Fertiliser in openLCA- negative CO2 emissions

Question

Hi- I am trying to model the emissions of feeding crickets. One of the byproducts is insect frass (which is basically insect excrement). In my project, the frass is generally used as fertilizer. I've expanded my system boundary to include it as an output in my process, and clicked the "Avoided Product" checkbox. As for the frass itself, I divided the mass being produced equally between nitrogen fertiliser, as N; potassium fertiliser, as K2O;phosphate fertilizer, as P2O5.

The database I used is ecoinvent 3.6 (cut-off), and the LCIA method I used was Recipe Midpoint (H) v 1.13.

When I ran my system, I was very surprised to see that I had negative emissions from my system, mainly from the avoided fertilizer production.The negative emissions from the fertilizer were much larger (on an absolute scale) than the positive emissions from the corn and soy that are being fed to the crickets. As a result, the total CO2 emissions turns out negative.

Just wanted a sanity check to see if this is correct. Does avoided fertilizer usually result in such a large credit?

Thank you!
Rubi

Answer 1

Dear Rubi,

this is perfectly possible, if the avoided emissions from the fertilizer production are larger than the emissions from the cricket model, then the result would be negative. Maybe you can check the literature and see if other studies about fertilizer production yield similar results. When using avoided products, it is important to make sure that the processes you choose for the avoided burdens are a realistic model of how the substituted products are actually produced. Thus, you could check how the results change if you use different fertilizer production processes as your avoided processes. I hope this helps a bit.