From the above discussions, there are at least four major WtE technologies available, i.e., incineration, anaerobic digestion, gasification and pyrolysis. These technologies will now be compared. The comparison includes the energy potential and suitability of each technology in the context of New Zealand.
4.1. Energy Potential from Municipal Waste in New Zealand
In 2015, New Zealand generated 3.221 million tons of municipal waste [26
]. To get an estimation of the energy that could be generated from this waste, a calculation was carried out. Table 3
shows the amount of each waste type generated annually in the New Zealand (data are adapted from Reference [27
]), the heating value of each kind of waste (values are approximated based on Reference [47
]) and the corresponding available energy. In total, there is more than 30.8 PJ of energy available per year from waste in New Zealand. In comparison, the total energy demand of the country was 577.6 PJ in 2016, of which 25.5 PJ, 270.9 PJ and 81.3 PJ was supplied by coal, oil and natural gas, respectively [30
]. Therefore, the available energy from municipal waste in New Zealand is comparable to the energy demand provided by coal.
To estimate the potential energy that can be generated from WtE plants, the total amount of waste energy is multiplied by the various efficiency values of the different WtE technologies. The calculated values for incineration, anaerobic digestion, gasification and pyrolysis are tabulated in Table 4
. It should be noted that for each technology, the amount of energy generated varies for two main reasons: (1) the different efficiencies, and (2) the different kind of waste that it can treat. For example, anaerobic digestion can only treat organic waste, which explains the low value of energy generated as compared to other technologies, since plastic waste has a high heating value and represents around 12% of waste generated in New Zealand. As a consequence, anaerobic digestion needs to be combined with a good recycling strategy, if it is to be adopted nationwide. As expected, the most sophisticated technologies (WtE-GT integrated and advanced gasification) offer the most energy generation, and further studies in this area will allow the energetic optimization of future WtE plants. However, these technologies are less mature at the moment.
4.2. Suitability Comparison of the Technologies for New Zealand
To analyze the suitability of the available WtE technologies in the context of New Zealand, a comparison is carried out in the following aspects: air pollution, cost (capital and maintenance), side product, capacity of production, commercial maturity, energy efficiency and type of waste to be treated.
Studies of gas emissions from each type of plants are available in the literature [32
]. There are technologies to treat the various gas emissions to reduce air pollution. These, however, have financial implications. For example, although gasification plants emit less carbon dioxide than incineration plants [54
], it is more expensive to treat the emission from gasification plants than that from incinerators [55
]. Among all the technologies, anaerobic digestion is the least air polluting since all gases are captured to produce methane [56
]. Pyrolysis is less polluting than incineration because of the absence of oxygen during the process and the lower temperature used [57
]. It is also less polluting than gasification for the same reason [50
To compare the cost of each technology, Table 5
tabulates the costs of WtE plants based on data from GIZ (German Corporation for International Cooperation) [58
]. It can be seen that anaerobic digestion is the most economical option. Conventional incineration is relatively more economical than gasification/pyrolysis, while the advanced incineration plant is currently very costly.
Regarding side products, incineration units usually recycle the bottom ash to recover metals from it and the remaining ash can be used as construction materials (aggregates) [10
]. Anaerobic digestion’s main side product is digestate, which can be used for fertilizer [32
]. For gasification, only ashes remain from the process apart from the syngas are generated [54
]. The side products of pyrolysis depend on the parameters used. The range and proportions of products is then quite varied and is constituted by unconverted carbon, charcoal, ash, pyrolysis oil and syngas [59
Regarding the waste treatment capacities, the common scales of capacity of each technology are as follows [47
]: (1) incineration can treat 1500 tons of waste per day, (2) pyrolysis and gasification can treat 10 and 100 tons of waste per day, respectively, and (3) around 500 tons of waste per day can be treated by anaerobic digestion [58
]. The capacity of production obviously depends on the size of the treatment facility, but those figures give an idea of the current performances of the existing plants in the world.
In terms of the level of maturity of the technologies, waste incineration is the most mature, followed by anaerobic digestion, while pyrolysis and gasification are not yet mature [10
Another aspect to consider is the energy production efficiency of each technology. It is noted that the efficiency of each technology depends on many factors such as the thermodynamic cycle employed, the scale of the plant and all the techniques used for optimization that are different for each plant. For conventional incinerators, steam turbines are commonly used and the electrical energy production efficiency is typically around 15–30% [47
]. When used to provide heating, the efficiency of incinerators can reach 90% and for combined heat and power is 40% [47
]. For anaerobic digestion, the maximum efficiency of anaerobic digesters to produce biogas based on the heating values is 28% [48
]. Assuming a gas turbine with an efficiency of 30–40%, the overall efficiency is between 8.4–11.2%. The gasification technology has an efficiency of between 10–27% or 30–40% (advanced gasification), depending on the type of turbine used [50
]. Pyrolysis has an efficiency of between 16–25%, assuming a gas turbine for the electricity production process [50
In terms of the type of waste that can be treated, incineration is the most comprehensive as it can treat almost any type of waste. Gasification and pyrolysis plants may be able to treat all kind of waste with less environmental impact and at a lower cost as compared to incineration, but the technologies are less mature. Anaerobic digestion is the most limited option as it cannot treat non-biodegradable materials, such as plastic waste.
To summarize the comparison of the various aspects above, to find the most suitable technology for New Zealand, scores ranging from 0 (the worst) to 3 (the best) were assigned by ranking each of the technologies in each parameter. For example, anaerobic digestion is the most preferred option in the aspect of air pollution, followed by pyrolysis, gasification and lastly, incineration. Therefore, in the aspect of air pollution, anaerobic digestion was given a score of 3, pyrolysis got a score of 2, a score of 1 for gasification and incineration was given a zero score. Table 6
summarizes the scores of the four technologies in all the aspects considered without any special weightage for any of the parameters. The table shows that, without weightage, incineration and anaerobic digestion are generally the most attractive WtE options, followed by pyrolysis and, lastly, gasification. Incineration is attractive mainly because of its capacity and maturity. Anaerobic digestion is advantageous in terms of its environment-friendliness and cost.
In practice, New Zealand has two main concerns regarding its waste treatment strategy, i.e., environment-friendliness and economy. Therefore, more weightages should be assigned to the relevant parameters in comparison. The main parameter that is relevant to environment-friendliness is air pollution, while that for the economy is cost. A double weightage was therefore given to these two parameters. The results are presented in Table 7
. The table shows that anaerobic digestion seems to be the most attractive solution for New Zealand in general. It is both environment-friendly and economical. The technology is relatively mature. The inability of anaerobic digestion to treat non-biodegradable waste is actually consistent with New Zealand’s national waste management strategy to reduce, reuse and recycle waste. It should be noted, however, that the energy production efficiency of anaerobic digestion is relatively low as compared to the alternatives. Furthermore, it is seriously lacking in its waste treatment capacity. Therefore, in order for this option to be successful in its implementation nationwide, it is crucial to ensure the effectiveness of the existing national waste management strategy to reduce, reuse and recycle.