Consultation about boiler feed

Hello, the feed does not have enough heat in my process. When I add the code ```CHP_sys = create_coheat_and_power_system(combustible_slurries=stream_a, combustible_gases= stream_b, mockup=True)``` to create the boiler, I found that the boiler feed required natural gas (i.e., the natural_gas stream had mass flow), but in the Utilities worksheet of the output excel file, the cost of all units of electricity added up was negative, it seemed that there was surplus electricity produced by the boiler, but since the boiler feed required natural gas, why is there any surplus electricity? I looked at the source code (https://github.com/BioSTEAMDevelopmentGroup/biosteam/blob/master/biosteam/facilities/_boiler_turbogenerator.py) and it seems that the steam produced by natural gas should be used preferentially for heating and then for electricity, in which case, if the boiler needs natural gas, shouldn't the total cost of steam in the Utilities worksheet be 0 and the total cost of electricity 0?

Start with biosteam/facilities/_boiler_turbogenerator.py and reproduce the create_coheat_and_power_system call using the shown streams. Compare the boiler behavior with the Utilities worksheet's steam, natural-gas, and electricity costs. Done means establishing whether the reported surplus electricity and costs are expected, or documenting a reproducible defect with the relevant calculation path.

Written by the indexing model from the issue text.