A sophisticated tool for better boiler cleaning

Mirek Spicar introduces a sophisticated tool for boiler cleaning based on a novel concept

Incineration of solid organic materials generates flue gas, fly ash and slag. Fine fly ash particles are entrained with the flue gas, whereas coarse slag is removed at the discharge of the furnace. Fly ash and condensate, mostly metal salts, settle down on boiler surfaces, which hinders heat transfer through the tube wall to the water/steam cycle. All operators are faced with the problem of decreasing thermal efficiency of the boiler, caused by the settling down of fly ash, called fouling of the boiler.

Several methods of cleaning boiler surfaces are available, allowing cleaning of the heat transfer areas during boiler operation as well as during shutdown. The widely applied cleaning by soot blowers lets the driving media (superheated steam or compressed air) act in a linear or oscillating manner. The cleaning impact only reaches the areas of the direct media jet. Additionally, a considerable amount of the valuable superheated steam is consumed, and abrasion/erosion is caused to the protective shells or to the boiler tubes themselves. The idea of cleaning the boiler surfaces by using a spherical shock wave, which can remove fly ash deposits even from hidden surfaces, such as from rear sides of the boiler tubes, became the driving force for development of a compact and automated cleaning device at the turn of the century.

The predecessor of today's shock pulse generators involved the local ignition of a controlled portion of explosives or a gas mixture. The cleaning impact is sufficient for a certain time; however, the handling procedure requires strict safety precautions and specialist training for the operator. The so-called "manual online cleaning methods" are based on opening one of the inspection doors of the boiler during its operation and inserting a lance to the vicinity of the area to be cleaned. The lance tip is equipped with the device set to explode following the operator's initiation.

An expert in this technology, Explosion Power, has developed a DCS-controlled, online boiler cleaning solution, automatically generating shock pulses (SPs) by shock waves. Due to the spherical impact of the shock wave, deposits are removed from wall surfaces and bundle tubes upstream and downstream of the installation location. The shock waves are created by instant-combustion of a few grams each of a mixture of burnable gas (natural gas or methane) and pure oxygen. This act is carried out automatically and outside of the boiler in a pressure-resistant device. The resulting shock wave is jet shot into the boiler. Locally the flue gas, the internal boiler tubes and walls are put into a short oscillation and the deposits are cleaned off. The entire SP takes few milliseconds.

The shock pulse generators (SPGs) are designed for shock wave cleaning in a variety of industrial applications. Four different sizes can cover most application cases. The smallest unit of the EG series has a filling volume of the combustion chamber of 2.5 litres. The filling volume of gas comprises 16g of methane and equivalently 34g of oxygen required for the ideal combustion per SP. Approximately 350 SPs can be generated from a standard 200 bar gas cylinder. If an SP is generated every two hours, the gas cylinders last for approximately one month before needing to be exchanged.

The SPG is mounted on the outside of the boiler wall. Installation is easy, because the unit is installed on a nozzle of DN 125 and the whole installation and maintenance space requires approx. 1.5m3. The larger units are designed with larger combustion chamber of 3.5 litres, 4.4 litres and 7.7 litres. The power of the shock wave is effective between 3m and approximately 15m, meaning that one SPG can typically clean a complete radiation pass, or two neighbouring tube bundles. Should the boiler width be larger, then two SPGs can be installed at opposite walls and the SPs can be activated simultaneously or alternately.

The first installation was carried out in 2009. At three boilers of the waste-to-energy (WtE) plant at Lucerne in Switzerland, the eight installed SPGs proved to clean the boilers to satisfaction. Due to this positive result, the previously installed cleaning systems were completely dismantled, and the boilers were since then cleaned by SPGs only and activated from the plant's central control system. The travelling time of the boilers increased, which means more energy may be used for power production and no manual intervention into the boiler for cleaning during operation is needed.

Since 2009 nearly 700 units have been installed worldwide for cleaning of different boiler types and sizes. They have been used for cleaning horizontally and vertically arranged tube bundles as well as finned tubes and boiler walls. There is no abrasion of the inside of the boiler. Even refractory-lined boiler internals are not damaged and can be efficiently cleaned.

Explosion Power is continuously improving the product. In 2017, a new development was initiated, where instead of oxygen, compressed air is used. Compressed air can either be supplied in gas cylinders or generated by an auxiliary air compressor unit in the plant itself. And in August 2020 new SPG types, SPGr10 and SPGr16, were launched.

Mirek Spicar is with Explosion Power