosynthesis and therefore to plant growth.

To emphasize this one more time, CO₂neutrality in the atmosphere would be present if in a given time interval the rate of carbon entry (in Gt C)/year or Pmol/year) into the atmosphere were equal to the rate of carbon exit (in (Gt C/year or Pmol/year) from the atmosphere into a sink such as photosynthesis. It is impossible, to yield such a result, which can be mathematically formulated as (for CO₂ neutrality), simply from the carbon cycling associated with wood combustion alone. In the presence of other CO₂ sources, it is imperative that these are also included in the neutrality analysis.

Of course, all CO₂ molecules are identical and therefore indistinguishable, regardless of which carbon-contaiN_ing fuel source they originate from. This applies to their uptake in photosynthesis as well as their distribution in (sea) water.

The following example should serve to further explain the relationships for the CO₂ changes in the atmosphere, using data from Friedlingstein, et al. [1].

For the atmospheric amount of CO₂ a value of NO_atm = 74.827 Pmol or 898.75 Gt C has been found using a ppm value of 423. The quantity of carbon bound in the vegetation is then NO_wood = 0.514 74.827 = 38.483 Pmol or 462.22 Gt C [1]. To a first approximation, the atmosphere can be regarded as an ideal gas of reference volume $V_{a t m}^{r e f}$ (288.15 K).

According to L. Boltzmann, all particle types of gas are arranged in such a way that the energy distribution with the maximum probability is adopted spontaneously [2,3]. For an ideal gas in a given volume, this means that the maximum multiplicity is achieved when the entire volume is homogeneously filled with the respective particles. This applies to all particle types in the atmosphere, including CO₂ molecules from different sources. Consequently, all CO₂ molecules must be present as a complete mixture. This in turn means that CO₂ molecules enter the efflux reactions of the atmosphere in the same proportions, as they are present in the atmospheric mixture. These proportions are given by the known influxes from different sources.

All emissions (in Gt C/yr) are constant, and the total CO₂-efflux (absolute values for effluxes) with E_Lsink = 2.9 (land sink) and E_ocsink = 3.0 Gt C/yr (ocean sink) is given by J_out = E_Lsink+ E_ocsink Gt C/yr. E_Lsink contains mainly the efflux J_ps via photosynthesis. The two influxes are Efos = 9.3 (fossil emission) and J_woodb = 0.8 E_Luc Gt C/yr (wood burN_ing of 0.8 E_Luc is an arbitrary assumption). E_Luc = 0.9 (land use change) contains J_woodb plus Jdef (deforestation). The total influx then is given by J_in = Efos+ J_woodb Gt C/yr. The respective masses of CO₂ in the atmosphere after a given time t are, for example, N_fosb = Efos×t and N_woodb = N_woodb×t Gt C. The total amount that has entered the atmosphere during time t is given by N_in = N_fosb+ N_woodb Gt C.

For the derivation of the following results, it was assumed that due to the indistinguishability of the CO₂ molecules originating from different sources, their probability of entering the photosynthesis reaction, for example, depends on their relative frequency in the atmospheric mix. If, for instance, there is about 13 times more CO₂ of fossil origin in the mix than from wood combustion (with N_woodb/N_im= J_woodb×t/(Efos+J_woodb)×t, N_fosb/N_in = J_fosb×t/(Efos+J_woodb)×t yielding N_fosb/N_woodb = J_fosb/J_woodb = 12.917), then in a given time interval 13 times more fossil CO₂ than CO₂ from wood burN_ing will be removed from the atmosphere via this pathway.

The time-related change of the total mass of CO₂ in the atmosphere (G_ATM in Friedlingstein's publication [1]) is given by

$\frac{Δ C O_{2}}{Δ t} = E_{f o s} + 0.8 E_{L u c} - E_{L \sin k} - E_{o c e a n} (in Gt C / yr), (1)$

the effluxes e.g. via photosynthesis (with J_woodb = 0.8 E_Luc) are

$J w o o d_{p s} = \frac{N_{w o o d b}}{N_{i n}} \times J_{p s} = \frac{J_{w o o d b} \times t}{(E_{f o s} + J_{w o o d b}) \times t} \times J_{p s} = 0.284, and (1a)$

$J f o s_{p s} = \frac{N_{f o s}}{N_{i n}} \times J_{p s} = \frac{E_{f o s} \times t}{(E_{f o s} + J_{w o o d b}) \times t} \times J_{p s} = 2 . 6916 Gt C / yr, (1b)$

(the photosynthesis flux J_ps equals E_Lsink, Jwood_ps+Jfos_ps must add up to J_ps = 2.9 Gt C/yr).

These equations result from the fact that each efflux pathway takes up CO₂ molecules from different sources in the same proportions as they occur in the atmospheric mixture. These are identical to those of the constant influxes, so that, for instance for the photosynthetic efflux pathway, the following proportionality equation is obtained, Jwood_ps/J_ps= J_woodb/J_in.

In order to assess CO₂ yield, these effluxes are now compared with the respective influxes from the corresponding sources. This is done by calculating quotients (taking absolute values for effluxes) according to

$\frac{J w o o d_{p s}}{J_{w o o d b}} = \frac{J_{p s}}{E_{f o s} + J_{w o o d b}} = 2.9 / (9.3 + 0. 72) = 0. 2894, and (1c)$

$\frac{J f o s_{p s}}{E_{f o s}} = \frac{J_{p s}}{E_{f o s} + J_{w o o d b}} = 2.9 / (9.3 + 0. 72) = 0. 2894 . (1d)$

Although both influxes are very different, their yield quotients (via photosynthesis) are identical.

Since there is an additional efflux possibility via E_ocsink, this must also be taken into account. The yield quotients via this pathway are given by

$\frac{J w o o d_{o c}}{J_{w o o d b}} = \frac{E_{o c s i n k}}{E_{f o s} + J_{w o o d b}} = 3 .0 / (9.3 + 0. 72) = 0. 2994, and (2a)$

$\frac{J f o s_{o c}}{E_{f o s}} = \frac{E_{o c s i n k}}{E_{f o s} + J_{w o o d b}} = 3 .0 / (9.3 + 0. 72) = 0. 2994 . (2b)$

Both pathways (“ps” and “oc”) differ only slightly because their respective effluxes (2.9 and 3.0 Gt C/yr) have similar values. For the entire process, the result is

$\begin{array}{l} \frac{J w o o d_{p s} + J w o o d_{o c}}{J_{w o o d b}} = \frac{J_{p s} + E_{o c s i n k}}{E_{f o s} + J_{w o o d b}} = \frac{J_{o u t}}{J_{i n}} (3a) \\ = 0. 2894 + 0. 2994 = 0. 5888, and \end{array}$

$\frac{J f o s_{p s} + J f o s_{o c}}{E_{f o s}} = \frac{J_{p s} + E_{o c s i n k}}{E_{f o s} + J_{w o o d b}} = \frac{J_{o u t}}{J_{i n}} = 0. 5888 . (3b)$

A quotient value of 1.0 would mean that the total CO₂ efflux is equal to the total CO₂ influx (J_out = J_in). Then CO₂ neutrality would be present and the CO₂ yield of the atmosphere would be zero Gt C. The sum of all effluxes is then equal to the sum of all influxes, that is, J_out = J_in, or J_out / J_in = 1.0. If J_out / J_in < 1.0, no CO₂ neutrality can occur and CO₂ would accumulate in the atmosphere. If, on the other hand, J_out / J_in > 1.0, the atmosphere would lose CO₂.

Under current conditions, only about 29% of the CO₂ produced by wood burN_ing (first influx pathway) is removed from the atmosphere via photosynthesis (first efflux pathway). The same applies to the CO₂ produced from fossil fuels (second influx pathway). Again, only 29% of this source is removed via photosynthesis. About 30% of the total CO₂ is taken up by seawater (second efflux pathway). This is followed by a drop in pH, which may be lethal for many marine organisms. The remainder of about 41% remains in the atmosphere and adds up to the CO₂ accumulated over previous years.

The results show that the flux ratios of efflux and influx for a given efflux pathway, such as photosynthesis, are the same for all CO₂ sources. This can be verified by adding further CO₂ sources and/or sinks. For example, an additional combustion of synthetic gasoline (e-fuel) produced from atmospheric CO₂ and burned instead of fossil gasoline (a third efflux pathway “sy” is thus created in concert with the additional efflux J_syn and influx J_synb, J_syn = J_synb = 0.1 J_ps =0.29 Gt C/yr) yields,

$\frac{J w o o d_{p s}}{J_{w o o d b}} = \frac{J f o s_{p s}}{J_{f o s b}} = \frac{J s y n_{p s}}{J_{s y n b}} = \frac{J_{p s}}{J_{i n}} = 0. 2894, (4a)$

$\frac{J w o o d_{o c}}{J_{w o o d b}} = \frac{J f o s_{o c}}{J_{f o s b}} = \frac{J s y n_{o c}}{J_{s y n b}} = \frac{J_{o c}}{J_{i n}} = 0. 2994, and (4b)$

$\frac{J w o o d_{s y}}{J_{w o o d b}} = \frac{J f o s_{s y}}{J_{f o s b}} = \frac{J s y n_{s y}}{J_{s y n b}} = \frac{J_{s y}}{J_{i n}} = 0.0 289 . (4c)$

This results in a slightly increased flux quotient formed from all effluxes and influxes,

$\frac{J_{o u t}}{J_{i n}} = \frac{J_{p s} + J_{o c} + J_{s y}}{J_{i n}} = 0. 2894 + 0. 2994 + 0.0 289 = 0. 618 . (4d)$

The same result would be produced if no synthesized fuel was burned and no fossil fuel was replaced.

The simplest way to achieve a quotient of 1.0 (CO₂ neutrality) may be realized by reducing drastically J_fosb. Reducing this CO₂ influx by a factor of 0.557 yields,

$\frac{J w o o d_{p s}}{J_{w o o d b}} = \frac{J f o s_{p s}}{J_{f o s b}} = \frac{J_{p s}}{J_{i n}} = 0. 4915, (5a)$

$\frac{J w o o d_{o c}}{J_{w o o d b}} = \frac{J f o s_{o c}}{J_{f o s b}} = \frac{J_{o c}}{J_{i n}} = 0. 5085, and (5b)$

$\frac{J_{o u t}}{J_{i n}} = \frac{J_{p s} + J_{o c}}{J_{i n}} = 1 .0.$

The same result can be also produced by adding an CO₂ efflux pathway (“sy”). Increasing the CO₂ efflux by the addition of a CO₂ utilizing reaction (J_syn=1.421 J_ps, J_synb=0) yields

$\frac{J w o o d_{p s}}{J_{w o o d b}} = \frac{J f o s_{p s}}{J_{f o s b}} = \frac{J_{p s}}{J_{i n}} = 0. 2894, (6a)$

$\frac{J w o o d_{o c}}{J_{w o o d b}} = \frac{J f o s_{o c}}{J_{f o s b}} = \frac{J_{o c}}{J_{i n}} = 0. 2994, (6b)$

$\frac{J w o o d_{s y}}{J_{w o o d b}} = \frac{J f o s_{s y}}{J_{f o s b}} = \frac{J_{s y}}{J_{i n}} = 0. 4113, and (6c)$

$\frac{J_{o u t}}{J_{i n}} = \frac{J_{p s} + J_{o c} + J_{s y}}{J_{i n}} = 1 .0. (6d)$

As has been shown [4,5], such a reaction could be realized by electrochemical reduction of atmospheric CO₂ to graphene-like carbon. Such a CO₂-consuming reaction seems particularly suitable for reducing the CO₂ content of the atmosphere on a large scale for two reasons: the reduction only to C requires significantly less energy than the reduction to e-fuel (hydrocarbon), for example, and the resulting carbon is non-hazardous and hence storable.

With regard to wood burN_ing for energy supply, it seems particularly worth mentioN_ing that this process emits about twice the amount of CO₂ compared to natural gas combustion with the same energy output. In addition, wood combustion is known to be extremely harmful to humans and possibly also to many aN_imals and plants.

In summary, it can be stated that fossil fuels and fuels of plant origin, such as wood, do not differ in terms of their atmospheric CO₂ yield. Consequently, their respective ability to influence CO₂ neutrality given the same CO₂ production rate must be also identical. This is not surprising, as it is always the same molecule that is released into and removed from the atmosphere, even if the involved processes take place via several different CO₂ input sources and output pathways. Only the influxes and effluxes of this molecule over the same time interval are decisive for the temporal change in the atmosphere $(Δ C O_{2}^{a t m} / Δ t)$ and consequently also for CO₂ neutrality.

The statement that wood burN_ing can only release that amount of CO₂ that it has already absorbed from the atmosphere through photosynthesis over the years as it grows, as catchy as such a statement may be for many scientists, bears absolutely no relationship to the amount of CO₂ in the atmosphere. It is simply inappropriate and leads to the erroneous conclusion that wood burN_ing is CO₂ neutral.

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