Prediction of recovery energy from ultimate analysis of waste generation in Depok City, Indonesia

Refuse derived fuel (RDF) is an environmentally friendly renewable fuel developed to reduce waste generation. RDF can consist of various kinds of waste such as paper and gardens. One of the critical parameters is the chemical element and calorific value. The purpose of this study was to determine the potential for waste reduction and the relationship of ultimate longevity in RDF to the calorific value. This study's paper and garden waste mixture were P0 (100% paper), P25 (75% paper and 25% garden), P50 (50% paper and 50% garden), P75 (25% paper and 75% garden), and P100 (100% garden). The calorific value of the mixture can reach 3.6-5.2 kWh/kg. Simultaneously the relationship of ultimate elements nitrogen (N), hydrogen (H), oxygen (O), and ash affects the heating value of RDF. Sampling the application in Depok City can reduce waste by 6.67%, with the potential for electrical energy from paper and garden wastes of 358,903.8 kWh and 48,681 kWh, respectively. This shows that this energy waste can supply 0.1% of the total daily electricity demand in Depok City.


METHOD
The first step in making this pellet is the preparation of the materials needed. The raw materials prepared as raw materials for RDF pellets are garden waste (wood twigs and dry leaves) and paper waste Figure 1. The wooden twigs are pre-cut about 10 cm to make them easier to put into the chopper. Next is the process of mixing paper and garden waste with an interval ratio of 25%, namely 100%:0% (100% paper waste), 75%:25% (75% paper waste and 25% garden waste), 50%:50% (50% paper waste and 50% garden waste), 25%:75% (25% paper waste and 50% garden waste), 0%:100%, (100% garden waste) respectively. It takes the addition of 1,000 mL of water and 10% tapioca flour by 200 grams. After that, mixing is carried out until all the ingredients become homogeneous.

Raw Garden Waste
Raw Paper Waste  Calorific value analysis is carried out to describe the energy content of a material. Calculation of the calorific value is carried out using a bomb calorimeter device. Bomb calorimeter is a tool used to measure the amount of heat liberated on complete combustion of a compound, food, fuel. A number of samples are placed in an oxygenated tube immersed in a heat-absorbing medium (calorimeter), and the sample will be burned by an electric flame from a metal wire attached to the tube. The method used for the calorific value analysis is the American Standard Test Method for Gross Calorific Value D5865-11a. The calorific value test procedure is a sample in dry conditions (heating to 105 o C) as much as ±0.5 grams is placed in a cup and automatically inserted into the bomb calorimeter. The calorific value results will come out of the tool. Several samples are placed in an oxygenated tube immersed in a heat-absorbing calorimeter, and the sample will be burned by an electric flame from a metal wire attached to the tube.

RESULTS AND DISCUSSION
The ultimate analysis was carried out to determine the chemical composition of elements, including levels of C, H, O, and N. The main components that are most important in combustion are the elements carbon and hydrogen. Carbon and hydrogen are the main combustion elements and correlate with heating values [23], [24]. An exothermic reaction between carbon and hydrogen with oxygen produces CO2 and H2O during combustion. Meanwhile, the nitrogen element indicates combustion emissions because it reacts with air to become NOx [25]. Table 2 shows the ultimate test result data for each variation.
The highest carbon value was found in P0 pellets at 42.25. The addition of garden waste to the pellets can increase the percentage of carbon. The higher garden waste carbon content is 41.93%, while the paper's carbon content is lower, namely 34.96% [26]. Carbon value correlates with heating value. The higher the carbon value, the higher the heating value and the better the pellet quality. So, mixing the two waste compositions is an effective way to optimize the properties of pellet fuel.
Biomass energy with high potential is fuel in pellets from raw materials of twigs, stems, branches, leaves, and others [27], [28]. Pellets are biomass energy from the pressing or densification process using high pressure. Biomass generally consists of cellulose, hemicellulose, and lignin [29], [30]. High temperature and pressure during the molding process will affect the softening of lignin, increasing the ability to bind biomass and increasing its energy density [31], [32]. Table 3 is the result of the calorific value test for the variation of RDF made. Table 3 shows the caloric value in each variation of the pellets used. Each variation has its own calorific value.  The advantage of biomass pellets is that they produce low emissions [29], [33], [34] because biomass is carbon neutral, reduces greenhouse gas emissions, and has low ash and sulfur content. Indonesia has also started to support the development of biomass pellets to be applied in the industry as a substitute for coal (cofiring) and households as an alternative to LPG [35]. Paper is a flammable material, which indicates that paper has high energy value [36]. Paper is a material derived from wood fibers that consist of cellulose and hemicellulose [13]. This wood fiber provides a good density quality in the fuel, and the wood fiber that is burned will provide a calorific value. The presence of paper content will make the pellet structure more substantial and more durable so that paper has the potential as raw material for RDF pellets.
Independent T-Test is a comparative or different test to determine whether there is a significant difference in the mean or mean between each RDF parameter. It can be seen that all parameters have different averages, but in the parameter Nitrogen, the significance is only 0.12. This shows that Nitrogen in RDF is not too different from other variations. Table 4 shows the value of the independent T-Test on each variable. Waste processed into RDF fuel has essential water content, ash, and calorific value criteria [22], [32], [37]. The grade value can be different for each city or country because it depends on the source of the waste, such as from households, offices, buildings, and the waste collection and processing system. Mixed waste will be different from waste that has been separated from the source of the waste [38]- [41]. Based on Gendebien et al. [42], RDF technology has been used for a long time by European countries, namely England, Italy, and Finland. Europe has its RDF quality standards that must be met in the manufacture of RDF fuel. Therefore, the correlation for each ultimate value shows a different correlation Table 5. The parameters that have the most significant effect on increasing the calorific value are all variables simultaneously except for the composition carbon. The variance inflation factor (VIF) value>10.00 for the parameters nitrogen, oxygen, and ash shows the occurrence of multicollinearity in the regression model Table 6. Garden and paper waste utilization in Depok City can minimize waste generation by 6.67% Table 7. Along with public concern for the environment, the cement industry implements an eco-label program by reducing fossil fuel consumption and zero waste [43], [44]. In addition, the use of RDF can supply electrical energy of 0.1% of the total electricity demand of Depok City. The utilization of RDF is used as a fuel for coal substitution with biomass or co-firing in the cement industry [14], [45], [46]. The cement industry uses high energy because it requires large amounts of coal as fuel used for the combustion process in the rotary kiln [47]. The tendency to use high coal causes an increase in the cost of fossil fuels, so the cement industry must have a policy of using alternative fuels that can provide benefits for the efficiency of the use of coal and have benefits for the environment to reduce CO2 emissions [48].

CONCLUSION
The results of this study indicate that the ultimate analysis test results for each variation of the mixture of paper and garden waste simultaneously affect the calorific value of RDF pellets. The total potential for waste reduction with a case study of Depok City, Indonesia, can reduce waste by 6.67%. The energy recovery potential is 358,903.8 kWh for paper waste RDF and 48,681 kWh for garden waste RDF. This energy recovery potential can supply 0.1% of the total daily electricity demand in Depok City.