WASTE TO ENERGY: INVESTIGATION, TECHNOLOGIES AND EVALUATION SYSTEM

Supported by the on going project on "Sustainable Solid Waste Landfill Management in Asia" under the Asian Regional Research Programme on Environmental Technology funded by Swedish International Development Cooperation Agency and Coordinated by Asian Institute of Technology, Thailand, a combined waste-to-energy (WTE) process has been developed in China. In this research two fresh and five aged municipal solid wastes (MSW) samples were investigated and characterized. Based on the analysis of results, suggestions were proposed for the proper WTE system with aged MSW from dumpsites and landfills and fresh MSW considered together. The WTE process included hierarchy of recycling plastics from fresh MSW as raw material and recycling combustibles from aged MSW as RDF. To recycle the plastics from fresh MSW, separation and purification system have been set up and their energy consumption was counted. Two analysis tools including life cycle assessment (LCA) and cost benefit analysis (CBA) have been adopted for the WTE system evaluation. Based on the necessary data collected during investigation process, LCA was carried out for RDF production from aged MSW and utilization in fresh MSW incinerator; and CBA was performed for plastics recycling from fresh MSW; good results have been obtained from those evaluations suggested the proposed WTE processes are promising.


INTRODUCTION
Waste to energy (WTE) is a common and essential method to dispose municipal solid wastes (MSW). But considering the characteristics of wastes are different from country to country and from region to region, there is non-universal method for effective and economical WTE process. Incineration is a good method for disposal of non-recoverable wastes, in some developed countries such as Denmark and Germany, MSW incineration is an important source of compensatory energy to replace the common fuels' consumption. But in most developing countries, incineration is difficult to popularize due to the expensive investment as well as the low level of lower heat value (LHV). In addition to incineration, gasification and pyrolysis are also important thermal processes involved in WTE process. But apparatuses only running on gasification and pyrolysis mechanism are very rare in developing countries, either in most developed countries, Biogas production from anaerobic processes is another alternative to WTE, but this is only applicable to degradable organics like garden and food wastes, In China MSW are not sorted at collection, and for long time landfill and dumpsite are the places for MSW storage, Proper WTE process may be chosen only after basic data of those newly generated and stored MSW are available, In this paper, an investigation was performed first and then WTE processes were suggested with proper evaluation tools to judge their feasibilities,

INVESTIGATION OF MSW
Two fresh MSW samples and five aged MSW were sampled and investigated, The investigation method included sampling, separation, weighing and drying, etc, Composition data for those MSW were listed (see Tables 1, 2 and 3): When analyzing the composition of the MSW, the system shown (see Figure I) was adopted, The system including sampling operation for aged wastes was not only used to analyze the sample composition, but also used for economic data investigation,

WTE process for fresh MSW
The common WTE process for fresh MSW is incineration accompanied with power generation or heating supply. But as the main energy from MSW comes from plastics (see Figure 2), it may be more energy effective to recycle those plastics than to incinerate them due to fossil CO 2 emission and dioxins formation involved in their incineration process [I]. Table 4): From Table 4 it can be seen that most of the plastics in the fresh MSW are soft PE & PP plastic film; plastics bottles of PET material are the main objective for informal recycling, so its content is low. In juice & milk package plastics is filmed together with paper and aluminum, this kind of plastics is hard to be recycled. The part that can be easily recycled in the separation system showed in Figure 1 is the soft plastics. And their recycling rate is around 80% [2]. If the plastics content in fresh MSW is around 22% by dry basis (see Table  J

WTE process for aged MSW
Although the plastics in the aged MSW are easier to be separated in system shown in Figure I than those in fresh M SW, the quality is not good enough to be recycled [3). And to recycle all of the combustibles including plastics to produce RDF is the better choice for carrying out WTE process [4).
RDF process included separation of combustibles, producing RDF with additives and RDF transportation and combustion in fresh MSW incineration plants (see Figure 1). When fresh and aged MSW treatment consider together, the comprehensive WTE process can be shown in Figure 3. Compared to other WTE process, application of system in Figure 3

SYSTEM EVALUATIONS
There should be some evaluation tools to judge whether the WTE process is beneficial environmentally and economically, two useful tools that can be adopted for this purpose arc life cycle assessment (LCA) and cost benefit analysis (CSA). LCA is a holistic approach that takes account every necessary factor from "cradle to grave". When used to evaluate WTE process it covers all main activities related to the energy recovering system at the same time and translates the information into resource consumptions and potential environmental impacts. In CSA method the planned project and the baseline scenario arc compared when the project costs including the cost of investment and operation, the project income including direct and indirect income are considered. If the net present value (NPY) is positive, then the project is acceptable from economy. However, to carry out those LC A and CSA evaluations, the basic database should be built up. And LCA and CSA evaluations can be applied to the overall process as well as the singles steps with view to optimize the step. In this study we have collected the necessary data based on characterizing investigation process and pilot

Life cycle assessment of RDF process from aged wastes
Here LCA analysis is carried out to evaluate RDF utilization system with help of the newly developed program EASEWASTE (Environmental Assessment of Solid Waste Systems and Technologies), Framework and structure of EASEWASTE was described in detail by [5], Table 5 presents the important categories related to waste management technology and the nonnalization reference used to convert the individual potential impact categories into person equivalents (PE), which is an average value for the yearly contribution to that impact category by all the activities and consumptions of one person in Europe and can be replaced by those data from the country or the region being considered,  Figure 4 gives the calculated potential impacts of different categories connected with RDF production, RDF incineration in person equivalents (PE). The positive value in PE means the harmful impacts to the environment; while negative values mean saving the environmental impacts. It can be seen that RDF production process causes hannful impacts to environment due to energy and material Consumption, especially for category of human toxicity via soil (HTs). But due to the power generated from RDF utilization systems, their potential impacts to the environment are generally negative if transportation considered. To keep negative PE data, RDF transportation should be less than 29.2 km [4], this can be easily controlled if used in the same city.

Cost benefits analysis of plastics recycling from fresh MSW
In the planned project, the plastics were separated from fresh MSW by the mechanical separation plant shown (see Figure 1 )

Project costs
The project costs include the cost of the separation plants (separation equipments as well as the pollution control equipments), the imputed value of the land. Table 6 shows the project costs when recycling plastics from fresh MSW incineration plant.
, I

Present benefits of the projects
The economical benefits of the project include I) the recycled plastics material; 2) the pollutants reduced. Table 7 shows the benefits resulted from plastics in fresh and old MSW respectively. For fresh MSW it is assumed that tonnage recycling rate of plastics is 38.88 kg plastics of PE & PP material, The indirect benefit (sec Table 7) includes environmental benefits such as global warming potential reduction because of recycling plastics as raw material instead of incineration. For per ton of CO2 emission, the cost is US$ 6.9; and the mass content of mineral carbon in plastics is around 65%; however, when the plastics recycled, auxiliary fuel such as RDF may need, therefore the reduction of CO 2 emission will be weakened. Other indirect benefit includes reduced pollution due to crude oil production, where the plastic raw material comes from; and other valuable materials recycled at the same time. Generally the indirect benefit is presumed to be 25% of the direct benefit.

Calculation of net present value
The main index in CBA is net present value (NPV), it is defined as: Where Bt and Ct are the benefit and cost at the t-th year. R is discount rate, and t is the calculation year. If N PV>0, the project is acceptable. If the project lasts for 15 years, and the discount rate is 12°/4,. The yearly cost, benefit and present value of benefits of the projects are shown (sec Table 8).
From Table 8 it can be seen that investment cost of plastics recovery is the most expensive part of cost; yet NPV of plastics recovery has the positive value from the second year till the last service year. The total positive NPV suggested that the recycle project deserves to set up.