Waste to Energy : Fuelling the Asian Dragon: WtE challenges in China

In 2007, China became the world's largest CO2 emissary, putting the USA in second place with 1.59 billion tonnes of CO2 or 16% of global emissions. This was largely a result of a more affluent population, predominantly located in first and second tier cities, copying a newly attainable Western lifestyle. China's emissions meanwhile, which currently stand at around 18%, are set to grow to around 20% of the world total by 2025. Only fair, you might say, given that China also accounts for 20% of the global population.

In an effort to stabilise growth, the Chinese government has set 'harmony' as the top most priority in social and economic development. Apart from developing an independent Chinese R&D industry and promoting sustainable manufacturing, the reduction of resource utilisation, an increase in recycling, and the efficient operation of waste management will be key to driving stable, harmonious, long-term economic growth. Efficient resource utilisation, energy saving technologies, and alternative forms of energy generation, environmental protection, recycling and waste management are all areas where international research and knowledge sharing can make significant contributions.

MSW generation, composition and disposal

Economic and social progress leading to new consumption highs have also brought growth and increasing diversity of waste. The total volume of municipal waste collected in 1981 amounted to 26.1 million tonnes. By 2002, more than 20 years later, that figure had quadrupled to 110 million tonnes. This represents annual growth of 8.2%, compared to an annual population growth of around 4.4%.

In 2002 the statistical parameters of the waste collection database were changed, so historical comparisons cannot be accurately made, however they show a 4.6% growth in municipal waste from 2003 (148 million tonnes) to 2005 (155 million tonnes) and a decrease in real terms by 2007 (148 million tonnes), showing that the propagation of "3Rs" has made a noticeable entry. From 2003 to 2006 the number of waste treatment and disposal facilities also shrank by 27% to 419 facilities. Total capacity however, rose by 17.5% to 258,000 tonnes a day, indicating that more modern and efficient disposal units were put into operation, according to the China Statistical Yearbook 2010.

Research in that field showed that the current statistics only cover the large municipalities, whereas the rural back country is not accounted for. The United Nations and the World Bank estimate that rural waste generation is 0.8 kg to 1 kg per person per day, adding some 360 kg to each head of the rural population. Considering that 52% of China's population lives in the countryside, this adds another 65 million tonnes per year. Thus the total waste volume that should be dealt with in a sanitary way is more than 225 million tonnes per year.

Waste composition studies have shown that the organic fraction of municipal solid waste is above 60% after the sorting of recyclables. Implications are that serious environmental burdens caused by landfill gas and leachate emissions. Relevant government authorities have acknowledged that environmental protection and modern waste management systems hold key roles in sustainable and responsible development. To drive this, China is actively pursuing an exchange of experience and know-how with the developed world.

Against this background China has made great efforts to establish waste incineration plants (WIPs) in recent years. Within the framework of a study including Chinese WIPs, 30 plants were shortlisted for visits. Telephone calls and direct contact to operators during the annual convention of the WIP Association in Shenzhen showed that many of the plants do not allow visitors. As a result, only 15 of the 77 plants currently in operation (19.5%) could be visited. Many of the plants that do not allow visitors and thus could not be viewed belong to operators who have reference plants which they proudly display. Operators are quite selective about whom to show which plant, because most of the plants are run with a much higher coal co-firing content than officially admitted.

China currently has approximately 100 WIPs in operation, using three main technologies. Most large cities use the stoker grate technology, which was imported at the end of the 1980s. As a response to implementation problems China adapted fluidized bed furnaces to implement waste co-firing. Stoker grate holds approximately a 60% market share and fluidized bed furnaces holds approximately 33%, with rotary kiln technology accounting for the final 7%. Where implemented, rotary kilns tend to have a capacity under 100 tonnes per day and are used in hazardous waste applications (e.g. Hangzhou DADI). Stoker grate WIPs typically have a daily capacity of 1000 to 1500 tonnes, each line having a maximum throughput of 500 tonnes per day.

Fluidized bed furnaces have a smaller capacity ranging from 100 to 500 tonnes per day. Interestingly the larger stoker grate plants do not generate more electric power. Indeed, early plants faced a series of operational troubles, which in turn led to a series of adaptations. Discussions with plant operators gave valuable insight into operational methodology as well as problems and solutions. For example, most of the plants were designed for a calorific value of 5 to 7 MJ/kg. In actual practice a value of < 5 MJ/kg is achieved due to the high water and organic content. Research showed that even today, due to the high content of wet organic fractions, the average lower heating value of Chinese MSW ranges from 3 to 5 MJ/kg, as opposed to the 6 to 7 MJ/kg typically required to obtain a smooth combustion process. MSW bunkers are therefore crucial to pre-dry waste. Within five to seven days, up to 20% of water content is lost to leakage, which puts an enormous strain on the water treatment facility, where installed. Other developments to combat the challenges presented by the high water content of the MSW are an extension to the length of the stoker grates to allow for further drying and combustion time. Last but not least, insulation walls have been reinforced to keep losses at a minimum.

Despite these measures, most of the waste's heating value is used to evaporate the MSW's high water content, rather than superheating the turbine's steam circles. Given that water reduction during firing accounts for 80% of weight and volume loss, it is not surprising that fluidized bed technology came into favour. By co-feeding MSW into the coal-fired combustion process, power generation units can claim 'renewable energy status' and receive the subsidised energy price of 0.55 RMB/kWh (8.6 U.S. cents/kWh), as opposed to the standard 0.33 RMB/kWh (5.1 U.S. cents/kWh) paid to wholly coal-fired power stations. District heating is unknown, as heating is not common in southern China (south of the Yangtze River) and tariff and supply regulations are missing.

All plants cover their costs through tipping and co-generation fees. With regards to problems mentioned during WIP operation, these were corrosion within the flue gas system (40% of plants), treatment of leakage water from the bunker (20% of plants), internal energy consumption (electric efficiency: 20% of plants), service and maintenance (13% of plants) and problems caused by the high water/organic content (grate furnaces).

Bottom ash and slag residues from incineration are used as road and construction materials. Typically slag is first sifted for any remaining metals and then sold on to construction companies. Fly ash from the exhaust gas stream is considered to be hazardous waste, which the Chinese State Environmental Protection Agency guidelines require to be stabilised (through cement) and disposed of in secure landfill sites. The waste to energy facility in the Nanshan district of Shenzhen handles 800 tpa This rule is however still in the implementation stage, which means that not all plants are yet in compliance. In terms of air pollution and residues, most Chinese WIPs use air pollution control systems (APC) comprising of an active carbon adsorption system, dry or semi-dry scrubbers, and textile filters. These should be sufficient to properly treat exhaust gases, however due to the corrosive and abrasive materials, they require frequent servicing and maintenance. As the latter is generally neglected, many plants operate without the required filtering equipment, or with only limited (and insufficient) results.

Conclusions

The study outcome showed that while still at an early stage, China is making great efforts to establish incineration plants for MSW treatment. There are however a few technical and organisational problems to resolve. Among them the urgently needed improvement to the flue gas cleaning systems, and the disposal of the solid waste incineration residues. The main problem is that most WIPs combust waste with a high organic and water content, which is ecologically and energetically unrewarding. Two possible solutions are to either implement the separate collection of the biodegradable waste for utilisation in biogas and/or compost plants, or to treat the mixed wet residual waste by pre-drying and mechanical conditioning in mechanical biological treatment plants, which allows an optimised thermal utilisation.

Prof Michael Nelles is the associate professor, department of waste management, University of Rostock, Rostock, Germany.

Co-author Thomas Dorn is from the faculty of agricultural and environmental sciences at the university.email: michael.nelles@uni-rostock.de

The University of Rostock investigated the status of waste incineration in China and the main results of 2009 and 2010 are represented here.

Though waste incineration does not yet play a major role in China's disposal strategy, currently 16% of MSW is combusted while more than 80% still goes into landfills. The limitation of land resources, water table contamination, fire and explosion hazards as well as residential unrest due to foul odours has led to a growing number of WIPs.

The combustion of MSW through incineration plants offers several advantages over landfilling: volume is drastically reduced to 10% to 30% of initial values; the material is rendered inert; and, provided high-calorific material is burnt, resulting energy can be recovered and transformed into heat and electricity.

The major drawbacks are the high investment costs for the incineration plant; the need for trained manpower and the need to treat flue gas, bunker leakage water and ash, as these contain highly toxic elements.