by producing electricity SSTP is in the power market while USSE is in the fertilizer/biofuel market.
SSTP is really a different (but related) market than USSE.
USSE is the supplier for SSTP.
By producing electricity, (rather than just selling the biofuel for the transportation industry), USSE/SSTP can avoid all the complications with getting approvals from auto engine manufacturers, warranties, distribution of fuel to all the retail outlets, etc.
I'm guessing they could get revenue from the green tags regardless of the market they cater to, and the value of the green tags boosts profitability. With an increasing number of states boosting the mandated % of energy coming from green sources, value of green tags will likely go up with the increased demand for them. If a company can't use/produce green energy they will likely be forced to purchase green tags.