1. Introduction
In recent years, the energy sector has been experiencing an increasing diffusion of renewable energy sources, e.g., solar or wind, that contribute to the electricity grid [
1]. These sources are strongly dependent on environmental factors, such as local weather phenomena. Therefore, their contribution is prone to fluctuations which could result in local grid instabilities [
2]. In this context, decentralized power generation with combined heat and power (CHP) units becomes increasingly popular. On the one hand, such plants feature a very high energy efficiency by simultaneously generating thermal and electric power as close as possible to the demand site. On the other hand, CHPs can offer power balancing services to stabilize the electricity grid [
3,
4]. In addition, particularly for CHPs of the mini and micro power class (<50
el), there is a tendency towards a more demand-driven operation based on the actual energy consumption [
5,
6,
7,
8,
9]. However, both the power balancing services and the demand-driven operation lead to more frequent start-ups of the plant and thus to an increasing number of cold-start processes. Research in the automotive sector has widely shown that cold-start procedures cause a high share of the total pollutant emissions of a system with an internal combustion engine (ICE) [
10,
11,
12,
13,
14]. As mini and micro CHPs (mCHP) generally feature low pollutant emissions under steady-state conditions, an increasing number of cold-starts would mitigate this property to a great extent.
In this paper, we investigate strategies to reduce the share of pollutant emissions emitted by an mCHP during the cold-start phase. The mCHP is conceptually based on a full-load operation of a one-cylinder internal combustion engine which is fueled by natural gas and utilizes a conventional three-way catalytic converter (TWC) as an aftertreatment system. In this context, we focus on the pollutants nitric oxide (NO) and nitrogen dioxide (NO
2), in the following jointly considered as NO
x, as well as carbon monoxide (CO). These are already regulated by several emission regulations for stationary energy generation units. Additionally, we consider the total hydrocarbon emissions (THC), because they expected to be regulated in future amendments. As a reference, we employ emission directives that are widely accepted in Europe, the Technical Instructions on Air Quality Control (Germany, herein referred to as
TA-Luft) [
15] and the Swiss Clean Air Act (Luftreinhalte-Verordnung, Switzerland, herein referred to as
LRV) [
16].
Recent research on reciprocating internal combustion engines has proven that the cold-start emissions of a system with a TWC as an aftertreatment system can be reduced by either of two approaches [
17]:
Strategies that reduce the raw emissions originating from the combustion of the air-fuel mixture inside the cylinder. Such strategies can be either mechanical measures, e.g., the design of in-cylinder components or valves, or the application of combustion control approaches.
Strategies that accelerate the heat-up process of the TWC until it reaches a specific light-off temperature. Only after TWC light-off, THC, CO and NOx are significantly converted. Again, such strategies may utilize either specific hardware components or dedicated combustion control techniques that, in this case, generate high exhaust gas temperatures.
Numerous approaches have been described in the literature that fall within these categories. They range from dedicated control systems (e.g., for the ignition timing [
18], the air/fuel ratio (AFR) [
19,
20] and the charge motion [
21,
22]) via controlled hardware solutions (e.g., electrically-heated catalysts [
23], secondary air injection solutions [
24] and exhaust gas recirculation [
25]) to uncontrolled auxiliary devices (e.g., latent-heat storage [
26]). However, the majority of combined heat and power plants is fueled by natural gas or liquefied petroleum gas rather than by gasoline or diesel. Furthermore, the operating scheme greatly differs from the automotive context, e.g., full-load and constant speed operation.
Therefore, within this study, we adapt existing approaches to the distinct features of combined heat and power plants and experimentally investigate their impact on the cold-start emissions. We focus on solutions that can be realized by the CHP manufacturer. As the engine is typically obtained by a third-party manufacturer, we thus do not consider any engine component design solutions. However, interfaces for applying certain combustion control techniques are usually provided in order to coordinate all subsystems to achieve a desired plant characteristic. We chose to develop strategies based on variations in the ignition timing as well as in the reference air/fuel ratio and we subsequently investigate their potential to reduce the cold-start emissions of an mCHP. Both concepts are software-based and therefore require only a marginal adaptation/integration effort. As a third solution, we investigate the effect of an electrically-heated catalyst (EHC). This device is typically mounted directly in front of the TWC where it heats up the exhaust gas, thereby accelerating the heat-up process of the TWC.
The paper is structured as follows:
Section 2 provides details on the mCHP test bench used in this study.
Section 3 sheds light on the emission reduction principles for the concepts selected. On this basis, the strategic control approaches are derived.
Section 4 provides details on the underlying test procedure. In
Section 5, the experimental results are presented, compared, and evaluated on the basis of a set of criteria.
5. Results and Discussion
In order to compare the effectiveness of the different strategies for reducing the cold-start pollutant emissions, the results are discussed with respect to the following measures:
Time in [] until a quasi-stable conversion rate is reached. In the following discussion this condition is assumed to be fulfilled as soon as the mass concentration falls below for CO and NOx, and below for THC.
The mean emission concentration over the entire cold-start duration in [].
The cumulated emission mass for the entire cold-start in []
The specific emission mass with respect to the electrical energy produced in [//]
The strategies investigated are compared to the reference case, i.e., the case with MBT timing at a spark advance , an AFR setpoint of , and a disabled EHC.
Figure 7 shows an excerpt of the evolution of the emission concentrations of NO
x, THC and CO during a cold-start procedure with respect to time. Each of the strategies shows a different influence on the emission characteristics. First, the impact of the EHC is only significant for the reference case
REF as well as the CO emissions of the remaining cases. This fact suggests that the EHC accelerates the TWC warm-up such that pollutant emissions are effectively converted earlier than with a disabled EHC. However, the application of the
SA and
AFR strategies outperforms the effect of the EHC on the emissions of NO
x and THC. The remaining influence on the CO emissions results from the fact that, in contrast to the conversion of THC and NO
x, CO is already oxidized at TWC temperatures as low as 150
–250
. This effect is observable throughout the entire set of strategies. Second, the application of the
SA strategy reduces the overall concentration levels for all pollutants and simultaneously shifts the peak in mass concentration to earlier points in time for CO and NO
x. Third, in contrast to the
SA strategy, the
AFR strategy causes a change in the overall characteristic.
Figure 7 shows that the concentration level for case
AFR is almost identical to the reference case during the first two minutes. However, it follows a pronounced dip in the NO
x and THC concentrations between 2.5
to 5
. This dip also exists in case
COMB and can therefore be attributed to the application of the
AFR strategy. As both effects, the reduced pollutant emissions level due to the
SA strategy and the characteristic drop in THC and NO
x emissions due to the
AFR strategy can be observed in case
COMB, we conclude that the
AFR and
SA strategies do not interfere or interact, but superimpose with each other.
Figure 8 shows
, the accumulated emission mass in [
mg] from startup until
as well as the cumulated emission mass until
relative to the cumulated emission mass for the entire cold-start procedure of 60
for all cases. In this context,
acts as a measure of how quickly each emissions concentration decreases, whereas
represents the total emission mass during this time frame. The value of
indicates how much of the total emission mass is produced during
, thus highlighting the significance of the emission behavior during the cold-start phase. The time
is generally very short for CO and varies in a narrow range of 1.6
(case
SAEHC) to 2.7
(case
REF). For the emissions of THC and NO
x, a more clear distinction between the strategies is observed. The maximum reduction occurs for the case
COMB where the value of
amounts to 3.4
(THC) and 3.5
(NO
x), respectively. This result represents the maximum relative reductions compared to the case
REF of about
% (THC) and
% (NO
x). Notably, the case
AFR seems to bring the emissions below the threshold for
more quickly than the case
SA. However, the emission mass
is lower for the case
SA. This confirms the assumption made at the beginning of the chapter that the
AFR strategy changes the temporal characteristic of the emission reduction. In contrast, the
SA strategy reduces the raw engine out emissions and accelerates the TWC warm-up. Thus the overall level of emission concentrations is reduced and conversion effects in the TWC start earlier.
The value of
represents the cumulated emission mass for each individual case, i.e., until the respective
. Calculating the cumulated emission mass for each case until
(15.2
, 2.7
and 12.5
for NO
x, CO and THC, respectively) sheds light on the emission mass reduced compared to the reference case. In case
COMB the largest relative reduction in emission mass can be achieved with
% (NO
x),
% (THC) and
% (CO). The ratio
represents the share of the emission mass released during the cold-start phase, i.e., during
, relative to the emission mass released during an operation of 60
.
Figure 8 shows that these values decrease from 97% (NO
x), 975% (THC) and 74% (CO) for the reference case down to 70% (NO
x), 83% (THC) and 51% (CO) for case
COMB. This demonstrates that the cold-start emissions are still dominant, but that the large disparity to the steady-state emissions could be attenuated by applying the strategies developed. The EHC shows little influence on the pollutant emissions compared to the strategies applied. However, CO is converted earlier, i.e., at lower temperatures, inside the TWC than NO
x and THC [
27]. As the EHC is switched on at startup, its additional heat energy accelerates the warm-up of the TWC such that there is a positive effect on the reduction of CO emissions. However, the effects of the
SA strategy to reduce the raw engine out emissions and accelerate the TWC warm-up due to higher exhaust gas temperatures surpasses the effect of the EHC, thereby mitigating its benefits to a large extent.
Figure 9 shows the accumulated emission masses
for the entire cold-start procedure of a duration of 60
. The relative reductions compared to the reference case are nearly identical to the results obtained until
. This confirms that
is a valid criterion for evaluating the effectiveness of a cold-start strategy. As before, the most substantial improvement (case
COMB) is achieved in the reduction of the NO
x emissions where a total of 91% less emission mass was released to the environment. This strategy also reduces the THC mass released by about 71%. Both the significant decrease in NO
x mass produced and the reduction of THC emissions can be attributed (1) to lower raw emissions and a faster warm-up of the TWC (
SA strategy) and (2) to the adaption of the exhaust gas composition to the state of the TWC and therefore a substantially lower
(
AFR strategy).
The electric power output is an important benchmark for comparing combined heat and power plants. Particularly in the mini and micro power classes, losses of several hundreds of
in electrical power potentially create large differences in terms of electrical efficiency and subsequently the efficiency of the entire plant. The developed
SA strategy and the EHC are two approaches that directly affect the electrical output power of the mCHP. The
SA strategy leads to a loss in electrical power by about 6%, whereas enabling the EHC results in a reduced electrical power by about 9% for the time of activation.
Figure 10 shows the calculated energy-specific pollutant masses. For the reference case with EHC,
REFEHC, the increase in specific emissions due to EHC operation is only observable during the first 8
. Once the EHC is disabled after approximately 10
, the specific emissions of the case
REFEHC are already lower than those of the case
REF. As both produce about the same level of raw engine-out emissions, this reduction can be attributed to the faster warm-up of the catalyst due to EHC operation. In case of late spark timing, the peak value of the specific emissions of the case
SA is 56% lower than in the reference case. As
is also reduced drastically, the positive effect on the reduction of pollutant emissions due to late spark timing exceeds by far the drawback of the loss in electrical power.
To place these results into context with the legal framework for pollutant emissions permitted,
Figure 11 shows the mean emission concentrations for the entire cold-start procedure. Additionally, the legal limits in effect for Germany (
TA [
15]) and Switzerland (
LRV [
16]) are listed. These limits are defined for steady-state operation with a warm TWC. Furthermore, in the case of NO
x emissions, the considerably stricter Swiss city limit of
mg/m
[
38], denoted as
LRV+, is specified instead of the limit that is set in the federal regulation (
LRV). The CO concentrations are far below the regulatory limits for all cases. As of 2020, THC emissions are not regulated. However, as the best case (
COMB) produces a mean concentration of just
mg/m
during the cold-start procedure and of about
mg/m
during steady-state operation, these emission levels are likely to meet future regulations. The most substantial reduction concerns the NO
x emissions. This is reflected in
Figure 11 in a reduction of about 91%. Therefore, with the most profitable case
COMB, the mCHP investigated emits a mean NO
x concentration during a cold-start procedure that falls below the strict limit
LRV+ of
mg/m
that is effective for steady-state conditions.
For the system under consideration, the THC emissions produced are almost exclusively unburned methane (CH
4), which is a greenhouse gas (GHG). A common method to evaluate the environmental impact of these GHG emissions is to convert the THC emissions, which in this context are presumed to be pure CH
4 emissions, to CO
2 equivalents and compare them to the unavoidable CO
2 emissions resulting from the sheer combustion of fuel. For this comparison, a mean fuel mass flow of
/
is assumed. Multiplied by a specific mass of CO
2 of
CO
2/
CH4 this results in a production rate of CO
2 of
/
. A second multiplication by the time period considered (
or 60 min) yields the CO
2 mass due to fuel combustion. The equivalent CO
2 mass is calculated with the cumulated THC-emission for the same two time periods, until
and for the entire test procedure of 60 min, respectively (see
Figure 8 and
Figure 9). The CO
2 equivalence factor used in this context is the global warming potential (GWP) of CH
4 over 100 years (GWP = 34) [
39].
Table 4 lists the calculated results with respect to the time period used. The equivalent CO
2 mass from the CH
4 emissions, i.e., THC emissions, amounts to a maximum of about
% (case
AFR) and to a minimum of
% (case
SAEHC) of the CO
2 mass due to fuel combustion during the time period
. However, as
is different for each case (see
Figure 8), it is more reasonable to compare the strategies with the uniform operating time of 60 min. For this time basis, the CO
2 equivalent of the CH
4 emissions constitutes from
% (case
REF) down to
% (case
COMB and
COMBEHC). These shares are very low such that the global warming effect due to emission of a CO
2 equivalent can be deemed negligible. Even when using the higher GWP of CH
4 for a horizon of 20 years (GWP = 86) [
39], the minimum and maximum values increase to just
% and
% for
and
% and
% for
, respectively. Therefore, the THC emissions due to cold-start operation do not significantly enlarge the environmental footprint of the mCHP under consideration. However, the strategies investigated contribute to reduce these emissions even further.
To summarize the system-level results of this study, we can state that a combination of the SA and AFR strategies bears great potential to reduce the pollutant emissions released during a cold-start. However, an application of an electrically heated catalyst cannot be recommended, because of a high price and relatively small benefit compared to both software-based control solutions. The SA strategy is found to be defined by system boundaries. In general, the spark advance should be as low as possible, i.e., the gas mixture should be ignited very late in the compression phase because our results show that the degree of emission reduction justifies the loss in mechanical power. However, the lower bound for the spark advance at engine start is defined by either knocking constraints or combustion instabilities which lead to misfires or even an unsuccessful engine start. As a late ignition generates very high exhaust gas temperatures, temperature constraints on the exhaust path material define the further course of the spark advance trajectory during the warm up process. We recommend to end the SA strategy as soon as the TWC reaches its steady-state operation temperature, i.e., set the ignition timing to MBT timing to maximize the power output of the CHP. The AFR strategy is flanked on one end by the optimal steady-state reference air/fuel ratio that generates the lowest steady-state pollutant emissions. The exact value has to be calibrated. On the other end the AFR strategy is defined by a reference air/fuel ratio setpoint at startup. This value leaves some flexibility depending on the application. In our study, we chose this value to generate a little higher CO emissions in favor of very low NOx emissions, as there are more stringent limits on nitrogen oxide emissions in most regulations. This guideline can be adapted to CHPs with a similar operation during startup and coasting modes which are characterized by pure air being pumped through the exhaust path such that the O2 storage of the TWC is filled to capacity. The results further show that the CO2 equivalence of the methane slip during a cold-start is negligible compared to the CO2 mass due to sheer combustion.