UCC Dry Sorbent Injection System

Enhanced In-Line Mill Technology Saves Utilities Millions

Technology BRIEF 

Coal-fired power plants are being driven by the EPA to assess various 

technologies and capabilities for enhanced SO3

 removal. Since 2005,

United Conveyor Corporation-Dry Sorbent Injection has conducted 

numerous Dry Sorbent Injection (DSI) tests to demonstrate SO3

 removal 

performance versus trona particle size. The test described in this 

technology brief confirms that low SO3

 stack emissions are achievable 

with dry sorbent injection technology. A key discovery is that milling 

technology, specifically in-line pneumatic milling systems, provides the 

only way to achieve very low SO3

 stack emissions when injecting trona. 

Additionally, results show that a fine milled particle size can reduce 

sorbent usage by half.

Dry Sorbent Injection Test Overview 

The DSI test compared SO3

 removal performance of as-delivered 

unmilled trona, in-line coarse milled trona and in-line fine milled trona. 

DSI is an economical and effective technology that injects selected 

sorbents into the flue gas to control SO2

, SO3, mercury and other acid 

gases. When trona is the appropriate sorbent choice, in-line milling 

decreases trona usage by increasing surface area per ton injected to 

enhance reactivity and improve dispersion throughout the flue gas. 

The test had three primary objectives:

1. Verify known removal performance of as-delivered unmilled trona 

and UCC fine milled trona

2. Determine the SO3

 removal performance of in-line coarse-milled 

trona

3. Use the SO3

 removal curves to compare the life cycle sorbent costs 

of each approach

Implementation and System Configuration

Trona injection was performed with one positive pressure pneumatic 

conveying system splitting into eight injection lances located 

downstream of the air preheaters and upstream of the electrostatic 

precipitator (ESP) – a common configuration for injection systems. The 

United Conveyor Corporation (UCC) splitter system was designed to 

ensure ±10 percent sorbent distribution across each injection lance, and 

was verified to achieve this performance during laboratory testing. The 

lance placement was determined using computational fluid dynamics 

(CFD) modeling. The patent-pending UCC in-line mill system, was used 

to mill the trona. The UCC in-line mill system operates with the positive 

pressure pneumatic conveying system and utilizes only the conveying air 

stream in the milling process. The mill size reduction is adjustable, and 

was reduced by design during some test runs, intentionally increasing 

the sorbent particle size to closely match competing mill performance 

and corresponding SO3

 removal rates. 

The median trona particle sizes evaluated:

• As-delivered unmilled trona: 30-40 microns

• Coarse milled trona: 16-18 microns

• UCC fine milled trona: 12-14 microns

Plant Operation

The UCC test was performed at a plant with a 700MW unit with the 

following environmental controls: 

■ Selective catalytic reduction (SCR) system for nitrogen oxide (NOx) 

removal

■ Cold side ESP for particulate removal

■ Wet flue gas desulfurization (FGD) for SO2

 control

The baseline for the plant SO3

 concentration levels were:

■ The boiler and SCR converting approximately 2 to 2.5 percent total 

SO2

 to SO3

■ ESP outlet SO3 concentration in the range of 28-33ppm without DSI 

injection

Injection Point
Sampling Methodology and Testing Tools

SO3

 concentration was measured by both a dew point monitor and 

controlled condensate testing (EPA Method 8A). All testing samples 

were taken between the ESP and FGD. The dew point monitor 

provided continuous monitoring of the flue gas dew point which was 

converted to an approximate SO3

 concentration. SO3 concentrations 

calculated indirectly from dew point compared conservatively to SO3

concentrations determined by EPA Method 8A. 

Note: SO3

 concentrations well below 5ppm were measured using 

interval and event sampling techniques. However, UCC acknowledges 

that at concentrations well below 5ppm, the measurement tolerance 

of current technology and test methods may not provide the accuracy 

necessary to consistently validate emission limits due to testing 

tolerance bands exceeding absolute measurements.

As-delivered unmilled, coarse milled and fine milled trona samples were 

taken passively by a pitot tube placed in the center of the conveying 

stream. This sampling method was validated by taking 100 percent 

material samples at multiple injection lance sites concurrent with the 

pitot tube samples. All samples were analyzed using a Beckman Coulter 

laser analyzer.

Test Results and Analysis

The test results showed:

■ Predictable SO3

 removal performance curves for each particle size 

measured/tested

■ Smaller particle size allows lower SO3

 removal while significantly 

lowering trona usage

■ Fine milled trona is projected to save $4.5 million and $2 million over 

five years compared with as-deliver unmilled trona and coarse milled 

trona respectively

Proven Return on Investment

Figure 1 shows a comparison of the annual trona costs and five year 

aggregated savings associated with achieving 5ppm SO3

 concentrations 

at the ESP outlet using as-delivered unmilled, coarse milled (competitive 

mills) and UCC fine milled trona. The annual trona cost comparison 

assumes a capacity factor of 90 percent for a mid- to large-sized power 

plant and a price of $180/ton for as-delivered unmilled trona.

Conclusion

DSI systems are low capital cost and suitable for retrofit into units with 

SCRs and/or wet scrubbers. UCC DSI technology delivers effective SO3

control and is capable of achieving SO3

 emission limits well below 5ppm 

as a result of in-line milling technology producing sorbent particle size of 

12 to 14 micron. This small particle size will reduce sorbent usage up to 

50%. Industry leading UCC DSI technology provides the coal-fired power 

industry the most efficient and economical method to meet current and 

future SO3

 emission limits and regulations.

www.unitedconveyor.com

 UCC is a registered trademark of United Conveyor Corporation

©2010 United Conveyor Corporation TB11-01

Sorbent

(Trona)

Particle Size

(median)

Injection 

Rate

(lb/hr)

Annual 

Sorbent 

Costs

5-Year 

Sorbent

Savings

As-delivered 

Trona

25 - 40 

micron 2,300 $1,631,988 n/a

Coarse 

Milled

16 - 19 

micron 1,600 $1,135,296 $2,483,100

UCC Fine 

Milled

12 - 14

micron 1,000 $ 709,560 $4,464,000

Figure 1 - Cost Comparison to Achieve <5ppm SO3

Note: Typically less than 5ppm is necessary to avoid a visible SO3 Plume

Figure 2 - Milled Trona Injection Rate vs. Outlet SO3 Concentration 

 Median Trona Particle Size

 As-Delivered – 25-40 micron 

 Coarse Milled – 16-19 micron

 UCC Fine Milled – 12-14 micron

SO3

(ppm)

Trona Injection Rate (lb/hr)