Increasing energy efficiency in Asia’s fleet of heat recovery steam generators

According to the latest international energy outlook, global power consumption is expected to increase by 53% from 2008 to 2035.With the growing affluence of Asian economies, particularly China andIndia;increased appetite in consumption of goods and services has resulted in soaring demand for energy in this part of the world.

The need for highly efficient fossil power generation remains, as alternative energy sources like wind, hydro and solar are insufficient. Gas-fired power plants, in particular heat recovery steam generation (HRSG) plants, provide the quickest method of generating large amounts of power. Combined cycle gas power stations deliver electrical power with net efficiency approaching 60% making them the most efficient power generation systems available today.

Morgan Thermal Ceramics has worked with leading companies in combined cycle technology to supply high performance, high temperature thermal insulation materials for HRSGs over the past 25 years and the company remains at the forefront of new technologies. This article looks at advancedthermal insulation, its application in HRSGs and how power plants can increase the efficiency of their power generation systems.

Insulation systems
There are two main types of insulating fibre – refractory ceramic fibre (RCF) and alkaline earth silicate fibre (AES). Both are man-made vitreous fibres, with RCF being manufactured predominantly from alumina and silica and AES being predominantly manufactured from calcium, magnesium and silicon oxides.  Both insulating fibre products are available in many formats; from bulk, blanket and boards to monolithic modules.

SUPERWOOL® Plusfibre, anew AES wool from Morgan Thermal Ceramics, enablepower plants to operate more efficiently as itprovides excellent insulation in applications up to 1200°C.SUPERWOOL® Plusdelivers high performance with less mass and blanket thickness than alternative products, thus, offering weight and cost savings.

AES woolsalso have significant health and safety advantages compared to RCFs. They have a lower bio-persistence than RCFs and are exonerated from the carcinogen classification in the European Union under the terms of Nota Q of Directive 67/548/EEC.

Morgan Thermal Ceramics’ Pyro-Bloc® moduleslikewise offer alternative insulation.  They are the only “monolithic” insulating fibre modules in the market and combine high insulating properties with rapid furnace installation.  The monolithic module is easy to cut and fit around openings and modify in the field. In addition, the modules are lightweight, have low heat storage and provide durable service.

The overall performance expectation for insulation systems in many HRSGs is typically 10 years or more.
The following examples demonstrate where AES and RCF insulation are used and how they are helping power plants to increase efficiencies.

Insulation in the boiler

Good insulation is critical for the lining of the boiler in a combined cycle HRSG, however, the environment is harsh.  Temperatures are between 600-700?C and gases move at high velocities of up to 20-30 m/s.  Tubing and corners are also present, which result in local turbulence and this increases the difficulty in managingthe gas velocity.

When an insulating blanket is directly exposed to the gas velocity, it could experience some erosion and develop hot spots in the lining.  The insulation becomes weak at these spots, eventually failing and heat and energy is lost.For that reason, a stainless steel lining is installed to protect the insulation from direct contact with the high gas velocity.  The stainless steel lining is necessary in a traditional insulation system but has several disadvantages such as extra costs and heat and vibrations being transferred to the structural steel casing.

Pyro-Bloc modules provide a shroudless system which eliminates the need for studs and the stainless steel shroud protector, thereby providing a robust and more thermally efficient insulation, at lower weight and cost.

Pyro-Bloc modules can withstand higher gas velocities than traditional blankets thanks to a combination of interlocking fibres and their orientation to the gas flow.  In traditional fibre blankets, the fibres are orientated parallel to the direction of travel of the hot gases.  It is therefore relatively easy for individual fibres to be removed and the surface of the blanket to be eroded.  With Pyro-Bloc modules the fibres are end on to the gas flow and are rooted firmly into the monolithic structure.  For this reason, Pyro-Bloc modules are much more resistant to erosion by high gas velocities and are able to work reliably without the protection of shrouds.

In one application, a plant found its insulation was wearing out after only four years.  They had a basic industry standard configuration in place.  An outer skin of carbon steel was insulated with a ceramic fibre blanket on studs which was in turn protected with a stainless steel shroud.  The shroud was supposed to protect the blanket from the tremendous gas-turbine exhaust velocities.  However, despite constantly upgrading the materials, the customer found that they were replacing the lining more frequently than planned.

After investigation, the problem was found to be the insulation system design, rather than the product itself.  As there is a significant difference in firing temperature and the radiant temperature the initial design specifications needed to be upgraded. After consideration, the traditional insulation was replaced with a monolithic weld-on ceramic fibre module – Pyro-Bloc HS.  The primary benefit to the customer is the enhanced velocity resistance provided by the module.

Pyro-Bloc HS modules become extremely hard after the burnout of the organic lubricant.  This hardening is enhanced by the addition of a proprietary agent during the manufacturing process, to ensure penetration through the full thickness of the module.  As a result of the unusual hardness, the customer was able to install the insulation without the stainless steel shroud required by the original ceramic fibre blankets, thus providing a long-life efficient system.

Another advantage of the Pyro-Bloc HS modules is that they could be installed quickly without a pre-laid-out stud pattern.  In addition, their cost was only one third of the previous insulation ceramic fibre and shroud system.  The company was so pleased with the results, it retrofitted four HRSGs.

The next generation AES Superwool Plus™ fibre insulation blankets are also significantly more efficient than traditional AES insulating blankets.

Steve Chernack, Manager, Applications Engineering, Morgan Thermal Ceramics