Why does the production of sulphuric acid give a negative CO2 result in Open LCA using ecoinvent IPCC 2021

Question

LCA Version: 2.0.3 

Database: Ecoinvent_391_consequential_upr_n2

Characterisation method: Ecoinvent IPCC 2021

Impact Category: Climate change GWP 100

So I am modelling a process that uses sulfuric acid. In the contribution tree, it shows that the production of sulfuric acid stores CO2 instead of releasing it. (The value is negative). By further looking into the process it shows that the negative value is coming from the process 'market for steam, in the chemical industry' 

Now, I am a bit confused because how can the production of sulfuric acid store carbon? I would appreciate any insights as I must defend my results before class.

Answer 1

This result is a characteristic of the consequential system model. When you use the "Allocation, cut-off by classification" (Cutoff) system model, the result will be positive and it will be what you maybe expect intuitively. In this process for sulfuric acid, also "steam, in the chemical industry" is produced. Whereas in Cutoff this process is split in two processes by allocation, in Consequential this heat "steam, in the chemical industry" can substitute the "heat" somewhere else and therefore gets subtracted (a credit) from the initial process.