The last few years have seen a dramatic change in attitudes towards pollution in China, both in terms of public opinion and government action. Enforcement has been greatly strengthened at the same time as maximum emission limits have been reduced.
Given the scale and technical capabilities of its cement industry, China provides a benchmark case study of how the industry can improve worldwide, in particular when it comes to reducing NOx (nitrogen oxide) emissions.
The production of NOx by the cement industry is related to combustion processes in the cement kiln, with NOx concentrations in the flue gas being relatively high compared to other industries, due to the high kiln temperature.
NOx is detrimental to both human health and the environment: they are the principal components of smog and acid rain, which can cause pulmonary and respiratory issues, as well as damage to buildings, vegetation and plant growth.
They also play a key role as the precursor to the production of ground level ozone, which has a plethora of associated problematic health effects. NOx reduction is therefore a pressing challenge facing power plants, as well as the steel, cement, glass and other manufacturing sectors.
China’s national NOx emissions limits for the cement industry are set at 320mg/Nm3. However, in January 2016 the limit in Beijing was set at 100mg/Nm3, with Jiangsu, Henan and a number of other provinces following suit in 2017. Some cities and regions have even implemented NOx caps as low as 50mg/Nm3. If companies cannot comply with these emissions limits, plants are required to shut down.
The approach taken by the cement industry to reach the low levels required in China and some other jurisdictions is to use a combination of technologies including:
- Self-denitration calciners
- Nitrogen-Rich AF
Multi-staged combustion self-denitration calciner technology makes use of reducing intermediate substances produced during calciner combustion, such as carbon monoxide (CO), which in turn reduces NOx formed in the calciner and kiln. This can typically reduce NOx emission levels from 800 mg/Nm3 to 300 mg/Nm3.
This occurs when a strong reducing atmosphere is created between the first stage and second stage burners in the cone and column of the calciner. In this area, excess air coefficient is less than 0.5 and a large amount of CO gas is generated by anoxic combustion of pulverised coal. The weak reducing atmosphere between the two inlets of the tertiary air duct generates additional CO. The CO generated in the calciner reduces NOx generated in the kiln and suppresses the generation of NOx in the calciner.
Selective noncatalytic reduction (SNCR) is a post-combustion emissions control technology for reducing NOx, which operates by injecting an ammonia-type reactant into the calciner. This technology is often used for mitigating NOx emissions since it requires a relatively low capital expense for installation, albeit with relatively higher operating costs.
SNCR has been installed in almost all Chinese cement plants and can typically reduce NOx emission levels by up to 70%, depending on the amount of ammonia used. However, as a result of the increase in ammonia dosage, the concentration of ammonia that is emitted will be a challenge for the industry.
Selective catalytic reduction denitration technology (SCR) reduces NOx in the flue gas to nitrogen and water with an amino reducing agent, under the action of the catalyst. This technology was once considered infeasible in cement plants, but its application in thermal power plants and coal-fired boilers is now mature and has demonstrated the possibilities for cement producers. However, SCR is still a relatively immature technology in the cement industry and further optimisation work is required.
Tests of low-temperature SCR in two installations in China show that reductions of NOx concentrations of 80 – 95% or more are possible. This, in combination with other technologies, means that the emissions limit set by some Chinese provinces of 50 mg/Nm3 can be met.
Burning alternative fuels which contain volatile organic compounds (VOCs) and are rich in nitrogen, like refuse-derived fuel (RDF) or sewage sludge (SS), can also reduce NOx emissions. These fuels release aminos, hydrocarbyls and other reductive compounds when burnt, which react with NOx, reducing it to nitrogen.
NOx reductions of 20 – 30% are possible with 50 – 100kg SS per t of clinker. Use of suitable alternative fuels allows a reduction of SNCR ammonia dosage of up to 70% to achieve the same emission levels, resulting in significant cost savings.
WCA’s role in reducing NOx emissions
In order to help our members to tackle the challenge of reducing NOx emissions across their operations, the WCA hosted a member forum in December 2019 at the Conch Jining Plant in Shandong, China.
This involved showcasing Conch’s low NOx project and more generally reviewing de-NOx technology in the Chinese cement industry, as well as in-depth group discussions on the innovation opportunities and challenges that implementing such technologies will bring.
At this site, the first three technologies outlined above are combined to achieve NOx emissions of less than 50mg/Nm3, while the concentration of ammonia in emissions is less than 5mg/Nm3.
Even with the 2.5kWh/t.clinker power consumption increase due to SCR usage, the specific power consumption is within 50kWh/t.clinker, and specific heat consumption is around 700kCal/kg.clinker.
As lowering NOx emissions becomes an increasingly important issue for the industry, WCA plans to host further events on this topic.
About the authors
Ian Riley has been CEO of WCA since September 2019. Prior to this, from 2014 to 2019 he was China Country Head for LafargeHolcim. From 2006 to 2014 Ian was a member of the Huaxin top management team, responsible for international business, strategy, business development, sustainability and equipment manufacture. Ian was responsible for Huaxin’s first overseas venture in Tajikistan.
Manon Burbidge has been working for WCA since October 2019 as Communications Coordinator. She has a background in geography, environmental studies and social sciences, and has a particular interest in improving sustainability and gender diversity in the cement and concrete industries.
Read the article online at: https://www.worldcement.com/special-reports/31012020/the-wca-discusses-de-nox-technologies-and-application-in-cement-plants/