A TECHNICAL EVALUATION OF MODULAR INCINERATORS WITH HEAT RECOVERY

Transcription

1 A TECHNICAL EVALUATION OF MODULAR INCINERATORS WITH HEAT RECOVERY RICHARD E. FROUNFELKER and NED J. KLEINHENZ Systems Technology Corporation (SYSTECH) Xenia, Ohio ABSTRACT The use of small modular incinerator systems for solid waste disposal and energy recovery in municipal and industrial applications is studied. The technical evaluation presented herein is based on the results of 3 oneweek field tests at each of two sites. Test data was used to calculate the following for each system: a mass balance, an energy balance, the heat recovery efficiency, the environmental effects, and the overall effectiveness of the system as a solid waste disposal facility. As a result of a nationwide survey, two facilities which best satisfied selection criteria were chosen. These selection criteria included that the sites consist of modular incinerators with heat recovery having a capacity of 50 tons (45 t) or less per day and that the systems be representative of current technology, designs, and operational procedures. The two facilities selected for this study included a municipal incinerator plant with a Consumat system in North Little Rock, Arkansas, and an industrial incinerator facility with a Kelley system in the plant of the Truck Axle Division of the Rockwell International Corporation in Marysville, Ohio. Since the two systems differ in many respects and operate on dissimilar waste streams, direct comparisons of the two systems are not included. modular incineration systems sponsored by the United States Environmental Protection Agency (EPA*).1 The overall objective of the evaluation was to determine the feasibility of modular incinerator usage for solid waste disposal and heat recovery in municipal and industrial environments. The two units evaluated were the North Little Rock, Arkansas, municipal, 100 tons/day (91 tid) facility and an industrial, 12 tons/day (10.9 t/ d) facility in Marysville, Ohio. Each facility was monitored and tested over three Iweek periods within a 6month span. The industrial facility was tested in April, July, and August The municipal facility was tested in March, May, and October The individual waste and residue streams were sampled and analyzed. The energy inputs and outputs were recorded. The systems' emissions to atmosphere, water, and land were sampled and analyzed. The test data was used to calculate: 1. system mass and energy balances; 2. combustion, heat recovery, and overall system efficiencies; and 3. criteria pollutant e'mission factors. One weeklong test considered to be most representative of each facility is reported in this paper. The test period during July 1978 was chosen for the industrial facility because of the more uniform level of boiler loading and consistent energy de 'This project has been financed with Federal Funds from INTRODUCTION This paper presents the results of a technical, environmental, and economic evaluation of small the Environmental Protection Agency under Contract No from the Office of Solid Waste, Washington, D.C. The contents do not necessarily reflect the views and policies of the Environmental Protection Agency. 73

2 mand. The test period during October 1978 was chosen for the municipal system because of system performance and completion of equipment modifications. fraction was combustible; percent of the total residue was combustible; and 4. the residue's average heating value was 940 Btu/lb (3.2 MJ /kg) with 2 and 6.7 percent of the residue being fixed carbon and volatile rna tter, respectively. MUNICIPAL SYSTEM WASTE AND RESIDUE CHARACTERIZATION The waste stream was sampled and categorized daily. The waste compositions were determined by hand sorting at North Little Rock, and the results are shown in Table 1. At the municipal facility the categories of paper and plastics comprise 62 percent by weight of the waste on an asreceived basis and 87 percent of the total energy available in the solid waste on a dry weight basis (see Table 1). The moisture on an asreceived basis was 22 percent. The heating values for the refuse samples averaged 6151 Btu/lb (14.3 MJ/kg) on a dry basis and 4777 Btu/lb (11.1 MJ/kg) on an asreceived basis. The residue was sampled hourly and composited into daily samples. The following residue characteristics on a dry basis were determined: percent of the residue passed a Iin. (25 mm) sieve; percent of this less than Iin. (25 mm) SYSTEM BALANCES The system mass and energy balances were calculated and are shown in Figs. 1 and 2, re pectively. The mass inputs, refuse, combustion air, auxiliary gas, residue cooling water, and aspirator fan air were measured and calculated to be 4567 tons (4141 t). The mass outputs, residue, and stack flue gas were measured to be 4630 tons (4198 t). The difference, namely 63 tons (57 t) or about 2 percent of the output, was not accounted for. For the energy balance, the inputs were refuse, electricity, and auxiliary fuel, while the outputs were steam, sensible he t and remaining energy in the residue, heat lost by radiation and convection, and the sensible heat in the flue gases. The inputs totaled 2179 million Btu (2297 GJ), and the outputs totaled 2229 million Btu (2352 GJ) for a difference of 50 million Btu (53 GJ) or 2 percent of the output. TABLE 1 NORTH LITTLE ROCK DAILY REFUSE COMPOSITION FOR OCTOBER 1978 Daily Percentage Standard Total Weekly Wt Deviation (kg) (lb) (s) Category 10/6 10/9 10/10 10/11 10/12 Avg Food waste ,880 30,600 Garden waste ,307 11,700 Paper waste , ,960 Plastics ,774 39,150 Textiles ,878 8,550 Wood ,837 4,050 Ferrous ,150 Aluminum ,123 13,500 Glass ,738 36,900 Inert ,350 Fines ,798 21, , ,450 74

3 Mass balance 118.5hour test* Input Output Mg per Mg refuse % of Total Mg per Mg refuse % of Total Source or or Ton per ton refuse Ton per ton refuse Refuse Natural gas Residue, cooling water Aspirator air Blower air Residue, wet Flue gases Total * Total refuse input204 Mg (225 tons). t DUMP STACK FLUE GAS t BOILER STACK FLUE GAS BOILER GAS A.B. ASPIRATOR BLOWER PRIMARY REFUSE l. RES IDUE '. WATER PIT FIG.l MASS BALANCE FOR INCINERATIONHEAT RECOVERY PROCESSES IN NORTH LITTLE ROCK FACI L1TY DURING THE 118.5HR OCTOBER FIELD TEST 75

4 Energy balance l18.5hour test* Input Output GJ per (_tu per) % of Total GJ per (MMBtu pej % of Total Mg of ton of Mg of ton of Source refuse refuse refuse refuse Refuse (9.56 ) Electricity.09 (.08 ) 0.83 Natural gas.052 (.044) 0.46 Unburned combustibles.661 (.569) 5.74 Steam Flue gases 4.30 Radiation and 5.99 (5.15 ) (3.70 ) convection 0.56 (0.48 ) 4.85 Total (9.684) (9.909) * Total refuse input 204 Mg (225 tons) t DUMP STACK FLUE GAS I RiC LOSSES t I BOILER STACK FLUE GAS I GAS_ A.B. BOILER STEAM ELECTRICITY REFUSE.. PRIMARY l UNBURNED COMBUSTIBLES.. FIG.2 ENERGY BALANCE FOR INCINERATIONHEAT RECOVERY PROCESSES IN NORTH LITTLE ROCK FACILITY DURING THE 118.5HR OCTOBER FIELD TEST 76

5 PERFORMANCE As a solid waste disposal facility, the system performance was as follows: 1. for each 25 tons/ day (22 tid) module the charging rate averaged 1900 lb/hr (860 kg/h); and 2. refuse reduction through combustion was 55 percent by weight and 95 percent by volume based on the ratio of wet residue to asreceived refuse and 70 percent weight reduction on a dry residue basis. As an energy recovery system, the facility was divided into two subsystems, the combustion chamber and the heat recovery unit. The efficiencies of each and of the system as a whole was calculated. Efficiency of Incinerator System The efficiency of the incinerator system is that percentage of the total energy in the refuse and combustion air which, through the combustion process, was made available to the heat recovery system. Using the heat balance method, the efficiency was computed by: where E = boiler effectiveness temperature of flue gas entering the boiler = temperature of flue gas exiting the boiler = temperature of feedwater entering the boiler During the third field test, the Th,Th and 1 1, Tc temperatures averaged 1800, 450, and 280 F 1 (982, 232, and 138 C), respectively. Consequently, the boiler effectiveness was 89 percent. Thermal Efficiency of Boiler While the effectiveness is based on the theoretical maximum performance of the boiler, the actual operational efficiency was calculated as follows: 1)1 1 Or + Qw + Qrc Qt 1)B heat transferred to steam sensible heat of gases into boiler where 1)1 Qw Qrc Qt incinerator efficiency = sensible heat and remaining energy in the residue because of unburned combustibles latent heat of vaporization of moisture in the refuse plus hydrogen combustion heat lost by radiation and convection up to the boiler section total input energy The substitution of the energy balance values in the above equation yields an incinerator efficiency of 76 percent. Effectiveness of Boiler The effectiveness of the boiler is the ratio of the actual heat transfer rate in the exchanger to the thermodynamically limited maximum possible heat transfer rate (which could be realized only in a counterflow heat exchanger of infmite heat transfer area). This can be expressed as follows: or where M w (h2 hi) 1)B Mg C p (TI T2) M w mass of water entering h2 hi Mg C p TI T2 enthalpy of water leaving enthalpy of water entering mass of flue gases entering mean specific heat of flue gases temperature of flue gases entering temperature of flue gases leaving The calculations determined the heat exchanger efficiency to be 71 percent. System Energy Recovery Efficiency The system energy recovery efficiency is that efficiency defmed by ASME PTC 4.1. In this test it is represented by the output steam energy of the boiler divided by the input refuse energy to the incinerator. During the evaluated test period, steam 77

6 was generated during 95 percent of the test time. Consequently, to represent the optimum system performance, the input energy should be adjusted to 95 percent of its total value. Accordingly, the efficiency was calculated as follows: T/ steam energy = 0.56 input energy (0.95) Therefore, the system efficiency during the October field test was 56 percent. The manufacturer designed each module to burn 2000 lb/hr of 4500 Btu/lb (906 kg/h of 10.5 MJ/kg) waste. Hot gases from two modules were combined to one heat recovery unit which was designed to produce 10,000 Ib (4530 kg) of 150 psi (1.0 MPa) saturated steam per hour. Computations based on these numbers indicate a designed system efficiency of about 56 percent. SYSTEM EMISSIONS The stack emissions were monitored for primary gas species and particulates along with the gaseous SOx, NOx, cr, F1, and hydrocarbons. The criteria pollutant emissions are given in Table 2 in terms of pounds per ton charged. Under a program sponsored by the State of California Solid Waste Management Board, the emissions were analyzed for condensibles and specific elements. Five tests for total particulates, including the wetcatch particulates, averaged grjdscf (0.076 gjm3) with a maximum of grjdscf (0.096 g/m3) and a minimum of gr/dscf (0.052 g/m3) when corrected to 12 percent CO2; several Method 5 filter catch particulates were composited, and an atomic absorption analysis was run with the results shown in Table 3. Dry catch particulate averaged gr /DSCF (0.057 g/m3) at 12 percent CO2, The unit was designed to meet a stateapproved particulate standard of 0.2 gr/dscf (0.088 g/m3) corrected to 12 percent CO2, A cascade impactor sizing analysis of the particulate showed 90 percent by weight less than 7 11m. The concentrations of chlorine and fluorine averaged 130 ppm and 2.0 ppm, respectively. INDUSTRIAL SYSTEM WASTE AND RESIDUE CHARACTERIZATION The weight and composition of the solid waste burned were monitored. The total weights burned during the singleweek tests in April, July, and August were 28,9941b, 56,552Ib, and 28,481 Ib (13.2 t, 25.7 t, and 12.9 t), respectively. Table 4 TABLE 2 NORTH LITTLE ROCK POLLUTANT EMISSION RATES FOR OCTOBER 1978 TEST Emission rate Pollutant Maximum Average Minim1lm lb/ton refuse chargedt (kg/mg) Particulate 0.231* gr/scf 0.130* gr/scf 0.067* gr/scf so x <10 ppm < NO x 99 ppm 82 ppm 69 ppm CO 36 ppm 29 ppm 16 ppm HC 40 ppm 28 ppm 20 ppm Pb 4.49 mg/m * t Corrected to 12 percent CO2 Based on an average flow of 15,198 DSCFM including aspirator air and a feed rate of 1.9 TPH. 78

7 TABLE 3 NORTH LITTLE ROCK SUMMARY OF ELEMENTS DETECTED IN STACK EMISSION FILTERS Emission rate Element Concentration in gas (j.lg/m3)* Emission factor g/mg of refuset Silicon Iron Aluminum Sodium Calcium Potassium Magnesium Lead Zinc Cadmium Beryllium Titanium Chromium Copper Nickel Manganese Lithium Antimony Boron Tin Vanadium , ,280. 9, * Concentrations based on a composite of six filters from October test period. t g/mg: 500 = lb/ton. 79

8 TABLE 4 MARYSVILLE MEASURED REFUSE COMPOSITION AND ENERGY CONTENT FOR AUGUST 1978 TESTS Category Percent of total by weight Measured heating value MJ/kg (Btu/hr) Wood 65.7 Paper (7,758) 17.9 (7,693) Plastics <.1 Paint Grease <.1 Inert.1 Textiles <.1 Rubber <.1 shows the average composition and heat content of the waste burned. Over the three tests weeks, 65 percent of the waste was wood having a moisture content of 9 percent, while 33 percent was paper with a moisture content of 8 percent. During August the heating value on an asreceived basis was 7758 Btu/lb (18.1 MJ/kg) for the wood and 7693 Btu/lb (17.9 MJ /kg) for the paper. The residue was removed from the incinerator, weighed, and sampled. The combustible content of the residue on a dry basis ranged from 010 percent and averaged 3 percent. The volatile matter ranged from 16 percent, and the fixed carbon ranged from 010 percent for the residue. SYSTEM BALANCES The system mass and energy balances were computed and are shown in Figs. 3 and 4. The combustion air input could not be measured and had to be computed from the refuse input and flue gas output. The energy inputs totaled 413 million Btu (436 GJ), and the outputs totaled 381 million Btu (402 GJ). The difference, 8 percent of the energy, was no t accoun ted for. PERFORMANCE As a solid waste disposal facility, the system performance was as follows: 1. a charging rate average of 600 Ib/hr (272 kg/h) as operated; and 2. a refuse reduction by weight through combustion of 95 percent on a dry basis. As an energy system, the facility was divided into a combustion unit and an energy recovery unit. Using formulas described in previous sections, the efficiency of the incinerator system was found to be 75 percent. The energy recovered as measured by a Btu integrator was 142 million Btu (150 GJ) and was calculated to be 173 million Btu (182 GJ) by the heat loss method using the values in Fig. 4. The actual value probably lies somewhere in between. The effectiveness of the boiler, with Th,T h,and Tc temperatures averaging 1400, , and 160 F (760,135, and 71 C), respectively, was 90 percent. The boiler thermal efficiency was found to be 80 percent. SYSTEM ENERGY EFFICIENCY The heat recovery energy efficiency was calculated by the following equation for the heat loss method: heat losses T} = 1 total heat input From the energy balance in Fig. 4, the heat losses were 233 million Btu (246 GJ), and the total input was 413 million Btu (436 GJ). Consequen tty, 80

9 Mass balance l20hour test* Input Output Mg per Mg refuse % of Total Mg per Mg refuse % of Total Source or or Ton per ton refuse Ton per ton refuse Refuse Cooling spray water Natural gas Combustion air Flue gases Residue, dry Total * Total refuse input 25.6 Mg (28.3 tons). Flue Gas Air Gas. THERMAL REACTOR ""\ Water PRIMARY BOILER Refuse Residue FIG. 3 MASS BALANCE FOR INCINERATIONHEAT RECOVERY PROCESSES IN MARYSVILLE FACILITY DURING THE 120HR l75.5hr HEAT RECOVERY) JULY FIELD TEST 81

10 Energy balance 120hour test* GJ per Mg of Input ('MMBtu per) ton of % of Total GJ per Mg of Output \MMBtu per) % ton of of Total Source refuse refuse refuse refuse Refuse Electricity (14.07) ( 0.10) Natural gas Heat recovery 0.29 ( 0.25) 1.7 Burndown Residue 0.21 ( 0.18) ) 0.2 Radiation and Convection Heat recovery Burndown Flue gases ( 1. 94) 14.4 ( 0.71) 5.3 Heat recovery Burndown ( 3.60) 26.7 ( 1.95) 14.5 Hot watert (measured) 6.09 ( 5.24) (14.60) (13.47) * Total refuse input 25.6 Mg (28.3 tons). t Hot water output by difference 7.41 GJ/Mg refuse (6.37 MMbtu/ton). t R/C Losses I t Flue Gas I Gas THERMAL REACTOR E1ectricity Refuse PRIMARY Residue BOILER Hot Water FIG. 4 ENERGY BALANCE FOR INCINERATIONHEAT RECOVERY PROCESSES IN MARYSVILLE DURING THE 120HR (75.5HR HEAT RECOVERY) JULY FIELD TEST 82

11 the asoperated heat recovery efficiency was 44 percent for the July test week. For the period that the system was being used for hot water recovery (75 hr), the energy inputs were: 1. gas, 7 million Btu (7.5 GJ); 2. electricity, 3 million Btu (3 GJ); and 3. refuse, 343 million Btu (362 GJ); and the losses were: 1. flue gases, 102 million Btu (107 GJ); 2. radiation and convection losses, 55 million Btu (58 GJ); and residue, 1 million (1 GJ). Substituting these values in the heat loss equation yields a potential thermal efficiency of 55 percent. The manufacturer designed the unit for 65 percent system efficiency when burning 1200 Ib/hr of 8500 Btu/lb (543 kg/h of 19.7 MJ/kg) waste. Use of the manufacturer's equation for derating the unit to lower feed rates and Btu values yielded a predicted heat recovery efficiency of 48 percent. SYSTEM EMISSIONS The criteria pollutants emissions are given in Table 5 as pounds per ton charged. The particulate loading averaged gr/scf (0.021 g/m3) at 12 percent CO2, Cascade impactor sizing analysis of the particulate showed 85 percent by weight less than 7 11m. The concentrations of chlorides averaged 54 ppm, and fluorides averaged 1.1 ppm. Several filter catch particulates were combined for atomic absorption analyses of the particulates. The results are presented in Table 6. TABLE 5 MARYSVILLE EMISSION RATES FOR JULY 1978 TEST Pollutant Maximum Emission rate 1b/ton refuse Average Minimum charged (kg/mg) Particulate gr/scf* gr/scf* gr/scf* so x 31 ppm 8 ppm <5 ppm NO x 125 ppm 30 ppm 6 ppm co <1000 ppm 240 ppm 17 ppm HC 2285 ppm 765 ppm 21 ppm Lead 624 ].lg/m * Corrected to 12 percent CO2, 83

12 TABLE 6 MARYSVILLE SUMMARY OF ELEMENTS DETECTED IN STACK EMISSION FILTERS Emission rate Concentration in gas Emission factor Element (jjg/m 3 )* g/mg of refuset Silicon Iron Aluminum 2, Sodium 13, Calcium 1, Potassium Magnesium Lead Zinc 2, Cadmium Beryllium Titanium Chromium Copper Nickel Manganese Lithium Antimony Boron Tin Vanadium * Concentrations based on a composite of five filters from July test period. t g/mg = lb/ton. SUMMARY Both of the small modular incinerator systems were found to be an effective means of reducing solid waste volumes and of recovering 4256 percent of the heat released in the process. The particulate emissions of the municipal incinerator with automatic feed control and continuous ash removal are within the local standards. The particulate emissions contained lead and many other trace metals. The industrial unit was subject to operational cycling due to sporadic loading. A uniform feed rate closer to the unit's designed capacity would increase the heat recovery efficiency from the measured 44 percent to an estimated 65 percent. Key Words Emission Energy Incineration Refuse Waste Heat 84