Negative values for the LCIA

Question

I am modelling an industrial process and now I get negative values for certain steps... I am using Ecoinvent v3.9 and OpenLCA v1.11.0. For the LCIA I used ecoinvent-CML v4.8 2016. When I just run the calculation everything is fine and all numbers are positive, but if I want to know the contribution of each substep, I get some negative values. For example: for the CO2-eq: step 12: 74.6325 and for step 13: 73:6858. So if I want to know the share of step 13, logically I do Step 13 - Step 12 = 73:6858 - 74.6325 = -0.9467. This can't be right since these are consecutive steps in an industrial process and chemicals like ethyl acetate and liquid nitrogen are used but not recovered.

Is this even possible to have negative values for this?

Thanks in advance for your answer.

Greetings,
Eline

EDIT:

These are the results I get when following the advice of Sjamil. However, when calculating with the "Project" option, I get the exact same numbers. This is a problem because into step 10 go Step 6 and Step 9 (screenshots also added below) and as you can see, step 10 - step 6 - step 9 = less than zero. (In the original question I spoke of step 12 and step 13 and I also still have the same problem there.

Screenshots of step 10:

Screenshots of step 9:

Screenshots of step 6:

Comments

Right, so I think I did as you suggested (I openend up the "product system" I made for step 10 and then calculated using the green triangle on the left above the "navigation" tab). But I still get the same results.
It appears I can't put screenshots in this comment, so I'll put in in the original question. Maybe I misunderstood your suggestion. If so please correct me because I really want to figure this out :-)

Answer 1

Hello Eline, this is very interesting. I am not sure what the solution is, though can you mention if the value 74.6325 represents the CO2 emissions? That seems like a huge number. Thanks

Comments

Yes, that is the CO2 emissions indeed. The product I am calculating are the production of 1000 rubber stoppers.

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Okay, that makes sense. I suppose each step is a separate process with specific input and output flows. And the 'the CO2-eq: step 12: 74.6325 and for step 13: 73:6858' represent the impact analysis results for each process.

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Yes, indeed, but I did the flow of a 15-step process and I want to know the emissions of each seperate step. So then I should do step 13-step12 to know what the emissions of step 13 are, no? So how can they be negative when the steps are consecutive and you just add things?

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That is a valid point. I have a similar LCA going on with 11 processes. Some processes only have direct emissions while some have background processes (electricity, chemicals etc.). A good example is the electricity grid mix. The grid is combination of different sources: hydro, coal, wind, nuclear etc. To estimate the impact of each source and the impact of the grid mix altogether, I tried modelling hydro, coal, wind etc. individually along with the entire grid mix. The results seems quite according to the flow impact factors. Exporting to an excel file is always an option for further calculation.

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Really? I know how to export to excel but how do you further calculate yourself that way?