Waste to energy is proving to be an unstoppable technology, as its growth in China shows us. Here, we look at the extent of this growth plus the environmental issues concerned
by Nickolas Themelis and Zhixiao Zhang
The need for intelligent waste management has led to the concept of the ‘hierarchy of waste management’ that places the various means for dealing with MSW in order of environmental preference.
Of the estimated one billion tons (907 million tonnes) of global ‘post-recycling’ MSW, close to 200 million tons (181 million tonnes) are processed in Waste-to-Energy (WTE) plants that recover the energy content of waste in the form of electricity or heat. The dominant WTE technology involves combustion of MSW on an inclined or horizontal grate. There are over 500 WTE plants of this type operating in 35 countries.
Most of the global urban MSW, i.e. over 800 million tons (725 million tonnes), is landfilled. The Earth Engineering Center of Columbia University has estimated that one square metre (about 10 square feet) is used up, forever, for every ten tons (nine tonnes) of MSW landfilled. True sustainable development requires that only inorganic residues be landfilled, as is already the practice in several countries. However, this would require us to considerably increase the present global WTE capacity of about 200 million tons (181 million tonnes) and this is a very costly proposition, especially for developing nations.
Obviously, the need is greatest in large nations with rapidly growing cities, such as China and India, where existing dump sites are overfilled.
China has the largest population (1.33 billion) on Earth and is experiencing rapid economic growth. This country has a GDP of $8.8 trillion in terms of Purchasing Power Parity (PPP), which is the third largest in the world after the EU and the US. However, its population is over four times that of the US so the actual per capita GDP is only $6.800 and corresponds to a fraction of the US GDP per capita.
Despite the relatively high capital cost of WTE, the central government of China has been very proactive with regard to increasing WTE capacity. One of the measures brought in provided a credit of about $30 per MWh of electricity generated by means of WTE rather than by using fossil fuels.
‘Harmless treatment’ of MSW in China
The term ‘harmless treatment’ in China means the disposal of MSW by recycling, composting, WTE and sanitary landfilling. The ‘harmless treatment’ rate is defined as the percentage of the weight of total MSW treated with these methods. The generation of MSW, and also the ‘harmless treatment’ fraction have been increasing over the past 30 years in China.
Table 1<sup>2</sup> shows the reported data from 2001 to 2007 and also the number of WTE plants and their total capacity. The Chinese WTE capacity has increased steadily from 2.2 million tons in 2001 to nearly 14 million tons by 2007. However, landfilling remains the dominant means of waste disposal in China.
Most WTE plants are located in eastern China, especially in the districts of the Changjiang and Pearl River Deltas. As of 2007, three provinces in these two districts, Guangdong, Zhejiang and Jiangsu had fifteen, fourteen and nine WTE plants, respectively. These plants constitute 64 % of the existing WTE capacity in China. This is explained by the relatively high economic development in these provinces.
China’s 11th Five-Year Plan (2006-2011) is very ambitious, showing expected construction of many new WTE facilities across the country, as shown in Table 2.WTE technologies used in China
Stoker grate incinerator and circulated fluidized bed (CFB) incinerator are the main types of technology used in WTE plants in China. According to a preliminary survey of 100 WTE plants in operation or under construction, most of the MSW incinerators are of the grate combustion type (‘mass burn’), and are based either on imported or domestic technologies. The CFB incinerators co-fire MSW with coal (up to 15 % coal by weight) and have been developed by Chinese academic research centers, such as Zhejiang University, Chinese Academy of Sciences (CAS), and Tsinghua University. Most of the new plants are based on the stoker grate design.
The capacity of the WTE plants built in earlier years was generally less than 800 tons/day (725 tonnes/day). However, recent WTE plants are larger, typically over 1000 tons/day (907 tonnes/day). The capacity of a single line within a plant has also increased, from the 200 tons/day (181 tonnes/day) in early years to over 500 tons/day (453 tonnes/day) in recent years.Air pollution control systems
Most of the air pollution control systems built in the Chinese WTE plants are similar to the predominant gas control systems in the US: a combination of semi-dry scrubber, activated carbon injection (to remove volatile metals and organic compounds) and fabric filter baghouses (to remove particulate matter). In some WTE plants, selective non-catalytic reduction is included to remove nitrogen oxides, such as, for example, the WTE plants under design for Guangzhou, Shantou, and Chongqing.
A major problem that faced the western incinerators in the late 1980s was the discovery of high dioxin emissions. For example, the US WTE plants in 1989 emitted a total of 10,000 grams of toxic equivalent dioxins (grams TEQ), corresponding to 100 nanograms TEQ per standard cubic meter of stack gas. This led to the USEPA regulation of Maximum Achievable Control Technology (MACT) that resulted in the retrofitting of about 90 WTE plants in the US and the closing of nearly 50 small plants. As of 2002, this retrofit resulted in decreasing WTE dioxin emissions by a factor of 1000 to less than 10 grams TEQ. 3Figure 1: Combustion of MSW on a moving grate.
The emissions of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (dioxins) from 19 MSW incinerators in China were investigated by the Chinese Academy of Science (CAS). Sixteen stoker grate and three circulating fluid bed incinerators with capacities from 150-500 tons/day (136-453 tonnes/day) were examined. The air pollution control systems of nine of the grate combustion WTE plants consisted of semi-dry scrubber, activated carbon injection and fabric filter baghouse; the other seven plants did not use activated carbon injection. The results of this study showed that the dioxin emissions of these 19 MSW incinerators ranged from 0.042 to 2.461 nanograms TEQ /Nm3; the average value was 0.423 ng TEQ/Nm3.
The dioxin emission levels of three MSW incinerators were higher than the 1.0 ng TEQ/Nm3, which is the emission standard in China. Only six MSW incinerators had dioxin emission levels below 0.1 ng TEQ/Nm3, which is the emission limit in Europe, the US and other developed countries. Therefore, the average emissions of dioxins from Chinese incinerators ranged from being as low as European and US plants to being 24 times the western standard. Considering the significant amount of MSW generation in China, the dioxin emissions from some poorly-operated WTE have been a severe problem and caused an adverse public reaction against all WTE facilities.
The dioxin emission factors to the atmosphere from these 19 MSW incinerators were calculated to range from 0.169-10.72 μg TEQ for per ton MSW with an average 1.728 μg TEQ per ton MSW.Conclusions
The generation of billions of tonnes of solid waste by humanity presents both a challenge and an opportunity to developing nations. The information presented in this article shows that China, more than any other developing nation, is taking major steps to increase its WTE capacity.
Since the beginning of the 21st century, China has increased its WTE capacity from 2 to 14 million tons of municipal solid wastes. This makes China the fourth largest user of waste-to-energy (WTE), after the EU, Japan, and the US. There were 66 WTE plants in China by 2007 this is projected to increase to one hundred by 2012. Two thirds of these plants employ either imported or domestic versions of combustion on a moving grate; and the other third various forms of a home-developed technology, the circulating fluid bed reactor.
Fuzhou Hongmiaoling EfW plant
Credit: Sanfeng Covanta Environmental Company, Chongqing, China
This study also examines in detail the environmental performance of Chinese WTE plants. Using as a yardstick the emission of dioxins from a group of 19 Chinese WTE plants, we found that seven operate below the EU dioxin standard (0.1 nanograms TEQ per standard cubic meter of stack gas) and 12 above this standard. The fact that several WTEs in China are able to control dioxin emissions to the very strict EU standard (which is 10 times lower than the present Chinese standard for dioxins) is very encouraging and indicates that Chinese operators and air pollution control systems can be as good as those in the west.
Prof. Zhixiao Zhang is Associate Professor at the University of Dianzi Hangzhou and member of the management team of WTERT-China
This article is based on a paper presented at the WasteEng 2010 International Conference (Beijing, May 2010), co-chaired by Professor Ange Nzihou, Ecole des Mines d’Albi, France.
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