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Semi-industrial biomethane pilot plant

FOREWORD

Climate change requires us to reduce consumption from fossil fuels by replacing them with renewable fuels such as biomethane, which can play an increasingly important role in the future, especially if produced as more sustainably as possible.

Sustainability in biomethane production means developing facilities that can have low self-consumption, scalable to small sizes so that they can be built where biomass is available at zero km, optimizing the nitrogen production cycle by minimizing digestate spillage, and especially seizing present in the biogas, to avoid

climate changing gas emissions into the atmosphere, resulting not only in the production of a green gas (biomethane) with a zero CO2 balance, or better even with a negative balance.

In addition, sustainability means building plants that are well embedded in the territory, having minimal size, no odor, with optimized nitrogen fluxes, and with the lower possible impact on the road system.

Oltre a questo, sostenibilità significa realizzare impianti ben inseriti nel territorio, con minime dimensioni, assenza di odori, con una ottimizzazione dei flussi azotati e con il minor impatto possibile sulla viabilità.

Bearing this in mind, several years ago A.S.A.C already decided to focus its efforts toward optimizing the biomethane production cycle through a patent aiming to solve all the problems arising from the anaerobic digestion production process. Specifically:

  • Create a steady anaerobic digestion process not subject to imbalance
  • Getting the highest possible yield from biomass inputs
  • Eliminate emissions to the atmosphere in terms of CO2, CH4, H2S, and odorigenic emissions
  • Develop a simple, cost-effective, and scalable upgrading process even on small sizes
  • Rationalize the nitrogen flow by minimizing digestate to be spread
  • Significantly reduce anaerobic digestion volumes by making the plant as small and less impactful as possible

Following the release of a specific, highly innovative patent for the production of biomethane, ASAC has built a semi-industrial pilot plant that enables the treatment of various types of waste, ensuring the production of methane with no CO2 emission (CO2 seizing with an extremely low carbon footprint). In particular, the pilot plant is authorised under Art.211 of Legislative Decree 152/06 as amended for the following EER codes 02.03.00, 19.08.05, 19.08.05, 20.01.08, 20.02.01, 20.03.02 and has a capacity of 4 tons/d.

The plant solution proposed by the above patent ensures the following advantages:

  • The process can be applied to existing biogas facilities by converting them to bio methane production.
  • The process can be applied to new grass roots facilities.

Bio methane production without CO2 emissions. This advantage is not achievable with the other available technologies (molecular sieves, absorption with amines or cryogenic condensation) unless complex catalytic synthesis processes powered by green hydrogen (electrolytic from photovoltaic systems) are used. About that, it should be recalled that the use of CO2 for industrial purposes (technical gas) or even more for food applications is unlikely, given the quantities involved and the absence of a certain and continuous market demand. In fact, carbon dioxide has represented the greenhouse gas since 70 years. Solutions related to underground storage are not currently applicable in our country because of the high margins of uncertainty in the systems.

GENERAL PLANT DESCRIPTION AND OPERATING DATA

The ASAC pilot plant consists of a biomass feeding section where the biomass is diluted with recirculating water and properly homogenized, obtaining in such a way a pumpable sludge having the desired dry concentration.

biomassa

This sludge is sent to the hydrolysis section, where it undergoes a thermo-chemical treatment that allows the more complex organic matter to be transformed into simpler molecules solubilized in the sludge, and then into a sludge having ideal conditions to be readily available to the bacteria that operate anaerobic digestion producing a very good quality biogas, very rich in methane.

The next step, of the ASAC patent consists of a system to convert the CO2 present in the biogas into bicarbonate. The process includes an acid scrubber for the removal of the ammonia stripped during the hydrolysis phase, by converting it into ammonium sulphate and residual sulfur compounds. Ultimately, a very pure bio methane is obtained, suitable to be introduced into the network and to be used as a biofuel, presenting values of CH4 > 96-97% and CO2<0.1% against the minimum 2.5% required by the UNI standard.

The pilot plant does not have a connection to the gas grid. In this case, the bio methane is compressed inside a series of cylinders powering an endothermic engine with “island” operation.

The pilot plant has been running for more than 2 years, with a capacity of 4 tons/d of biomass feed: During these two years. It has treated mainly sewage sludge, but also particularly difficult biomasses such as poultry manure and wet municipal waste.

The operation gave exceptional results, as it could have be verified that it is possible to achieve all the goals set forth with a simple but particularly smart system, especially in the biomass hydrolysis and CO2 absorption sections. In particular, the effects of the patented hydrolysis include not only an optimal pre-treatment of the organic matter, allowing to facilitate anaerobic digestion with reduced time, but it also allows us to stripe all the excess ammonia nitrogen, and simultaneously operate a CO2 and H2S absorption in a simple and absolutely reliable way.

CONCLUSIONS

During the two year testing, the main plus of the patent were proved, namely the production of high-quality bio methane suitable to be fed into the grid, the total seizing of the CO2 produced by anaerobic digestion and, therefore, no emissions to the atmosphere. This means that there is no need for the post combustion of gaseous CO2, as it is happens by utilising membrane systems, which can have even substantial CH4 losses.

The test also allowed to verify the ease of operation of the plant, which, first of all, operates at atmospheric pressure unlike other bio methane upgrading technologies do. Consequently the pilot plant proves to be a safe and reliable system with no leakage at all from the pipes; It proves also to be a simple system, without any sections of particular complexity and/or requiring special maintenance.

Self-consumption is extremely low, and biomass pretreated by the special ASAC hydrolysis step was much easier and faster to process, allowing to have very small digesters compared those required by traditional technologies.

The whole system is characterized by an extreme plant engineering simplicity and requires very little space. It is also easily scalable, adapting well to both facilities with large Biogas production and to small plants.

In addition, the ASAC process, by removing CO2 from the biogas stream and fixing it in the aqueous phase as bicarbonate, provides the anaerobic digestion system with greater stability, ensuring a high buffering capacity that prevents any imbalances in the biological reactions converting biomass to CH4. The digestate produced is an easily handled digestate, as it has undergone a very severe reduction of organic matter and removal of ammonia nitrogen, and having values suitable for direct discharge to the sewer with no need of final composting.

ASAC technology is applicable for:

Plant facility

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Reconversions and upgrading

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Microunits

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Biogas plants in cogeneration

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