Can EnviroScrub clean up in the multi-pollutant control business?

The Pahlman process "allows a coal-fired power plant to operate as cleanly as a natural gas-fired plant using SCR." This is the ambitious claim made by Minneapolis-based EnviroScrub Technologies, owner and developer of the technology.

EnviroScrub says the process removes over 99 per cent of NOx and SOx from the gaseous emissions of coal, natural gas, and other fossil fuel fired power plants. It also removes 60-70 per cent of vapour phase mercury and will capture PM-2.5 particulates.

EnviroScrub points out that the process is a "tailpipe" technology, therefore suited to retrofits because it treats the gases after the combustion process and before they travel up the stack, meaning that there are no alterations to the boiler or modifications of the fuel.

At 99 per cent, the Pahlman multi-pollutant control process promises greater efficiency than flue gas desulphurisation (90-98 per cent) and selective catalytic reduction (85-95 per cent), but at a total lower total cost than using a combination of wet flue gas desulphurisation for SOx reduction, selective catalytic reduction for NOx reduction, and activated carbon for Hg reduction. EnviroScrub estimates a Pahlman installation would cost about 30-50 per cent less to construct, operate, and maintain over a 20-year period as compared with the total costs of providing the three separate systems.

"The Pahlman process uses no ammonia and generates no contaminated end-products, such as limestone sludge, which results from a wet FGD process. Instead our regeneration process yields valuable commercial chemicals, including fertilisers, acids and explosives," says Kurt Steinbergs, EnviroScrub's executive vice president.

Predictably, EnviroScrub says the process removes multiple pollutants more effectively than the competitors, which the company lists as BOC, Catalytica, Fuel-Tech, Mobotec, and Powerspan.

EnviroScrub estimates that in the case of a 500 MW coal fired plant, the Pahlman system would realise total savings of more than $126 million (net present value) relative to competing technologies over a 20-year period. This example includes such economic benefits as: lowering the delta-P; reducing the parasitic load of the pollution control system; allowing the plant to maximise the amount of energy extracted from the fuel during combustion; trading pollution credits; and selling the byproduct chemicals.

"A further major benefit to coal-fired plant owners and operators is that the Pahlman process offers complete fuel diversity, allowing plants to burn whichever sort of coal is economic," says Kurt Steinbergs.

A fine idea

The Pahlman process is a single-stage, dry, closed-loop process, with the gas capture process operating at temperatures between ambient and about 350°F.

The key to the technology is a sorbent called Pahlmanite, named after Dr Jack Pahlman, formerly a scientist with the US Bureau of Mines, who conducted early tests on the sorbent, but died in 1999, just as it began to show promise.

In 2000, John von Steinbergs, EnviroScrub's chairman and CEO, launched a campaign to commercialise the Pahlman process. After two years of effort the company believes the process is now ready for commercial deployment.

Minnesota Power, a major shareholder in EnviroScrub, is likely to host the first commercial scale application of the process. This is planned for Minnesota Power's 1000 MWe Boswell plant, where the installation will be sized to handle 40 MWe worth of flue gases and is due to go into operation next year. Burns & McDonnell, of Kansas City, has completed the majority of the third-party engineering work for this project.

Pahlmanite sorbent compounds are low density fine black powders with very high specific surface area and physical and crystal structures optimised for gas sorbent applications. The particle sizes "are large enough to be ideal for filtration by fabric bag house filters, and small enough to assure efficient gas-solid contact," says Charles Hammell, EnviroScrub's chief technology officer - although the company, at this crucial stage in development of the technology, is sparing on details of exact composition and process conditions.

The process removes pollutants "under dry or humid gas conditions, with or without gaseous oxygen", says Richard Boren EnviroScrub's senior emissions engineer, "with best performance when oxygen is present in the flue gas."

Tests have been done at humidity levels between 7 per cent and 15 per cent and have demonstrated good removal efficiency in this range, with the humidity apparently having no detrimental effects.

Both fluidised bed and baghouse filter types of reactor have been evaluated in conjunction with the Pahlman process and found to be ideal gas/solid contact devices. Transport, cyclone and various combinations of reactor types are also expected to be useable.

Loaded sorbent is captured by a baghouse filter, or possibly a cyclone/filter combination and removed for regeneration.

The Pahlman sorbents are made by proprietary processes and do "not require highly specialised equipment", according to Charles Hammel. The Pahlman process technology includes: sorbent manufacture; contact and reaction with flue gas; removal of gas reaction byproducts; and regeneration of the sorbent for re-use (the sorbents being regenerable for an unlimited number of cycles).

Manufacture of the initial start-up charge of sorbent may be done on site using the system's regeneration equipment, prior to gas removal and continuous sorbent regeneration operations.

When Pahlmanite is regenerated on site, it yields raw nitrates and sulphates that can be used to manufacture commercial-grade chemicals such as ammonium nitrate, potassium sulphate, and sodium sulphate.

Does it work?

During December 2000 and from February to September 2001, EnviroScrub performed a series of initial engineering tests designed to determine the operational parameters of the prototype Pahlman process demonstration unit. The scope and function of the testing was two-fold. The first goal was to quantify the demonstration unit's ability to remove SO2 and NOx from an actual exhaust gas stream. The second was to gather critical design and mechanical operational data for further process optimisation, in order to continue with a pilot-plant engineering project.

The first series of tests were conducted at Ameren Energy's Hutsonville power station in Illinois. Hutsonville currently operates its steam boilers on high sulphur coal, without emissions control for SO2 and NOx, thus resulting in emissions concentrations several times greater than those commonly seen at comparable industrial sites. This plant provided an extremely high emission exhaust gas stream where both SO2 and NOx emissions were available for removal.

For the Hutsonville test, a slipstream in excess of 1000 standard cubic feet per minute (scfm) was diverted from the facility's exhaust stack and ducted to the demonstration unit for emissions removal. The stack gas contained on average 1750 ppm SO2 and 300 ppm NOx. During the test programme, documented steady state reductions of 99.8 per cent were achieved for SO2, while at the same time removing up to 75.3 per cent of NOx emissions.

Minnesota Power's Boswell Energy Center in Cohasset provided a second large-scale industrial site seeking substantial reductions in SO2 and NOx emissions. The 1000 MWe Boswell Energy Center consists of four coal-fired steam boilers burning PRB (Powder River Basin) coal. Demonstration unit performance tests were conducted on the exhaust gas stream of unit 1, a 75 MW boiler which currently operates without any form of back-end emission control systems for NOx and SO2.

Over several weeks of testing at Boswell, steady state emissions reductions of 99.98 per cent of SO2 and 91.6 per cent of NOx were achieved while processing 1000 SCFM of exhaust gas. Unit 1 emission levels were sampled at the inlet to the demonstration unit and found to be typical of coal-fired boilers operating on PRB coal.

Significant NOx emission reduction improvements were obtained following the Hutsonville demonstration tests by making improvements to the Pahlman process technology.

Subsequently, independent tests were carried out on the improved technology at Boswell by Interpoll Laboratories of Circle Pines, Minnesota, under contract to Minnesota Power to verify the NOx and SOx removal efficiencies of the Pahlman process.

In its published report, Interpoll did indeed confirm that EnviroScrub's technology was able to remove over 96 per cent of NOx and over 99 per cent of SOx.

To demonstrate the ability of the Pahlman technology to achieve higher levels of NOx reduction in the absence of SO2, a third round of testing was conducted at Potlatch Corporation Northwest Paper Division's facility located in Brainerd, Minnesota. Boiler 1, a Zurn Industries 75 000 lb/h steam boiler, operates exclusively on natural gas and exhibits a relatively high NOx concentration output. The ability to conduct performance testing on a high concentration NOx-only emission source provided EnviroScrub with a perfect opportunity to optimise the NOx removal aspect of the Pahlman process, the company says.

Following four weeks of extensive performance testing at Brainerd, dramatic NOx reductions were achieved. While processing exhaust gas flow rates ranging from 1500 to 500 scfm, NOx inlet values as high as 133 ppm were reduced by up to 95.7 per cent, or to 5.4 ppm. Engineering data were collected that resulted in an improved understanding and demonstration of the commercial potential and viability of the Pahlman process, EnviroScrub reports. Included among these data were the relatively low system pressure differentials required to achieve and maintain commercially attractive levels of NOx reduction.

Monitoring mercury

To further evaluate the multi-pollutant aspects of the Pahlman process, GE Mostardi Platt was contracted to perform mercury emissions removal efficiency testing on the mobile Pahlman process plant operating at the Boswell station.

The testing, done in December 2001, involved a slipstream of about 1000 dry scfm extracted from the reheat gas stream, downstream of the electrostatic precipitator (ESP) of the unit 4 boiler - a 570 MW tangentially-fired unit burning low sulphur western coal.

The test programme consisted of triplicate US EPA Method 29 sample trains and a blank conducted simultaneously on both the inlet and outlet ducts of EnviroScrub's Pahlman pilot scrubber. The results suggest mercury removal rates up to 67 per cent for vapour phase mercury (combined elemental and oxidised forms). "We are excited with these initial results", commented Charles Hammel, "if we are able to capture high rates of the more difficult elemental and oxidised forms of mercury, we feel our total mercury removal rates will be among the industry's best. A baghouse is a key component of our system, so we expect to capture virtually all of the particulate mercury present in the emissions."

What is the value?

Scaling up the process and demonstration of good solid performance at Boswell are the next major steps for EnviroScrub, and this is the crucial development stage where a number of promising looking technologies have come unstuck in the past. But success would of course bring colossal rewards. EnviroScrub quotes the following figures for the power plant emissions reduction markets: SOx reduction - $50 billion over the next eight years; NOx reduction - $3 billion per year in the USA, growing by $1 billion per year; worldwide capital expenditure on FGD and SCR systems - $8 billion per year; and mercury - reaching $1 billion per year in the USA by 2007.

As the company likes to ask potential investors "What is the value of a technology that makes coal, the world's most plentiful yet dirtiest fuel, clean?"

Meanwhile, EnviroScrub is offering at-plant demonstrations of the Pahlman process using its mobile demonstration unit. Built on a 48 ft semi-trailer, it can be transported virtually anywhere in the USA or Canada and can scrub emissions flows ranging from 500 to 3000 ft3/min. As a tailpipe system, Enviroscrub says it requires only 4-7 days on-site to fully demonstrate the virtues of its technology.