These factors helped drive an increase in adjusted EBITDA of nearly 25% compared to last year, and we produced our strongest first half financial results in six years.
We have built a solid order book for the third quarter at an all-equivalent price of $285 per ton, though prices remain at a significant discount to urea for the reasons Tony mentioned.
As you can see on Slide 9, energy costs in these regions have increased to over $14 per MMBtu and Eastern European producers have become the global marginal producer for the time being.
For the first half of 2021, the Company reported net earnings attributable to common stockholders of $397 million or $1.83 per diluted share.
EBITDA was $994 million and adjusted EBITDA was $997 million.
The trailing 12 months net cash provided by operating activities was approximately $1.2 billion and free cash flow was $700 million.
First, we raised our estimate for capital expenditures for 2021 from around $450 million to approximately $500 million.
Going forward, we expect capital expenditures to return to the range of $450 million per year.
With this additional maintenance project, the high level of previously planned maintenance, and the additional maintenance from severe weather in February, we estimate that gross ammonia production and sales volume will be around 9.5 million tons and 19 million product tons respectively; both at the low end of our forecasts earlier this year.
Looking into 2022, we have a more typical maintenance schedule and we'd expect to return to approximately 10 million tons of ammonia production and sales volume of 19.5 million to 20 million product tons.
We have announced that we will redeem $250 million of our senior notes due June 2023, which will reduce our gross debt to $3.5 billion.
We expect to lower our gross debt to $3 billion by or before the maturity of the 2023 notes.
Most importantly, we did this safely with our recordable incident rate at the end of June at just 0.28 incidents per 200,000 labor hours, significantly better than industry averages.

Summary:
For the first half of 2021, the Company reported net earnings attributable to common stockholders of $397 million or $1.83 per diluted share.
First, we raised our estimate for capital expenditures for 2021 from around $450 million to approximately $500 million.