Greenhouse gas emissions from anaerobic digestion plants - Nguyen Thanh Phong

Journal of Science and Technology 54 (4B) (2016) 208-215 GREENHOUSE GAS EMISSIONS FROM ANAEROBIC DIGESTION PLANTS Nguyen Thanh Phong1, *, Joachim Clemens2, Carsten Cuhls3 1Faculty of Science and Technology, Hoa Sen University, 8 Nguyen Van Trang, District 1, HCM city, Vietnam 2Faculty of Agriculture, University of Bonn, Karlrobert-Kreiten Str.13, 53115 Bonn 3Ingenieurgesellschaff für Wissenstransfer, Gewitra mbH, Im Moore 45, 30167 Hannover *Email: phong.nguyenthanh@hoasen.edu.vn Received: 15 August2016; Accepted for publication: 10 November 2016 ABSTRACT This study investigated emissions of CH4, N2O and NH3 from nine anaerobic digestion plants that treat biowaste. The treatment is in form of mechanical pre-treatment, anaerobic digestion followed by a composting with or without intensive aeration. The exhaust gases from the mechanical and anaerobic steps are treated by biofilters. The emission sources at the plants consisted of biofilters, combined heat and power units (CHP), liquid digestate treatment systems (LTS) and open composting windrows of the solid digestate. Overall, the emission factors were 0.4 - 16 kg (Mg biowaste)-1 for CH4, 7 - 170 g (Mg biowaste)-1 for N2O and 41 - 6,032 g (Mg biowaste)-1 for NH3. Open composting windrows of solid digestate resulted in high emissions of CH4 and N2O. Intensive aeration of the solid digestate could reduce greenhouse gas emissions. Keywords: greenhouse gas, emissions, anaerobic digestion, windrows, organic waste, methane. 1. INTRODUCTION Anaerobic digestion for treatment of biowaste is rapidly gaining interest in developed and developing countries [1, 2]. The treatment is essentially based on the activities of microorganisms that transform organic substances into biogas [3]. Biogas is used as renewable energy source, and nutrients in the residue can be recovered in agriculture as fertilizer or soil conditioner [4]. In addition, AD of biowaste is attracting attention as an effective method to reduce Greenhouse gas (GHG) emissions according to Kyoto protocol [4]. According to the life cycle analysis (LCA), AD results in negative GHG emissions. The total greenhouse gas (GHG) emissions for AD can reduce up to one tonne CO2 equivalent/ Mg separated organic waste [5]. Actually, many studies have been conducted to show the benefits of AD treatment, for instance the works of [2, 4, 6 and 7]. However, there is still missing an overall evaluation of GHG emissions during treatment. For example, the GHG emissions associated to the pre-treatment and post-treatment of AD were often excluded in previous studies. In fact, AD plants may have fugitive emissions of CH4, N2O and NH3. The aim of the study was to investigate emission Greenhouse Gas Emissions from Anaerobic Digestion Plants 209 factors of CH4, N2O and NH3 g (Mg biowaste)-1 and to compare emission sources in the plants. Additionally, the efficiency of biofilters was taken into account. Nine operating AD plants, two wet digestion plants, four dry digestion plants and three solid digestion plants, were evaluated. 2. MATERIALS AND METHODS 2.1. Measured locations and emission determinations The emission sources at the plants consisted of biofilters, combined heat and power units (CHP), liquid digestate treatment systems (LTS) and open composting windrows of the solid digestate. The detailed inventories of the AD plants are listed in Table 1. Table 1. Processing parameters of the anaerobic digestion plants. Wet digestion Dry digestion Solid digestion TS30 % Plant AD 1 AD 2 AD 3 AD 4 AD 5 AD 6 AD 7 AD 8 AD 9 Pre-treatment yes yes yes yes yes yes yes yes yes Feeding Cont. Cont. Cont. Cont. Cont. Cont. Batch Batch Batch Temp. meso thermo thermo thermo thermo thermo meso meso meso HRT (day) - 20 15-30 15-20 15-20 21 28 21 21 Digetate Separation yes yes yes yes yes yes no no no Composting yes yes no yes yes yes yes yes yes 2.1.1. Biofilter The gas inlet and outlet of biofilter was analysed at each plant for 1 week. At capsuled biofilters the treated air left the biofilter in a chimney. Here the gases were measured (biofilters at plants 1, 4, 6 and 9). At open biofilter (at plants 2, 3, 5, 7 and 8), 16 m2 of the biofilter (4x4 m) was covered by a thin foil. Concentrations of the treated gases were measured under the foil. Continuously monitored parameters included TOC, CH4 and N2O. TOC was measured by flame ionisation detector (Bernath Atomic 3006) while CH4 and N2O were measured by an infrared gas analyser. Gas concentrations in the treated and untreated exhaust air were recorded every minute. To control the accuracy of the infrared gas analyser, exhaust gases were sampled manually by evacuated headspace vials and subsequently analysed on CH4 and N2O by GC (ECD/FID) in the laboratory. A manual discontinuous analysis was applied for NH3 measurement: NH3 was extracted from the waste gas stream by absorbing it in sulfuric acid and subsequently measured Nguyen Thanh Phong, Joachim Clemens, Carsten Cuhls 210 colorimetrically in the laboratory. NH3 samples of treated and untreated gases were collected twice. Air fluxes to the biofilter were measured by an anemometer (testo 435) or micromanometer (Müller Instruments EPM-300-BA, Germany). It was assumed that the volumes of treated and untreated air were the same. 2.1.2. Open composting windrows To measure GHG emissions from composting windrows, a tunnel covers an area of around 50 m2 with a length of 10 m and a width of 5 m. The height of the tunnel may vary from 1.5 to 2 m. Two ventilators are used to ventilate the tunnel from one side. The ventilation rate is fixed at 1000 m3 h-1. Fresh air enters the tunnel from the front. In the tunnel, gas is emitted into the fresh air and leaves the tunnel at the rear. At the outlet, a Teflon tube (4 mm in diameter) is installed 0.5 m above the windrows and used for gas sampling. The gas is pumped via a cooler to an infrared gas N2O and CH4 analyser (Uras, ABB). The infrared detector has a sensitivity of 0.1 mg/m3 for N2O and 1 mg/m3 for CH4. When the tunnel was installed, it took ten to twenty minutes for GHG concentrations to be constant. GHG concentrations were then recorded every minute for one hour. Air fluxes were determined using an anemometer (testo 435) or a micromanometer (Müller Instruments EPM-300-BA, Germany). In parallel, 60L of outgoing air were flushed through two flasks containing 40 mL of a 0.05 M H2SO4 solution. NH3 was trapped in the solution as NH4+ and subsequently analysed colorimetrically in the laboratory. 2.2. Other measuring points (e.g. CHP, receiving and pre-treatment hall, liquid digestate treatment systems (LTS)) Other emission sources were point sources with preinstalled sampling points. For one hour the TOC concentrations were recorded every minute by FID. In parallel, gas samples were taken regularly using evacuated headspace vials for CH4 and N2O. For NH3, samples were taken by absorbing it in sulfuric acid solution. Air fluxes were also determined by measuring velocity (m/s) and cross section area (m2). LTS: After anaerobic digestion, the digestate is dewatered by a second centrifuge. The solid digestate is mixed with green waste and used for composting. The liquid is treated in nitrification and denitrification tanks. CHP: CHPs consist of a combustion engine and a generator. Biogas is used to generate electricity and heat in these combustion engines. 2.3. Calculations of emissions factors for anaerobic digestion plants The emission factors of CH4, N2O and NH3 g (Mg biowaste)-1 were calculated using the aeration rates and concentrations of gases. The emission rates and emission factors for each gas were calculated using the following formula: 1000 QEEMF × = (g h-1) [8]; w MF f M EE )724( ××= g (Mg waste)-1 [8] with: E: concentration (mg x m-3), Q: air flow (m3 x h-1), EMF: emission mass flow (g × h-1), Mw: total mass of incoming waste (Mg per week), Ef: emission factor g (Mg waste)-1 The emissions were calculated in form of CO2 equivalent according to Intergovernmental Panel on Climate Change (IPCC) in 2007. N2O and CH4 are potential GHG with respective Greenhouse Gas Emissions from Anaerobic Digestion Plants 211 global warming potentials 298 and 25 times higher than that of CO2 respectively. Additionally, it was assumed that the CO2 equivalent of NH3 is 2.98 [8]. )98.229825( 3242 ×+×+×= ∑ fNHOfNfCHequivalentfCO EEEE [8] Overall GHG emissions from AD plants were calculated by the sum of emissions of CH4, N2O and NH3 from open emission sources such as biofilter, CHP, open composting windrows and liquid digestate treatment systems. Emissions from machinery and energy used in the plants were not considered in the calculations. )(1 LTSOWBFplant EEEE ++= ∑ )(2 OWBFplant EEE += ∑ )(3 CHPBFplant EEE += ∑ )(4 CHPBFplant EEE += ∑ )(5 ∑= BFplant EE )(6 ∑= BFplant EE )(7 CHPBFplant EEE += ∑ )(8 ∑= BFplant EE )(9 OWBFplant EEE += ∑ 3. RESULTS AND DISCUSSION 3.1. Emission factors of CH4, N2O and NH3 from open emission sources in AD plants The emission factors of CH4 varied from 16 to 819 g (Mg biowaste)-1 for liquid treatment system (LTS), from 50 to 1500 g (Mg biowaste)-1 for CHP, and from 0.4 to 15.4 kg (Mg biowaste)-1 for open windrows (Figure 1). Liquid digestate still contains potential to form CH4 [6]. Thus, CH4 emissions still occur in treatment systems of liquid digestate. Biogas produced at the AD plants is burned in CHPs to produce electricity and heat. Since the combustion process is not 100 %, some CH4 escapes unburned into the atmosphere. By this way, CHP contributes to CH4 emissions. The emission factors of N2O were in the range of 1.22 to 37.57 g (Mg biowaste)-1 for LTS, 0.1 to 2.7 g (Mg biowaste)-1 for CHP, and 56 to 201 g (Mg biowaste)-1 for open windrows. The emissions of N2O at the CHP were insignificant, while the N2O emissions from LTS and open windrows need to be considered. The results are in line with the findings of [4, 8]. The emission factors of NH3 were in the range of 0.1 to 0.16 g (Mg biowaste)-1 for LTS, 0.03 to 1.16 g (Mg biowaste)-1 for CHP and 65 to 3327 g (Mg biowaste)-1 for open windrows. The emissions of NH3 from the LTS and CHP were low, while open windrows had high emissions of NH3. With plantE : overall emission factor of an AD plant BFE : emission factor of biofilter OWE : emission factor of open windrows LTSE : emission factor of liquid treatment system CHPE : emission factor of CHP Nguyen Thanh Phong, Joachim Clemens, Carsten Cuhls 212 LTS CHP BF OW E m is si on fa ct or o f C H 4 g (M g w as te )-1 10 100 1000 10000 100000 LTS CHP BF OW E m is si on fa ct or o f N 2O g (M g w as te )-1 0,1 1 10 100 LTS CHP BF OW Em is si on fa ct or o f N H 3 g (M g w as te )-1 0,01 0,1 1 10 100 1000 10000 Figure 1. Emission factors of CH4, N2O and NH3 from emission sources in AD plants. Error bars show min and max values. Liquid treatment system (LTS) (n = 2), Combined heat and power units (CHP) (n = 6), biofilter (n = 15), open windrow (n = 3). 3.2. Emissions factors of CH4, N2O and NH3 at the AD plants The CH4 emission factors from AD plants were from 444 to 1,5713 g (Mg biowaste)-1 (Figure 2). The median CH4 emission factor was 3,397 g (Mg biowaste)-1. The plants 1, 2 and 9 with open composting windrows showed highest CH4 emissions. The CH4 emission factors from composting windrows were 15,452, 5,763 and 10,254 g (Mg biowaste)-1 which contributed relatively to 95 %, 73 % and 96 % and of the total CH4 emissions at the plants 1, 2 and 9 respectively. Emission factors of CH4 from CHPs were measured only in the plants 3, 4 and 7. CH4 emission factors from CHPs varied from 52 to 2,040 g (Mg biowaste)-1. The results were higher than a previous study: [4] reported that the emission factors of CH4 from CHP ranged from 16 to 819 g (Mg biowaste)-1. The emission factors of CH4 from biofilters varied from 236 to 5,237 g (Mg biowaste)-1. The results are in line with the findings of [4, 8] but comparatively higher than the results of [9], who found that the emission factors of CH4 were about 100 g (Mg waste)-1. plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9 Ei ss io n fa ct or : g C H 4 ( M g w as te )-1 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Biofilter CHP Open Composting Windrow Liquid treatment Figure 2. Median emission factors of CH4 g (Mg biowaste)-1 from different AD plants. For plants 1, 2, 4, 5, 6, 8 and 9, CHP emission data are missing. Greenhouse Gas Emissions from Anaerobic Digestion Plants 213 The emission factors of N2O were in a range of 7-170 g (Mg biowaste)-1 (Figure 3). Median N2O emission factor was 67 g (Mg biowaste)-1. N2O emissions from open composting windrows contributed significantly to the total N2O emissions of the AD plants. The contributions of open composting windrows to the total N2O emissions were 76 %, 80 % and 94 % at the plants 1, 2 and 9 respectively. The N2O emissions from CHPs were from 0.5 to 5 g N2O (Mg biowaste)-1 and contributed only 2-7 % to the total N2O emissions. The N2O emission factors from biofilters ranged from 6.7 to 78 g (Mg biowaste)-1. The N2O emissions from LTS contributed in the range of 2-27 % of the total N2O emissions. plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9 E is si on fa ct or : g N 2O (M g w as te )-1 0 50 100 150 200 Biofilter CHP Open Composting Windrow Liquid treatment plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9 Ei ss io n fa ct or : g N H 3 ( M g w as te )-1 1 10 100 1000 10000 Biofilter CHP Open Composting Windrow Liquid treatment Figure 3. Median emission factor of N2O g (Mg biowaste)-1 from different AD plants. Figure 4. Median emission factors of NH3 g (Mg biowaste)-1 from different AD plants. The emission factors of NH3 were in the range of 41-6,032 g (Mg biowaste)-1 (Figure 4). Median NH3 emission factor was 101 g NH3 (Mg biowaste)-1. Open composting windrows contributed 91 %, 86 % and 99 % to the total NH3 emissions at the plants 1, 2 and 9 respectively. High NH3 emissions at the plant 3 were due to conversion of NH4+ in digestate to NH3 at a high pH and a high temperature in the belt dryer. 3.3. The contribution of CH4, N2O and NH3 from AD plants to global warming potential The emission factors were transferred into CO2 equivalents according to IPCC (2007). The overall CO2 emissions were in a range from 31 to 435 kg (Mg biowaste)-1. The CH4 emissions from AD plants were more important than the emissions from N2O and NH3 (Figure 5A). The emissions of CH4 accounted from 36 - 92 % while the emission of N2O and NH3 contributed from 6.9 - 30 % and from 0.08 – 58 % respectively to the overall CO2 emissions. The median CO2 equivalent emission was 105 kg CO2 (Mg biowaste)-1. The results were in line with a previous study. [4] reported that an AD plant contributed up to 111 kg CO2 equivalent (Mg waste)-1. The AD plants with open composting windrows (1, 2 and 9) showed higher CO2 equivalent emissions than the AD plant without open composting windrows. Figure 5B shows the net total of CO2 equivalent from different emission sources at AD plants. The open composting system resulted in high GHG emissions accounting from 73 to 96 % to the total emissions at plants 1, 2 and 9. CHP contributed from 5 to 50 % to the total emissions at plants 3, 4 and 7. The liquid treatment system resulted in insignificant (3.7 %) to the total CO2 equivalent emissions at plant 1. Nguyen Thanh Phong, Joachim Clemens, Carsten Cuhls 214 plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9 Em is si on fa ct or : C O 2 e qu i k g (M g w as te )-1 0 100 200 300 400 500 CH4 N2O NH3 Σ 435 Σ 31 Σ 85 Σ 105 Σ 203 327 197 3806 98 Σ 74 Σ 40 Σ 149 Σ 305 A plant 1 plant 2 plant 3 plant 4 plant 5 plant 6 plant 7 plant 8 plant 9 Em is si on fa ct or : k g C O 2 e qu iv (M g w as te )-1 0 100 200 300 400 500 Biofilter CHP Open Composting Windrow Liquid treatment Σ 435 Σ 31 Σ 85 Σ 105 Σ 203 Σ 74 Σ 40 Σ 149 Σ 305 B Figure 5. A: The contribution of CH4, N2O and NH3 in form of CO2 equivalent emissions at AD plants. B: The contribution of different emission sources at AD plants. For plants 1, 2, 4, 5, 6, 8 and 9, CHP emission data are missing. 4. CONCLUSIONS Anaerobic digestion plants are a source of GHG emissions. Emission sources are biofilter, open windrows, CHP and liquid digestate treatment system. 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