La Biomassa

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Biomassa

Con il termine biomassa si indica il materiale di origine organica, vegetale o animale. Nel contesto della biomassa a fini energetici,  biomassa viene spesso usato per indicare il materiale a base vegetale, ma la biomassa può applicarsi sia a materiali derivati vegetali che animali.

Composizione chimica.
La biomassa ha una base di carbonio ed è composta da una miscela di molecole organiche che contengono idrogeno, atomi di ossigeno, azoto e spesso anche piccole quantità di altri atomi, tra cui alcalini, alcalino-terrosi e metalli pesanti. Questi metalli sono spesso presenti nelle molecole funzionali come le porfirine, che comprendono clorofilla che a sua volta contiene magnesio.

Materiale vegetale.
Il carbonio utilizzato per costruire la biomassa viene assorbito dall’atmosfera, come anidride carbonica (CO2), nel corso della vita delle piante che utilizzano l’energia dal sole.
Le piante possono poi essere mangiate da animali e così trasformate in biomassa animale. Tuttavia l’assorbimento principale avviene dalle piante.
Se il materiale vegetale non viene consumato è generalmente riassorbito da altri organismi oppure viene bruciato:

  • Se riassorbito il carbonio torna all’atmosfera, principalmente sia come anidride carbonica (CO2) che come metano (CH4), a seconda delle condizioni e dei processi coinvolti.
  • Se bruciato il carbonio viene restituito all’atmosfera sotto forma di CO2.

Questi processi fanno parte del ciclo del carbonio.

Combustibili fossili.

I combustibili fossili come carbone, petrolio e gas derivano da materiale biologico, con stoccaggio di CO2 assorbita dall’atmosfera molti milioni di anni fa.
I combustibili fossili offrono un’alta resa energetica, ma fare uso di questa energia comporta bruciare il combustibile, con l’ossidazione del carbonio in anidride carbonica e l’idrogeno in acqua (vapore). A meno di non essere immagazzinati questi prodotti di combustione sono normalmente rilasciati in atmosfera, rimettendo in circolo il carbonio stoccato milioni di anni fa e contribuendo così ad un aumento delle concentrazioni atmosferiche.

Differenza tra biomasse e combustibili fossili.

La differenza fondamentale tra biomassa e combustibili fossili è su scala temporale. La biomassa preleva carbonio dall’atmosfera mentre è in crescita, e la restituisce quando viene bruciata. Se è gestita in modo sostenibile, la biomassa viene prelevata come parte di un raccolto costantemente rifornito e dunque rinnovabile. Ad esempio nel processo di gestione dei rifiuti vegetali di un bosco o della potatura di colture arboree. Questo mantiene un ciclo chiuso di carbonio, senza un aumento netto dei livelli di CO2 nell’atmosfera.

Categorie di materiali di biomassa.

Nell’ambito di questa definizione, la biomassa per la produzione di energia può includere una vasta gamma di materiali.
Il legno è un materiale di alto valore e non è adatto, a livello di mercato, ad applicazioni energetiche su larga scala. Tuttavia ci sono enormi risorse provenienti dai residui, da co-prodotti e rifiuti che potrebbero diventare disponibili, in grandi quantità, a costi relativamente bassi, o addirittura ad un costo negativo, ad esempio laddove ci sia una richiesta di pagamento per lo smaltimento.

Ci sono cinque categorie di base del materiale:

  • Legno vergine, dalla silvicoltura, attività di arboricoltura o di trasformazione
  • Colture energetiche: le colture ad alto rendimento coltivate appositamente per la produzione di energia
  • I residui agricoli: scarti provenienti dall’agricoltura
  • I residui alimentari, dalla produzione degli alimenti e delle bevande e rifiuti post-consumo
  • Rifiuti da produzione industriale

Poiché vi è grande diversità delle caratteristiche e proprietà di queste diverse classi di materiali, e dei loro diversi sottogruppi, c’è anche una vasta gamma di tecnologie di conversione, che comprendono sia la conversione termica che chimica.

Per approfondire:

[Fonte: Biomassenergycenter]

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