The blast furnaces used today in the U.S. are similar in form and function to those built about 260 years ago, but the energy efficiency is incomparably better. Through steady engineering improvements, the smelting process has become nearly 20 times more energy-efficient since the mid-1700s.[I] The challenge now is to better understand how to use alternative fuel sources in order to continue to decrease the carbon intensity of blast-furnace ironmaking.
Chris Pistorius joins us to talk about his research to improve energy efficiency in blast-furnace ironmaking.
What is the current focus of your blast-furnace research?
In ironmaking, there is an immovable baseline of energy that will always need to be supplied for the process to work. To make the process more energy-efficient, we are up against the laws of nature – a hard battle to win. By focusing on introducing a lower-cost, lower-carbon fuel such as natural gas into the process, however, we can prevent the creation of billions of tons of carbon emissions a year. Natural gas contains about 15% less carbon by mass than coke, the traditional fuel for blast furnaces.
My group is focused on studying how best to introduce natural gas into the blast-furnace ironmaking process, comparing coke replacement ratios, carbon intensity and furnace productivity. Just this past fall, we uncovered new data around the best natural gas utilization methods. We now have better estimates of how much natural gas can be injected through the tuyères. This is upward of 150 kg of natural gas per metric ton of hot metal, which is more than one-third of the total fuel requirement of blast-furnace ironmaking. The most effective use of natural gas to displace coke, though, would be to use the gas to reduce the iron ore to metallic iron in a separate shaft furnace. Use of metallic iron in a blast furnace increases productivity and gives a large decrease in coke rate – effectively saving around 1.8 kg of coke for every kilogram of natural gas used in such a shaft furnace. This is also an effective way to export the energy advantages of natural gas. Austrian company voestalpine is currently finishing construction of a $740 million shaft furnace plant in Corpus Christi, Texas, to pre-reduce iron ore for use in their blast furnaces in Europe.
We are looking forward to presenting this work at a symposium titled “Applications of Process Engineering Principles in Materials Processing, Energy and Environmental Technologies: An EPD Symposium in Honor of Professor Ramana G. Reddy” at the Minerals, Metals & Materials Society (TMS) Annual Meeting & Exhibition in February.
How does the long history of the blast furnace impact your research?
It’s very interesting to work with a technology that has been used in the U.S. for so many years. Through our research, my colleagues and I in the Center for Iron and Steelmaking Research (CISR) have found a deep respect for just how tightly optimized blast-furnace ironmaking is. That’s what comes from over 250 years of engineering the process!
One key challenge of studying blast furnaces is that optimizing the technology is a complex balancing act. Coke is a necessary component of blast-furnace ironmaking, and although we can minimize its impact, we can’t completely eliminate the component from the process. The key question for researchers is: How much coke is needed? The goal is to minimize the amount of coke needed in order to decrease both the carbon intensity and the cost of the ironmaking process.
If we can never eliminate coke out of blast furnaces completely, why do we still use them?
As a global steelmaking community, we are committed to blast furnaces right now because they are the most cost-competitive process, but in the future we may use something else. About 25 years ago, there was a big push by steel companies and government organizations to investigate alternatives to blast-furnace ironmaking because of the fact that you can’t run the process without coke. It was seen as a major constraint to increasing iron production because coke is a relatively rare material.
My colleague Richard Fruehan (U.S. Steel Professor of Materials Science and Engineering and Co-Director of Center for Iron and Steelmaking Research) led a project at the time to develop a new ironmaking process called bath smelters. The basic idea of bath smelters is you have a big pot of molten metal and slag into which you inject the reductant. Versions of this kind of process are still being tested right now. In the future, this new ironmaking capacity could potentially overtake the blast-furnace field.Click here for a podcast where Dr. Pistorius discusses his research on blast furnaces.
What do you see for the future of blast furnaces in the U.S.?
The future for blast furnaces will be focused on remaining competitive compared to other processes, such as recycling steel. With research from Carnegie Mellon and other groups around the world, I see a continuation of this incremental trend of continued energy and cost savings. This will definitely rely on being able to use different fuels, such as natural gas, in a more flexible way. The other interesting future possibility includes using the blast furnaces as recycling units for materials such as plastics. (However, there are currently a set of environmental regulations against this.)
A deeper dive into Dr. Pistorius’ research
Tuyère injection of natural gas needs to be balanced with oxygen enrichment of the blast air to maintain a sufficiently high adiabatic flame temperature (AFT) at the tuyères. However, too much oxygen enrichment causes the off-gas temperature at the top of the furnace to drop, because less nitrogen is available to transfer heat to the furnace burden. These two constraints define an “operating window” of allowable combinations of natural gas injection rates and oxygen enrichment of the blast air – shown as the shaded region in the graph. The data points are the 2015 operating conditions of actual North American blast furnaces with natural gas injection; the points cluster around a “TCE” of 1 GJ/THM (THM = metric tons of hot metal). The TCE quantifies the sensible heat and heat of reaction supplied by gases produced in the lower part of the furnace. The results show that injecting more than 150 kg methane per metric ton of hot metal is theoretically possible, and some furnaces are already operating close to this limit.