Energy from Waste: Pure Physics and Chemistry

25 of September of 2011

Energy-from-waste (EfW) takes advantage of the energy potential of the fractions of waste that cannot be reused (either because of the current state of technology, or because the cost is too high). As with all waste processing installations, EfW facilities tend to arouse considerable opposition among “greens” and ordinary citizens. In general, that opposition is purely emotional and is not supported by scientific knowledge of the processes involved. In fact, some of the arguments actually clash with the laws of nature.

When I check the list of subjects my children taken in primary and secondary school, I find that very few of them match the ones I took when I was young. Among the few that haven’t changed are Physics and Chemistry. Those two subjects are so deeply rooted in the education system that they have been adopted as the name of a popular series about high school on Spanish TV.

I don’t know why Physics and Chemistry have survived unscathed through numerous changes in education policies and syllabuses. It may be because they deal with basic Science (with a capital S), the laws and equations of nature, which hold universally, regardless of the location, the time or the ideology of the person applying them.

The conclusion, therefore, is that generations of students have received the same education in Physics and Chemistry. Therefore, in debates over waste in general, and EfW in particular, it’s surprising to find that a significant number of the people representing the public (either elected or self-appointed) evidence absolute and total ignorance of the laws of physics and chemistry. The situation is all the more serious since some of the arguments they raise belong properly in the realms of science fiction or alchemy.

I will try to set out the basic issues here, and they will be elaborated on in future posts. Waste contains basically two things that we can use: matter and energy (i.e. physics and chemistry, linked by Einstein’s famous equation E=mc2). Let’s start with the chemical side: matter. It’s clear (and required by law) that the first step in waste management must be to recover any materials that are susceptible. In this way, the material rejoins the production process because it has been given value. Although numerous technologies are used for this purpose, in the end up to 50% of the waste cannot be reused, not even in the countries that invest most in this area. Reasons: either the waste is intermingled, making it impossible to separate; or no technology exists as yet for recovering the material; or the process is simply so expensive that it is not cost-effective.

What do we do with this apparently useless waste? Humanity found the solution in prehistoric times: we throw it away in a specific place (midden or, more recently, landfill). However, our remote ancestors were aware that the waste that remained after removing the useful components by hand could still be used as fuel. Unfortunately, many people in some countries are still forced to perform this waste triage process by hand.

The more developed countries have designed complex technologies and equipment to extract all possible energy from this “residual waste” at modern facilities that are fully equipped to protect the health of people and the environment. The processes that take place in such facilities (combustion, pyrolysis, gasification, etc.) have been known for centuries and are not at all exclusive to EfW facilities. To serve as a fuel, a substance must be composed primarily of carbon and hydrogen. For that reason, chemistry tells us that fuels must always be organic in origin. That is simple chemistry, and it does not matter whether the carbon atom comes from timber, oil, waste plastic or a piece of potato peel. Combustion is a chemical process in which carbon and hydrogen combine with oxygen, releasing the energy contained in the molecular bonds. That is simple physics (specifically: thermodynamics) and it happens regardless of whether the bonds are in a molecule of methane (natural gas) or residual waste. Therefore, combustion is one of the phases of the natural cycle of carbon.

Among the many criticisms levelled at EfW, two are clear evidence that the speaker has forgotten what they learned in school. One such fallacy is the claim that waste is not a homogeneous material and, therefore, contains a wide variety of substances whose combustion may create more dangerous compounds. The magic words here are dioxin and volatile heavy metals, such as mercury. What nobody says is that dioxins and mercury are emitted by many other processes (both natural and anthropic), that the amounts are tiny, and that EfW plants are the only ones with specific legislation and specific equipment for avoiding these emissions. Forest fires, barbecues and motor vehicles also produce these emissions (in very small amounts, admittedly, but from millions of sources), and nobody seems to be concerned. The atoms and molecules produced by those sources are chemically indistinguishable and, if dioxin or mercury are going to kill us, as the critics of incineration claim, our body has no way of knowing if they came from an EfW plant or somewhere else.

The second argument against EfW is that it produces CO2, the greenhouse gas. This is true and undeniable, but it is only a partial truth, presented in a biased way. Combustion of organic material has supported life on Earth for millions of years. Your breathing as you read these lines is merely a form of low-temperature combustion of the food you eat. Combustion was also the first external energy source used by our remote ancestors. What we need to to know here is that all the CO2 that comes out of the chimney of an EfW plant is produced by burning organic matter, but it has two origins that we must distinguish clearly:

1) Biomass, i.e. organic matter from living organisms (garden trimmings, food leftovers, paper, etc.). This waste was created because plants captured CO2 from the atmosphere, and were subsequently eaten by animals (including humans). Therefore, that CO2 was already in the atmosphere and the final balance is zero, i.e. no net contribution to the atmospheric concentration of CO2. What critics do not say is that it is the inevitable fate of all organic matter on Earth to be converted into CO2 and water, regardless of what humanity may do. This is because the atmosphere is 23% oxygen (in chemical terms, our atmosphere is oxidising) and, therefore, all plants and animals literally become oxidised both during their lives and after they die. Therefore, even if the biomass were composted (the critics’ preferred alternative) and assuming there were enough citizens willing to make use of the compost (not the case at present), the carbon in the compost would end up in the atmosphere in the form of CO2 within a few years. The reason is clear: part will be oxidised directly by the oxygen in the air, and the remainder will be metabolised by organisms in the soil. I repeat: all molecules of CO2 are identical and create the same greenhouse gas effect, whether they come from a chimney, our lungs or the compost scattered on a farmer’s field.

2) Non-recoverable plastics (the proper term is “polymers”), which were obtained from oil. Although many people don’t know this, oil simply consists of the remains of living organisms that lived on Earth millions of years ago. Therefore, oil and coal were made from atmospheric CO2. Nevertheless, we won’t count that in this analysis and will accept that the emissions in this category are new emissions.

However, since the plastics and the fuels we use (gasoline, gasoil, coke, etc.) come from oil and coal, the combustion of plastic in EfW plants avoids the combustion of an equivalent amount of fuel in a thermal power plant, which would produce an equivalent amount of CO2. That is, the total amount of gas emitted per unit of energy produced is the same—only its origin differs. However, since all CO2 molecules are identical, the final effect is the same regardless of their source.

What critics of EfW fail to mention is the energy that would be required (and the resulting emissions of CO2) to recover, clean and transport all the plastic from the mass of waste. The net outcome would actually be negative for the environment.

So long as we continue to produce energy by combustion, we will need fuels rich in carbon and hydrogen. It seems more logical to use oil first as a raw material for producing plastics and, when the plastics have ceased to be useful, to burn them as fuel, rather than burning oil directly.

Energy-from-waste is a solution for the proper, environmentally-sound management of waste and is a complementary source of energy at a time when fossil fuels are expensive and are becoming scarcer. All the countries that actively manage their waste use a significant portion as fuel; Spain cannot be an exception. As Einstein said, “Education is what remains after one has forgotten everything he learned in school.” I don’t know if we in Spain are more educated, but we do seem to have forgotten a lot of the physics and chemistry we learned.

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