Pedros, Philip

Dr. Pedros is a Vice President and Principal Process Engineer at Mott MacDonald. He has over twenty-seven years of experience as a process engineer with eight patents and experience in process design and operation of biological nutrient removal treatment plants. He has a thermodynamcis and combustion background and is interested in the thermochemical processing of sewage biosolids, the thermal destruction of nitrous oxide emissions from BNR processes and convection in porous media filters for low temperature nitrification.

Titles from this speaker

Intensification of Pyrolysis by Autothermal Operation

Abstract

INTRODUCTION Pyrolysis was previously considered by the wastewater industry and is now experiencing a renewed interest. The reasons are: a) mounting evidence that PFAS are removed from the resulting biochar, b) the volume of the final material may be reduced by 90% and c) pyrolysis systems are easier to permit than incinerators. Pyrolysis is subdivided based on the primary product which is a function of heating rate and operating temperature. Slow pyrolysis favors the production of a solid fuel, such as charcoal, and occurs at lower heating rates and temperatures from 570 - 750 F. Fast pyrolysis involves high heating rates and temperatures of 900 F or higher and is often followed by rapid cooling to produce the oils from the condensable vapors. The conventional definition of pyrolysis is the thermal decomposition of solid organic substances, in the absence of oxygen, to produce char, non-condensable gases and condensable vapors, which when allowed to condense become oil, referred to as either py-oil or bio-oil. This definition is reasonable, since the primary product of fast pyrolysis is the bio-oil and any oxygen within the system would react with the vapors produced and thus decrease the bio-oil yield (Brown, 2021). A more accurate definition of pyrolysis is the 'thermochemical decomposition of biomass into a range of useful products, either in the total absence of oxidizing agents or with a limited supply that does not permit gasification to an appreciable extent.' (Basu, 2008) Fast pyrolysis of sewage sludge biosolids (SSB) is designed to remove PFAS from the resulting char and to reduce the volume of the SSB by 85 - 90% while minimizing external energy required to dry the material from 25 - 30% solids to the requisite 80 - 90% solids for pyrolysis. requirements. This is achieved by burning the non-condensable and condensable gaseous fuels in a thermal oxidizer and recovering the thermal energy (heat) for drying. See Figure 1. INTENSIFICATION OF PYROLSYIS FOR LARGE SCALE APPLICATIONs The reactions of pyrolysis are endothermic and thus require an input of energy. In the absence of oxygen, this energy demand is met by the transfer of external thermal energy (i.e. heat transfer) via either the reactor surface (auger reactor) or the media (fluidized bed reactor). However, providing this thermal energy at typical pyrolysis temperatures of approximately 930 F (500 C) can be challenging, especially as the reactor becomes large. Hot gases can be employed for this purpose using tubular heat exchangers, but the low specific heat of gases requires large volumetric flows to meet the thermal energy requirement. Granular media is more suitable for direct heating due to their refractory properties and high heat capacity. However, both heat carriers require ancillary equipment to heat and circulate them. As the reactor becomes larger the energy required for pyrolysis, which is a function of reactor volume, increases faster than the surface area for heat transfer, which makes heat transfer rate limiting in allothermal pyrolyzers. Under these conditions, assuming a fixed diameter to length ratio, the reactor size scales to the square of the diameter. The cost of the reactor also increases approximately with the square of the diameter cost increases are roughly linear with no economy of scale. This is the reason that the currently available pyrolyzers, for the treatment of SSB are restricted to relatively small quantities of sludge and therefore small wastewater treatment plants. Allothermal pyrolysis is not economically feasible to scale to large applications; however, autothermal pyrolysis is. Autothermal pyrolysis refers to the addition of a small amount of oxygen to partially oxidize some of the vapor products of pyrolysis to provide the energy for pyrolysis within the reactor. The amount of energy required to provide the energy for pyrolysis is a function of the feedstock and may be relatively small (7 - 10% of the energy content of the feedstock). The exothermic reactions of the oxidation of these vapors meets the energy demand of the endothermic reactions and the parasitic load of maintaining reactor temperature. The chemical reactions control the process and the capacity scales as the cube of the reactor diameter. Therefore, autothermal pyrolysis would provide a several fold increase in capacity over conventional pyrolysis in the same size reactor. APPARATUS AND RESULTS A schematic of the bubbling fluidized bed pyrolizer, shown in Figure 1, can operate as either a conventional or autothermal pyrolyzer. In this research the quality and quantity of the bio-oil produced as the throughput increased was evaluated. However, this does not have the same significance in the wastewater industry since the desired product is the thermal energy, recovered from the combustion of both the condensable and non-condensable gases in a thermal oxidizer, for drying the dewatered biosolids. The details for the bio-oil production and characteristics are discussed in detail in the paper by Polin et al. 2019. Operating in allothermal (conventional) mode, the pyrolyzer throughput was 253 lb/day (4.78 kg/hr). For autothermal operation, 11.4% of the stoichiometric oxygen requirement was introduced. The result was an increase in throughput to 814 lb/day (15.4 kg/hr), or 3.2-folds increase. Figure 2 shows the effect on throughout as the reactor diameter increases. Autothermal pyrolysis increases the throughput of a given reactor by the cube of the diameter (D3). Although there was a shift in the constituent products, the specific energy yield remained constant which indicates that the energy available for drying remains the same for either mode of operation. The second significant advantage of autothermal pyrolysis is that it provides a reduction in scale reducing capital costs, whereas allothermal pyrolysis depend on economies of scale. For autothermal pyrolysis capital costs of the reactor are a fraction of allothermal pyolyzers at the same throughput as seen in Figure 3. CONCLUSIONS AND RECOMMENDATIONS Autothermal pyrolysis is currently being demonstrated at scales up to 50 tons/day using feedstocks such as corn stover and red oak. The benefits of autothermal pyrolysis can be scaled for application at large wastewater treatment plants. The next step is a demonstration scale facility operating on one hundred percent sewage sludge biosolids at a large wastewater facility.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerPedros, Philip

Presentation time

14:30:00

15:00:00

Session time

13:30:00

15:00:00

SessionCarbonization

Session number19

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicAdvanced Thermal Conversion, Intensification, pyrolysis

Author(s)

Pedros, Philip

Author(s)P. Pedros1, T. Daugaard2, S. McKelvey3

Author affiliation(s)Mott MacDonald 1; Iowa State University 2; Philadelphia Water Department 3;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159419

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count7

Description: Reduction of Nitrous Oxide Emissions from Biological Nutrient Removal Processes by...

Reduction of Nitrous Oxide Emissions from Biological Nutrient Removal Processes by Thermal Decomposition

Abstract

PURPOSE The purpose of this research is to demonstrate the concept of thermal destruction of nitrous oxide as a technology that fits into the current goal of resource recovery and utilization at wastewater treatment plants while simultaneously providing increased flexibility with which to operate a given treatment process. This paper will present the computer simulated results indicating the thermal decomposition of nitrous oxide emissions from a biological nutrient removal (BNR) process. INTRODUCTION Two separate sources of gaseous emissions at wastewater treatment plants are those from the aeration basins for the biological treatment and from the combustion processes within the facility. The combustion processes may include one or more of the following: boilers, operating on fossil fuel or natural gas, combined heat and power (CHP) units, operating on digester biogas or a sludge incinerator. The objective of this research is to link the gaseous emissions of the biological processes to the air intake of the existing combustion processes with the net effect of reducing the total emissions of harmful gases from the facility. The emission of nitrous oxide (N2O) from wastewater treatment plants is of concern due to its impact as a greenhouse gas [i.e 298 times that of carbon dioxide (CO2)] and is a consideration in the ongoing research regarding the nitrite shunt and Anammox processes. By using the emissions from the biological reactors as the inlet gas stream to the combustion process the nitrous oxide is removed by thermal decomposition; see Figure 1. Thermal decomposition of N2O occurs at approximately 565 ºF (296 ºC), well below the combustion temperature, as N2O splits to nitrogen (N2) and oxygen (O). The oxygen is consumed as an oxidant. Thermal decomposition of nitrous oxide has been implemented to reduce the unavoidable N2O emissions from the manufacture of adipic acid for the nylon industry (Reimer et al. 1994 and Shimizu, Tanaka and Fujimori 2000). In addition to the reduction of nitrous oxide, any hydrogen sulfide (H2S), methane (CH4) or VOCs that may be emitted is also removed. A secondary benefit of using the gaseous emissions from the biological processes as the inlet to the combustion process is that the CO2 acts as a thermal sink thereby reducing the thermal NOx emissions produced by the combustion process. This technology removes any constraints on how the biological process may be operated because it eliminates the need to consider the N2O emissions. For example, the goal of maximum pollutant removal, lowest energy consumption or resource recovery may be pursued without concern for the gaseous emissions that result. For this study, gaseous emissions from the aerobic portion of a BNR process treating a flow of 30 million gallons per day (MGD) with influent parameters of biochemical oxygen demand (BOD5) = 250 mg/l and NH3-N = 35 mg/l were estimated. Six cases, for which, the nitrous oxide emissions were estimated as a percentage of the influent ammonia value were simulated. The values of 0.2, 0.5, 1, 2, 5, and 10% were chosen based on published literature (Kampshreur, et al. 2009). Methane, hydrogen sulfide and VOCs emissions were also estimated. Estimates of methane emissions were based on data from a municipal plant treating wastewater at a rate of 1 MGD that showed the average annual emission rate from the aeration tanks was 2.2 x 105 g-CH4/year (Czepiel, Crill and Harriss, 1993). Hydrogen sulfide emission estimates were based on data collected from three municipal treatment plants (Devai and DeLaune, 1999). The VOC emissions were estimated based on the EPA 40-City Study, (1982) and the Joint Emissions Inventory Program (JEIP), (1993). Computer simulations using STANJAN© to run chemical equilibrium calculations were conducted (Reynolds 1986). Each of the six cases (i.e. the gaseous emissions with different concentrations of N2O) was run at different fuel to air ratios (equivalence ratios). The primary objective of the study was to determine the fate of the nitrous oxide emissions from the biological process. The secondary objective was to determine the effect of carbon dioxide (CO2) and nitrogen gas (N2) on the production of thermal NOx (i.e. NO and NO2) formed in the combustion process. The formation of nitrogen oxides (NOx) primarily nitric oxide (NO), by the oxidation of atmospheric nitrogen is a function of flame temperature and is termed thermal NOx (Turns 1996 and Warnatz, Maas and Dibble 2001). The final objective was to determine the fate of several other gaseous constituents that may be present: methane (CH4), hydrogen sulfide (H2S) and ten of the most common VOCs found in municipal treatment plants. RESULTS In all six cases nearly complete thermal decomposition of N2O was achieved. Equilibrium conditions were assumed given the adiabatic flame temperature of methane is approximately 2000 K. In Figure 2 the influent and effluent mass flow rates of N2O, for three of the gas streams, (those with 0.1, 0.2 and 0.5% of NH3-N as N2O) are plotted against the equivalence ratio. The effluent mass flow rate (shown in red) is negligible indicating significant thermal destruction of nitrous oxide. The results are similar for the higher concentrations, not shown. In Figure 3 thermal NOx produced, for each of the individual gases, the complete gas stream and the standard gas stream (air) is plotted against equivalence ratio. The results indicate that combustion with the gas stream from the biological process, (i.e. complete gas stream) reduces the thermal NOx emissions by 18.4% over combustion of methane with air (i.e. standard gas stream). The reason for this is that the carbon dioxide (CO2) produced from the biochemical reactions acts as a thermal sink. This is analogous to exhaust gas recirculation (EGR) technology used to reduce NOx emissions from internal combustion engines. Figure 3 also indicates that the nitrogen gas (N2) produced from the thermal decomposition of the N2O had no effect on NOx emissions when compared to the standard gas stream. This is due to the small quantity of N2 compared to the amount of CO2 produced by the biological reactions. Figure 3 also indicates that the equilibrium concentration of NOx is high at temperatures near stoichiometric combustion and decreases rapidly from that point (Flagan and Seinfeld 1988). Most combustion process used for power generation are operating slightly at an equivalence ratio between 0.8 -0.95, near the peak of the plot in Figure 3. CONCLUSIONS The results indicate that the thermal decomposition process reduces N2O emissions as well as methane, hydrogen sulfide, and VOC emissions (results, not shown here). A secondary benefit is the reduction in thermal NOx emission from the combustion process. This process mitigates greenhouse gas emissions without significant impact on the existing WWTPs. It also provides more flexibility in the process decisions by removing the constraints that results when considering the gaseous emissions that result from the biological process.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerPedros, Philip

Presentation time

09:00:00

09:30:00

Session time

08:30:00

11:45:00

SessionGreenhouse Gas Emissions at Wastewater Treatment Plants

Session number11

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicAdvanced Thermal Conversion, incineration, Nitrous oxide