SCR myth refuted

20 August 2001

RJM Corporation of Norwalk, Connecticut, has announced contracts for the installation of its ARIS™ selective catalytic reduction (SCR) technology on over 100 diesel and gas engines in the USA, one early adopter being Block Island Power Company of Rhode Island.

The system meters a reducing agent – 32.5 per cent urea in aqueous solution – into the engine’s exhaust stream. The urea decomposes to ammonia, which reacts with NOx across a catalyst located downstream of the injection point.

The widely held view that SCR is prohibitively expensive for diesels is robustly challenged in a recent paper by Ravi Krishnan of RJM.

He points out that many power producers in the US Pacific Northwest are resorting to diesel generation as low water levels have reduced hydro generating capacity. In Washington state, for example, newly permitted diesel and natural gas engines amount to 396 MW and 75 MW respectively, with average unit sizes of around 1.5 MW and 1.05 MW respectively. The demand for diesel generation has also increased elsewhere in the USA.

Fuelling this rapid growth, argues Krishnan, is the realisation among regulators and air boards that diesel generation with SCR can achieve lower emissions than an untreated natural gas engine, but at the same time diesels are cheaper, have short lead times with no order backlog and can be sited quickly.

Krishnan also notes that while SCR can prove a crippling cost burden for fossil fired boilers, it has a minimal effect on the cost of generation for diesels, particularly at higher load factors. Table 1 shows his calculations for a 2 MW engine.

In the Pacific Northwest, load-intensive users on interruptible rates pay as much as $0.60/kWh for electricity. This is used to calculate the savings column, which represents the difference between with-SCR generation costs and grid prices in the region.

The table suggests that, for example, a large energy-user planning to displace, say, 8 million kWh of electricity can generate his own clean power (with SCR technology) at $0.17/kWh (assuming 4000 operating hours per year) as opposed to buying it in from the grid at $0.60/kWh. A marketer in the same scenario could install SCR technology but still generate a profit in excess of $1.1 million or $0.147/kWh, according to Table 1.

Ravi Krishnan’s paper also analyses the capital and operating cost of the ARIS technology over a 15-year lifetime for a 2 336 hp (1 752 kW), four-stroke diesel engine.

Assuming 8000 hours operation per year, and capital costs for the ARIS system of $157 590 (90 per cent NOx reduction), $149 940 (75 per cent reduction) and $142 290 (50 per cent reduction), Table 2 shows annualised capital and operating costs for the three levels of NOx reduction.

The cost/ton of NOx reduced is very dependent on the number of annual operating hours.

This is shown in the graph above. The figures range from $4 554/ton (1000 h operation, 50 per cent reduction) to $738/ton (8000 h operation, 90 per cent reduction).

In summary, argues Krishnan, the primary driver for a large energy-user or power producer considering selective catalytic reduction should be the cost differential between prevailing grid prices and the cost of generation and not the cost of SCR technology. “The installation of SCR technology has a negligible effect on the cost of diesel- fired generation,” he concludes.

Table 1. Cost impact of SCR on a 2 MW diesel
Table 2. Annualised capital and operating costs for the ARIS SCR ($)

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