FIRST 18 MONTH PERFORMANCE SUMMARY FOR ANAEROBIC DIGESTION OF DAIRY COW SLURRY AT AFBI HILLSBOROUGH

8/10/2019 FIRST 18 MONTH PERFORMANCE SUMMARY FOR ANAEROBIC DIGESTION OF DAIRY COW SLURRY AT AFBI HILLSBOR

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INTERIM TECHNICAL REPORT

FIRST 18 MONTH

PERFORMANCE SUMMARY FOR

ANAEROBIC DIGESTION OF

DAIRY COW SLURRY AT

AFBIHILLSBOROUGH

Figure 1: Photograph of AD plant at AFBI (digester tank left of centre)

Peter Frost and Stephen GilkinsonAugust 21

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AFBI

Hillsboroug

INTERIM TECHNICAL REPORT

FIRST 18 MONTH

PERFORMANCE SUMMARY

FOR ANAEROBIC DIGESTION OF DAIRY COW SLURRY

AT AFBI HILLSBOUGH

Peter Frost

and

Stephen Gilkinson

August$ 21

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SUMMARY OF AFBI DIGESTER PERFORMANCE

Follo%ing intensi&e 'onitoring of the on!far' anaeroic slurr digester at AFBI$ *illsorough during of

operation %ith dair co% slurr as the input$ AFBI has oser&ed that$ on a&erage:!

1+ 1 tonne of dair co% slurr at ,2 g-kg dr 'atter produced 1#+# cuic 'eters of iogas containing

". k/h of energ

2+ 1 tonne of organic 'atter in slurr produced 2" cuic 'eters of iogas (+2" '0-kg)

0+ 01 k/h energ (as heat) per tonne of input slurr %as reuired to 'aintain digester te'perature at

0, (0,3 of gross energ produced)

4+ 5he a&ailale nitrogen concentration in digestate %as 23 greater than in ra% slurr

#+ Digestate did not reuire 'i6ing efore land spreading and did not crust

.+ 5he a&erage *2S concentration of the iogas %as 1$., pp'+

,+ 5he dr 'atter concentration of digestate %as 173 lo%er than the ra% slurr

"+ 5he 8D of digestate %as 2"3 less than that of ra% slurr

7+ Digester operation reuired an a&erage of appro6i'atel 2 person hours per da

INTRODUCTION

Anaeroic digestion of ani'al slurries on far's in&ol&es acteriological reakdo%n of organic 'atter to

produce iogas and digested effluent (digestate)+ Digestate is lo%er in pollution potential$ has less odour$

contains fe%er &iale %eed seeds$ has fe%er pathogens than the input slurr and is an e6cellent iofertiliser+Biogas is a 'i6ture of gases:

9ethane #,#3

aron dio6ide 2##3

;itrogen 13

*drogen 13

*drogen sulphide 03

86gen1 23

1Air 'a e added to iogas in the digester to lo%er the hdrogen sulphide concentration$ other%ise there should e no

o6gen present+

5he calorific &alue of iogas is &ariale (depending on 'ethane content) at 2!2. 9


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Anaeroic digestion reuires a gastight tank %ith dra%!off points for iogas in the headspace$ a heating

sste' to 'aintain opti'u' digester te'perature (0#!4 for 'esophilic digesters)$ a 'ethod of loading

inputs and unloading digestate+ 9i6ing of digester contents is necessar to pre&ent settling of solids and

crust for'ation$ as %ell as to ensure an e&en te'perature %ithin the digester+ 5picall$ 'i6ing is carried

out 'echanical stirrers or iogas recirculation+

5o e &iale$ far' digesters reuire a regular suppl of slurr %ith greater than . 3 total solids (5S) content$

%hich should produce a iogas ield of aout 1. ! 2 '0iogas per tonne of slurr+ 5herefore$ dilution of

slurr should e a&oided+ 8n 'an far's the re'o&al of dilution %ater fro' slurr %ould in&ol&e e6tensi&e

ci&il %ork+ At the other e6tre'e$ a high 5S content of slurr (>123) 'akes for poor flo% characteristics and

for difficult pu'ping+

Digester heating is nor'all carried out circulating hot %ater through a heat e6changer located inside or

outside the digester+ *ot %ater can e produced utilising so'e of the iogas produced through a iogas

oiler (,#!73 efficient)$ or through a co'ined heat and po%er unit+ Alternati&el$ heat can e pro&ided

electrical heating$ an oil oiler or fro' so'e other source of energ+ Insulation of the digester is

i'portant to 'ini'ise heat loss+ It is esti'ated that$ depending on cli'ate and te'perature of slurr

inputted$ 2!.3 of the gross energ a&ailale in the iogas produced through anaeroic digestion of cattle

slurr is reuired to 'aintain digester te'perature+

DETAILS OF THE AFBI ANAEROBIC DIGESTER

5he anaeroic digester at AFBI!*illsorough %as specified AFBI and %as designed$ supplied and

constructed Greenfinch ?td (no% BiogenGreenfinch)$ ?udlo%$ Shropshire+ onstruction took place

et%een Septe'er 2, and 9arch 2"+ 5he ti'e fro' co''ence'ent of feeding the digester %ith

slurr until stead state production of iogas %as appro6i'atel . 'onths (end of 9arch 2" until the

eginning of 8ctoer 2")+ 5he AD plant at *illsorough is detailed in 5ale 1 and as a photograph in

Figure 1 on the front co&er of this report+

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5ale 1: Details of AD plant at AFBI

Digester tank ..'0ao&e ground epo6 coated sealed steel tank %ith 1'' 'ineral %oolinsulation and 1'' plastic coated steel outer protection+ ontinuousl stirred tank

reactor (S5@) operating at 'esophilic te'perature (0,)+

Secondar digester

tank

..'0ao&e ground epo6 coated sealed steel tank$ continuousl stirred$ ut not

insulated

Feedstock tank 2 '0ao&e ground epo6 coated open top steel tankDigestate store 1$# '0ao&e ground epo6 coated open top steel tankDigester feed Fed hourl %ith a positi&e displace'ent loe pu'p

Digester and secondar

digester discharge

Discharged hourl %ith positi&e displace'ent loe pu'ps

Digester 'i6ing Biogas recirculation 0 6 # 'inutes per hour

Digester heating 6ternal 1k/ heat e6changer %ith circulation of digestate positi&e

displace'ent loe pu'p+ *ot %ater supplied district heating sste'

Solids feeder 1 '0open top hopper co'plete %ith 'i6er$ transfer auger and feed augersDigester feed

'acerator

All digester inputs 'acerated to a no'inal particle sie of 12''

Biogas oiler *o&al 1k/ no'inal heat output

*P 5edo' 20k/ no'inal electrical output

Sste' control Progra''ale logic controller (P?)

Design andconstruction

Greenfinch ?td (no% BiogenGreenfinch)$ ?udlo%$ Shropshire to AFBIspecification

PERFORMANCE OF THE AFBI ON$FARM DIGESTER

8&er the ti'e period (24


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slurr to the digester and spreading of digestate$ the a&erage rene%ale energ gain fro' iogas utilised

through the *P %as 01 k/h-t of slurr input$ %ith ero use of fossil fuel energ+ Cse of a iogas oiler

ga&e an a&erage net rene%ale energ gain of 44+, k/h-t of slurr input for a #+4 k/h-t input of fossil fuel

energ (fossil fuel: rene%ale energ of 1: "+0)+ ;ote that higher iogas ields per tonne of fresh input

%ould e otained fro' using higher dr 'atter slurr or fro' co!digestion %ith for e6a'ple$ grass silage+

/ork to deter'ine the iogas ield fro' dair co% slurr co!digested %ith grass silage is currentl ongoing

in AFBI+

/hen co'pared %ith the &olu'e of slurr fed$ the &olu'e of digestate reco&ered %as al'ost the sa'e$

%hilst che'ical o6gen de'and (8D) and dr 'atter of the digestate %ere lo%ered 2"3 and 173

respecti&el+ 5he total uantities of nutrients (;$ P and ) in the digestate %ere al'ost identical as those in

the input ra% slurr+ *o%e&er$ due to the process of digestion$ the plant a&ailale ; (a''onia!;) content

of the digestate %as 23 greater than in the ra% slurr (5ale 2)+ Field trials are ongoing %ithin AFBI to

deter'ine the effect of this enhanced a&ailale nitrogen content on crop ields and nitrogen use efficienc+

It is possile that greenhouse gas e'issions ('ethane and nitrous o6ide) during digestion$ post digestion

storage and after land spreading of digestate could e less than fro' con&entional storing and spreading ra%

slurr+ 5rials are ongoing %ithin AFBI to 'easure e'issions of these gases+

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5ale 2: Perfor'ance of AFBI anaeroic digester o&er ,, %eeks (24


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Dairy cow slurryDairy cow s lurry

1 tonn e1 ton ne

7.2% dry ma tter7.2% dry matter

DigesterDigesterHeat

31 kWh

HeatHeat

31 kWh31 kWh

BiogasBiogas

15.515.5 mm33

8kWh energy8 kWh energy

DigestateDigestate

!."" tonne!."" tonne

5.8% dry matter5 .8% dry matter

Bio #ertiliserBio#ertiliser

$ield cros$ield cros

&H'&H'

(lec tric ity(lectricity aa

23.2 kWh23.2 kW h

)et)e t

(lec tric ity(lectricity

17.8 kW h17.8 kWh

(lectricity(lectricity

gridgrid

HeatHeat **

++.1 kW h++ .1 kWh

)et he at)et h eat

13.1 kWh13.1 k Wh

,##ices,##ices

Biogas *oilerBiogas *oiler cc

HeatHeat

75.7 k Wh75.7 kWh

)et heat)et he at

++ .7 kWh++.7 kWh

,##ices,##ic es

$rom

(le ctricity grid

(lectricity

5.+ kWh(lectricity(lectricity

5.+ kW h5.+ kWh

5ale 0: AFBI o'ined *eat and Po%er outputs o&er .$0 hours et%een August 27 and


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Summer versus winter production

Perfor'ances of the digester during su''er (


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(appro6i'atel 0 person hours per ear)+ Dail checks of the plant could e co'pleted in 1# 'inutes$

assu'ing no other tasks are in&ol&ed+

5ale 0: ?aour reuire'ents for AFBI AD plant

8peration ;u'er

of

people

5i'e

(hours)

Annual ti'e

(appro6i'ate

person hours

per ear)9anage'ent-checking

Dail

/eekl

9onthl

uarterl

1

1

1

1

1

H+#

H+#

H1+

0.#

H2.

H.+

H4+

/eekl ! transfer of ra% slurr to digester 1 4 2"

8ccasional (e+g+ once per 2 %eeks elo% ground tank su'ersile

pu'p clear lockage)

0 1 ,2

&er 1 das ! change acti&ated caron 2 1 ,0

To)+l $ $ 3%#

COMMENTS

Anaeroic digestion can pla a 'aor role in production of rene%ale energ as %ell as helping to 'anage

and reccle organic nutrients as fertiliser+ Further'ore$ anaeroic digestion is &er effecti&e at lo%ering the

pathogen load in digestate$ though there is inco'plete kill of so'e acterial pathogens such as sal'onella$

listeria$ coli and ca'ploacter+ /ork %ill e untaken in this area to uantif the e6tent of these

reductions+ *o%e&er$ digestate can e pasteurised to eli'inate pathogens and help reak an e6creta related

disease ccle on a far'+ In addition$ digestate can e 'echanicall separated$ in the sa'e 'anner as %hole

slurr$ to create a firous fraction %ith a higher dr 'atter concentration and a liuid fraction %ith a lo%er

dr 'atter concentration than the inputted 'aterial+ It is likel that thorough aeroic co'posting of the

separated fire %ould kill 'an of the pathogens originall in the fire+ 5he ad&antages of 'echanical

separation include:

?o%ering the &olu'e of liuid reuiring storage

reating potential to e6port plant nutrients contained in the separated fire off far'

I'pro&ing the efficienc in nitrogen uptake fro' the liuid fraction

Pro&iding a greater %indo% of opportunit for application of the liuid fraction

?o%ering the reuire'ent for 'i6ing of the liuid fraction prior to spreading+

9echanical separators co''onl used on far's include rotar screens$ elt presses$ and scre% presses+

he'icals can e used to i'pro&e separator efficienc and to help differentiall partition plant nutrients

(particularl phosphorous) to the separated firous fraction+ 5he separated fire has a potential 'arket &alue

that 'a e further enhanced co'posting to produce a Jpeat sustituteK containing ;$ P and that could

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e used as co'ined soil conditioner and fertiliser for horticulture+ ;ote that separation creates t%o outputs$

a liuid and a firous 'aterial$ that need to e stored and handled separatel+

Anaeroic digestion$ either on its o%n or %hen follo%ed 'echanical separation$ has thus a significant

potential in helping nutrient 'anage'ent at far' le&el+

5he digestate fro' the AFBI digester %as 2"3 lo%er in 8D than the ra% input slurr (5ale 2) and

therefore it is concluded that AD partl reduced the pollution potential of digested slurr+ *o%e&er$ the

digestate %as still highl polluting and %as not suitale for direct discharge to %ater!courses+ 6perience at

*illsorough suggests that the s'ell of digestate produced fro' dair co% slurr %as si'ilar to that fro'

%hole slurr$ though the s'ell %as less intense and did not last as long after land spreading+ In addition$ the

digestate at *illsorough did not reuire in!store 'i6ing$ %as free flo%ing and %as easil spread trailing!

shoe tanker+ Cnder so'e conditions these positi&e en&iron'ental enefits fro' anaeroic digestion could

e &er i'portant+ It should e stressed that digestion to reduce pollution potential is an inco'plete process

and that other processes such as aeration 'a e used to further reduce odour and 8D+ A co'parati&e

stud %ould e reuired to assess the relati&e costs and enefits fro' the &arious options a&ailale+

8ne tonne of dair co% slurr at AFBI (,+2 3 dr 'atter concentration) produced appro6i'atel 1#+# '0

iogas$ eui&alent to appro6i'atel " 1itres of diesel oil (4+2 to 1+" 1itres depending on hdraulic retention

ti'e$ te'perature and e6act calorific &alue of iogas)1+ /hen this iogas %as utilised a iogas oiler or

a *P it produced:

Biogas oiler ;et 4+2 litres oil eui&alent after digester heating %ith an additional

reuire'ent for #+4 k/h of electricit for digester plant operation

*P ;et 1,+" k/h electricit plus 1+2 litres oil eui&alent as net heat after digester

heating and operation

5he net energ produced digesting one tonne of dair co% slurr (,+23 dr 'atter) is %orth

appro6i'atel 1 %hen the iogas is co'usted in a iogas oiler$ or appro6i'atel 0+# %hen the iogasis utilised through a *P unit (electricit %ith doule @8Ls plus heat)2+ Should the current proposal to

issue uadruple @8s go ahead$ the net energ &alue of the electricit and heat produced fro' iogas

utilised through a *P %ould increase to appro6i'atel #+# per tonne of slurr digested+ 5hese &alues are

o&er and ao&e the &alue of digestate as a iofertiliser+

1Assu'es the energ &alue for diesel oil is 1+, k/h-litre2Assu'es cost of heating oil 4p-litre= cost of electricit 10p-k/h= re&enue fro' electricit sale of 10+#p-k/h (4+#p-k/h H264+#p-k/h doule @8s= perfor'ance data listed in this pulication

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o!digesting other organic 'aterials along %ith slurr can greatl increase iogas ield+ For e6a'ple$

adding 0 tonnes grass silage %ith the slurr$ the AFBI digester could$ in theor$ 'ore than doule iogas

production+ /ork is ongoing %ithin AFBI to deter'ine the enefits fro' co!digestion %ith grass silage+

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Documents

FIRST 18 MONTH PERFORMANCE SUMMARY FOR ANAEROBIC DIGESTION OF DAIRY COW SLURRY AT AFBI HILLSBOROUGH