ENERGY VALORISATION OF DIGESTED SEWAGE SLUDGE THROUGH GASIFICATION

Mocanu Gabriel; Ion V. Ion

УДК 662.767

ENERGY VALORISATION OF DIGESTED SEWAGE SLUDGE THROUGH

GASIFICATION

G. Mocanu, IV. Ion «Dunarea de Jos» University of Galati, Romania

iion@ugal.ro

Abstract: The general purpose of a sludge management strategy in wastewater treatment plants is to achieve a concept of sludge recovery, so as to avoid the negative effects of sludge on human health and the environment with low costs and energy consumption. In this paper it is analysed from an energetic and economic point of view the digested sewage sludge disposal by gasification and the use of syngas for the generation of electricity and heat. It was chosen gasification because it eliminates almost all environmental problems (does not produce pollutants and waste), is flexible in terms of plant size, has high energy efficiency, is self-sustaining and because it produces a fuel that can be easily used in energy cogeneration plants. The recovery of sewage sludge with electricity and heat production is of greater interest due to the recent increase in energy prices. THE PURPOSE. The public administrations must adopt sludge management strategies resulting from municipal wastewater treatment plants in accordance with the new environmental restrictions. This requires economic and energy analysis of existing options on the market. This paper welcomes those interested by providing information and a model for analyzing a sludge recovery technology. METHODS. The sewage sludge recovery through gasification was simulated by using the Cycle Tempo software and the Felicia Fock and Kristine Thomsen gasifier simulator.RESULTS. Integration of digested sewage sludge gasification plant with an internal combustion engine based combined heat and power generation unit in a wastewater plant with capacity of 97,000 m3/d may cover 17.04 % of the electrical energy consumption and the investment can be recovered in 2-6 years.CONCLUSIONS. The combination of sewage sludge digestion with digested sewage sludge gasification is a promising option for sewage sludge treatment and energy recovery.

Keywords: sewage sludge; gasification; syngas; combined heat and power generation; capital cost, payback period.

ЭНЕРГЕТИЧЕСКАЯ ВАЛОРИЗАЦИЯ ПЕРЕРАБОТАННЫХ СТОЧНЫХ ВОД ПРИ

ГАЗИФИКАЦИИ

Г. Мокану, И.В. Ион Университет «Дунареа де Жос», г. Галац, Румыния

iion@ugal.ro

Резюме: Общая цель стратегии управления осадком на станциях очистки сточных вод состоит в том, чтобы реализовать концепцию рекуперации осадка, чтобы избежать негативного воздействия осадка на здоровье человека и окружающую среду с низкими затратами и потреблением энергии. В данной статье анализируется с энергетической и экономической точки зрения удаление сброженного осадка сточных вод путем газификации и использования синтез-газа для производства электроэнергии и тепла. Эта газификация была выбрана потому, что она устраняет почти все экологические проблемы (не производит загрязняющих веществ и отходов), гибка с точки зрения размера установки, имеет высокую энергоэффективность, является самодостаточной и потому, что она производит топливо, которое можно легко использовать в энергетике. когенерационные установки. Рекуперация осадка сточных вод при производстве электроэнергии и тепла представляет больший интерес в связи с недавним повышением цен на энергию. ЦЕЛЬ. Государственные администрации должны принять стратегии управления осадком, образующимся на муниципальных очистных сооружениях, в соответствии с новыми экологическими ограничениями. Для этого необходим экономический и энергетический анализ существующих на рынке вариантов. Этот документ приветствует тех, кто заинтересован, предоставляя информацию и модель для анализа технологии восстановления ила. МЕТОДЫ. Улавливание осадка сточных вод посредством газификации было смоделировано с помощью программного обеспечения Cycle

39

Tempo и симулятора газификатора Felicia Fock и Kristine Thomsen. ПОЛУЧЕННЫЕ РЕЗУЛЬТАТЫ. Интеграция установки газификации сброженного осадка сточных вод с установкой комбинированного производства тепла и электроэнергии на базе двигателя внутреннего сгорания в установку для очистки сточных вод производительностью 97000 м3 / сут может покрыть 17,04% потребления электроэнергии, а инвестиции могут окупиться через 2-6 лет. ВЫВОДЫ. Комбинация сбраживания осадка сточных вод с газификацией сброженного осадка сточных вод является многообещающим вариантом для обработки осадка сточных вод и рекуперации энергии.

Ключевые слова: отстой сточных вод; газификация; синтез-газ; комбинированная выработка тепла и электроэнергии; капитальные затраты; срок окупаемости.

Introduction

Managing sewage sludge from municipal wastewater treatment is a financial and environmental challenge. Wastewater treatment sludge is a semi-solid residual material, i.e. a suspension of microorganisms, harmful organic matter and mineral mass in water. It represents about 250 mg/L of municipal treated wastewater [1], or (1-2)% of the volume of treated water and its treatment represents (20-60)% of the total operating cost of the wastewater treatment plant [2 ].

The amount of sludge generated for a year by a person is about 33 kg [3], or about 25 kg of dry matter [4]. Sewage sludge is a waste with specific characteristics (high water content, high content of nutrients, such as nitrogen and phosphorus; high content of hazardous contaminants, content of organic pollutants, organic toxins, pharmaceutical residues, pathogens, etc.) that complicates the operations of treatment.

Currently, the most used treatment methods consist of recycling in agriculture (44% in EU27, in 2020), incineration (32% in EU27, in 2020), landfill (7% in EU27 in 2020) and others (16% in EU27 in 2020).

Landfilling is acceptable when accompanied by landfill gas recovery for energy production. Use in agriculture as soil conditioning and fertilizer remains the main way of removing sludge [5]. Following the adoption of the Circular Economy Action Plan on 11 March 2020, the revision procedure of the Sewage Sludge Directive 86/278/EEC began, which encourages the correct use of sewage sludge in agriculture and its regular use to prevent adverse effects on soil, vegetation, animals and on people.

The recycling of materials supported by the circular economy must be done in such a way that what is used as a source is not contaminated so as not to increase soil, water, and air pollution. For this reason, efforts are being made to find the most suitable/effective sewage sludge (SS) treatment strategy. Although the elimination of SS using thermochemical treatment technologies is less attractive due to the high cost of generating electricity from SS, in the current conditions of rising electricity prices worldwide, it could become the first option in eliminating SS. Thermochemical treatment of SS is done by co-combustion, incineration, pyrolysis, gasification and hydrothermal liquefaction. The main advantages of thermochemical treatment are: reduction of volume and mass, destruction of toxic organic compounds, recovery of nutrients (as phosphorous) and energy recovery. The choice of SS thermochemical treatment technology must be made taking into account technical and economic, social and environmental aspects.

To be thermochemical treated, SS must be subjected to the dewatering/drying operation. It consumes almost all the heat generated by thermochemical processing and increases the cost of the energy recovery system. From an energetic point of view, the thermochemical treatment of SS is generally considered autonomous (energetically self-sustained). Even if incineration requires lower costs (when high-capacity installations are used), because it is not well seen/received by the citizens, more efficient energy recovery systems such as pyrolysis and gasification have been developed in recent years.

Pyrolysis is the process of thermochemical decomposition in the absence of oxygen, which results in combustible gases, bio-oil and bio-char in proportions that depend on the conditions of the process. Pyrolysis requires a heat input of 100 kJ/kg dry matter and pretreatment of the raw material in the form of drying.

Gasification is similar to pyrolysis, except that it takes place in the presence of a substoichiometric amount of oxidant and produces only combustible gases and to a lesser extent bio-char. The process does not require external heat input, it is self-sufficient, but requires pre-treatment by drying the raw material.

The choice of SS treatment technology must take into account several factors that affect energy efficiency, treatment cost and environmental impact.

There are many studies on methods of selecting treatment technology [2, 4-9]. Some of them [8] indicate SS incineration as the most suitable in terms of cost, energy efficiency, nutrient

recovery and flexibility in terms of SS moisture content, and pyrolysis as the most suitable in terms of market value of generated products and flexibility in regarding plant size. Other studies [2, 10-12] indicate SS gasification as the most suitable in terms of technological complexity and hazardous emissions.

In the paper [13] pyrolysis is identified as the most appropriate method for treating SS in Greece in terms of low emissions, waste generated, energy efficiency, generated products.

The properties of SS vary greatly, depending on its origins, wastewater treatment system, environmental requirements, and seasonal variations. The high water content, toxic inorganics, organic pollutants and content of pathogens lead to the complication of the thermochemical treatment process (pre-treatment: drying; ash disposal or reuse strategies; gas treatment), to the increase of costs and to the reduction of energy efficiency. For this reason, the choice of treatment technology with all processes must be tailored to suit individual location and a detailed technical and economic analysis. Coupling biochemical treatment with thermochemical treatment is a way to optimize energy recovery [14].

In this paper is analysed from an energetic and economic point of view valorisation of sewage sludge through gasification.

Sewage sludge gasification

The organic substances from sewage sludge are converted to producer gas, in the largest proportion, and biochar during the gasification process. Some amount of ash is generated from the inert material. The optimum value of the air ratio (corresponding to the LHV maximum value of producer gas) being equal to 0.18. The water content of the sewage sludge must be (10-20)%, depending on the gasification technology: updraft fixed bed; downdraft fixed bed and fluidized bed. The heat required for drying of mechanically dewatered sewage sludge is produced by the utilization of the producer gas [4].

There are many studies on sewage sludge gasification [15 - 22]. The problems that occur with gasification sewage sludge are related to the high moisture content (it raises the energy consumption), high nitrogen content (it is transformed into ammonia or nitrogen cyanide during gasification process), high ash content and varying chemical composition (it creates difficulties in process control). These problems can be alleviated by co-gasification with coal or other biomass [23].

The scale of gasification plants ranges from (5-20) MW for downdraft fixed bed gasification up to 100 MW for circulating fluidized bed gasification [4].

The evaluation of the energetic performances of the gasification process (LHV and composition of producer gas, cold efficiency of gasification) can be done with the help of software developed for this purpose.

The overall gasification reaction can be written as follows:

CmHnOpNqSr + z (O2 + 3.76N2

^ x1CO + x2H2 + x3CO2 + x4H2O + x5CH4 +

+ z3.76^

N2 + x6H2 S

(1)

where CmHnOpNqSr is the chemical formula of biomass. The oxidant-fuel ratio is:

m

air .

-; m.a

m

= ER • ml = ER

321 — + H + — - ° 12 4 32 32

0,23

(2)

where ER is equivalent ratio defined as the ratio of actual air fuel and stoichiometric air. The cold gas efficiency of biomass gasifier is:

^ceff

VLHvsg

mb • LHVb

where: Vsg- is producer gas flow rate (Nm /s); mb- biomass flow rate (kg/s);

LHVsg - lower heating value of producer gas (kJ/Nm3):

HHV = 12760 • H2 +12630 • CO + 39760 • CH4

xOF =

LHV = HHV - 2008 ( H2 + 2 • CH4 )

(4)

H2, CO and CH4 represent volume participation of H2, CO and CH4 in the producer gas. LHVb - higher heating value of biomass (kJ/kg). It can be calculated by using the Dulong and Petit equation [24]:

O

HHV = 33823 • C +144249 • ^H - — I + 9418 • S LHV = HHV - 2250 (9H + W)

(5)

H, C, O, S and W represent mass participation of hydrogen, carbon, oxygen, sulphur and water in biomass.

A case study

A European strategy for sewage sludge management has as general objective the long-term improvement of environmental quality factors by minimizing the adverse effects of inadequate sludge management and is based on Directive 86/278 EC on environmental protection and especially soil, in the case of agricultural use of sludge. The legislation on waste establishes the gradual reduction of waste disposal in landfills. Under these conditions, the county administrations are looking for long-term solutions for the disposal of sludge based on the principles of safety and reliability and economic and environmental efficiency.

Currently, in order to be disposed of, the sludge is stabilized as specified in EU legislation on the storage of sludge in landfills and possibly its use as a fertilizer in agriculture (Directive 86/278 / EEC of 12 June 1986 on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture) (Fig. 1). Stabilization is done by anaerobic fermentation ((30-50)% degradation rate of volatile solid (VS)) followed by dehydration of fermented sludge to 65% dry substance (DS) by weight. The biogas generated from the anaerobic fermentation process is used in a boiler to heat the anaerobic digestion tank and the buildings related to the municipal wastewater treatment plant. Digested sewage sludge after dewatering (due to the fact that there were no requests for taking over the sludge in agriculture) is in proportion of 14% co-incinerated in cement factory and 86% landfilled.

From the options for eliminating the digested sludge for the studied case, the gasification and use of the generated gas in a cogeneration system based on internal combustion engine was chosen (Fig. 2). The gasifier is of the downdraft fixed bed type and the gasification is done with air (the cheapest technology) (Fig. 3).

The elemental composition and Lower Heating value (LHV) of dewatered digested sewage sludge is given in Table 1.

Municipal wastewater 97,000 m3/d 0.28 kg ill-3 DS

Wastewater treatment

Primary + secondary sludge 2,910 m3 d; 1.5% DS; 71%VS.TS

Treated water to river

0.035 kg rwDS

Heat

Digested sludge, 233 пГ/Л, 5% DS

Water

Sludge dewatering

Dewatered sludge, 4.4 t/d; 65% DS

"^iSÓ"

14% Co-incineration ^6%Landfillmg

Fig. 1. Scheme of wastewater treatment plant with Рис. 1. Схема очистных сооружений с anaerobic digestion of sewage sludge (DS - dry анаэробным сбраживанием осадка сточных вод substance; VS - volatile solid; TS - total solid) (DS - сухое вещество; VS - летучие твердые

вещества; TS - твердые частицы).

Table 1

Elemental composition and calorific value of dewatered sewage sludge

Parameter Value

C 32.70

O 15.85

H 4.55

N 5.19

Moisture (W) 13.04

Ash 27.40

Lower heating value, LHV (kJ/kg) (wet basis) 16310

Dewalered sludge, 4.4 t'd; 65% DS

Ail

Sludge drying j

Dryed sludge 3.68 t'd: 1.3% water

Producer gas

I

Gas cleaning -heat recovery

iНе можете найти то, что вам нужно? Попробуйте сервис подбора литературы.

Heat

290 kWt +

Syngas_

11045 m3 d

ICE based CHP

Fig. 2. Scheme of valorisation of digested sewage sludge through gasification

I

Electricity 150 kWe

Рис. 2. Схема валоризации сброженного осадка сточных вод путем газификации.

The equilibrium syngas composition was estimated by using the Cycle Tempo software [25] and the Felicia Fock and Kristine Thomsen gasifier simulator [26].

Sewage sludge gasification

Pelletized SS LHV=12937 kJ/kg Mass flow rrate=0.051 kg/sK Moisture=13.1 % Ash=27.4%(T>\ C=32.7% H=4.5% N=5.2% S=1.2% O=15.9%

Syngas

Fow rate=0.127 m3

LHV=5173 kJ/m3

CO=22.9%

CO2=9.5%

H2=15.3%

CH4=1.5%

H2O=11.2%

N2=39.6%

Downdraft Gasifier

Reaction pressure = 1.013 bar Reaction temperature = 850°C Oxidant-fuel ratio, xOF= 1.8 kg air/kg fuel

Air preheating

Gas cooling

®

Gas cleaning

10 д2

Hot water 98°C

Dust and particulate

Mass flow rate = 0.092 m3/s

Fig. 3. Cycle-Tempo scheme of sewage sludge Рис. 3. Схема Cycle-Tempo газификации осадка gasification сточных вод.

s

In Table 2 are given the estimated syngas compositions. It can be seen small differences.

By gasifying with air in a downdraft gasifier of digested sewage sludge results syngas with LHV of about 5200 kJ/Nm3, the gasifier having the cold gas efficiency of 71.9%. The syngas is used in a gas engine to generate electricity (190 kWel) and heat (290 kWt). It was chosen the internal combustion engine plant because it is suitable for small size CHP plant (100 - 1000 kWel) as the heat produced can be exploited at the local scale [27]. Syngas-powered internal combustion engines are less efficient than those powered by natural gas, gasoline or diesel, due to the low

calorific value of syngas. To increase the efficiency, it is necessary to reduce the air/fuel ratio. The heat generated in the CHP plant is used for drying the sewage sludge before the gasification.

Table 2

Composition and calorific value of syngas obtained from digested sewage sludge.

Producer gas composition Cycle-Tempo software [25] Felicia Fock simulator [26]

CO 22.9 19.5

CO2 9.5 10.2

H2 15.3 17.9

CH4 1.5 1.8

n2 39.6 39.9

H2O 11.2 10.7

Lower heating value, LHV (kJ/Nm3) 5173 5250

Cold gas efficiency - 71.9

A municipal wastewater treatment plant may consume between 20 kWh/PE/year (for large plants serving > 100,000 PE) and 45 kWh/PE/year (for plant serving around 10,000 PE [28] (1 PE equals 200 litres of sewage per day). For the studied case it means an energy consumption of 9,700,000 kWh/year. The CHP power of 190 kWe will cover only 17.04% of the needs of the studied wastewater treatment plant.

The total investment cost of the entire capitalization plant by gasification of sewage sludge (thermal dryer, gasifier with producer and gas cleaning, CHP unit) reaches 471,500-1,212,500 € [24]. The revenue of sewage sludge gasification plants is derived from the sale of the end products and avoided cost of sewage sludge transportation to the cement factory (0.08 €/m3/km).

If we take into account the operation and maintenance costs [24], the operating period of 8700 h/year and the actual cost of electricity of 235 €/MWh, the payback period is between 2 and 6 years.

Conclusions

The feasibility of using gasification for the treatment of sewage sludge generated by the municipal wastewater treatment plant has been analysed. The environmental cost of improper disposal and stricter environmental regulations (landfilling restriction) represent reasons for improving the current sewage sludge disposal practices in Europe.

The combination of anaerobic digestion of primary and secondary sewage sludge with gasification of digested sewage sludge is a feasible strategy of sewage sludge treatment given that fuel and energy prices are constantly rising. Applied to the wastewater treatment plant with a capacity of 97,000 m3/day, it can fully cover the plant heat demand and about 15.8% of the electricity demand. The investment in the technology of gasification and capitalization of syngas in a CHP unit based on internal combustion engine can be recovered in about 2-6 years.

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Authors of the publication Gabriel Mocanu - «Dunarea de Jos» University of Galati, Romania. Ion V. Ion - «Dunarea de Jos» University of Galati, Romania.

Литература

1. Metcalf & Eddy Inc., Tchobanoglous G, Franklin LB, Ryujiro T, Stensel HD. Wastewater Engineering: Treatment and Resource Recovery. 5th ed. New York, NY: McGraw-Hill Professional; 2013.

2. Durdevic D, Trstenjak M, Hulenic I. Sewage Sludge Thermal Treatment Technology Selection by Utilizing the Analytical Hierarchy Process. Water. 2020;12;1255.