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Biogas: Biogas collection system study in a waste storage facility

abor

Fig. 1 – Principles and cross-section of a storage compartment

The energy production related to alternative and renewable energies is a major deal with continually increasing energy demands, environmental constraints and ecological constraints. Biogases provide answers to these questions and new technological challenges at the same time. Within the existing solutions which enhance biogases, we present the «skid» case in the present paper which monitors the biogas quality delivered to the engine generating current.

Introduction
Storage facilities receive garbage which are compacted and buried in specific compartments especially designed to contain the biogases produced (cf figure 1).
The biogas caught by the drains is collected and then purified before being used in an engine for producing electrical energy (cf figure 2).
The collecting system design whose extension may reach several kilometers with pipes is a complex task. Indeed, biogas production fluctuations with time imply regular and significant modifications of the biogas system and the valorization equipments.



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biogas

Fig. 2 – Biogas valorization principles

The pressure losses in the biogas collection network are difficult to estimate because of its long branched system. However, the linear pressure loss has to remain very low in order to obtain acceptable ventilated energy consumption. Therefore it is of major interest to optimize the ventilation system design and to assess the impact of the system’s modifications.
The software Flowmaster allows rapid and reliable simulation of an existing branched system and a rapid assessment of the modifications’ impact projected by the storage facility exploitation.
This 1D CFD tool enables the user to optimize a collecting system continuously, i.e. the collecting pipes’ diameter and the related ventilation systems.

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Fig. 3 – Block diagram of the valve “skid”

VALORIZATION PROCESS DESIGN
As part of the biogas energetic valorization study, we present the case of a valve “skid” which regulates the pressure loss of two storage compartments and collects biogas from these, supplying a suppressor and an engine (cf figure 2).

The “skid” has been designed with Flowmaster from the block diagram provided by Veolia – Propreté (cf figures 3 & 4).
By monitoring the upstream pressure regulation valves, the valve skid has to maintain:

  • constant quality of biogas (i.e. at the network exit with Flowmaster);
  • The suppressor entrance pressure above a prescribed lowest value

Biogas quality is decreased by parasite air intake in the storage compartment and pressure losses are induced in the system through valves and collectors. The design purpose consists in characterizing all the system pressure losses. This design will be able to identify system areas where pressure losses and parasite air intake risks are the highest.

SIMULATION RESULTS
A compressible steady state calculation has been performed with the Flowmaster model. Pressure losses and distribution flows are both represented in figures 5 and 6.

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Fig. 4 – Modélisation Flowmaster du «skid» de robinetterie

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Fig.5 – Pressure losses from valve “skid” at distribution-level towards suppressor and engine

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Fig. 6 – Valve “skid” flow at distribution-level towards suppressor and engine

The highest pressure losses in the low branch “skid” are shown in figure 5. Therefore biogas quality is reduced by dilution with parasite air. This implies the system has to be re-designed biogas quality improvements purposes.

biogas

Fig. 7 – Optimization workflow with modeFRONTIER minimizing pressure
losses on the “skid” low branch

CONCLUSIONS AND PERSPECTIVES
The valve “skid” suggested by Veolia which allows biogas distribution to a suppressor and an engine has been designed with Flowmaster.
Compressible steady state calculations have been done. Calculations have shown the low branch of the valve “skid” is subjected to pressure losses thereby causing possible biogas quality degradation.

As a consequence, a system optimization could be possible by looking for valve designs minimizing the low branch pressure losses on the Flowmaster network. Therefore this optimization has also to minimize the installation price. To make it possible, calculations coupling Flowmaster for the “skid” with the optimization software modeFRONTIER can be done (cf. figure 6) in order to minimize pressure losses and to reach required performance.

In the modeFRONTIER workflow presented in figure 7, three dimensions of the low branch are optimized to reduce installation costs and related pressure losses. As part of upcoming studies, power production as well as consuming power ventilators will be designed in the skid of Flowmaster network. Two extra objectives will be added to the modeFRONTIER workflow. The energy consumption will be reduced and energy production will be maximized.





Articolo pubblicato sulla Newsletter EnginSoft Anno 10 n°3

B. Piton
EnginSoft

 
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