1 SO2 CONTROL

The solution to SO2 pollution

Many cement plants are facing compliance challenges related to consent decree requirements and air permits. These require a number of existing plants to install air pollution control devices for the management of SO

2 emissions.

n by Gerald Hunt, Lhoist North America, USA

INTERANTIONAL CEMENT REVIEW JULY 2016

Dry sorbent injection (DSI) is a mature technology that has been applied

for acid gas removal for decades and has gained significant acceptance by coal-fired utilities as well as a multitude of industrial applications for HCl and SO3/H2SO4 capture. DSI provides an attractive solution for the following reasons:• low installed capital cost• relatively easy to retrofit to a majority of facilities (only injection lances are in direct contact with exhaust gas)• system has good process flexibility for various sorbents and ability to easily modulate based on unit load and/or different fuels• small equipment footprint (typically one or two silos and blower building)• relatively short schedule: around one year from contract award to commercial operation• low consumable requirements (ie, air and water) as well as low parasitic power requirements.While DSI system design had advanced

during this time, limitations remained in the ability to use DSI with hydrated lime (calcium hydroxide, Ca(OH)2) to successfully control sulphur dioxide (SO2) emissions. There was a perception that a hydrated lime sorbent would not be an effective solution for higher levels of SO2 removal in conjunction with DSI technology. However, there have also been advances in the manufacture and application of hydrated lime sorbents that have been successful in altering this perception in a multitude of applications such as cement.

Sorbacal® – engineering an enhanced hydrated lime sorbentPrior to the 1980s the standard hydrated lime sorbents consisted of hydrated lime with a surface area of 10-20m2/g. Lhoist then developed hydrated limes for flue gas treatment by increasing the surface area of such limes significantly above this limit, producing its first generation of

Sorbacal® sorbents. The high surface area combined with a small particle size resulted in a significant performance boost compared to standard hydrated lime. During the acid removal reaction, the rate is slowed down because the reaction products, such as calcium sulphate (CaSO4), form a diffusion layer on the fresh unreacted Ca(OH)2 material. More importantly, the reaction product CaSO4 has a higher molar volume and thus gradually fills up the porosity of the sorbent. This means that to improve the performance, increasing the surface area alone is not sufficient – the pore volume also needs to be increased.

Extensive research performed by the Lhoist group in the 1990s showed that both the acid gas capture capacity and the reactivity of hydrated lime were directly proportional to the pore volume. In contrast, the surface area was found to be contributing to a lesser extent to the acid gas removal efficiency. This research led to the development of a second generation of sorbents with both a higher pore volume (>0.2cm3/g), which is almost three times greater than standard hydrated lime and twice the surface area (> 40m2/g), which Lhoist designated as Sorbacal® SP. Numerous laboratory-, pilot- and commercial-scale tests have demonstrated

Typical dry sorbent injection storage silo (left) and sorbent metering systems (right)

Table 1: various hydrated lime sorbents and their properties

Sorbent Standard hydrated lime

FGT-grade Sorbacal H

Sorbacal SP

Sorbacal SPS

Typical available Ca(OH)2 (%) 92-95 93 93 93

Typical surface area (m2/g) 14-18 20 40 40

Typical pore volume (cm3/g) ~0.07 0.08 0.20 0.20

Typical D50 (µm) 5-7 5-7 8-12 8-12

©Copyright Tradeship Publications Ltd 2016

2SO2 CONTROL

JULY 2016 INTERNATIONAL CEMENT REVEIW

that the reactivity of Sorbacal® SP can be up to twice that of high-quality standard hydrated lime.

The third generation of enhanced hydrated lime sorbents, referred to as Sorbacal® SPS, combines the engineered pore structure of Sorbacal® SP with a chemical reaction enhancement, which augments the sorbent’s reactivity. Table 1 documents the different hydrated lime products including a summary of the critical chemical and physical properties.

Key performance and optimisation factorsTo maximise DSI performance to achieve the desired SO2 removal, while minimising the sorbent consumption, there are a number of considerations that must be evaluated. Process conditions to be considered on a case-by-case basis include exhaust gas temperature and composition, plant configuration, particulate control device as well as sorbent properties. Not only does each site provide specific process conditions that make its solution unique, but cement plants pose a challenge for DSI design given the variability of the process conditions when the raw mill operation cycles on and off. When the raw mill operation changes the exhaust gas temperature, moisture content and SO2 concentration changes, which will all impact on sorbent consumption.

Additionally, cement plants generate a high dust load from the production process to the particulate control device, which carries a unique implication to DSI performance for SO2 control. Generally, the dust generated from the cement production process is 1-2 orders of magnitude greater than the sorbent injected by the DSI system. Since the sorbent is such a small percentage of the overall dust loading into the particulate control device, lower degrees of reactivity have been observed compared to other applications where the sorbent-to-dust ratio may be nearly equal for SO2 control. As a result, maximising the sorbent dispersion in the ductwork for optimal sorbent-to-SO2 contact should be considered when designing the injection grid (ie, injection lance quantity, spacing, penetration depth, etc). However, improved mixing technologies and injection lance designs are commercially available in the market today and offer the potential to reduce the DSI operating costs associated with sorbent consumption with relatively minimal capital investment.

Case studyA cement plant looking to reduce SO2 emissions to comply with a future consent decree requirement wanted to perform a proof of DSI concept trial to determine if Sorbacal® SPS was a viable solution. The plant needed to comply with an SO2 emission of 0.85lb SO2/st clinker produced and the primary challenge was doing so with the raw mill not operating. The back-end of the plant consisted of a four-stage preheater tower, gas cooling tower, ID fan, raw mill with bypass and fabric filter. Five DSI injection ports were installed immediately downstream of the ID fan and SO2 emissions were continuously measured by an existing SO2 CEMS in the main gas stack. A temporary DSI skid was brought on-site to store, meter and measure the sorbent injection rates by loss-in-weight measurement.

The cement plant was able to comply with the SO2 emission requirement while the raw mill was operating. Therefore, DSI testing was primarily conducted when the raw mill was turned off.

However, the plant design and operation only enabled a few hours’ period to test while the raw mill was down. Once the raw mill was turned off it took around 45min for SO2 emissions to stabilise to determine the average pretest baseline of 4.40lb SO2/st clinker produced. Following the stable baseline SO2 reading, Sorbacal® SPS injection commenced at around 2250lb/h and after nearly an hour the SO2 emissions averaged 1.27lb SO2/st clinker produced. Subsequently, the Sorbacal® SPS injection rate was increased

to 3400lb/h and after 30min the SO2 emissions were reduced to 0.72lb SO2/st clinker produced, comfortably meeting the SO2 compliance target. The DSI skid was then turned off to enable a 45-min period to measure a post-DSI test baseline SO2 emission before the raw mill had to be turned on. Figure 1 is a real-time snapshot of the raw mill off-testing with Sorbacal® SPS demonstrating successful consent decree SO2 compliance.

The activities illustrated in Figure 1 represent a successful short-term proof of concept DSI trial with Sorbacal® SPS achieving 80 per cent SO2 removal at a cement plant while the raw mill was not operating.

This case study demonstrates that moderate to high degrees of SO2 removal are achievable with DSI technology while using Sorbacal® SPS contrary to preconceived negative perceptions about the technology.

ConclusionIn recent years cement plants have had to find a solution to control their SO2 emissions due to limitations on air permits and consent decrees. DSI technology with more reactive hydrated lime products like Sorbacal® SP/SPS has proven to be a flexible, reliable solution to control SO2 emissions from cement plants.

However, understanding how the process conditions impact DSI performance is important, especially at cement plants where the process conditions are altered by the raw mill mode of operation. n

Figure 1: snapshot of real-time DSI performance with Sorbacal SPS while raw mill was off

©Copyright Tradeship Publications Ltd 2016