Knocking out NOx EMISSIONS

Compared with the massive investments by the power industry on the flue gas desulphurisation projects of the early 1990s, tackling NOx emissions might not appear to be such a major problem. But current and proposed EU regulations on allowable NOx levels are forcing not just the power utilities, but also the process industries to look again at the operation of their boilers, fired heaters and kilns.

With most potential sources of pollution, there are two distinct ways of tackling the problem. First, minimise production of the pollutant from the start. And second, when all else fails, put in an `end-of-pipe' clean-up process to prevent the pollutant being liberated.

NOx emissions are no different. On page 45 we report on some still quite novel approaches to removing nitrogen oxide compounds from flue gases - novel, but `end-of-pipe' nevertheless. Before that, however, we consider how best to minimise the formation of NOx in flue gases in the first place. Essentially that means by improved burner design and operation - and that itself means a continuous programme of development on the part of burner manufacturers such as Hamworthy Combustion Engineering (HCE).

At its main site in Poole, Dorset, HCE develops, models and tests burners for specific applications in power generation and process plants. Typical of the latter is the recently completed contract to supply 216 customised low-NOx burners, and a ground flare system, to IBP's Numaligarh refinery in Assam, India. To meet the refinery's requirements on NOx the Radol radiant burners, to go into a new hydrogen reformer, had to be designed from new.

The project was carried out through HCE's joint-venture partner Airoil-Flaregas India and reflects the global nature of HCE's business. HCE acquired Airoil-Flaregas in the UK in 1993 and Peabody Engineering in the US, the year before. The combination of Peabody's power generation experience and Airoil's refining, petrochemical and utilities expertise considerably broadened the scope of HCE's operations. It also gave the company the capability to respond quickly to changing customer demands, particularly in areas such as NOx reduction.

ONE STEP AHEAD OF THE LAW

According to HCE's sales and marketing director Michael Wignall, `our development teams have been given virtually a free hand at the moment so they can try to "second guess" what the legislators and regulators may have in store for the future.' Already this approach is said to have produced several design features that should be appearing in future products. More importantly in the short term, though, it has helped in the company's policy of obtaining in-country approvals of its burners in new and developing markets.

A case in point is the recent approval granted by the Russian legislature for Hamworthy to supply process burners and associated equipment direct to contractors and end users operating in the country. `We are now in a position to submit relevant approval documentation within the sales bid package,' explains Wignall, `which helps our customers when supplying into the former USSR'. The approval includes chemical, oil and gas industries and covers the Russian Federation and 14 FSU countries from Ukraine to Kazakhstan.

While not every country demands the same standards over emission levels, the techniques employed to inhibit NOx formation are fairly standard - the commercial trick being to apply them in the right combination to suit the process conditions. For instance, basic chemistry dictates that less NOx will result from burning natural gas instead of fuel oil with a high nitrogen content.

That could be an easy solution, or at least a partial solution, to cutting NOx emissions, though it is hardly a viable option for plants on interruptible gas contracts, or for refineries that prefer to use the wide variety of fuels available to them from their own processes.

Dual-fuel burners are commonplace, of course, and they answer some of the objections. More efficient NOx reduction, however, can be achieved by looking at how a burner operates, rather than what it burns. Again, chemistry - or more accurately stoichiometry - indicates that running a burner at low levels of excess air will reduce NOx formation, as will reducing the temperature.

One way of achieving the latter is by flue gas recirculation (FGR) in which the exhaust gases from the boiler/process unit are ducted back to the air intake of the burner as a preheated diluent. This is a popular technique on natural gas-fired boilers and can cut NOx formation by 70 per cent.

Another way of cutting NOx formation is staged combustion. There are two approaches here. One is the staged-air technique, in which the combustion air is separated into two streams. The first, or primary stream mixes with the total amount of fuel flowing through the burner nozzles to produce a fuel-rich flame, relatively cool and low in excess oxygen - conditions that inhibit `fuel' NOx formation.

The secondary air stream is introduced downstream of this flame zone to complete the combustion at a temperature low enough to limit `thermal' NOx formation (that is, the combination of nitrogen and oxygen in the air, as opposed to the NOx formed by oxidation of nitrogen compounds in the fuel).

STAGE BY STAGE REDUCTION

The other type of staged combustion is fuel staging, in which a portion of the fuel gas is injected into the burner flame through secondary injectors to create a gas-rich zone within the flame. A successful example of this approach is Hamworthy's Walrad design of radiant wall gas burners for applications such as ethylene cracking furnaces. Under normal operating conditions, these burners run at an excess air level of 5-10 per cent and are achieving NOx levels of less than 100mg/Nm3.

The Walrad was a product of the Airoil-Flaregas input to HCE's portfolio, as were the Enviromix 2000 series of forced draught (for gas and oil duties) and natural draught (gas only) burners. These are staged-air units designed for a variety of process fired-heater applications and to meet current and expected European NOx emission standards.

More recently HCE has introduced an ultra-low NOx burner for boiler applications. This is the Envirojet series, which is said to provide the very low emission levels more commonly associated with flue gas recirculation, but without the additional hardware of ductwork, fans and controls. Typical NOx levels achieved to date are 45mg/Nm3 (at 3 per cent O2) when firing natural gas, and 120 mg/Nm3 for gas oil on firetube boilers. The burners feature a `slip stream' motor/fan that allows smaller drive units to be used, so saving energy and helping to reduce noise levels - just two more factors for process equipment manufacturers to take heed of in these environmentally enlightened times.A novel end-of-pipe solution with plasma processing is described on page 45