Many cement plants utilise a spray tower for temperature control and the removal of acid gases such as SO2 and HCl. Mississippi Lime has developed a high reactivity hydrated lime that allows cement plants to inject the hydrate as a dry sorbent (dry sorbent injection - DSI) into the flue gas instead of making a hydrate slurry and feeding to the spray tower as a liquid. Avoiding the slurry make-down minimises plugging and erosion of the spray nozzles while simplifying operation of the feed system in freezing environments.
Introduction
Cement plants have had to control SO2 for many years, typically with a rolling average. The most common control method is the use of spray towers that incorporate a slurry of hydrated lime. Future environmental regulations may also require cement plants to control HCl in the flue gas. When added to the flue gas in the spray tower, hydrated lime reacts with acidic components such as SO2 and HCl in the gas to form calcium sulphite and calcium chloride:
SO2 + Ca(OH)2 → CaSO3 + H2O
2HCl + Ca(OH)2 → CaCl2 + 2H2O
For slurry systems, hydrated lime is delivered in pneumatic trucks and off-loaded into a silo at the plant site. The hydrated lime is then added to a slurry tank and mixed with water to make hydrated lime slurry. This mixture is pumped to the top of a spray tower and injected into the flue gas through small atomising nozzles. The nozzles can plug, significantly reducing the distribution and efficacy of the neutralising sorbent being added. Nozzles can also erode, resulting in the wrong size droplet being formed, reducing the effectiveness of the tower. Because the hydrate is applied as a slurry, all of the transport lines and storage tanks must be heat traced and insulated so that they do not freeze in the winter months. All of these challenges can reduce the amount of SO2 removal that a plant can achieve and make it difficult to maintain compliance.
Case studies
Two cement plants in midwestern USA wanted to explore switching from their slurry feed systems to DSI feed systems. Plant A used hydrate made into a slurry at the plant site and then pumped it through the spray tower. Plant B used a liquid slurry made in a highly-controlled process at a local hydrated lime manufacturer. Both plants struggled with the slurry application of the hydrated lime. They had plugging and erosion in the spray tower nozzles, which resulted in significant maintenance costs and less efficient SO2 removal. During cold periods in the winter, freezing slurry lines also presented challenges. When slurry transfer lines would freeze, the plants couldn't get the liquid hydrate into the spray tower and couldn't maintain environmental compliance.
Both plants requested that Mississippi Lime work with them to evaluate dry sorbent injection (DSI) as a means of improving SO2 capture while eliminating the challenges associated with operating their respective slurry systems. The plants also wanted to compare the performance of different grades of hydrated lime. Mississippi Lime's flue gas treatment (FGT) grade and the new high reactivity hydrate (HRH) were both used in the testing. HRH was evaluated extensively during its development and found to offer 25 - 50% performance improvement over FGT hydrate.
Mississippi Lime utilised its temporary feed system consisting of a silo, rotary air lock, blower trailer and associated control equipment (Figure 1). The system was installed at the plant sites for several weeks to evaluate the performance of different hydrates under different conditions.
Plant A
Plant A had an on-site hydrated lime silo and fed its hydrate from the bottom of the silo into a water mixing tank. Here, it is mixed into a slurry and then pumped to the nozzles in the slurry tower. Plant A is required to maintain a rolling 30-day SO2 average. To accomplish this, it feeds hydrate slurry on a continuous basis. With the raw mill off and resulting higher SO2 levels, the scrubber tower struggled to keep the SO2 below the targeted threshold.
Because the slurry feed system was the limiting factor in how much SO2 the plant could remove, supplementing the slurry system with DSI when the raw mill was off allowed the plant to maintain compliance during raw mill outages. The other advantage of the DSI 'overlay' of the slurry system is that the plant can feed more or less hydrate at its convenience. This allows it to stay within the 30-day target at all times and gives it much greater flexibility in operating its system while maintaining
regulatory limits.
Plant B
Plant B utilised a finely-mixed hydrated lime slurry that was delivered as a liquid slurry. This material was manufactured under exacting standards by a local lime hydrate producer and stored at the plant in a large liquid bulk storage tank. The slurry was then transferred to a day tank where it was pumped up to the spray nozzles in the scrubber tower. The plant experienced regular erosion and pluggage issues with the spray nozzles. Additionally, the slurry feed system was a challenge to run during cold weather periods due to freezing of the process lines. Plant B expressed a desire to evaluate DSI as an alternative.
With the raw mill off and the slurry to the tower locked out, hydrated lime was fed through the temporary DSI system for SO2 control. After start up, the hydrate was fed at two dosages to see the effect on SO2 reduction (Figure 2). At the first dosage, a 45% reduction of SO2 was achieved. With an increase in the sorbent dosage, a 60% reduction of SO2
was achieved.
After verifying that the DSI would provide the additional SO2 control needed, two different grades of DSI hydrate were compared at two different dosages. The HRH demonstrated a significant improvement in SO2 capture compared to the traditional FGT grade (Table 1). The improved performance of HRH allowed Plant B the option to feed less hydrate, reducing its annual reagent costs or to achieve lower SO2 levels with roughly the same amount of feed.
In anticipation of future HCl limitations, both the FGT and HRH grades were evaluated for HCl removal. The plant observed greater than 75% reduction in HCl from the baseline (Figure 3). The HCl sensor takes some time to reach equilibrium, which is why the figure shows a gradual decline in HCl instead of an instantaneous reduction. As the feed of hydrate continues, the detection limit of the sensor is approached.
Feed rate (kg/hr) | SO2 removal (%) | |
FGT | HRH | |
680 | 11 | 30 |
1000 | 25 | 42 |
Above - Table 1: SO2 removal of different grades of FGT and HRH at different feed rates.
Conclusion
In both of the case study plants, the use of DSI provided an alternative to the existing hydrate slurry systems. Even if the plant was using a slurry manufactured specifically for the scrubbing towers, DSI application of FGT grade or higher quality hydrated lime could achieve the same acid gas removal rate. In some cases, the DSI feed approach allowed the plant to feed more hydrate without plugging the nozzles, so that even greater acid gas removal could be achieved.
Eliminating the use of the slurry drastically reduces the plugging and erosion of the tower nozzles. This not only reduces the maintenance and operational costs associated with the plugging, but also provides more consistent sorbent distribution in the system. By feeding the sorbent in a consistent fashion, better acid gas removal can be maintained. Transitioning to DSI also prevents the freezing challenges that the case study plants historically faced in the winter months.
The use of Mississippi Lime's HRH showed a significant improvement in the acid gas capture of the hydrate. In both of plants described here, a 20% greater capture with the HRH compared to the FGT grade of hydrate was observed. This allowed the plants to either feed the same amount of hydrate and achieve greater acid gas capture or to meet the
current regulations using significantly less sorbent.