Thermal Power

05 Dec 2019

Maintaining Thermal Power Plants Beacuse the End for Fossil Fuel Energy Haven't Come

05 Dec 2019  by Power Engineering   

The global energy mix is continuing to diversify and evolve as the market for renewable energy production has become more competitive and cost-efficient in comparison to traditional fossil-fuels. As governments, industry and consumers work to reduce greenhouse gas emissions in the decades to come, we will continue to see significant investment in the development of renewable energy infrastructure and technologies worldwide, and a changing role for fossil fuels.

This does not yet spell the end for fossil fuel energy sources. On a global basis, we will continue to rely primarily on traditional power generation for some years to come, in order to meet an insatiable demand for energy worldwide and to provide greater energy security in the interim.

These thermal power plants need to be carefully maintained, in order to prolong their working life, as new investment in coal and oil-fired power generation is going to be increasingly hard to come by, due to increased environmental awareness. Both developed and developing economies will focus new investments on more environmentally efficient energy solutions, including wind, solar and natural gas. The emphasis here is on developing the most reliable and economically efficient power sources and related infrastructure possible.

Tailored energy solutions

Most projections suggest that as more renewable energy facilities come online, gas-fired power plants will be utilised on a more flexible basis, to supplement and support a broader matrix of energy inputs across national power grids. We already see this in many green economies, such as Denmark, where wind energy provides most of the power generation for the national grids, but gas-fired plants are used to supplement the grid on the less productive (i.e. less windy) days.

From an environmental standpoint, this makes sense. Natural gas emits 50 to 60 percent less carbon dioxide (CO2) when combusted in a new, efficient natural gas power plant compared with emissions from a newly built coal plant. In today’s competitive market, liquified natural gas (LNG) is attractively priced, and global infrastructure for its extraction, transportation and storage has been developing worldwide at pace. As such, we will continue to see investment in the development and upkeep of gas-fired power plants and infrastructure, with further investment in new facilities, and a growing emphasis on running gas-fired power stations on a needs must basis. They will, in short, be used at variable capacity rates.

Flexibility doesn’t come easily

From an operational standpoint, the use of gas-fired power plants as flexible energy sources creates additional challenges for owners and managers which must be overcome. Ensuring that the equipment, pipework and structures are physically prepared for flexible working is essential to ensuring profitability for asset owners. An important consideration for reducing maintenance costs is managing the risk (and considerable expense incurred) of corrosion under insulation (CUI). The nature of the intermittent operation of these types of facilities means that more frequent changes in process temperature occur than with other types of thermal power plant (coal and oil), which operate at constant base loads.

CUI occurs when moisture becomes trapped underneath insulation around pipework, ductwork, valves and other insulated equipment throughout the plant. This can be external water, which seeps through leaking and poorly maintained joins in the outer cladding, or condensation formed when the plant is heated and cooled, passing equipment through a temperature range of 60 – 150 degrees Celsius with increasing frequency. As it is beneath the insulation, the trapped water does not have the opportunity to evaporate and so settles on the metal surfaces in the space between the insulation and the equipment substrate, subsequently leading to corrosion.

This phenomenon can be very problematic for gas-fired power plants. A problem which rests out-of-sight, the cost of detection, removal of insulation, remediation and re-instatement of insulation can be a considerable drain on the operating expenditure of these assets. Within these power plants, the newer of which utilise steam systems as well as the gas turbine (combined cycle), to generate power the risk of CUI is ever-present but can be mitigated with the right protections in place. Addressing the risk of CUI proactively makes absolute sense. This is where paints and coatings come in.

Traditionally, the thermal power generation industry has used zinc silicates, sometimes overcoated with aluminium silicones on pipework, valves and other insulated equipment. While these coatings perform very well at extremely high temperatures, they are not designed to handle cyclic conditions through a wide temperature range. In addition, when zinc silicates without topcoats or with topcoats that have degraded are exposed to repeated cycling in warm wet areas, (such as those found beneath thermal insulation) the zinc is consumed rapidly, and corrosion quickly occurs. These coatings are no longer the ideal choice.

Until recently, whilst there are many standards and guidance documents around combating CUI, there are no international standards for testing a coating’s ability to operate in CUI conditions. As a result, many ‘ad-hoc’ test programmes have been devised but are often designed with the oil and gas and process industries specifically in mind, not necessarily thermal power. To better regulate the coatings solutions, ISO standard 19277 was developed in 2018.

New thinking for preventing CUI in power plants

The simple solution to CUI is to overcoat the zinc with other coatings to prevent it from coming into contact with the warm wet environment. However, this approach has limitations. The topcoat used must be able to resist the temperatures concerned, provide good barrier properties in all temperature ranges where moisture is present and must, of course, be thoroughly compatible with the zinc silicate.

A better solution is to use inert multi-polymeric matrix type materials, such as Hempel’s Versiline CUI 56990, which has been proven on multiple projects around the globe. These coatings contain a silicone backbone, they are applied in thicker films and have better barrier properties than thin­film aluminium silicones, which offer limited corrosion protection. As a result, they provide far better CUI resistance during the time that the plant is in the CUI temperature zone. Inert multi-polymeric matrixes have other advantages. They have a high temperature resistance, beyond the 400°C of zinc silicates. They are also faster drying than silicone aluminium coatings, and so can reduce downtime during maintenance and increase productivity when applied from new.

The final advantage with inert multi-polymeric matrix type materials is simplicity. They were initially developed for the oil & gas industry, because oil & gas applications involve a very wide temperature range. Inert multi-polymeric matrixes can be specified for a number of temperature and performance categories, making them ideal for bulk items such as pipework and valves. By introducing these coatings, the thermal power generation industry has a ready-made solution for the CUI challenge.

Looking below the surface

Addressing the pressures of a changing energy mix worldwide requires a pragmatic approach to managing existing and new assets. In order to meet continued demand for reliable and affordable energy, ensuring the viability of gas-fired power plants is vitally important. This means doing everything possible to ensure that these extremely expensive assets, which are vital to every economy, can remain viable, safe and fit-for-purpose. Doing so requires energy providers to work proactively to fortify every component of a power plant, and to search out efficiency savings. Looking below the surface and applying ready-made solutions will help to maintain balance into the future.

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