Biogas from waste and landfill gas production phenomenology

Biogas collection and recovery plants are present in many industrial sectors such as anaerobic waste treatment plants, landfills, purification plants, the agricultural and livestock industry etc.

For illustrative purposes only and as an extremely simplified summary, we take into consideration LANDFILL plants where waste is deposited for which recycling or other form of recovery has not been possible.

In this place, the residues of many types of waste, especially the residues of biodegradable waste, remain active for over 30 years and, through natural decomposition processes, they produce biogas and leachate.

Here, the decomposition of the organic component of the waste takes place by means of different processes, often characterized by various, complex aspects. These are mainly physical, chemical and biological processes acting simultaneously and which we will now list below in an extremely simplified form:

• PHYSICAL DEGRADATION involves the modification of the physical characteristics of the waste itself, such as the reduction in volume and the precipitation, release and absorption of substances;
• CHEMICAL DEGRADATION involves the activation of reactions between the various substances making up the waste; for example they are significant in the quality of leachate (variation of pH, redox potential, solubility);
• BIOLOGICAL DEGRADATION involves the transformation of the material that constitutes the waste by microorganisms such as bacteria; biological degradation takes place mainly in three stages (aerobic stage, anaerobic stage and anaerobic methanogenic stage) by means of which the biogasification process of the waste takes place.

The biogas production phenomenology is only hinted at here through the following summary:

• Aerobic stage: this occurs immediately after the deposition of waste by aerobic microorganisms and depends on the availability of oxygen present in the matrix. It is usually short-lived (from a few hours to a few months) and is linked to the type of waste. The aerobic process is significantly exothermic (heat production that can reach temperatures of 70 °C) and is characterized by emissions of carbon dioxide, water and partially degraded organic substances.
• Anaerobic (acidic) stage: this occurs when the availability of oxygen is reduced to the point where an aerobic process is no longer possible. In this context, aerobic microorganisms prefer to use free oxygen but, in its absence, they can use bound oxygen. In this stage, the production of carbon dioxide takes place. There is less generation of thermal energy compared to the aerobic process and a considerable production of partially degraded organic substance. Most of the partially degraded organic substance consists of organic acids. These acids, with dissolved carbon dioxide, are found in the leachate, to which they give a certain level of acidity.
• Anaerobic methanogenic stage: (consisting in turn of the ‘non-stationary anaerobic methanogenic stage’ and the ‘stationary anaerobic methanogenic stage’): this is the final stage of the decomposition of organic waste. In this stage, the microorganisms convert the organic substance partially degraded by the aerobic organisms into methane, carbon dioxide and other micro-components. Once again, this stage is characterized by the production of thermal energy (lower than the aerobic stage), the use of dissolved organic matter and the production of methane and carbon dioxide, as well as the increase in pH with values close to neutrality.

The methanogenic stage occurs after a period that varies between 3-6 and 9-12 months from the disposal of the waste in the landfill. Once this stage has begun, biogas is produced for many years (over 30), according to a trend that highlights the maximum production in the first years and a progressive asymptotic exhaustion until the organic substance is completely degraded or for as long as there are the environmental conditions suitable for the process.

Fig.1: Overall biogas production: publication by Filippo Dal Pastro – Degree dissertation in Industrial Process Engineering – University of Padua “Landfill biogas collection: biogas analysis and optimal plant choice”

Essentially, the production of landfill biogas takes place starting from the initial cultivation stage of the waste and grows during the progressive cultivation, reaching its peak at the closure of the landfill (i.e. at the end of the cultivation of the landfill) and production continues, albeit progressively decreasing, even in the initial post-closure stage (for 8-10 years) until there is a more evident decrease in the advanced post-closure stage for over twenty years. This means that even post-closure landfills contribute to the production of biogas.

It goes without saying that there are numerous studies and technical publications that reach often conflicting conclusions on the differentiation of the stages of waste decomposition: this is because the phenomenon in a real landfill differs in complexity from that seen in the laboratory.

In landfills, there are many variables that intervene and influence the production of biogas. These are related to waste management (e.g. management and construction characteristics of the landfill and of the plants connected to it, compaction and coverage of waste as well as any infiltration), environmental conditions (e.g. temperature, windiness, barometry, precipitation, insolation), the characteristics of the waste (e.g. product composition, size, density, water content, pH and temperature inside the waste heap, presence and distribution of microorganisms, concentration of nutrients).

There are, in fact, various types of ‘biogas’, and they are related to the decomposition stages described above. The main macro-components are made up of methane (CH₄) and carbon dioxide (CO₂) but oxygen (O₂) and nitrogen (N₂) must also be considered alongside them. There are also a certain number of micro-components (H₂O, H₂S, NH₃, CO etc.) which have a variable volumetric impact on the composition of the biogas, and which, albeit with a lesser impact, provide the biogas with particular characteristics of hazardousness, aggressiveness and/or odour.

The formation of the “bad smell” occurs during all the waste decomposition stages until the conditions exist for the degradation process to take place.

In biogas, the volumetric incidences of the various gases listed above are very variable; some researchers and experts have published data which, however, are often contradicted by on-site checks. We nevertheless believe it useful to show a concise (and not exhaustive) representation of the landfill biogas compositions, as resulting from technical-scientific literature from the sector:

For all the chemical and physical characteristics of the biogas (with specific reference both to the chemical and physical characteristics of the gases making up the ‘biogas’ mixture, and to the combustion and explosion characteristics of the biogas, the density of the biogas depending on its composition, the temperature of the biogas, the biogas differential, barometric and collection pressures, the moisture present in the biogas, the sedimentability and the smell of the biogas etc.), please refer to specific discussions.

In order to prevent disorientation and confusion, and having already highlighted how problematic it is to define all the biogas variables precisely (since even in a single landfill, several types of biogas characterized by different chemical and physical characteristics can coexist at the same time) we will list the STANDARD PARAMETERS CHARACTERIZING ‘TYPICAL’ BIOGAS defined by technical literature:

Normally, a biogas mixture characterized by the presence of 50% methane is internationally defined as LFG50(Landfill Gas).

Having a quantitative indication of the biogas production of a landfill is crucial for all life stages of the landfill itself, starting with design and construction through to management and post-management as well as for monitoring and for biogas collection activity.

If the biogas is not collected, in fact, it is dispersed into the atmosphere in the form of fugitive emissions which intensify the greenhouse effect since its two main macro-components are methane and carbon dioxide, both important GHGs.

Furthermore, as the collected biogas is composed of the methane macro-component (combustible gas with a flammability range between 5 and 15% in air), it is considered a flammable gas which, if sent for energy recovery treatment, guarantees economic income.