Cheap technology for recycling CO2 from flue gas? 

Cheap? When you hear this word in the context of carbon dioxide capture, you immediately want to check what exactly the interlocutor is talking about. Often under "cheapness" understand low capital costs, forgetting about operating costs, or vice versa. Or they generally mean reducing the cost of relatively expensive amine scrubbers. Let's understand without illusions.

Where does the “cheapness” lie? Taking it apart piece by piece

Marketing aside, the key to the economy is the source of gas.Flue gasfrom a thermal power plant or a cement plant - this is not pure CO2. There is 10-25% carbon dioxide, the rest is nitrogen, oxygen, moisture, and most importantly - impurities: SOx, NOx, dust. The first and most expensive stage of any ?cheap? technology is pre-cleaning. If you ignore it, you may not go further: the catalysts will be poisoned, the absorbents will degrade. I have seen installations where attempts to save money on cleaning led to the fact that after six months the adsorbers turned into a useless mass. Capital investments went to zero.

Therefore, when they talk about a low price, I always ask: “What is included in the price?” Often in pilot projects, the cost of disposal is considered only at the absorption/adsorption stage, “forgetting?” about gas preparation, compression, storage and logistics of the resulting product. The complete chain is where the main costs lie. Low-cost technology is one that minimizes costs throughout the chain, rather than at one link.

Another point is energy consumption. Amine scrubbing is expensive due to the enormous heat required to regenerate the solution. So, “cheap?” the alternative must either radically reduce this energy or use waste heat from the same plant. For example, use low-grade heat to regenerate new types of absorbents or work on the principles of pressure swing adsorption (PSA/VSA), which, however, is also “gluttonous?” for compression.

Mineralization: a lot of noise and a stone result

A very fashionable trend, which is often presented as a panacea. The idea is simple: bind CO2 into carbonates using waste (slag, ash) or natural silicates. The technology can actually be inexpensive to operate if the raw materials are lying around. But here we run into kinetics. The natural process of geological carbonation lasts thousands of years. To accelerate it to an industrial scale, you need either high pressure and temperature (energy again!), or expensive catalysts/activators.

We participated in a project to recycle CO2 using steel slag. Laboratory tests were encouraging. But when scaling up, problems emerged: the heterogeneity of the slag composition from batch to batch, the need for its finest grinding (energy consumption), and most importantly, the difficulty of organizing continuous contact of gas with solid material in the reactor. The result was either a low output or a huge, “expensive” one. reactor. The product - carbonates - can theoretically be sold, but the market for such volumes in the region turned out to be illusory. The project stalled at the pilot installation stage. Valuable experience, but not a technological breakthrough.

Conclusion on mineralization: This is a potentially low-cost method of disposal, but not capture. It is good for point application, where there is a CO2 source, a silicate source, and a carbonate consumer nearby. For typical flue gas from a thermal power plant, it is still difficult and not always profitable.

Biological pathways: algae and more

This is perhaps the most ?natural? and media attractive way. Grow algae using CO2, then use it for biofuel, feed, fertilizer. Sounds like a perfect cycle. Reality is harsher. The main cost item is not the bioreactor itself, but gas preparation. Algae are very sensitive to impurities, especially sulfur and nitrogen oxides. Serve them straightflue gas- means killing culture. Almost the same deep cleaning is required as for chemical methods.

Next is the light. For high productivity you need a large area and good illumination (artificial light eats up the entire economy). Plus control of temperature, pH, nutrients. As a result, the cost of capturing a ton of CO2 through algae in temperate climates is prohibitive. The economy can only be saved by the high cost of the final bioproduct (for example, for pharmaceuticals). For mass carbon utilization from thermal power plants, this is not yet an option.

There are more mundane biological methods, such as using CO2 in greenhouses to intensify plant growth. This is a really working and relatively cheap practice, but the scale of recycling is limited by the area of ​​greenhouses and seasonality.

Membranes and adsorbents: a quiet race of materials

This is where the main research work is currently taking place, aimed specifically at reducing costs. The idea is to replace energy-intensive amine recovery with easier separation using new materials. Ceramic and polymer membranes, MOFs (metal-organic frameworks), porous carbon materials - the list is long.

Hybrid systems are of practical interest. For example, do not try to separate pure CO2 from flue gas, but use membranes to obtain a rich mixture (say 50-70% CO2), which can then be used in technological processes that do not require high purity. This reduces finishing and compression costs. I am familiar with the work of Chinese colleagues, for example, fromChengdu Yizhi Technology Co.(their website ishttps://www.yzkjhx.ru). This design institute, established on the basis of Huaxi Technology, is actively working on gas separation and resource recovery technologies. Their portfolio includes solutions where membrane pre-enrichment is combined with a final stage of post-treatment, resulting in a total energy gain. They don't promise ?cheapness? like a magic word, but they talk about optimizing the total cost of ownership for a specific customer. This is an honest approach.

The problem with new adsorbents and membranes is aging and scaling. Laboratory efficiency in grams and a pilot plant processing thousands of cubic meters per hour are two very different things. How will the material behave after 10,000 adsorption-desorption cycles in a flow of real, unpurified gas? Often the answer comes only through lengthy industrial tests. And this is a risk area for the investor.

So what's the bottom line? View from the workshop

Universal ?cheap? technology for anyoneflue gasno and probably won’t. It all comes down to location. A cheap solution is a custom one, tailored for a specific pipe. Somewhere there is access to cheap heat for regeneration - you can think about advanced fluids. Somewhere nearby there is a quarry and a crushed stone market - it is worth considering mineralization. Somewhere there is a network of gas pipelines - we can consider membranes for producing commercial CO2.

The biggest practical lesson I've learned is: Don't start by choosing a technology. Start with a thorough analysis of the gas (not according to the passport, but based on real measurements in different operating modes of the boiler) and with a clear understanding of what you will do with the resulting CO2. Sell, download, store or use locally? The economy depends 80% on this answer.

And one more thing. Often ?cheap? can be found not in breakthrough technology, but in competent integration. Utilization of low-grade heat, use of existing infrastructure, synergy with other plant processes. Sometimes a simple modernization of heat exchangers and optimization of the combustion mode gives a greater effect in reducing emissions per ruble of cost than a complex capture system. But for some reason they talk less about this.

Therefore, I would answer the question in the title this way: there are cheap technologies, but they are not lying on the shelf. They are created by engineers and technologists for a specific task, combining known solutions, taking into account local conditions and the real, not paper, economy. And in this process, the experience of such applied institutes as the one mentionedChengdu Yizhi Technology Co., which has been operating since 2013 and has a serious authorized capital, is often more valuable than high-profile laboratory discoveries. They look at the problem from the end - from the product and its cost, and this is the right path to that very “cheapness”.