From biowaste to energy Valorização energética de resíduos orgânicos Energy valorization of organic residues 4. Briquettes and Dark Fermentation Jorge C. Oliveira Invited Professor, Instituto Superior Tecnico Head of Dpt., University College Cork E-mail: j.oliveira@ucc.ie From biowaste to energy So let’s start to see how biowaste can be turned to energy. For this hour I bundled together two things, drying & compressing to make solid biofuels, and dark fermentation to produce biohydrogen

Just burn the stuff... Note the relevance of water in the economics. If we spend more energy evaporating the water than we get from burning the VS... We must also note the problem of burning things stream must be well known and with composition that doesn’t vary significantly for potential in contaminants We’re concerned with: nitrogen sulphur potassium magnesium phosphorus any other nasty stuff that can be generated by burning? You’ve talked about combustion and electricity production with CHP units in this module, I don’t need to repeat that. But we can think about selling solid biofuels from dried and compressed biomatter. Of course, it doesn’t make sense to do it from waste streams with too much water. The other problem is that in industrial combustion units you can add the post-treatments to eliminate the emissions of toxic components (nitrogen, sulphur, in the case of biomaterials, dioxins and carcinogenics would also be a concern); however, if you compress and sell for you client to burn, then this sold biofuel cannot have emissions of this stuff at all.

How about briquettes? Example: Florafuel® , H.J.X.W. GmbH, Munich (D) critical issues: reduction of ash, chlorine, nitrogen, sulphur & other contaminants There are environmental regulations as to what can be present in what quantities in a solid fuel. The first 2 columns of the table in this slide show you the criteria in Germany for Class A and Class B fuels (you can only use Class A in your fireplace). The next 3 columns are analysis of 3 waste streams collected in Munich and Bavaria in 2015. The middle one, grass cuttings from the English Garden (if you know Munich, it’s a very nice and big park northeast of the city., along the Isar river) is usable as is, but the other 2 (from elsewhere in the city, likely to be more exposed to car exhaust fumes) and from another small town in Bavaria are not (too much Nitrogen and Chlorine in particular). This information comes from a company that developed a technology they call Florafuel that they patented which is generically described in the slide above. You can see that they do a series of pre-treatments before drying and compressing, and as the last 3 columns of the table show, this reduces the contaminants significantly, meeting environmental regulations (in the example above from 2015 there is still too much ash to be class A solid fuel, but the technology is continuing to improve)

What can we use to make them? But not all biowaste streams can be used like this. They may have too much water, too much nitrogen, etc. The above list identifies which biowastes were deemed appropriate by the Florafuel technology (it means that after applying the processing treatments that this company has developed, they are now meeting environmental regulations). So whatever is a “no” above cannot be simply dried, compressed, and then become a solid fuel – too many contaminants. The list of waste streams that are usable with this technology is shown in order of preference. Although at the bottom of the list, note that digestates from biogas production are usable ! That means that we could turn to biogas to produce fuel in that way, and then the digestate can continue its progress to become energy – it now fits the criteria to be dried and compressed to solid pellets. Green waste/garden prunings Organic fraction of Municipal Solid Waste Food market waste/vegetable waste Animal faeces, urine and manure Digestate residue from biogas production that’s interesting, don’t you think?

let’s fill up our knowledge sink info source for your study: Extract from HJXW study Some waste can be turned into solid fuel maybe it works better in some cases But that includes digestate itself, hence we can digest biowaste to produce biogas or ferment it to produce biogas and the leftover can be further used to produce briquettes or then, be used as fertilizer We could even have: algae producing biodiesel the left over digested to produce biogas the left over dried and compressed to produce solid fuel Let’s now reflect on what we captured in this part of the journey. Some waste can be turned to a solid fuel just by drying and compressing. Such fuels will be better or worse depending on the actual source. That includes digestates from biogas production, so if some waste is not suitable for this purpose, it is possible to take it to biogas, and still use the digestate left-over to produce solid fuel. It seems that we can start thinking that much of what we are finding could be tied up together to maximise energy production, if that was our objective. For instance, we could have algae purposedly grown to produce biofuel, that we sell. The algae biomass left over after removing the valuable fuel could go to produce biogas out of it, which we also cell. And the digestate left over could be turned into solid fuel. I leave on this subject the extract of the report (feasibility study) of the HJWX company that patented the Florafuel technology, which i already mentioned earlier about waste characterisation.

How about hydrogen? Should be first on our mind because hydrogen has the highest energy content per mass of any fuel (142 kJ/g) burning hydrogen produces nothing but water vapour There already are some car models that run on hydrogen fuel cells Toyota, Mercedes, Honda There are different ways of producing hydrogen, but dark fermentation is best for waste So let’s now see what we can do in terms of producing biogas from organic waste. We start with the production of hydrogen, where I will just give a brief overview. Hydrogen is a great fuel, as burning it is totally harmless to the environment, producing just water vapour. it also has the highest energy content per mass of any fuel, so to turn kg to kJ, this is the best molecule. There are car models already get using hydrogen fuel cells, and companies continue to work with it. The jury is still out on whether this is better than electric engines. Just LPG has gone out of fashion, and hydrogen does have some of the reasons for LPG not being so successful, like lack of places to fill up tanks, safety restrictions for underground parking, so it is still not a certainty it will be a winner. There are different ways of producing hydrogen (including simple hydrolysis of water), but the best to do it out of organic matter is by dark fermentation

Producing biohydrogen Overall there are two main pathways by which organic tissue produce hydrogen. To start with, photolysis is the process by which some algae produce hydrogen out of water which is a sort of biocompetitor of electrohydrolysis of water. Photosynthetic bacteria also consume carbon monoxide and produce hydrogen and CO2 in the presence of light. There are two “dark” processes, the most promising for biowaste is dark fermentation – I won’t explore microbial electrolysis cells, although in the last section I will leave you with a text that talks about this process.

Achievable outcomes This table show some of the yields in hydrogen and equivalent energy that have been obtained with these various processes; note the higher yields of dark fermentation. If you turn these numbers into electric energy, see for instance in the first column of power that a 5W portable generator requires only 1.49 L volume for a municipal sewage dark fermentation process (last in the list), with compares with hundreds to thousands of L for the same energy production, with photolysis (firsts on the list)

Biohydrogen from dark fermentation Bacteria converting carbohidrates to organic acids and hydrogen (most are cyanobacteria) Clostridium spp. Enterobacter spp. Bacillus spp. Complex process, but overall 𝐬𝒖𝒈𝒂𝒓+𝒘𝒂𝒕𝒆𝒓→𝒐𝒓𝒈.𝒂𝒄𝒊𝒅+𝑪𝑶𝟐+𝑯𝟐 e.g. glucose gives max. molecular yield 𝑪 𝟔 𝑯 𝟏𝟐 𝑶 𝟔 +𝟐 𝑯 𝟐 𝑶→𝟐𝑪 𝑯 𝟑 𝑪𝑶𝑶𝑯+𝟐𝑪 𝑶 𝟐 +𝟒 𝑯 𝟐 but yields can be lower; in this example: 𝑪 𝟔 𝑯 𝟏𝟐 𝑶 𝟔 →𝑪 𝑯 𝟑 𝑪 𝑯 𝟐 𝑪 𝑯 𝟐 𝑪𝑶𝑶𝑯+𝟐𝑪 𝑶 𝟐 +𝟐 𝑯 𝟐 Alas, the bacteria that produce hydrogen when digesting biomass are cyanogens, that is, they are highly toxic to humans. There are studies using Clostridium, Enterobacter and Bacillus species. The metabolic pathways are complex and I won’t explore them here (you can check for details in the texts that I’ll leave with this material if you are curious). Overall, sugars from the biomass (may come from hydrolysis of carbohydrates) and eventually water (not always needed as reagent) “turn into” organic acids, carbon dioxide and hydrogen. In terms of H2 yields, the best is the pathway from glucose to acetic acid, producing 4 moles of hydrogen for one of glucose. Other yields from other sugars may be less good, and even from glucose there are other possible pathways less generous for hydrogen, for instance, when the carboxylic acid generated is butyric acid, there are only 2 moles of hydrogen produced per mole of glucose.

What types of waste work well? Many different types of biowastes have been tried with various microorganisms and operating conditions. This review table shows that yields vary quite substantially, the highest reported being 346 L of H2 per kg of volatile solids in the waste, however, overall values of over 100 L/kg seem to be already as good as one can generally get from this technology

Problems Hydrogen needs to be purified more processing, more costs No real commercial application yet, because low H2 yields lack of robust microbial cultures If you wish to sell hydrogen, then it needs to be purified of all other gases, and with cyanobacteria as producers, this means the “usual suspects” like sulphur, but also nastier cyanogenic components. You may however run your own CHP plant with this hydrogen to produce electricity and save on some purification costs (though you still need to treat the gaseous emissions). In spite of the interest of producing H2, this technology is not mature yet, it is deemed to produce still insufficient amounts of hydrogen to be feasible on its own, and there are some doubts on the robustness of the cyanogen cultures, that is, they are not so easy to maintain healthily, which for an engineer means higher maintenance/replacement costs than with typical bioprocessing organisms.

let’s fill up our knowledge sink info source for your study: Dark Fermentation reviews (2) Cyanobacteria can produce hydrogen out of sugars/carbohydrates in absence of light This may require some hydrolysis of longer molecules The process is complex, so operating conditions must be optimised to maximise the H2 yield This is at present the most effective way of producing biohydrogen, specially from waste However, its economics are not brilliant but don’t through it away just yet... wait till the end when we’ll discuss biorefineries So in this brief overview we noted that cyanobacteria can produce hydrogen out of organic matter by converting carbohydrates. While being the most effective way to produce hydrogen from biowaste, the technology is still under development as yields would need to improve. But we will find later on that this capability to produce hydrogen is going to have more applications ... I leave 2 recent reviews on the subject.