Alternatives to Hydrazine in Water Treatment
at Thermal Power Plants
Hydrazine is generally used as an oxygen scavenger for corrosion control in thermal power plants. Although hydrazine is very effective in this application, it is a genotoxic carcinogen. The use of alternative chemicals such as nontoxic oxygen scavengers and new oxygen scavenger-free water treatment technologies is recommended or required in foreign countries. Even in Japan, such new approaches to water treatment would contribute to environmental protection, increased confidence, and a reduction in power plant operating costs.
In recent years, our company has received orders for power plants from overseas customers. The specifications for some of these plants prohibit the use of hydrazine, requiring the use of nontoxic alternatives or different methods of water treatment technology without oxygen scavengers. Hydrazine-free water treatment provides the following advantages: ・ Reduction in environmental impact and improvement of the work environment; ・ Reduction in deposition, which in turn reduces the frequency of chemical cleaning for through-flow boilers;
・ Reduction in pipe wall thinning due to flow-accelerated corrosion (FAC); and ・ Reduction in startup time and water consumption in the drum boilers and heat recovery steam generator (HRSG) boilers.
In this paper, we introduce our approach to water treatment technologies.
|2. Role of Hydrazine in Water Treatment
What is hydrazine?
There are two types of hydrazine: anhydrous hydrazine (N2H4) and hydrazine hydrate (N2H4・H2O).
Hydrazine hydrate is mainly used as a raw material for plastic foaming agents, with 28% of hydrazine used in water treatment and pH control.
Corrosion-control effects of hydrazine
Oxygen dissolved in water causes corrosion. Because hydrazine removes this oxygen via the reaction N2H4 + O2 → N2 + 2H2O, it is useful in preventing corrosion. It decomposes at a temperature of 200°C or more to form ammonia (3N2H4 → 4NH3 + N2) , which acts as a pH adjuster.
This scavenging reaction proceeds very slowly near room temperature, and the formation of ammonia by decomposition is not great. However, hydrazine is also used to control corrosion during the storage or cooling of water due to its action as a pH adjuster. Figure 1 shows an example of corrosion-control tests on metallic material using hydrazine. When the metal was soaked in 100 mg/l hydrazine, no rust was produced even after 500 hours had elapsed.
Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009) 44
Figure 1 Corrosion reduction using hydrazine
|3. Restrictions on the Use of Hydrazine
Trends in chemical substance management
In recent years, an international framework for the control of chemical substances has been created.
(1) In 1992, the United Nations Conference on Environment and Development adopted “Agenda 21” (Chapter 19), a global action plan for sustainable development in the 21st century.
(2) In 2002, the World Summit on Sustainable Development (WSSD) adopted the “Johannesburg Plan of Implementation” containing guidelines on the management of chemical substances to minimize major adverse effects on human health and the environment by 2020. (3) The International Conference on Chemicals Management in 2006 adopted the “Strategic Approach to International Chemicals Management (SAICM)” designed to implement the Johannesburg Plan.
In 2007, the European Union implemented the Regulation on Registration, Evaluation, Authorization, and Restriction of Chemicals to achieve the WSSD goals by 2020. Studies on the SAICM-related approach are also under way in Japan.
3.2 Recent development of the use of hydrazine from Pollutant Release and Transfer Register data...