An Integrated High Pressure SOFC and Premixed

Compression Ignition (PCI) Engine System

Sage Kokjohn, University of Wisconsin - Madison

Development of a “plug and play” engine system for integration with

solid oxide fuel cell based distributed power generation systems

Project Vision

Project Overview

Project Context

Project brings together expertise in advanced combustion engines and

solid oxide fuel cell systems to harness synergies between the devices

Fed. funding: $1.7YM

Length 24 mo.

Team member Location Role in project

University of

Wisconsin -

Madison

Madison, WI Program Lead

Advanced Combustion Development

Caterpillar Peoria, IL Balance of Plant Component Development

Tech to market

United

Technologies

Research Center

East Hartford, CT Fuel Cell Modeling

System Layout

Wisconsin Engine

Research

Consultants

Madison, WI Engine Modeling

1

Phase I: Approach and Objectives

ApproachIntegration of an advanced, combustion

engine with a solid oxide fuel cell

system to enable operation on depleted

anode tailgas beyond what is possible

with current combustion technology

Task outline, technical objectives• Evaluate the needed combustion

strategy to efficiently consume

depleted anode tailgas

• Develop air and fuel handling system

(integrated with the engine) to supply

air and fuel to the engine and SOFC

• Evaluate approaches for load

following / transient operation

Tech-to-Market objectives

• Identify customers for a “plug and

play” engine system for the

distributed generation market

• Assess market potential and “value”

of a 70% distributed generation

system

2

System Overview

3

‣ System layout (Aspen HYSYS) developed demonstrating 70% electrical efficiency

at 1 MWe

‣ Key system components

– High efficiency, pressurized SOFC

operating near 80% fuel utilization

(~90% of net power 900 kW)

– Energy recovery using

reciprocating engine with

advanced combustion (~10% of

net power 100 kW and cathode

air flow assistance)

– Energy recovery using turbo-

machinery for both engine and

SOFC

– Fuel processing (desulfurization,

pre-reforming, etc…)

1 MWe System at 70% Electrical Efficiency

4

‣ System model used to evaluate

part-load operation and identify

pathways to maintain high

efficiency

‣ Increase in efficiency at part

load due to combination of lower

current density and lower BOP

losses

‣ Cathode airflow reduced, pre-

reforming increased, and anode

heat exchanger bypass used to

maintain stack temperature

Displacement [L] 2.5

Stroke [mm] 171

Bore [mm] 137

Con. Rod [mm] 261

Swirl Ratio 0.7

Compression Ratio 16.8:1

IVC [°ATDC] -154

EVO [°ATDC] 113

‣ Single-cylinder test cell setup to

represent operation on anode tailgas

‣ Engine represents one cylinder of the

6 cylinder, 15 L engine

‣ Fuel blending system allows

operation at SOFC fuel utilization

above 80% (CO, H2, CO2, H2O)

Engine Operation on Dilute Tailgas

5

Utilizing Dilute Anode Tailgas in a Recip.

‣ Operation on anode tailgas is challenging due to the low flame speed resulting

from dilute mixtures

‣ Two approaches investigated to enable operation on diluted anode tailgas

– Homogenous Charge Compression Ignition (HCCI) combustion

– Pre-chamber assisted spark-ignited combustion

Spark Ignition HCCI Combustion Pre-chamber SI combustion

Pre-chamber image from

Zigler DOE AMR 2019

Utilizing Dilute Anode Tailgas in a Recip.

‣ HCCI combustion

– Stable combustion is achievable with relevant operating temperature (~80°C)

– Peak gross indicated efficiency ~45%

– Engine out NOx less than 0.05 g/kW-hr NOx aftertreatment is not required

– Operation at intake pressures below 225 kPa is challenging requires SOFC pressure greater than ~250 kPa

– Combustion phasing is controllable using water injection

𝐺𝐼𝐸 =𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑃𝑜𝑤𝑒𝑟

𝑚 𝑎𝑛𝑜𝑑𝑒𝑡𝑎𝑖𝑙𝑔𝑎𝑠

𝐿𝐻𝑉𝑎𝑛𝑜𝑑𝑒𝑡𝑎𝑖𝑙𝑔𝑎𝑠

Increase

Water

All cases at 80% SOFC fuel utilization

Utilizing Dilute Anode Tailgas in a Recip.

‣ Pre-chamber SI combustion

– Stable operation possible at

intake pressures up to ~225

kPa at 80% fuel utilization

(85% is possible at intake

pressures up to 175 kPa)

– Peak gross indicated efficiency

approximately 45%

– Easily controllable combustion

phasing

– NOx below 0.1 g/kW-hr

NOx aftertreatment not

required

Stable

Combustion

Unstable

Combustion

Utilizing Dilute Anode Tailgas in a Recip.

‣ Peak gross indicated efficiency occurs under spark-assisted

compression ignition (SACI) conditions

– Early portion of burn is flame propagation

– Final burn is auto-ignition

Utilizing Dilute Anode Tailgas in a Recip.

‣ The impact of water

knockout was studied

‣Minimal benefit of

water removal

– Gross indicated

efficiency increases

by ~0.7%

– Engine efficiency

increase likely

outweighed by

system efficiency

penalty

Remove

Water

Add

Water

Utilizing Dilute Anode Tailgas in a Recip.

‣ HCCI and pre-chamber SI show similar

efficiency

‣ Evaluation of complete water removal shows

minimal benefit

‣ Stable combustion is possible using both

strategies, but the pressure range is different

– HCCI > 225 kPa

– Pre-chamber SI < 225 kPa

‣ System may require CO aftertreatment

‣ NOx aftertreatment is not required and NOx is

an order of magnitude below US electricity

generation average ultra dilute combustion

Other Developments

12

‣ Printed heat exchangers using a range of materials have been tested

and performance is similar to / or better than fabricated versions with

more material / geometry flexibility

‣ Load following studies using transient flow model have demonstrated

ability to add / remove power using the engine

Phase I Summary and Future Plans

‣ Effort has identified a system capable of achieving greater than 70% electric efficiency over

a wide range of operating conditions

‣ Engine combustion experiments show that a passive pre-chamber is suitable to enable high

efficiency operation up to 85% fuel utilization

– Combustion mode is spark-assisted compression ignition (SACI)

– Indicated efficiency near 45% (approaching diesel engine efficiency)

– Ultra-low NOx without aftertreatment NOx below 0.1 g/kW-hr (engine only) and

0.01 g/kW-hr (system)

– CO aftertreatment may be required (low cost, passive, mature technology)

‣ Cost assessments suggest stack cost dominates system cost, but overall cost-uncertainty

is high Market assessment suggests multiple regions where payback may be favorable

‣ Ongoing efforts will finalize engine testing and systematic evaluation of engine system and

refine cost-assessment

13